@article{abbott_yuan_2024, title={A simple image correlation technique for imaging subsurface damage from low-velocity impacts in composite structures}, ISSN={["1741-3168"]}, DOI={10.1177/14759217241228884}, abstractNote={A robust computer vision system is proposed to visualize subsurface barely visible impact damage (BVID) in composite structures through a simple image correlation technique together with a damage imaging condition. This system uses a digital camera to record a video of the surface motion, capturing micron-scale dynamic movement from guided waves propagating on the surface of the structure generated via a sweeping frequency excitation (chirp) up to the ultrasonic frequency range. As the excitation frequency changes during the chirp, waves become trapped within this damaged region, forming standing waves to generate local resonance at specific frequencies. This localized resonance accumulates high wave energy in the region, generating a higher transverse displacement at the damage site compared to the remaining part of the structure. In this work, a simple image correlation technique is proposed to correlate each filtered video frame with the temporal mean of the filtered wavefield video to highlight standing waves at localized damage. The proposed image correlation technique is distinct from digital image correlation (DIC) since it does not use correlation for subset matching to track the movement of surface patterns between two images (video frames). Instead, it uses correlation to directly quantify similarities between corresponding image pixels (windows). Utilizing a single camera greatly simplifies system complexity and hence enhances the practicality and potential for real-time performance over a recently developed technique that utilized 3D DIC with a stereo camera for vision-based BVID detection. To realize the aim, work was conducted in two sequential steps: (1) off-axis 2D DIC was employed rather than 3D DIC to examine the potential of employing a single camera to capture images and extract not only in-plane displacement but also a fraction of transverse displacement in which local resonance is dominant and (2) image correlation was then employed, supplanting the off-axis 2D DIC image processing involved in the first step, to highlight the damage with significantly less processing complexity. This proposed technique using a zero-mean normalized cross-correlation imaging condition, rather than the total wave energy used for DIC-based approaches, is efficient and effective for identifying regions of minute surface movement from local resonance within the damage region without the use of the computationally intensive interpolation to get the sub-pixel gray level information followed by subset matching employed in DIC-based image processing. Two geometrically identical CFRP composite honeycomb panels that had been subjected to low-velocity impacts were used for verification and validation, and two excitation location configurations were tested for each panel. Damaged images produced with correlation for a 100 mm × 100 mm field of view using a single 3-s video, or a total of 19,200 video frames, show accurate damage imaging capabilities regardless of excitation location that exceed that of DIC techniques and is comparable to benchmark damage images obtained from laser Doppler vibrometry, ultrasonic C-scans, and X-ray CT scans. This success of correlation-based imaging of subsurface BVID demonstrates substantial improvements in efficiency and practicality and shows high potential for in situ and real-time computer vision-based nondestructive inspection or structural health monitoring of subsurface BVID in composite aircraft and other critical structures.}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Abbott, T. Bryce and Yuan, Fuh-Gwo}, year={2024}, month={Feb} }
 @article{zargar_yuan_2024, title={Physics-informed deep learning for scattered full wavefield reconstruction from a sparse set of sensor data for impact diagnosis in structural health monitoring}, ISSN={["1741-3168"]}, DOI={10.1177/14759217231202547}, abstractNote={This paper presents a physics-informed deep learning framework for the reconstruction of full scattered spatiotemporal Lamb wavefields (video images) in plate-like structures from a sparse set of time-series sensor data. The reconstructed scattered wavefield contains a wealth of information about the wave propagation phenomenon including any interactions of the propagating wave with damage in the structure. This information is paramount for damage diagnosis as is demonstrated in this paper via impact diagnosis—a key structural health monitoring application. A physics-informed neural network (PINN) that encodes the underlying elastodynamic field equations into the learning/training process in the neural network is proposed for this purpose. This prior wavefield physics knowledge embedded in the loss function acts as a regularization agent for the minimization problem in the neural network training, thereby enabling the extrapolation of a sparse set of one-dimensional time-series signals into two-dimensional scattered wavefield. The wavefield reconstruction capabilities of the proposed supervised forward PINN framework are first verified both numerically and experimentally for a stiffened aluminum panel under a couple of narrowband ultrasonic-frequency excitations, and the results confirm its robustness to low spatial resolution and substantial noise in the measured sensor data. The PINN requires far fewer sensors for scattered wavefield reconstruction, thereby permitting for a higher sensor spacing or lower spatial sampling. To this end, it is shown that a sensor spacing of 5λ generates good wavefield reconstruction accuracy, which is a 10-fold increase over the Nyquist–Shannon sampling limit (λ/2). Two sets of experiments are then conducted on a long-stiffened aluminum panel to validate the proposed framework via low-velocity impact diagnosis in the near-ultrasonic frequency range. The first set of experiments, with the known excitation force incorporated into the PINN, allows the wavefields to be accurately reconstructed with the sensor spacing up to 5λ as expected. The second set of experiments assumes unknown impact force history—a classical case of impact diagnosis where the impact force history is not known a priori. It is shown that the wavefield reconstruction through PINN still provides good accuracy albeit with a less generous sensor spacing of 2λ. A convolutional neural network long short-term memory (CNN-LSTM) model then solves the mathematically inverse problem of inferring the impact location and impact force history by analyzing the reconstructed impact generated wavefield. The impact location is predicted well with 93% accuracy, and the impact force history is reconstructed with 90% accuracy, further validating the proposed framework.}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Zargar, Sakib Ashraf and Yuan, Fuh-Gwo}, year={2024}, month={Jan} }
 @article{jin_yuan_yu_wen_ren_2024, title={Size-dependent longitudinal-transverse mode interaction of fluid-conveying nanotubes under base excitation}, ISSN={["1573-269X"]}, DOI={10.1007/s11071-024-09345-w}, journal={NONLINEAR DYNAMICS}, author={Jin, Qiduo and Yuan, Fuh-Gwo and Yu, Dianlong and Wen, Jihong and Ren, Yiru}, year={2024}, month={Feb} }
 @article{gao_mu_yuan_liu_2023, title={A defect localization method based on self-sensing and orthogonal matching pursuit}, volume={128}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2022.106889}, abstractNote={In conventional structural health monitoring (SHM), a sensor array enables to localize a potential defect by using at least three lead zirconate titanate (PZT) patches. To reduce the vast number of patches needed for large-scaled structure, this paper presents an extremely sparse sensor array with only one single PZT patch, which could actuate and sense simultaneously. Firstly, a half-bridge circuit, referred as a self-sensing circuit is developed with a capacitor connected with the PZT patch, and the capacitance parameter and self-sensing performance are studied subsequently. Then, an orthogonal matching pursuit (OMP)-based sparse decomposition and dispersion removal algorithm is proposed to separate and reconstruct wave packets which are acutely overlapped. Subsequently, a matching strategy is proposed to determine the matching relationship between wave packets and wave paths. Finally, the ellipse-type imaging approach is employed to image the defect location. Two cases: one and two defects respectively are implemented to verify its efficacy. Experimental results illustrate that the proposed self-sensing unit and signal process method could erase the adverse effect of sensor-actuator interval and dispersion characteristic to the localization resolution and accuracy.}, journal={ULTRASONICS}, author={Gao, Yuqing and Mu, Weilei and Yuan, Fuh-Gwo and Liu, Guijie}, year={2023}, month={Feb} }
 @article{abbott_yuan_2023, title={Impact damage imaging for composite structures using guided wave techniques with 3D digital image correlation}, volume={12486}, ISBN={["978-1-5106-6079-3"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2659161}, abstractNote={This paper presents an integrated system capable of damage imaging of barely visible impact damage (BVID) in composite structures. This system applies guided-wave-based structural health monitoring using 3D digital image correlation, or GWSHM-3D DIC to produce a map of subsurface damage using a short video from a stereo pair of synchronized digital cameras. The proposed system overcomes many limitations of previous efforts of GWSHM that used 2D digital image correlation (DIC). First, 3D DIC can capture the higher-amplitude out-of-plane displacements associated with the anti-symmetric wave mode, lowering the spatial resolution requirements of the cameras. Second, a total wave energy (TWE) imaging condition is employed that uses the monogenic signal via a Reisz transform to obtain the local instantaneous amplitude as a contribution to wave energy. This condition can highlight local resonance in the damage region without the need for high frame rates to fully reconstruct the wavefield. With significantly lowered spatial and temporal resolution requirements of the cameras, high-stiffness materials like composites can be inspected or monitored with a larger field-of-view (FOV). Additionally, signal enhancement techniques intended to increase the effective resolution of the camera are no longer necessary, which reduces the data acquisition time from many hours to a few seconds. To demonstrate this integrated dual-camera concept with the TWE imaging condition, the system was used to image damage in a CFRP composite sandwich panel that had been subjected to a low-velocity impact. Initial damage maps produced for a 100-mm ´ 100-mm FOV using a three-second video pair show precise damage imaging ability that is comparable to benchmark ultrasonic and x-ray scans. This efficient and reliable integrated system demonstrated high potential for in-time damage inspection on composite aircraft and other critical structures.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2023}, author={Abbott, T. Bryce and Yuan, Fuh-Gwo}, year={2023} }
 @article{lyathakula_yuan_2023, title={Scalable and portable computational framework enabling online probabilistic remaining useful life (RUL) estimation}, volume={181}, ISSN={["1873-5339"]}, DOI={10.1016/j.advengsoft.2023.103461}, abstractNote={This work demonstrates a framework that enables online prognostics in adhesive joints by estimating the real-time probabilistic remaining useful life (RUL) using ANNs based hybrid physics models and vectorized sequential Monte Carlo (SMC) simulations. The framework is developed by integrating the physics-based damage degradation model and uncertainty quantification (UQ) techniques to estimate both probabilistic fatigue failure life and RUL. The fatigue damage growth (FDG) simulator, a hybrid surrogate model that simulates real-time fatigue degradation in adhesive joints, is used. In the initial set of results, the generalized framework is validated by estimating the probabilistic fatigue failure life using two UQ methods: Markov Chain Monte Carlo (MCMC) and SMC method. The computational results are successfully compared against experimental data. The conventional MCMC sampling methods are inherently serial, which limits the exploitation of the computational speed-up provided by the FDG simulator and hinders the real-time life predictions. The SMC method quantifies the uncertainties by parallelizing the sampling process, significantly reducing computational time and enabling real-time prediction. Next, the generalized framework is used to estimate probabilistic RUL from the fatigue crack propagation data. The parallel SMC method showed very good speedup compared to the MCMC method. To further enhance the computational speed-up with SMC method, vectorized FDG simulations are introduced into the framework and good scalability is achieved. Finally, the portability of the framework is demonstrated by deploying it on the portable Raspberry Pi cluster.}, journal={ADVANCES IN ENGINEERING SOFTWARE}, author={Lyathakula, Karthik Reddy and Yuan, Fuh-Gwo}, year={2023}, month={Jul} }
 @article{abbott_yuan_2023, title={Subsurface impact damage imaging for composite structures using 3D digital image correlation}, ISSN={["1741-3168"]}, DOI={10.1177/14759217231172297}, abstractNote={An integrated system is proposed to visualize subsurface barely visible impact damage (BVID) in composite structures using three-dimensional (3D) digital image correlation (3D DIC). This system uses a pair of digital cameras to record video frames in the field-of-view (FOV) of the structure’s surface, capturing the wavefield generated via chirp excitation in the near-ultrasonic frequency range. Significant pitfalls of previous efforts of damage imaging using two-dimensional DIC have been largely mitigated. First, 3D DIC enables capturing out-of-plane displacements, which are much larger in amplitude versus in-plane displacements that a single camera would be limited to sensing, thus increasing the signal-to-noise ratio. This enhancement in turn increases the sensitivity of the stereo-camera system. Second, a total wave energy (TWE) damage imaging condition is proposed to visualize the local damage region. The monogenic signal obtained via Reisz transform (RT) is employed to compute the instantaneous amplitude, with which the local wave energy can be calculated spatially over time. Since a high displacement amplitude and thus high wave energy will occur in the damage region due to the local resonance, the proposed TWE imaging condition can relax the Nyquist sampling requirement, unlike guided-wave-based structural health monitoring techniques which require fully reconstructing the wavefield and wave modes through sampling that satisfies the Nyquist criterion. As such, a much lower camera frame rate is adequate for the proposed system. Consequently, the maximum spatial resolution of the camera for a given FOV can be achieved at the expense of a reduced frame rate. With the maximized pixel resolution and reduced frame rate for employing the TWE imaging condition, composite structures can be inspected or monitored with a larger FOV. As a result, there is no longer any need to apply signal enhancement techniques, such as sample interleaving, image stitching, or averaging, to increase the effective performance of the camera. Rather than needing thousands of repeated videos for minimizing the incoherent noise, only a single stereo-video with a few seconds of sampling duration is necessary for damage imaging. The use of a powerful piezo-shaker also increases the wave signal amplitude and further enhances sensitivity without permanent adhesion. To demonstrate this stereo-camera concept with the TWE imaging condition, the system was used to image damage in two carbon fiber reinforced polymer composite honeycomb panels, which had been subjected to low-velocity impacts (2 J). For each panel, two excitation configurations were used to verify the robustness of the system. Initial damage maps produced for a 100 × 100-mm FOV using a three-second stereo-video show accurate damage imaging ability that is independent of excitation location and comparable to benchmark damage images computed from laser Doppler vibrometer data and those gathered from ultrasonic and X-ray computerized tomography scans. This efficient and reliable integrated system demonstrated high potential for in-time damage inspection on composite aircraft and other critical structures.}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Abbott, T. Bryce and Yuan, Fuh-Gwo}, year={2023}, month={May} }
 @article{wang_abbott_fong_xu_yuan_2023, title={Vibration-based hidden damage imaging using stereo cameras with digital image correlation}, volume={8}, ISSN={["1741-3168"]}, url={https://doi.org/10.1177/14759217231191102}, DOI={10.1177/14759217231191102}, abstractNote={This paper explores a full-field non-contact optical sensing technique using a stereo camera for imaging hidden damage based on vibration-based damage detection methodology in structural health monitoring. The technique utilizes a pair of digital cameras to capture dynamic operational deflection shapes (ODSs) over the region of interest (ROI) of a structure’s surface via digital image correlation (DIC) when subjected to vibrational excitation. This research overcomes bottlenecks in using high vibration modes for imaging the hidden damage area by (1) applying DIC to operational modal analysis with simple pick-peaking techniques to gather natural frequencies and operational mode shapes in plate structures, while (2) using wavelet analysis to reveal the image of the damage region as a means for baseline-free global damage quantification. In the feasibility study, four cases with two aluminum plates with large damage regions were investigated with a vibration shaker generating a frequency sweep up to 1 kHz. The stereo camera imaged the speckled surface of the plate with white light. Once the dynamic ODSs in the ROI were observed using DIC, the natural frequencies and associated operational mode shapes were extracted using a peak-picking technique in the frequency spectrum. Natural frequencies and operational mode shapes from finite element analysis correlated well with the experimental observations from three-dimensional DIC for all 12 vibration modes respectively. A wavelet transform mode shape curvature (WT-MSC) technique to obtain the modal shape curvature via a two-dimensional continuous wavelet transform with a Mexican Hat analyzing wavelet was then implemented on each of the first 12 vibration mode shapes. A damage image condition that incorporates all weighted wavelet coefficients is proposed to image the damage region. The hidden damage was visualized clearly with WT-MSC, as the technique is much less sensitive to noise than the use of MSC alone, and the use of high vibration modes exhibiting larger mode shape curvatures provided a greater sensitivity for imaging the damage region. Hidden damage regions were successfully visualized in all four cases.}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Wang, Shaohan and Abbott, Trenton Bryce and Fong, Rey-Yie and Xu, Cheryl and Yuan, Fuh-Gwo}, year={2023}, month={Aug} }
 @article{jin_ren_yuan_2022, title={Combined resonance of pulsatile flow-transporting FG nanotubes under forced excitation with movable boundary}, ISSN={["1573-269X"]}, DOI={10.1007/s11071-022-08148-1}, journal={NONLINEAR DYNAMICS}, author={Jin, Qiduo and Ren, Yiru and Yuan, Fuh-Gwo}, year={2022}, month={Dec} }
 @article{ge_xu_yuan_2022, title={Development of Viscoelastic Damper Based on NBR and Organic Small-Molecule Composites}, volume={34}, ISSN={["1943-5533"]}, DOI={10.1061/(ASCE)MT.1943-5533.0004339}, abstractNote={Viscoelastic (VE) damper is usually vulcanized by mixing polymer matrix, additives, and fillers, all of which have a great influence on the viscoelastic response of the material. In this study, a series of novel samples of VE materials have been produced by adding different kinds and amounts of organic small molecule modifiers (AO1035, AO60, and AO80) to neat nitrile-butadiene rubber (NBR). The mechanical properties of VE materials have been tested on a dynamic mechanical analyzer and universal testing tensile machine, and the most excellent formulation was selected for manufacturing VE damper. The dynamic mechanical properties of the VE material in the damper with amplitude and frequency are tested and analyzed. The results indicate that the VE damper has a high energy dissipation capacity. To clarify the dynamic mechanical properties of the novel VE material, a high-order fractional derivative model is proposed, which considers the effects of amplitude and frequency simultaneously based on the energy dissipation mechanism of the novel VE material. Finally, the calculated results using this model are compared with the tested data, which has verified the correctness of the mathematical model.}, number={8}, journal={JOURNAL OF MATERIALS IN CIVIL ENGINEERING}, author={Ge, Teng and Xu, Zhao-Dong and Yuan, Fuh-Gwo}, year={2022}, month={Aug} }
 @article{lyathakula_yuan_2022, title={Fatigue Damage Diagnostics-Prognostics Framework for Remaining Life Estimation in Adhesive Joints}, volume={5}, ISSN={["1533-385X"]}, url={http://dx.doi.org/10.2514/1.j060979}, DOI={10.2514/1.J060979}, abstractNote={This work presents an integrated damage diagnostics–prognostics framework for remaining useful life (RUL) estimation in the adhesively bonded joints under fatigue loading. A matching pursuit algorithm is proposed as the diagnostics technique for estimating the damage extent followed by the fatigue damage growth (FDG) simulator as the predictive model for simulating fatigue degradation. The framework calibrates the FDG simulator by quantifying uncertainties in fatigue model parameters using the damage extent data. Bayesian inference via the Markov chain Monte Carlo method is used to quantify uncertainties and estimate the probabilistic RUL from the quantified uncertainties. The FDG simulator encompasses a physics-based fatigue damage degradation model with an artificial neural network-based hybrid machine-learning model for tracing the damage progression. In the diagnostic technique, ultrasonic guided waves are excited into the structure using a pair of piezoelectric wafers, and the damage extent is quantified by reconstructing the reflected signal from the bond region. The proposed diagnostic technique is verified using the ultrasonic signal obtained from the finite element simulations. The damage prognostics part of the integrated framework is verified by estimating RUL in a mixed-mode failure joint specimen using the experimental fatigue damage progression data. In addition, the integrated framework is then verified by estimating RUL in two adhesively bonded joints: a single lap joint and a tapered single lap joint using Gaussian noise added synthetic data and diagnostic damage extent data.}, journal={AIAA JOURNAL}, publisher={American Institute of Aeronautics and Astronautics (AIAA)}, author={Lyathakula, Karthik Reddy and Yuan, Fuh-Gwo}, year={2022}, month={May} }
 @article{welzen_yuan_fong_2022, title={Hidden damage visualization using laser speckle photometry}, volume={131}, ISSN={["1879-1174"]}, url={http://dx.doi.org/10.1016/j.ndteint.2022.102700}, DOI={10.1016/j.ndteint.2022.102700}, abstractNote={This paper exploits laser speckle photometry (LSP), a full-field non-contact optical-based image analysis technique, for effectively and rapidly imaging hidden damage in structures, rather than with the complex setups in digital speckle pattern interferometry (DSPI) or shearography (SG)-based interferometry. This technique will demonstrate a promising potential for large-area inspection of composite structures in near real time to unearth barely visible impact damage (BVID) which would typically go unnoticed during routine inspections. Three image (processing) algorithms for localizing and then imaging the BVID area were explored: conventional mean squared error (MSE), normalized cross-correlation (NCC), and an index-centered algorithm known as structural similarity index measure (SSIM). When implementing these algorithms in LSP, a pre-processing step of selecting a window size (subregion size) for locally correlating the images to localize the damage and estimate its size was performed as an advancement on the previous pixel-by-pixel correlations made with LSP. A trade-off strategy between two perceptual-based metrics, image fidelity and image sharpness/blurriness, was implemented to evaluate an appropriate range for window sizes followed by the image algorithms for creating high contrast imaging of damage regions. From the correlation map, the strategy was carried out to mitigate image noise caused by the camera (image) sensors and speckle patterns dictated by the overall root-mean-square deviation (RMSD) while maintaining a high level of sharpness characterized by the magnitude of the third-level discrete wavelet transform. The proposed image algorithms in conjunction with the appropriately selected window size served as imaging conditions in the context of laser speckles for the first time and were tested on BVID in an impacted honeycomb composite panel under thermal excitation. A low coherence (high-power) laser for fast screening of a large area, if required, followed by a high coherence (low-power laser) for detailed imaging were used to demonstrate the efficacy of LSP. The damage image region agreed well with a baseline image from the well-established point-by-point CT-scan with all the three image algorithms. Overall, NCC and SSIM performed slightly better than MSE, with SSIM generally being the better of the two. Nevertheless, MSE has its merits with ease of interpretation and implementation. LSP with the proposed imaging conditions shows enormous potential as a real-time non-destructive inspection (NDI) technique not only in the aerospace industry but also in industries such as additive manufacturing where on-line in-situ monitoring is desired for prevalent defects. The real-time inspection using LSP will further allow immediate feedback for process controls.}, journal={NDT & E INTERNATIONAL}, publisher={Elsevier}, author={Welzen, J. and Yuan, F. G. and Fong, R. Y.}, year={2022}, month={Oct} }
 @article{fong_yuan_2022, title={Phase estimation via riesz transform in laser speckle interferometry for large-area damage imaging}, volume={132}, ISSN={["1879-1174"]}, url={https://doi.org/10.1016/j.ndteint.2022.102711}, DOI={10.1016/j.ndteint.2022.102711}, abstractNote={A direct phase estimation (DPE) via Riesz transform from a speckle pattern (images) for visualizing barely visible impact damage (BVID) occurring in layered composites is proposed in this paper. Rather than complex optical hardware setup for phase extraction in commercial digital speckle pattern interferometry (DSPI) or shearography (SG) systems, the proposed numerical (software) DPE implementation via Riesz transform with a log-Gabor filter enables a compact, portable, and cost-effective system for large-area real-time nondestructive inspection (NDI) due to its simplicity, stability, and efficiency. Using speckles in laser speckle interferometry (LSI) speckle pattern is intrinsically noisy; the correlation-based phase signatures derived from Riesz transform followed by the log-Gabor filter is implemented to supplant the conventional phase-difference algorithm to localize damage regions, especially in the case where the high-power diode laser (low coherence) can cause serious phase drift. By enveloping the monogenic signal through Riesz transform for direct phase retrieval is more intuitive than TPS or SPS. This yields visualization of the damage feature with higher fidelity. Consequently, a phase-correlation algorithm becomes a more robust operation to cope with speckle images from the interferometry. This full-field optical SG system was demonstrated and verified in an impacted damaged honeycomb composite panel using the proposed phase-correlation algorithm under thermal stress loading for large-area inspection in near real-time. The visualized damage images enhanced by principal component analysis (PCA) agreed with images from pulse laser/LDV and X-ray CT scan. The DPE algorithm has a promising potential to apply on any optical metrology for instantaneous phase estimation and can combine with selected imaging conditions for multi-damage detection.}, journal={NDT & E INTERNATIONAL}, publisher={Elsevier}, author={Fong, Rey-Yie and Yuan, Fuh-Gwo}, year={2022}, month={Dec} }
 @article{ge_xu_yuan_2022, title={Predictive Model of Dynamic Mechanical Properties of VE Damper Based on Acrylic Rubber-Graphene Oxide Composites Considering Aging Damage}, volume={35}, ISSN={["1943-5525"]}, DOI={10.1061/(ASCE)AS.1943-5525.0001385}, abstractNote={Because shock absorbers are an important component of high-rise buildings, it is essential to be able to detect damage to them. Viscoelastic (VE) dampers, as a common shock absorber, directly affect the safety and reliability of VE materials over the entire life of structures. This study is aimed at optimizing a high-dissipation VE material with excellent aging resistance. The VE damper was developed after aging of anti-aging and high-dissipation VE materials. To analyze the damping and mechanical properties of the VE dampers in detail, mechanical performance testing was conducted on VE dampers under various loading conditions, demonstrating that dampers had excellent mechanical properties and confirming the anti-aging properties of the VE materials. In addition, a fractional standard linear solid model based on damage modification is proposed, and aging is considered in the mathematical model of VE dampers as an indispensable factor. After the simulated value is compared to the experimental results, it is found that the fractional mathematical model that involves the aging factor precisely describes the real-time mechanical properties of the VE damper.}, number={2}, journal={JOURNAL OF AEROSPACE ENGINEERING}, author={Ge, Teng and Xu, Zhao-Dong and Yuan, Fuh-Gwo}, year={2022}, month={Mar} }
 @article{jin_yuan_ren_2022, title={Resonance interaction of flow-conveying nanotubes under forced vibration}, ISSN={["1619-6937"]}, DOI={10.1007/s00707-022-03425-x}, journal={ACTA MECHANICA}, author={Jin, Qiduo and Yuan, Fuh-Gwo and Ren, Yiru}, year={2022}, month={Nov} }
 @article{lyathakula_yuan_2021, title={A probabilistic fatigue life prediction for adhesively bonded joints via ANNs-based hybrid model}, volume={151}, ISSN={["1879-3452"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85108536903&partnerID=MN8TOARS}, DOI={10.1016/j.ijfatigue.2021.106352}, abstractNote={The paper is aimed at developing an efficient and robust probabilistic fatigue life prediction framework for adhesively bonded joints. This framework calibrates the fatigue life model by quantifying uncertainty in the fatigue damage evolution relation using a set of experimental fatigue life data. Probabilistic assessment of fatigue life is simulated through damage evolution along the bondline and Bayesian inference via the Markov chain Monte Carlo (MCMC) sampling method for inverse uncertainty quantification (UQ). To expedite the fatigue life simulation, a hybrid model composed of physics-based fatigue damage evolution relation and a data-driven artificial neural networks (ANNs) model is employed. The degradation of the adhesive is evaluated by the fatigue damage evolution relation which is then mapped to the strain redistribution along the bondline using the ANNs model. Once the mapping is learned by the ANNs, through data from FEA simulations, the probabilistic fatigue life prediction framework involves three successive modules: (I) fatigue damage growth (FDG) simulator, (II) uncertainty quantification (UQ), and (III) confidence bounds for fatigue life prediction. The FDG simulator can be used for simulating fatigue degradation rapidly for a given geometric configuration under any arbitrary fatigue loading spectra. The quantified uncertainties from the framework correspond to the intrinsic statistical material properties that can be used for probabilistic fatigue life prediction in any joint configuration with the same adhesive material. The probabilistic framework is verified using a single lap joint (SLJ) by quantifying uncertainties which are then used for probabilistic fatigue life prediction in laminated doublers in the bending (LDB) joint, that uses the same adhesive material as SLJ, and successfully compared with experimental data. The framework is also tested and validated by estimating probabilistic fatigue life in other joint configurations under constant and variable amplitude fatigue loading spectra.}, journal={INTERNATIONAL JOURNAL OF FATIGUE}, author={Lyathakula, Karthik Reddy and Yuan, Fuh-Gwo}, year={2021}, month={Oct} }
 @article{fong_yuan_2021, title={An In-situ Real-time Hidden Damage Inspection on C-17 Globemaster III Composite Aileron using LSP Technique under Thermal Excitation}, volume={11591}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85108636709&partnerID=MN8TOARS}, DOI={10.1117/12.2585247}, abstractNote={A non-contact, full-field vision-based non-destructive inspection (V-NDI) system was developed with multiple damages detection capabilities in composite structures under thermal excitation. In contrast to point-based nondestructive inspection (P-NDI) systems employing laser Doppler vibrometer (LDV) with discrete wavefield captured by pointwise scanning, the V-NDI system captures higher spatial resolution wavefield by a CMOS camera for every time instance without repeating the experiment tens of thousand times to reassemble the wavefield like P-NDI. An Advanced Damage Processing Network (ADPNet) was proposed with laser speckle photometry (LSP) employed in a V-NDI system for hidden damages inspection. The LSP/ADPNet system relies on observing the variation of speckle clouds in time sequence without a baseline and is very insensitive to ambient noise with statistics-based image processing where traditional holography/ESPI suffers greatly. Other advances of LSP/ADPNet system are its robust tolerance of laser coherence, larger illumination area, flexible choice of correlation functions, and more advanced post-processing techniques such as Bayesian updating/inference or unsupervised image segmentation that can be readily applied. Thermal excitation can have very large power throughput in hundreds of watts compared to traditional PZT actuator, merely in a few watt ranges. Laser speckle itself is the result of self-interference scatter field reflected from a rough surface, and each speckle can be treated as a sensing point from a randomly distributed speckle cloud. By observing the variation of speckle cloud on the structure surface, displacement related quantities in higher dimensions (e.g. hypercomplex envelope, phase between real-valued signal and its quadrature, phase congruency, etc.) can be deduced by the Riesz bp transform and correlated in time sequence to highlight the location of hidden damages in a very effective way. Another novelty of this paper is to make a super compact real-time LSP system on LabVIEW FPGA by applying ADPNet comprised of the Riesz bp transform, non-linear filter bank and unsupervised image segmentation to quantify/characterize barely visible impact damages (BVID) on a C-17 Globemaster III composite aileron. To conclude, the images processed by LSP/ADPNet of a V-NDI system show a very good agreement with ultrasonic C-scan and pulse laser/LDV wavefield reconstruction results. It is also demonstrated to be more accurate and robust than Digital Image Correlation (DIC) for minute deformation (sub-nano to nano meter) measurement and large area (meter by meter) inspection under industrial environment.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, author={Fong, Rey-Yie and Yuan, Fuh-Gwo}, year={2021} }
 @article{wang_zargar_yuan_2021, title={Augmented reality for enhanced visual inspection through knowledge-based deep learning}, volume={20}, ISSN={["1741-3168"]}, DOI={10.1177/1475921720976986}, abstractNote={A two-stage knowledge-based deep learning algorithm is presented for enabling automated damage detection in real-time using the augmented reality smart glasses. The first stage of the algorithm entails the identification of damage prone zones within the region of interest. This requires domain knowledge about the damage as well as the structure being inspected. In the second stage, automated damage detection is performed independently within each of the identified zones starting with the one that is the most damage prone. For real-time visual inspection enhancement using the augmented reality smart glasses, this two-stage approach not only ensures computational feasibility and efficiency but also significantly improves the probability of detection when dealing with structures with complex geometric features. A pilot study is conducted using hands-free Epson BT-300 smart glasses during which two distinct tasks are performed: First, using a single deep learning model deployed on the augmented reality smart glasses, automatic detection and classification of corrosion/fatigue, which is the most common cause of failure in high-strength materials, is performed. Then, in order to highlight the efficacy of the proposed two-stage approach, the more challenging task of defect detection in a multi-joint bolted region is addressed. The pilot study is conducted without any artificial control of external conditions like acquisition angles, lighting, and so on. While automating the visual inspection process is not a new concept for large-scale structures, in most cases, assessment of the collected data is performed offline. The algorithms/techniques used therein cannot be implemented directly on computationally limited devices such as the hands-free augmented reality glasses which could then be used by inspectors in the field for real-time assistance. The proposed approach serves to overcome this bottleneck.}, number={1}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Wang, Shaohan and Zargar, Sakib Ashraf and Yuan, Fuh-Gwo}, year={2021}, month={Jan}, pages={426–442} }
 @article{welzen_yuan_fong_2021, title={Comparison of Image Correlation Algorithms for Hidden Damage Laser Speckle Photometry}, volume={11591}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85108612487&partnerID=MN8TOARS}, DOI={10.1117/12.2585921}, abstractNote={This paper explores laser speckle photometry (LSP), a recent optical-based image analysis tool, as a method for detecting barely visible impact damage (BVID) in composite structures. This non-contact optical-based method provides the potential for large-scale scanning of aircrafts in real time to unearth BVID which would typically go unnoticed during routine inspections. In exploring LSP, various image correlation algorithms were tested to determine the most effective. The typical error-based correlation algorithm of mean squared error (MSE) was compared to two more advanced algorithms, normalized cross-correlation (NCC) and structural similarity (SSIM) index. Thermal LSP was conducted on a composite honeycomb panel with a surface dent of maximum depth 0.5 mm. The underlying damage was around 30 mm in diameter. When compared to the baseline experiments conducted using C-Scan and a laser Doppler vibrometer (LDV), it was found that only with limited cooling (around 2 seconds) did the algorithms produce consistently accurate results. Among the three imaging conditions, MSE and certain configurations of SSIM provided results that aligned the best with the baselines. Upon extended cooling, LSP produces too many false positives to be useful regardless of correlation algorithm used. Nevertheless, LSP shows great promise as a real-time non-destructive inspection tool not only in the aerospace industry but also in industries such as additive manufacturing where defects are prevalent.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, author={Welzen, J. and Yuan, F. G. and Fong, R. Y.}, year={2021} }
 @article{lyathakula_yuan_2021, title={Fatigue Damage Prognosis of Adhesively Bonded Joints via a Surrogate Model}, volume={11591}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85106139756&partnerID=MN8TOARS}, DOI={10.1117/12.2585280}, abstractNote={This paper demonstrates a diagnostic-prognostics framework to estimate probabilistic remaining useful life (RUL), in adhesively bonded joints subjected to fatigue loading, by calibrating the predictive model using the diagnostics data and quantifying uncertainty in the model parameters. The matching pursuit algorithm is used as the diagnostic method to measure the crack length and the rapid fatigue damage growth (FDG) simulator is used as the predictive model to estimate the remaining useful life (RUL). In the diagnostic method, Lamb waves are excited in the structure using piezo transducers, and the matching pursuit algorithm is used to quantify the damage from the reflected signal. The proposed diagnostic technique is verified using the signal obtained from finite element simulations and artificial noise is added to mimic the signal from a real structure. The diagnostic method is applied periodically to measure the crack length in the single lap joint (SLJ) subjected to fatigue loading and the crack length data is used to calibrate the parameters of the predictive model, which can estimate the RUL. However, the noise in the signal and assumptions in the diagnostic technique result in errors in the measured crack length. These errors in the crack length contribute to the parameter uncertainties during the predictive model calibration. To quantify the model parameter uncertainties, the Bayesian inference via the Markov chain Monte Carlo method is used, and to expedite the uncertainty quantification problem, the rapid FDG simulator is used as the predictive model. The approach is demonstrated using a fatigue damage growth simulation in the SLJ and promising results were achieved.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, publisher={SPIE}, author={Lyathakula, Karthik Reddy R. and Yuan, Fuh-Gwo}, editor={Zonta, Daniele and Huang, Haiying and Su, ZhongqingEditors}, year={2021} }
 @misc{zargar_yuan_2021, title={Impact diagnosis in stiffened structural panels using a deep learning approach}, volume={20}, ISSN={["1741-3168"]}, DOI={10.1177/1475921720925044}, abstractNote={Low-velocity impact on a structure emanates an elastic wave that propagates through the structure carrying a wealth of information about the impact event. This propagating wave can be visualized through a series of images (time-frames in the context of computer-vision) in the time–space domain collectively referred to as the wavefield. An approach for the autonomous analysis of these wavefields is presented in this article for the purpose of impact diagnosis, that is, identifying the impact location and reconstructing the impact force time-history. The high spatio-temporal dimensionality of the wavefield mandates the use of deep neural networks for analysis; however, unlike the traditional object detection problem in computer-vision, the nature of the impact diagnosis problem requires the capturing of context from the wavefield evolution. This necessitates learning across multiple time-frames of the wavefield simultaneously rather than focusing independently on each frame. While scanning simultaneously across multiple time-frames provides indispensable information about the wave propagation phenomenon in terms of its interactions with geometric features, boundaries, and so on, it mandates the use of deep learning models that can analyze this complex phenomenon in both spatial and temporal domains. A unified CNN-RNN network architecture is employed in this article to address this issue of spatio-temporal information extraction. The proposed approach is verified using simulated wavefields obtained from the finite element analysis of a five-bay stiffened aluminum panel. In order to demonstrate the generalization capabilities of the model, simulated wavefields corresponding to highly idealized impact scenarios are used for training, whereas for testing, the ones corresponding to more realistic impacts are used. It is shown that by incorporating the physics-based concept of time-reversal in the recurrent part of the network, better network performance can be achieved. The potential extension of the proposed methodology to an end-to-end vision-based impact monitoring system is also discussed at the end.}, number={2}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Zargar, Sakib Ashraf and Yuan, Fuh-Gwo}, year={2021}, month={Mar}, pages={681–691} }
 @article{lyathakula_yuan_2021, title={Probabilistic Fatigue Life Prediction for Adhesively Bonded Joints via Surrogate Model}, volume={11591}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85108669736&partnerID=MN8TOARS}, DOI={10.1117/12.2585281}, abstractNote={The paper is aimed at developing a probabilistic framework for fatigue life prediction in adhesively bonded joints by calibrating the predictive model, governing adhesive fatigue behavior, using the set of experimental data, and quantifying uncertainty in the model parameters. A cohesive zone model (CZM) is employed to simulate the fatigue damage growth (FDG) along the adhesive bondline and Bayesian inference is used for uncertainty quantification (UQ). The fatigue behavior predicted by FEA modeling for high cycle fatigue, in particular, is computationally intractable, not to mention the inclusion of UQ. To enhance the computational efficiency and yet retain accuracy, a rapid FDG simulator is developed for adhesively bonded joints, by replacing the computationally intensive strain field calculations with the artificial neural networks (ANNs) based surrogate model. The developed rapid FDG simulator is integrated with Bayesian inference and the integrated framework is verified by quantifying uncertainty in fatigue model parameters using the experimental fatigue life data of a single lap joint (SLJ) configuration under constant amplitude fatigue loading. The quantified parameter uncertainties are then used to predict the probabilistic fatigue life in the laminated doublers in bending joint configuration, fabricated using similar adhesive material as SLJ, and successfully comparing it with the experimental data.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, author={Lyathakula, Karthik Reddy and Yuan, Fuh-Gwo}, year={2021} }
 @article{ge_xu_yuan_2021, title={Study on Experiment and Modeling of Viscoelastic Damper Considering Interfacial Effect of Matrix Rubber/Carbon Black}, volume={143}, ISSN={["1528-8889"]}, DOI={10.1115/1.4050848}, abstractNote={
 Viscoelastic (VE) dampers are a kind of effective passive vibration control device and widely used to attenuate structural vibration. In this article, experimental study and multiscale modeling analysis on the VE damper for reducing wind-excited vibration are carried out. First, an experimental study on VE damper is conducted to reveal the dynamic properties of VE damper. The experimental results show that the dynamic properties of VE material are influenced by excitation frequency and insignificantly affected by displacement amplitude, and the VE material has good energy dissipation capacity. Second, the damping mechanism of VE damper is analyzed from micro-perspectives by considering the influence of cross-linked and free molecular chain networks. Then, a novel type spherical chain network model based on the chain network microstructure is proposed. The proposed model is verified by comparing the experimental data and the mathematical results, which indicates that the proposed model can accurately describe the dynamic properties of VE damper affected by different temperatures, frequencies, and displacements.}, number={4}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Ge, Teng and Xu, Zhao-Dong and Yuan, Fuh-Gwo}, year={2021}, month={Oct} }
 @article{wang_fong_yuan_2021, title={Vibration-based Damage Imaging via High-Speed Cameras with 3D Digital Image Correlation using Wavelet Transform}, volume={11591}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85108648763&partnerID=MN8TOARS}, DOI={10.1117/12.2585246}, abstractNote={Advances in computer prerformance, optical devices and digital imaging technology have allowed digital image correlation methods to measure the shape and deformation of the vibrating structures. This paper presents a novel vision-based damage detection framework which can eliminate the sinusoidal shaker step along with a magnified, precise modal shape data especially for higher modes that typical DIC algorithm can hardly detect. For this, an aluminum plate has been selected to make a comparative study on the performance of serval vibration-based damage detection techniques, namely, curvature mode shapes , Gapped smoothing techniques Generalized Fractal Dimension, Modal Strain Energy and Wavelet Transform.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, author={Wang, Shaohan and Fong, Rey-Yie and Yuan, Fuh-Gwo}, year={2021} }
 @article{kim_yuan_2020, title={Adaptive signal decomposition and dispersion removal based on the matching pursuit algorithm using dispersion-based dictionary for enhancing damage imaging}, volume={103}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2020.106087}, abstractNote={This paper aims to develop a method for high-resolution damage imaging for a sparsely distributed sensor network on a plate-like structure. Techniques for dispersion removal and signal decomposition are indispensable to accurate damage localization. By combining the dispersion-removed wave packets with the damage-imaging algorithm, a point-like damage can be precisely localized. In this article, a matching pursuit algorithm was utilized to decompose overlapping wave packets and then recompress the dispersion. The matching pursuit dictionary was constructed based on an asymptotic solution of the dispersion relation for Lamb waves in toneburst wave packets. The dispersion-based Hanning-window dictionary provided the parametric information for the extracted wave packets, such as propagation time-delay, dispersion extent, and phase. The parameters were leveraged for the dispersion-removal algorithm. Results of the simulation indicate that the proposed algorithm is capable of recompressing multiple dispersive wave packets with the different modes. Finally, the proposed approach was validated by the results of the experiment using a sparse array of piezoelectric wafers on an aluminum plate. Extracting the parameters of individual wave packets and removing the dispersion through matching pursuit, the algorithm for minimum-variance imaging produced a high-quality image with a fine spatial resolution. The image artifacts were significantly suppressed, and the accuracy was improved by 62.1% compared to conventional minimum-variance imaging.}, journal={ULTRASONICS}, author={Kim, Howuk and Yuan, Fuh-Gwo}, year={2020}, month={Apr} }
 @article{yuan_zargar_chen_wang_2020, title={Machine Learning for Structural Health Monitoring: Challenges and Opportunities}, volume={11379}, ISSN={["1996-756X"]}, DOI={10.1117/12.2561610}, abstractNote={A physics-based approach to structural health monitoring (SHM) has practical shortcomings which restrict its suitability to simple structures under well controlled environments. With the advances in information and sensing technology (sensors and sensor networks), it has become feasible to monitor large/diverse number of parameters in complex real-world structures either continuously or intermittently by employing large in-situ (wireless) sensor networks. The availability of this historical data has engendered a lot of interest in a data-driven approach as a natural and more viable option for realizing the goal of SHM in such structures. However, the lack of sensor data corresponding to different damage scenarios continues to remain a challenge. Most of the supervised machine-learning/deep-learning techniques, when trained using this inherently limited data, lack robustness and generalizability. Physics-informed learning, which involves the integration of domain knowledge into the learning process, is presented here as a potential remedy to this challenge. As a step towards the goal of automated damage detection (mathematically an inverse problem), preliminary results are presented from dynamic modelling of beam structures using physics-informed artificial neural networks. Forward and inverse problems involving partial differential equations are solved and comparisons reveal a clear superiority of physics-informed approach over one that is purely datadriven vis-à-vis overfitting/generalization. Other ways of incorporating domain knowledge into the machine learning pipeline are then presented through case-studies on various aspects of NDI/SHM (visual inspection, impact diagnosis). Lastly, as the final attribute of an optimal SHM approach, a sensing paradigm for non-contact full-field measurements for damage diagnosis is presented.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2020}, author={Yuan, Fuh-Gwo and Zargar, Sakib Ashraf and Chen, Qiuyi and Wang, Shaohan}, year={2020} }
 @article{chang_yuan_2020, title={Visualization of hidden damage from scattered wavefield reconstructed using an integrated high-speed camera system}, volume={10}, ISSN={["1741-3168"]}, url={https://doi.org/10.1177/1475921720940805}, DOI={10.1177/1475921720940805}, abstractNote={In this article, a feasibility study for the visualization of hidden damage using an integrated high-speed camera system was carried out. A thin, planar, and low-modulus high-density polyethylene plate with surrogate damage was chosen to represent a damaged structure for the proof of concept, and two different damage scenarios (mimicked by attaching lightweight rectangular/circular masses to the back of the plate) were investigated. The acoustic/ultrasonic guided waves were generated in the plate by a surface-mounted piezoelectric actuator under continuous sinusoidal excitation, and in-plane wavefield displacements on the surface of the plate were captured using a high-speed camera. In order to reconstruct the scattered wavefield, these in-plane wavefields which primarily include the fundamental symmetric wave mode S0 and fundamental shear horizontal wave mode SH0 (induced due to reflection/scattering of the incident S0 wave mode from the damage and plate boundaries) were then extracted using digital image correlation image analysis software. All the experimental parameters (e.g. material properties of the plate, excitation frequency, selection of lens, field-of-view, speckle size) were carefully designed, integrated, and optimized. In order to overcome the current hardware limitations (insufficient spatial/temporal resolution), sample interleaving was implemented to artificially enhance the frame rate and image stitching techniques were used to increase the total effective camera resolution. Together, these techniques provided a nearly 250-fold enhancement in the data acquisition capability of the high-speed camera. In order to fully demonstrate the efficacy of the sample interleaving technique, two frequencies were excited: 14 and 28 kHz, below and above the original Nyquist frequency, respectively. The first fundamental SH0 and S0 wave modes for both frequencies were successfully detected and identified, and the disturbances at the damage region were clearly observed in the scattered wavefield reconstructed with the SH0 mode in particular, as the SH0 mode has a shorter wavelength making it better suited for detecting smaller damage. The hidden damage was then visualized by employing a modified version of the phase-based damage imaging condition, wavenumber index, that was previously developed for visualizing hidden delamination damage in composites with a laser Doppler vibrometer scanning system.}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Chang, Huan-Yu and Yuan, Fuh-Gwo}, year={2020}, month={Oct} }
 @article{chen_cai_zhu_liao_qian_yuan_zhang_2019, title={3D printing of electroactive PVDF thin films with high beta-phase content}, volume={28}, ISSN={["1361-665X"]}, DOI={10.1088/1361-665X/ab15b7}, abstractNote={Bi-axially oriented electroactive PVDF thin films were prepared by a 3D printing process during which multiple parameters play a critical role in enhancing β-phase and crystallinity of the thin films. The PVDF molecular chains were rotated and stretched to form orderly arrangement by the pulling of in situ strong auxiliary electric potential (voltage) and the mechanical pulling force, and the aligned PVDF molecular chains improved the crystallinity of the thin films. Doping very small amount of multi-walled carbon nanotubes (MWCNTs) or graphene (GR) as a nucleating agent significantly increased the content of the β-phase in the films. Adding GR behaved is better than the MWCNTs in improving the β-phase due to the lamellar structure of graphene can generate a large number of micro electric fields stimulating the transformation of molecular chain from trans-gauche-trans-gauche(TG+TG−) to all-trans (TTTT). The content of β-phase in the PVDF/GR(0.03 wt%) composite thin film researched at 61.52%. Under the special environment of 3D printing, the printed PVDF thin films became very dense with high β-phase after depositing layer-by-layer. It exhibited good piezoelectric properties without post-poling treatment and mechanical stretching.}, number={6}, journal={SMART MATERIALS AND STRUCTURES}, author={Chen, Caifeng and Cai, Feixiang and Zhu, Yuan and Liao, Linchen and Qian, Jilong and Yuan, Fuh-Gwo and Zhang, Ningyi}, year={2019}, month={Jun} }
 @article{chang_yuan_2019, title={Damage Detection and Localization using Random Decrement Technique on Metallic Plates}, volume={10970}, ISSN={["1996-756X"]}, DOI={10.1117/12.2519313}, abstractNote={Technique with the capability of detecting and localizing damage of structures using naturally operating environments can provide a possibility of developing more efficient and simpler structural health monitoring systems. This passive sensing technique would eliminate the need of active actuation which requires power either from battery or ambients to generate controlled excitation source. In a recent study, self-Green’s functions (GF) were reconstructed using auto-correlation (AC), combined with a damage index by comparing the differences in GFs between damaged and pristine metallic panels to locate the damage. In this paper, random decrement (RD) technique is proposed to reconstruct GF with computational efficiency. While the RD has been widely used for damage detection and structure parameter extraction in civil structures, in the frequency usually below 1 kHz; this study explores using RD up to 15 kHz for transient wave reconstruction and then damage localization. The concept is first validated through simulation for a plate structure, and the results show that the reconstructed self-Green’s function match well with the one from the auto-correlation technique after approximately 10,000 averages of the RD signatures.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2019}, author={Chang, YuSheng and Yuan, Fuh-Gwo}, year={2019} }
 @article{liu_jiang_ren_qian_yuan_2019, title={Energy-absorption performance of composite corrugated plates with corrugated-shape ditch plug initiator}, volume={40}, ISSN={["1548-0569"]}, DOI={10.1002/pc.24924}, abstractNote={To improve the energy‐absorbing capacity of composite corrugated plate, the axial impact responses under axial quasi‐static crush loading is numerically studied and compared. Both the intra‐ply damage and delamination interface damage are considered. To accurately capture the progressive damage propagation, the failure material is modeled with a stiffness degradation method combined with the exponential damage evolution laws. To improve the crush performance, a novel corrugated‐shape ditch plug initiator (CDP) is proposed. The effects of extensive parameters including the ditch width and the inner geometry are carried out. Results show that well agreements between the numerical model and experimental results are identified on failure modes and energy‐absorption characteristics. The corrugated plate with CDP can increase the specific energy absorption by 20.3% than non‐plug initiator. The total of energy absorption is the decreasing function with respect to the width of ditch. The crushing performance can be improved due to the change in the inner geometry parameters. POLYM. COMPOS., 40:1708–1717, 2019. © 2018 Society of Plastics Engineers}, number={5}, journal={POLYMER COMPOSITES}, author={Liu, Zhihui and Jiang, Hongyong and Ren, Yiru and Qian, Zhansen and Yuan, Fuh-Gwo}, year={2019}, month={May}, pages={1708–1717} }
 @article{hudson_auwaijan_yuan_2019, title={Guided wave-based system for real-time cure monitoring of composites using piezoelectric discs and phase-shifted fiber Bragg gratings}, volume={53}, ISSN={["1530-793X"]}, DOI={10.1177/0021998318793512}, abstractNote={A real-time, in-process cure monitoring system employing a guided wave-based concept for carbon fiber reinforced polymer composites was developed. The system included a single piezoelectric disc that was bonded to the surface of the composite for excitation, and an embedded phase-shifted fiber Bragg grating for sensing. The phase-shifted fiber Bragg grating almost simultaneously measured both quasi-static strain and the ultrasonic guided wave-based signals throughout the cure cycle. A traditional FBG was also used as a base for evaluating the high sensitivity of the phase-shifted fiber Bragg grating sensor. Composite physical properties (degree of cure and glass transition temperature) were correlated to the amplitude and time of arrival of the guided wave-based measurements during the cure cycle. In addition, key state transitions (gelation and vitrification) were identified from the experimental data. The physical properties and state transitions were validated using cure process modeling software (e.g. RAVEN®). This system demonstrated the capability of using an embedded phase-shifted fiber Bragg grating to sense a wide bandwidth of signals during cure. The distinct advantages of a fiber optic-based system include multiplexing of multiple gratings along a single optical fiber, small size compared to piezoelectric sensors, ability to embed or surface mount, utilization in harsh environments, electrically passive operation, and electromagnetic interference (EMI) immunity. The embedded phase-shifted fiber Bragg grating fiber optic sensor can monitor the entire life-cycle of the composite structure from curing, post-cure/assembly, and in-service creating “smart structures”.}, number={7}, journal={JOURNAL OF COMPOSITE MATERIALS}, author={Hudson, Tyler B. and Auwaijan, Nicolas and Yuan, Fuh-Gwo}, year={2019}, month={Mar}, pages={969–979} }
 @article{she_ren_yuan_2019, title={Hygro-thermal wave propagation in functionally graded double-layered nanotubes systems}, volume={31}, ISSN={["1598-6233"]}, DOI={10.12989/scs.2019.31.6.641}, abstractNote={In this paper, wave propagation is studied and analyzed in double-layered nanotubes systems via the nonlocal strain gradient theory. To the author\'s knowledge, the present paper is the first to investigate the wave propagation characteristics of double-layered porous nanotubes systems. It is generally considered that the material properties of nanotubes are related to the porosity and hygro-thermal effects. The governing equations of the double-layered nanotubes systems are derived by using the Hamilton principle. The dispersion relations and displacement fields of wave propagation in the double nanotubes systems which experience three different types of motion are obtained and discussed. The results show that the phase velocities of the double nanotubes systems depend on porosity, humidity change, temperature change, material composition, non-local parameter, strain gradient parameter, interlayer spring, and wave number.}, number={6}, journal={STEEL AND COMPOSITE STRUCTURES}, author={She, Gui-Lin and Ren, Yi-Ru and Yuan, Fuh-Gwo}, year={2019}, month={Jun}, pages={641–653} }
 @article{she_yuan_karami_ren_xiao_2019, title={On nonlinear bending behavior of FG porous curved nanotubes}, volume={135}, DOI={10.1016/j.ijengsci.2018.11.005}, abstractNote={For the first time, the nonlinear bending behavior of porous functionally graded (FG) curved nanotubes is studied in this paper. The stiffness enhancement and stiffness reduction effects are described by the nonlocal strain gradient theory. The FG curved nanotubes have uniformly distributed pores in the radial direction. The material properties of the nanotubes corresponds to a modified power-law function. The asymptotic solutions of the curved nanotubes is obtained by using the two-step perturbation method. It shows that the nonlinear bending behavior of the curved nanotubes is influenced by the size effects, power law index, porosity distribution, temperature variation, boundary conditions, and physical dimension. Furthermore, the jump changes as well as snap-through buckling can take place when the FG curved nanotubes are under the influence of normal bending loads.}, journal={INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Karami, Behrouz and Ren, Yi-Ru and Xiao, Wan-Shen}, year={2019}, pages={58–74} }
 @article{lee_yuan_fay_chu_park_2019, title={Piezoelectric Characterization of Boron Nitride Nanotube-Polyurethane Composites}, volume={10982}, ISBN={["978-1-5106-2630-0"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2513999}, abstractNote={Boron nitride nanotubes (BNNTs) have exceptional thermal stability, thermal conductivity, mechanical properties, neutron radiation shielding, and piezoelectricity. Due to their multifunctional properties, BNNTs are potential candidates for sensory materials in harsh environments. Brittleness and non-conformity of conventional piezoelectric ceramics have limited their broad applications. Flexible and ultra-light piezoelectric sensors based on BNNTs could be an alternative solution in high temperature, high radiation, high shock, and severe vibration environments. In this study, BNNTPolyurethane (PU) composites were fabricated and their converse piezoelectric constant of d33 was assessed using a laser Doppler vibrometer (LDV). This study demonstrated that BNNT could be an excellent piezoelectric nanofiller for flexible sensor applications.}, journal={MICRO- AND NANOTECHNOLOGY SENSORS, SYSTEMS, AND APPLICATIONS XI}, author={Lee, Dongwon and Yuan, Fuh-Gwo and Fay, Catharine and Chu, Sang-Hyon and Park, Cheol}, year={2019} }
 @article{wang_li_yuan_bao_2019, title={Research on the Actuation Performance of 2D-Orthotropic Piezoelectric Composite Materials Linear Phased Array Transducer}, volume={19}, ISSN={["1533-4899"]}, DOI={10.1166/jnn.2019.16814}, abstractNote={Ultrasonic linear phased array actuator for electric-acoustic conversion is the key component of the ultrasonic phased array test system. Focusing on the existing deficiency of using phased array ultrasonic testing technology into complex engineering structure, using the 2D orthotropic piezoelectric composite materials (2D-OPCM) into the research of linear phased array actuator can achieve orientable exciting. The single OPCM element and its array actuator was designed and their mechanical properties of focusing were tested by experiment. The experimental results show that 2D-OPCM ultrasound phased array actuator can achieve precise phased array focusing and deflection, which might be of great promising in the nondestructive evaluation engineering application.}, number={8}, journal={JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY}, author={Wang, Ziping and Li, Xingjia and Yuan, Fuh-Gwo and Bao, Zhujie}, year={2019}, month={Aug}, pages={5205–5210} }
 @article{palagummi_yuan_2018, title={A bi-stable horizontal diamagnetic levitation based low frequency vibration energy harvester}, volume={279}, ISSN={["0924-4247"]}, DOI={10.1016/j.sna.2018.07.001}, abstractNote={A new form of bi-stable system based on the passive friction-free horizontal diamagnetic levitation mechanism is proposed in this article. The system is comprised of twelve magnets which create a bi-stable potential well for a centrally located floating magnet. The levitation is stabilized in the horizontal direction by the diamagnetic repulsion from pyrolytic graphite plates which are placed on either side of the floating magnet. Theoretical modeling is discussed involving the superposition of the magnetic fields and magnetic forces, which include semi-analytical equations, from the various magnets in the system to determine the characteristics of the bi-stable potential well. Stability equations for achieving static bi-stability and for maintaining stable levitation during intra-well and inter-well motion were discussed. An experimental prototype is presented whose frequency response characteristics were validated for varying input sinusoidal excitations. Once the dynamics of the bi-stable system were validated, copper coils are incorporated into the diamagnetic plates to enable the vibration energy harvesting capability of the levitation mechanism. The floating magnet underwent chaotic and interwell motions for a range of input sinusoidal excitation frequencies, 5.8 Hz – 8 Hz, at input accelerations of 1.99 m/s2 to 3.79 m/s2, and peak system efficiencies of close to 2.5% were achieved.}, journal={SENSORS AND ACTUATORS A-PHYSICAL}, author={Palagummi, S. and Yuan, F. G.}, year={2018}, month={Aug}, pages={743–752} }
 @article{yan_yuan_2018, title={A semi-analytical approach for SH guided wave mode conversion from evanescent into propagating}, volume={84}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2017.12.006}, abstractNote={Conversion of evanescent shear horizontal (SH) guided waves into propagating is presented in this paper. The conversion is exemplified by a time-harmonic SH evanescent displacement prescribed on a narrow aperture at an edge of a semi-infinite isotropic plate. The conversion efficiency in terms of the amplitude of the propagating SH mode converted from evanescent can be expressed in a very simple compact form. The magnitude of the conversion efficiency can be quantified through a derived semi-analytical form based on the complex reciprocity theorem in conjunction with a two-dimensional (2-D) finite element analysis (FEA). Through power conversion analysis, it can be shown that the power flow generated into the plate due to evanescent incident is complex valued. It is theoretically proved that the real part of the complex power flow is associated with the propagating SH modes, while the imaginary part is confined due to the evanescent modes at the plate edge. The conversion efficiency and converted modes are dependent on the geometric configuration of the aperture as well as the selection of the excitation frequency.}, journal={ULTRASONICS}, author={Yan, Xiang and Yuan, Fuh-Gwo}, year={2018}, month={Mar}, pages={430–437} }
 @article{harb_yuan_2018, title={Air-Coupled Nondestructive Evaluation Dissected}, volume={37}, ISSN={["1573-4862"]}, DOI={10.1007/s10921-018-0502-4}, number={3}, journal={JOURNAL OF NONDESTRUCTIVE EVALUATION}, author={Harb, Mohammad Said and Yuan, Fuh-Gwo}, year={2018}, month={Sep} }
 @article{chang_yuan_2018, title={Damage Detection and Isolation via Autocorrelation: A Step toward Passive Sensing}, volume={10599}, ISSN={["1996-756X"]}, DOI={10.1117/12.2303680}, abstractNote={Passive sensing technique may eliminate the need of expending power from actuators and thus provide a means of developing a compact and simple structural health monitoring system. More importantly, it may provide a solution for monitoring the aircraft subjected to environmental loading from air flow during operation. In this paper, a non-contact auto-correlation based technique is exploited as a feasibility study for passive sensing application to detect damage and isolate the damage location. Its theoretical basis bears some resemblance to reconstructing Green’s function from diffusive wavefield through cross-correlation. Localized high pressure air from air compressor are randomly and continuously applied on the one side surface of the aluminum panels through the air blow gun. A laser Doppler vibrometer (LDV) was used to scan a 90 mm × 90 mm area to create a 6 × 6 2D-array signals from the opposite side of the panels. The scanned signals were auto-correlated to reconstruct a “selfimpulse response” (or Green’s function). The premise for stably reconstructing the accurate Green’s function requires long sensing times. For a 609.6 mm × 609.6 mm flat aluminum panel, the sensing times roughly at least four seconds is sufficient to establish converged Green’s function through correlation. For the integral stiffened aluminum panel, the geometrical features of the panel expedite the formation of the diffusive wavefield and thus shorten the sensing times. The damage is simulated by gluing a magnet onto the panels. Reconstructed Green’s functions (RGFs) are used for damage detection and damage isolation based on an imaging condition with mean square deviation of the RGFs from the pristine and the damaged structure and the results are shown in color maps. The auto-correlation based technique is shown to consistently detect the simulated damage, image and isolate the damage in the structure subjected to high pressure air excitation. This technique may be transformed into passive sensing applied on the aircraft during operation.}, journal={NONDESTRUCTIVE CHARACTERIZATION AND MONITORING OF ADVANCED MATERIALS, AEROSPACE, CIVIL INFRASTRUCTURE, AND TRANSPORTATION XII}, author={Chang, Y. S. and Yuan, F. G.}, year={2018} }
 @article{xiang_yuan_he_2018, title={Damage Detection in Composite Structures Using Time-Reversal Migration Technique}, ISSN={["2166-5656"]}, DOI={10.1109/PHM-Chongqing.2018.00069}, abstractNote={The use of composites in aircraft manufacturing is growing dramatically. Ultrasonic guided waves (UGW) can propagate a long distance, and thus have been used in structural health monitoring (SHM) for large area composite structures like wings and fuselages. In this paper, a movable linear-array transducer along with time-reversal migration (TRM) technique has been adopted as a nondestructive inspection (NDI) tool to rapidly inspect and reconstruct the image of damage in composites. With this linear-array transducer, each time one element is used as the actuator and the rest of elements are used as receivers. Subsequently, a TRM image of damage (or the secondary sources) in the composite specimen is reconstructed through TRM technique. By sequentially switching the element in the linear-array transducer as the actuator, a set of TRM images of the same damage in the composite are reconstructed. Through stacking all images together, a final image of the damage with higher accuracy and lower noise rather than each individual image can be obtained. Experiment results confirm that the linear-array transducer and TRM technique can be a promising NDI tool for rapid inspection of damage in composite structures.}, journal={2018 PROGNOSTICS AND SYSTEM HEALTH MANAGEMENT CONFERENCE (PHM-CHONGQING 2018)}, author={Xiang, Dan and Yuan, Fuh-Gwo and He, Jason}, year={2018}, pages={369–374} }
 @article{kim_yuan_2018, title={Enhanced damage imaging of a metallic plate using matching pursuit algorithm with multiple wavepaths}, volume={89}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2018.01.014}, abstractNote={Waveforms received by sensors resulting from multiple wavepaths overlap and are hard to interpret. Because of this difficulty, they are usually intentionally ignored, thereby only the first arrival of wave mode being used for damage localization. This article proposes an imaging algorithm for damage localization by incorporating multiple wavepaths using piezoelectric wafers affixed on a metallic plate. Matching pursuit (MP) algorithm to enhance image quality is adopted for separating each wave packet individually. MP algorithm is an adaptive time-frequency signal decomposition technique that matches the best-fit elementary atom functions from an overcomplete dictionary. This study proposes a new dictionary composed of atom functions that constitute possible wave packets propagated by an excitation of Hann-windowed toneburst. The proposed dictionary converges faster and separates individual wave packets more accurately than typical Gaussian based dictionaries. Simulated studies first confirm the performance of MP algorithm with the proposed dictionary in comparison with those using conventional non-adaptive time-frequency analysis as well as MP with heuristic Gaussian-based dictionaries. The results of this study validate the proposed algorithm that multiple wavepaths can localize the damage with three to four piezoelectric wafers versus typical approaches employing only primary scattered waves.}, journal={ULTRASONICS}, author={Kim, Ho-Wuk and Yuan, Fuh-Gwo}, year={2018}, month={Sep}, pages={84–101} }
 @article{chang_yuan_2018, title={Extraction of guided wave dispersion curve in isotropic and anisotropic materials by Matrix Pencil method}, volume={89}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2018.05.003}, abstractNote={Guided wave dispersion curves in isotropic and anisotropic materials are extracted automatically from measured data by Matrix Pencil (MP) method investigating through k-t or x-ω domain with a broadband signal. A piezoelectric wafer emits a broadband excitation, linear chirp signal to generate guided waves in the plate. The propagating waves are measured at discrete locations along the lines for one-dimensional laser Doppler vibrometer (1-D LDV). Measurements are first Fourier transformed into either wavenumber-time k-t domain or space-frequency x-ω domain. MP method is then employed to extract the dispersion curves explicitly associated with different wave modes. In addition, the phase and group velocity are deduced by the relations between wavenumbers and frequencies. In this research, the inspections for dispersion relations on an aluminum plate by MP method from k-t or x-ω domain are demonstrated and compared with two-dimensional Fourier transform (2-D FFT). Other experiments on a thicker aluminum plate for higher modes and a composite plate are analyzed by MP method. Extracted relations of composite plate are confirmed by three-dimensional (3-D) theoretical curves computed numerically. The results explain that the MP method not only shows more accuracy for distinguishing the dispersion curves on isotropic material, but also obtains good agreements with theoretical curves on anisotropic and laminated materials.}, journal={ULTRASONICS}, author={Chang, C. Y. and Yuan, F. G.}, year={2018}, month={Sep}, pages={143–154} }
 @article{he_leser_leser_yuan_2018, title={IWSHM 2017: Damage-scattered wave extraction in an integral stiffened isotropic plate: a baseline-subtraction-free approach}, volume={17}, ISSN={["1741-3168"]}, DOI={10.1177/1475921718769232}, abstractNote={Ultrasonic guided waves enable long-distance inspection of structures for health monitoring purposes. However, this capability is diminished when applied to complex structures where damage-scattered waves are often buried by scattering from various structural components or boundaries in the time–space domain. Here, a baseline-subtraction-free inspection concept based on the Radon transform is proposed to identify and separate these scattered waves from those scattered by damage. The received time–space domain signals can be converted into the Radon domain, in which the scattered signals from structural components are suppressed into relatively small regions such that damage-scattered signals can be identified and extracted. In this study, a piezoelectric wafer and a linear scan via laser Doppler vibrometer were used to excite and acquire the Lamb wave signals in an aluminum plate with multiple stiffeners. Linear and inverse linear Radon transform algorithms were applied to the direct measurements. Currently, this method needs baseline measurements for comparison in the Radon domain, but avoids baseline subtraction. The results demonstrate the effectiveness of the Radon transform as an extraction tool for damage-scattered waves in a stiffened aluminum plate for a damage site in the bay area between two stiffeners and also suggest the possibility of generalizing this technique for application to a wide variety of complex, large-area structures.}, number={6}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={He, Jiaze and Leser, Patrick E. and Leser, William P. and Yuan, Fuh-Gwo}, year={2018}, month={Nov}, pages={1365–1376} }
 @article{chang_yuan_2018, title={Impact Damage Imaging in a Curved Composite Panel with Wavenumber Index via Riesz Transform}, volume={10599}, ISSN={["1996-756X"]}, DOI={10.1117/12.2302915}, abstractNote={The barely visible impact damages reduce the strength of composite structures significantly; however, they are difficult to be detected during regular visual inspection. A guided wave based damage imaging condition method is developed and applied on a curved composite panel, which is a part of an aileron from a retired Boeing C-17 Globemaster III. Ultrasonic guided waves are excited by a piezoelectric transducer (PZT) and then captured by a laser Doppler vibrometer (LDV). The wavefield images are constructed by measuring the out-of-plane velocity point by point within interrogation region, and the anomalies at the damage area can be observed with naked eye. The discontinuities of material properties leads to the change of wavenumber while the wave propagating through the damaged area. These differences in wavenumber can be observed by deriving instantaneous wave vector via Riesz transform (RT), and then be shown and highlighted with the proposed imaging condition named wavenumber index (WI). RT can be introduced as a two-dimensional (2-D) generalization of Hilbert transform (HT) to derive instantaneous phases, amplitudes, orientations of a guided-wave field. WI employs the instantaneous wave vector and weighted instantaneous wave energy computed from the instantaneous amplitudes, yielding high sensitivity and sharp damage image with computational efficiency. The BVID of the composite structure becomes therefore “visible” with the developed technique.}, journal={NONDESTRUCTIVE CHARACTERIZATION AND MONITORING OF ADVANCED MATERIALS, AEROSPACE, CIVIL INFRASTRUCTURE, AND TRANSPORTATION XII}, author={Chang, Huan-Yu and Yuan, Fuh-Gwo}, year={2018} }
 @article{girolamo_chang_yuan_2018, title={Impact damage visualization in a honeycomb composite panel through laser inspection using zero-lag cross-correlation imaging condition}, volume={87}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2018.02.014}, abstractNote={A fully non-contact laser-based nondestructive inspection (NDI) system is developed to detect and visualize damage in structures. The study focuses on the size quantification and characterization of a barely visible impact damage (BVID) in a honeycomb composite panel. The hardware consists of a Q-switched Nd:YAG pulse laser that probes the panel by generating broadband guided waves via thermo-elastic expansion. The laser, in combination with a set of galvano-mirrors is used to raster scan over a two-dimensional surface covering the damaged region of an impacted quasi-isotropic [60/0/-60]s honeycomb composite panel. The out-of-plane velocities are measured at a fixed location normal to the surface by a laser Doppler vibrometer (LDV). An ultrasonic full wavefield assembled from the three-dimensional space-time data matrix in the interrogated area is first acquired and then processed for imaging the impacted damage area. A wavenumber filtering technique in terms of wave vectors is applied to distinguish the forward and backward wavefields in the wavenumber-frequency domain. A zero-lag cross correlation (ZLCC) imaging condition is then employed in the space-frequency domain for damage imaging. The ZLCC imaging condition consists of cross correlating the incident and reflected wavefields in the entire scanned region. The condition not only images the damage boundary between incident and reflected waves outside the damage region but also, for longer time windows, enables to capture the momentary standing waves formed within the damaged region. The ZLCC imaging condition imaged two delaminated region: a main delamination, which was a skewed elliptic with major and minor axis lengths roughly 17 mm and 10 mm respectively, and a secondary delamination region approximately 6 mm by 4 mm, however, which can only be shown at higher frequency range around 80-95 kHz. To conclude, the ZLCC results were in very good agreement with ultrasonic C-scan and X-ray computed tomographic (X-ray CT) scan results. Since the imaging condition is performed in the space-frequency domain, the imaging from ZLCC can also reveal resonance modes which are shown in the main delaminated area by windowing a narrow frequency band sequentially.}, journal={ULTRASONICS}, author={Girolamo, Donato and Chang, Huan-Yu and Yuan, Fuh-Gwo}, year={2018}, month={Jul}, pages={152–165} }
 @article{ren_jiang_ji_zhang_xiang_yuan_2018, title={Improvement of Progressive Damage Model to Predicting Crashworthy Composite Corrugated Plate}, volume={25}, ISSN={["1573-4897"]}, DOI={10.1007/s10443-017-9610-z}, number={1}, journal={APPLIED COMPOSITE MATERIALS}, author={Ren, Yiru and Jiang, Hongyong and Ji, Wenyuan and Zhang, Hanyu and Xiang, Jinwu and Yuan, Fuh-Gwo}, year={2018}, month={Feb}, pages={45–66} }
 @article{she_yuan_ren_liu_xiao_2018, title={Nonlinear bending and vibration analysis of functionally graded porous tubes via a nonlocal strain gradient theory}, volume={203}, ISSN={0263-8223}, url={http://dx.doi.org/10.1016/J.COMPSTRUCT.2018.07.063}, DOI={10.1016/J.COMPSTRUCT.2018.07.063}, abstractNote={In this paper, the nonlinear bending and vibrational characteristics of porous tubes are analyzed for the first time. Within the framework of the nonlocal strain gradient theory, a size-dependent model for the tubes with radial inhomogeneity is formulated. It is assumed that the tube is made from functionally graded materials (FGM). Employed a new model for tubes which takes into account of the shear deformation effects, the motion equations are derived with the help of Hamilton variational principle and determined by the two-step perturbation technique. The validity and feasibility of the method are verified by actual examples. The effects of different parameters such as scaling parameters, porosity volume fraction, power law index and inner-to-outer radius ratio on the nonlinear bending and vibration behaviors of the porous tubes are particularly discussed.}, journal={Composite Structures}, publisher={Elsevier BV}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Ren, Yi-Ru and Liu, Hai-Bo and Xiao, Wan-Shen}, year={2018}, month={Nov}, pages={614–623} }
 @article{she_ren_yuan_xiao_2018, title={On vibrations of porous nanotubes}, volume={125}, ISSN={["1879-2197"]}, DOI={10.1016/j.ijengsci.2017.12.009}, abstractNote={In this study, the vibration behaviors of porous nanotubes are investigated for the first time. The nonlocal strain gradient theory in conjunction with a refined beam model are employed to formulate the size-dependent model. It is presumed that the porous nanotubes are made from functionally graded materials, and the material parameters of nanotubes relate to temperature variation and vary continuously in the radial direction. Employing a refined beam theory which includes the effects of transverse shear deformation, the equations of motion are derived based on Hamilton's variation principle and solved by the Navier solution method. Some comparisons are presented to valid the correctness of present solution method. The effects of the nonlocal parameter, strain gradient parameter, temperature variations, porosity volume fraction and material variation on the vibration characteristic of the nanotubes are discussed in detail.}, journal={INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE}, author={She, Gui-Lin and Ren, Yi-Ru and Yuan, Fuh-Gwo and Xiao, Wan-Shen}, year={2018}, month={Apr}, pages={23–35} }
 @article{she_yuan_ren_2018, title={On wave propagation of porous nanotubes}, volume={130}, ISSN={["1879-2197"]}, DOI={10.1016/j.ijengsci.2018.05.002}, abstractNote={An analytic model of porous nanotubes for the wave propagation analysis is formulated with the help of the nonlocal strain gradient theory. The dispersion relations between phase velocity and wave number is determined by solving an eigenvalue problem. It is found that the asymptotic phase velocity can be increased by increasing the strain gradient parameter or decreasing the nonlocal parameter. In addition, the heterogeneity of functionally graded materials and temperature variation have a substantial influence on the dispersion relations of nanotubes. The nonlocal parameter and strain gradient parameter have significant effects on the dispersion relation at high wave numbers, in contrast, this effects can be negligible at low wave numbers. Meanwhile, it can be inferred that the phase velocity can decrease or increase as the porosity volume fraction rises, which depends on the power law index.}, journal={INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Ren, Yi-Ru}, year={2018}, month={Sep}, pages={62–74} }
 @article{wang_xue_luo_yuan_2018, title={Standing surface acoustic wave technology applied for micro-particle concentration in oil}, volume={10599}, ISSN={["1996-756X"]}, DOI={10.1117/12.2297388}, abstractNote={Oil lubrication plays an important role in a variety of mechanical equipment. The traditional purification method is difficult to remove the tiny impurity size of 5-15 μm. Three different types of the transducers and its preparation methods were used in the experiment. The phenomenon that the impurity particles in viscous fluid by the acoustic radiation force was moved the wave node position and focused on the center line was observed by the super-depth microscope. The influence factors of the produced SSAW, particle force condition and movement track were analyzed. The experimental results show that the interdigital transducer can be used to generate SSAW, so as to achieve the separation effect of oil and suspended particles.}, journal={NONDESTRUCTIVE CHARACTERIZATION AND MONITORING OF ADVANCED MATERIALS, AEROSPACE, CIVIL INFRASTRUCTURE, AND TRANSPORTATION XII}, author={Wang, Ziping and Xue, Xian and Luo, Ying and Yuan, Fuh-Gwo}, year={2018} }
 @article{palagummi_yuan_2017, title={An enhanced performance of a horizontal diamagnetic levitation mechanism-based vibration energy harvester for low frequency applications}, volume={28}, ISSN={["1530-8138"]}, DOI={10.1177/1045389x16651152}, abstractNote={This article identifies and studies key parameters that characterize a horizontal diamagnetic levitation mechanism–based low frequency vibration energy harvester with the aim of enhancing performance metrics such as efficiency and volume figure of merit. The horizontal diamagnetic levitation mechanism comprises three permanent magnets and two diamagnetic plates. Two of the magnets, lifting magnets, are placed co-axially at a distance such that each attracts a centrally located magnet, floating magnet, to balance its weight. This floating magnet is flanked closely by two diamagnetic plates which stabilize the levitation in the axial direction. The influence of the geometry of the floating magnet, the lifting magnet, and the diamagnetic plate is parametrically studied to quantify their effects on the size, stability of the levitation mechanism, and the resonant frequency of the floating magnet. For vibration energy harvesting using the horizontal diamagnetic levitation mechanism, a coil geometry and eddy current damping are critically discussed. Based on the analysis, an efficient experimental system is setup which showed a softening frequency response with an average system efficiency of 25.8% and a volume figure of merit of 0.23% when excited at a root mean square acceleration of 0.0546 m/s2 and at a frequency of 1.9 Hz.}, number={5}, journal={JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES}, author={Palagummi, Sri Vikram and Yuan, Fuh-Gwo}, year={2017}, month={Mar}, pages={578–594} }
 @article{harb_yuan_2017, title={Barely visible impact damage imaging using non-contact air-coupled transducer/laser Doppler vibrometer system}, volume={16}, ISSN={["1741-3168"]}, DOI={10.1177/1475921716678921}, abstractNote={The aim of this study is to investigate the capability of the zero-lag cross-correlation imaging condition of an A0 Lamb wave mode in imaging a barely visible impact damage in a carbon fiber–reinforced polymer composite using a fully non-contact-guided wave-based non-destructive inspection system. A 16-ply (45/0/-45/90)2s carbon fiber–reinforced polymer laminate was impacted at three different locations with different impact energies using a drop ball at three drop heights causing three barely visible impact damages with different sizes. The A0 Lamb wave mode is generated inside the laminate using a circular air-coupled transducer and detected along the damaged region using a laser Doppler vibrometer. The measured wavefield is then decomposed into a forward and backward propagating wavefields by applying a frequency–wavenumber filtering post-processing technique. The decomposed wavefields are then cross-correlated in the frequency domain using zero-lag cross-correlation imaging condition producing a detailed cumulative damage image. The images obtained in frequency domain highlight the three damaged areas with higher zero-lag cross-correlation values compared to other parts of the inspected areas. The experimental investigation has shown a good correlation between the zero-lag cross-correlation imaging condition and C-scan images, which demonstrate a strong capability of guided wave zero-lag cross-correlation imaging condition technique in approximating the location and size of relatively small barely visible impact damages in thin composite structures.}, number={6}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Harb, M. S. and Yuan, F. G.}, year={2017}, month={Nov}, pages={663–673} }
 @article{kim_yuan_2017, title={Damage Imaging of an Isotropic Plate Using Matching Pursuit Algorithm}, volume={10168}, ISSN={["1996-756X"]}, DOI={10.1117/12.2258691}, abstractNote={A matching pursuit (MP) algorithm is effective tool to decompose the overlapped wave packets in a signal so that each wave mode can be identified. For the successful separations of the wave packets, an atom function should be properly designed, that can well resemble the physical features of the signal of interest. In this paper, a novel atom function for the MP algorithm is proposed based on the wave propagating model due to an excitation of a Hann-windowed toneburst signal, which performs very accurately compared to the MP algorithm with the existing Gaussian-type atom functions. The decomposed wave packets, including the directly scattered wave from damage as well as the reverberant waves from the free edges of the plate, via the MP method are employed in the damage imaging algorithm, highlighting the damaged location with higher intensity than the conventional algorithm utilizing only a direct reflected wave. The proposed approach is verified from the experiment where four piezoelectric wafers can accurately identify the damage location in a plate.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2017}, author={Kim, H. W. and Yuan, F. G.}, year={2017} }
 @article{wang_luo_zhao_yuan_2017, title={Design and optimization of an OPFC ultrasonic linear phased array transducer}, volume={13}, ISSN={["1573-8841"]}, DOI={10.1007/s10999-015-9317-y}, number={1}, journal={INTERNATIONAL JOURNAL OF MECHANICS AND MATERIALS IN DESIGN}, author={Wang, Ziping and Luo, Ying and Zhao, Guoqi and Yuan, Fuh-Gwo}, year={2017}, month={Mar}, pages={57–69} }
 @article{jiang_ren_gao_xiang_yuan_2017, title={Design of novel plug-type triggers for composite square tubes: enhancement of energy-absorption capacity and inducing failure mechanisms}, volume={131}, ISSN={["1879-2162"]}, DOI={10.1016/j.ijmecsci.2017.06.050}, abstractNote={To improve their energy-absorption capacity and induce failure mechanisms, the plug-type triggers of composite square tubes are designed and compared. A progressive failure model verified by the axial quasi-static loading test is proposed. Six different plug initiators including circle convex plug initiator, two types of square convex plug initiators, two types of groove-like plug initiators, and ditch-like plug initiator are given. To understand the triggering mechanism, the crashworthiness of chamfered square tubes is compared. Further, the influence of width of ditch plug-initiator towards energy-absorption characteristics and failure modes is investigated. Results show that well agreements in the failure and energy-absorption mechanisms between the simulation and experiment are identified. Three types of convex plug initiators can reduce the initial peak load and induce the out-ward petaling and splaying mode. The progressive inward-folding mode is similarly triggered by both two groove plug initiators. The ditch plug initiators are recommended as the best plug design with reasonably induced failure modes and an increase of 31.9% in total energy absorption. Comparing with non-plug initiator, square tubes crushed by narrower ditch plug initiator possess higher energy-absorption capacity, with an increase of 51.9% in SEA.}, journal={INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES}, author={Jiang, Hongyong and Ren, Yiru and Gao, Binhua and Xiang, Jinwu and Yuan, Fuh-Gwo}, year={2017}, month={Oct}, pages={113–136} }
 @article{hudson_hou_grimsley_yuan_2017, title={Imaging of local porosity/voids using a fully non-contact air-coupled transducer and laser Doppler vibrometer system}, volume={16}, ISSN={["1741-3168"]}, DOI={10.1177/1475921716668843}, abstractNote={This study exploits the feasibility of imaging zones of local porosity/voids simulated by introducing microspheres during layup of a unidirectional carbon fiber–reinforced polymer composite panel. A fully non-contact hybrid system primarily composed of an air-coupled transducer and a laser Doppler vibrometer was used for imaging the local porosity/void zones from the guided wave response. To improve image resolution, several preprocessing techniques are performed. The wavefield reconstructed from the laser Doppler vibrometer measurements was first “denoised” using a one-dimensional wavelet transform in the time domain followed by a two-dimensional wavelet transform in the spatial domain. From the total wavefield, the much weaker backscattered waves were separated from the stronger incident wave by frequency–wavenumber domain filtering. In order to further enhance the signal-to-noise ratio and sharpen the image, the attenuation of incident wave propagation to the damage site was compensated through two proposed weight functions. Finally, a zero-lag cross-correlation was performed for imaging the zone where the compensated incident and backscattered waves were in phase. This improved imaging condition, the “denoised” weighted zero-lag cross-correlation, was proposed and tested for defect imaging in the composite panel with eight intentionally introduced zones of high porosity/voids of varying diameters (1.59–6.35 mm) and depths (0.36–1.08 mm). As expected, the sensitivity of the non-contact air-coupled transducer/laser Doppler vibrometer hybrid system was limited by the wavelength of the excitation signal. The system incorporated with the denoised weighted zero-lag cross-correlation imaging condition for guided wave interrogation gave similar image quality in comparison with that by the immersion C-scan.}, number={2}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Hudson, Tyler B. and Hou, Tan-Hung and Grimsley, Brian W. and Yuan, Fuh-Gwo}, year={2017}, month={Mar}, pages={164–173} }
 @article{he_yuan_2017, title={Lamb Wave-based BVID Imaging for a Curved Composite Sandwich Panel}, volume={1806}, ISSN={["0094-243X"]}, DOI={10.1063/1.4974606}, abstractNote={Composite sandwich structures, consisting of a low density core sandwiched between two laminated facesheets, have been widely used in various aerospace structures. A new Lamb wave-based imaging condition, which will be referred to as the inverse incident wave energy (IIWE) imaging criterion, is proposed in this paper to resolve the situations where the incident wave energy weakly penetrates into the damaged area in the upper facesheet region. Current imaging conditions by analyzing wavefield reconstructed from laser Doppler vibrometer (LDV) scanning have been proven to be adequate for imaging damage in layered composite laminates. In this research, those current imaging conditions were applied and compared in the composite foam structures for barely visible impact damage (BVID). A piezoelectric wafer was used to excite Lamb waves into the structure and a LDV was used to scan the potential damaged areas in the upper facesheet of the panel. A BVID site in a curved composite sandwich foam aileron was inspect...}, journal={43RD REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION}, author={He, Jiaze and Yuan, Fuh-Gwo}, year={2017} }
 @article{he_yuan_2017, title={Lamb-wave-based two-dimensional areal scan damage imaging using reverse-time migration with a normalized zero-lag cross-correlation imaging condition}, volume={16}, ISSN={["1741-3168"]}, DOI={10.1177/1475921716674373}, abstractNote={This article presents a two-dimensional, non-contact, areal scanning system to image and quantify multiple sites of damage in isotropic plates using reverse-time migration with a normalized zero-lag cross-correlation imaging condition. The hybrid system composed of a single piezoelectric actuator mounted onto the structure and a laser Doppler vibrometer for two-dimensional scan. The laser Doppler vibrometer scanned a region in the vicinity of the lead zirconate titanate actuator to capture the scattered wavefield introduced by the damage. The proposed damage imaging technique takes into account the amplitude, phase, and all the frequency content of the single-mode Lamb waves propagating in the plate; thus, the size of multiple sites of damage can be imaged without bias, regardless of the damage locations. Damage image quality was used as a metric to compare two-dimensional areal scans and linear scans as well as to compare the proposed method with existing imaging conditions. The experimental results show that the two-dimensional reverse-time migration/normalized zero-lag cross-correlation technique is capable of imaging and quantification of multiple damage sites in an aluminum plate using a single lead zirconate titanate actuator and a nearby, areal laser Doppler vibrometer scan.}, number={4}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={He, Jiaze and Yuan, Fuh-Gwo}, year={2017}, month={Jul}, pages={444–457} }
 @article{she_yuan_ren_2017, title={Nonlinear analysis of bending, thermal buckling and post-buckling for functionally graded tubes by using a refined beam theory}, volume={165}, ISSN={["1879-1085"]}, DOI={10.1016/j.compstruct.2017.01.013}, abstractNote={Nonlinear bending, thermal buckling and post-buckling analysis for functionally graded materials (FGMs) tubes with two clamped ends by using a refined beam theory are investigated. The theory satisfies the traction-free boundary conditions on the inner and outer surfaces of the tube and also takes into account the transverse shear effects without artificially introducing shear correction factors. The material properties of FGM tubes are assumed to be temperature-dependent and vary in the radial direction. The asymptotic solutions of the FGM tubes under nonlinear bending and thermal post-buckling are solved by using a two-step perturbation method. The analytical solutions of Timoshenko beam and Euler beam are also presented. Detailed parametric studies are performed to investigate effects of inner-to-outer radius ratio, volume fraction as well as shear deformation on nonlinear bending, thermal buckling and post-buckling characteristics of the FGM tubes.}, journal={COMPOSITE STRUCTURES}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Ren, Yi-Ru}, year={2017}, month={Apr}, pages={74–82} }
 @article{she_yuan_ren_xiao_2017, title={On buckling and postbuckling behavior of nanotubes}, volume={121}, ISSN={["1879-2197"]}, DOI={10.1016/j.ijengsci.2017.09.005}, abstractNote={For the first time, the size-dependent thermal buckling and post-buckling behavior of nanotubes made of functionally graded materials (FGMs) with porosities is investigated by using a refined beam theory. This non-classical nanotube model is based on Eringen nonlocal elasticity model which incorporates the small scale effect. Two types of porosity distribution, including even and uneven distribution, are taken into account. The material properties of the nanotubes are temperature-dependent and vary in the radial direction. The size-dependent governing differential equations are derived by employing the generalized variation principle and solved by using a two-step perturbation method. The effects of small scale parameter, porosity volume fraction, the volume fraction index and boundary conditions on thermal buckling and post-buckling of FGM nanotubes are studied by several numerical examples. It can be concluded that the porosity volume fraction and small scale parameter change the buckling and post-buckling behavior of the nanotubes.}, journal={INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Ren, Yi-Ru and Xiao, Wan-Shen}, year={2017}, month={Dec}, pages={130–142} }
 @article{leser_hochhalter_warner_newman_leser_wawrzynek_yuan_2017, title={Probabilistic fatigue damage prognosis using surrogate models trained via three-dimensional finite element analysis}, volume={16}, ISSN={["1741-3168"]}, DOI={10.1177/1475921716643298}, abstractNote={Utilizing inverse uncertainty quantification techniques, structural health monitoring (SHM) can be integrated with damage progression models to form a probabilistic prediction of a structure’s remaining useful life (RUL). However, damage evolution in realistic structures is physically complex. Accurately representing this behavior requires high-fidelity models which are typically computationally prohibitive. In this paper, high-fidelity fatigue crack growth simulation times are reduced by three orders of magnitude using a model based on a set of surrogate models trained via three-dimensional finite element analysis. The developed crack growth modeling approach is experimentally validated using SHM-based damage diagnosis data. A probabilistic prediction of RUL is formed for a metallic, single-edge notch tension specimen with a fatigue crack growing under mixed-mode conditions.}, number={3}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Leser, Patrick E. and Hochhalter, Jacob D. and Warner, James E. and Newman, John A. and Leser, William P. and Wawrzynek, Paul A. and Yuan, Fuh-Gwo}, year={2017}, month={May}, pages={291–308} }
 @article{she_yuan_ren_2017, title={Research on nonlinear bending behaviors of FGM infinite cylindrical shallow shells resting on elastic foundations in thermal environments}, volume={170}, ISSN={["1879-1085"]}, DOI={10.1016/j.compstruct.2017.03.010}, abstractNote={This paper pays attention to predicting the nonlinear bending behaviors of functionally graded materials (FGM) infinite cylindrical shallow shells with a two-parameter elastic foundation by using a two-step perturbation method. The shells are subjected to uniform temperature rise and temperature dependency of the constituents is also taken into account. Two ends of the shells are assumed to be clamped or pinned and in-plane boundary conditions are immovable. The governing equations are derived based on physical neutral surface concept and high order shear deformation theory. The explicit expressions between the transverse load and the deflection are obtained by perturbation method. In numerical examples, some comparisons are shown to verify the correctness of the present research and solution method. It can be concluded that FGM cylindrical shallow shells subjected to uniform bending loadings will bring about snap-through buckling and jump changes, and the foundation can enhance the stability of the shells.}, journal={COMPOSITE STRUCTURES}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Ren, Yi-Ru}, year={2017}, month={Jun}, pages={111–121} }
 @article{wu_zheng_hu_zhang_yuan_2017, title={Study on Random Decrement Signature Under Different Triggering Level and Length of Time History}, ISBN={["978-3-319-54809-8"]}, ISSN={["2191-5652"]}, DOI={10.1007/978-3-319-54810-4_17}, abstractNote={Random Decrement Technique (RDT) is a widely used method to extract free decay response data by averaging time segments obtained under certain triggering condition from structural vibration response. And thus, different triggering condition will inevitably lead to different Random Decrement Signature (RDS) and different modal parameters. The quality of RDS has significant influence on the modal parameter identification results. Based on three assessments criteria for RDS’s quality, statistical analysis of change law of RDS with different triggering level and length of time history are carried out by a SDOF system and a 6-DOF system. A multi-triggering method is proposed, by which a better RDS can be obtained. Numerical simulation and the practical engineering application verified the recommended method and values.}, journal={TOPICS IN MODAL ANALYSIS & TESTING, VOL 10}, author={Wu, Jinzhi and Zheng, Xiujuan and Hu, Jie and Zhang, Yigang and Yuan, Fuh-Gwo}, year={2017}, pages={159–166} }
 @article{she_yuan_ren_2017, title={Thermal buckling and post-buckling analysis of functionally graded beams based on a general higher-order shear deformation theory}, volume={47}, ISSN={["1872-8480"]}, DOI={10.1016/j.apm.2017.03.014}, abstractNote={In the present work, attention is focused on the prediction of thermal buckling and post-buckling behaviors of functionally graded materials (FGM) beams based on Euler–Bernoulli, Timoshenko and various higher-order shear deformation beam theories. Two ends of the beam are assumed to be clamped and in-plane boundary conditions are immovable. The beam is subjected to uniform temperature rise and temperature dependency of the constituents is also taken into account. The governing equations are developed relative to neutral plane and mid-plane of the beam. A two-step perturbation method is employed to determine the critical buckling loads and post-buckling equilibrium paths. New results of thermal buckling and post-buckling analysis of the beams are presented and discussed in details, the numerical analysis shows that, for the case of uniform temperature rise loading, the post-buckling equilibrium path for FGM beam with two clamped ends is also of the bifurcation type for any arbitrary value of the power law index and any various displacement fields.}, journal={APPLIED MATHEMATICAL MODELLING}, author={She, Gui-Lin and Yuan, Fuh-Gwo and Ren, Yi-Ru}, year={2017}, month={Jul}, pages={340–357} }
 @article{lihua_xianghong_fuh-gwo_2016, title={A 3D collision avoidance strategy for UAV with physical constraints}, volume={77}, ISSN={["1873-412X"]}, DOI={10.1016/j.measurement.2015.09.006}, abstractNote={The main goal of this research effort is to find a flyable collision-free path for an unmanned aerial vehicle (UAV) in a dynamic environment. Given that the UAV path planning needs to adapt in near real-time to the dynamic nature of the operational scenario, and to react rapidly to updates in the situational awareness, a modified artificial potential field (MAPF) approach is utilized to provide collision avoidance in view of pop-up threats and a random set of moving obstacles. To ensure a practically reachable trajectory, this paper proposes a constraint reference frame to develop MAPF so that the decomposed forces from MAPF can be matched with the physical constraints of the UAV for online adjustment. Simulations and experimental results provide promising validation in terms of the efficiency and scalability of the proposed approach.}, journal={MEASUREMENT}, author={Lihua, Zhu and Xianghong, Cheng and Fuh-Gwo, Yuan}, year={2016}, month={Jan}, pages={40–49} }
 @article{he_yuan_2016, title={A quantitative damage imaging technique based on enhanced CCRTM for composite plates using 2D scan}, volume={25}, DOI={10.1088/0964-1726/25/10/105022}, abstractNote={A two-dimensional (2D) non-contact areal scan system was developed to image and quantify impact damage in a composite plate using an enhanced zero-lag cross-correlation reverse-time migration (E-CCRTM) technique. The system comprises a single piezoelectric wafer mounted on the composite plate and a laser Doppler vibrometer (LDV) for scanning a region in the vicinity of the PZT to capture the scattered wavefield. The proposed damage imaging technique takes into account the amplitude, phase, geometric spreading, and all of the frequency content of the Lamb waves propagating in the plate; thus, a reflectivity coefficients of the delamination is calculated and potentially related to damage severity. Comparisons are made in terms of damage imaging quality between 2D areal scans and 1D line scans as well as between the proposed and existing imaging conditions. The experimental results show that the 2D E-CCRTM performs robustly when imaging and quantifying impact damage in large-scale composites using a single PZT actuator with a nearby areal scan using LDV.}, number={10}, journal={Smart Materials & Structures}, author={He, J. Z. and Yuan, F. G.}, year={2016} }
 @article{zhou_li_yuan_2016, title={An anisotropic ultrasonic transducer for Lamb wave applications}, volume={17}, ISSN={["1738-1584"]}, DOI={10.12989/sss.2016.17.6.1055}, abstractNote={An anisotropic ultrasonic transducer is proposed for Lamb wave applications, such as passive damage or impact localization based on ultrasonic guided wave theory. This transducer is made from a PMNPT single crystal, and has different piezoelectric coefficients <TEX>$d_{31}$</TEX> and <TEX>$d_{32}$</TEX>, which are the same for the conventional piezoelectric materials, such as Lead zirconate titanate (PZT). Different piezoelectric coefficients result in directionality of guided wave generated by this transducer, in other words, it is an anisotropic ultrasonic transducer. And thus, it has different sensitivity in comparison with conventional ultrasonic transducer. The anisotropic one can provide more information related to the direction when it is used as sensors. This paper first shows its detailed properties, including analytical formulae and finite elements simulations. Then, its application is described.}, number={6}, journal={SMART STRUCTURES AND SYSTEMS}, author={Zhou, Wensong and Li, Hui and Yuan, Fuh-Gwo}, year={2016}, month={Jun}, pages={1055–1065} }
 @article{palagummi_yuan_2016, title={An efficient low frequency horizontal diamagnetic levitation mechanism based vibration energy harvester}, volume={9799}, ISSN={["1996-756X"]}, DOI={10.1117/12.2218914}, abstractNote={This article identifies and studies key parameters that characterize a horizontal diamagnetic levitation (HDL) mechanism based low frequency vibration energy harvester with the aim of enhancing performance metrics such as efficiency and volume figure of merit (FoMv). The HDL mechanism comprises of three permanent magnets and two diamagnetic plates. Two of the magnets, aka lifting magnets, are placed co-axially at a distance such that each attract a centrally located magnet, aka floating magnet, to balance its weight. This floating magnet is flanked closely by two diamagnetic plates which stabilize the levitation in the axial direction. The influence of the geometry of the floating magnet, the lifting magnet and the diamagnetic plate are parametrically studied to quantify their effects on the size, stability of the levitation mechanism and the resonant frequency of the floating magnet. For vibration energy harvesting using the HDL mechanism, a coil geometry and eddy current damping are critically discussed. Based on the analysis, an efficient experimental system is setup which showed a softening frequency response with an average system efficiency of 25.8% and a FoMv of 0.23% when excited at a root mean square acceleration of 0.0546 m/s2 and at frequency of 1.9 Hz.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2016}, author={Palagummi, S. and Yuan, F. G.}, year={2016} }
 @article{he_yuan_2016, title={An enhanced CCRTM (E-CCRTM) damage imaging technique using a 2-D areal scan for composite plates}, volume={9804}, ISSN={["1996-756X"]}, DOI={10.1117/12.2218684}, abstractNote={A two-dimensional (2-D) non-contact areal scan system was developed to image and quantify impact damage in a composite plate using an enhanced zero-lag cross-correlation reverse-time migration (E-CCRTM) technique. The system comprises a single piezoelectric actuator mounted on the composite plate and a laser Doppler vibrometer (LDV) for scanning a region to capture the scattered wavefield in the vicinity of the PZT. The proposed damage imaging technique takes into account the amplitude, phase, geometric spreading, and all of the frequency content of the Lamb waves propagating in the plate; thus, the reflectivity coefficients of the delamination can be calculated and potentially related to damage severity. Comparisons are made in terms of damage imaging quality between 2-D areal scans and linear scans as well as between the proposed and existing imaging conditions. The experimental results show that the 2-D E-CCRTM performs robustly when imaging and quantifying impact damage in large-scale composites using a single PZT actuator with a nearby areal scan using LDV.}, journal={NONDESTRUCTIVE CHARACTERIZATION AND MONITORING OF ADVANCED MATERIALS, AEROSPACE, AND CIVIL INFRASTRUCTURE 2016}, author={He, Jiaze and Yuan, Fuh-Gwo}, year={2016} }
 @article{wenqin_ying_aijun_yuan_2016, title={Damage Modes Recognition and Hilbert-Huang Transform Analyses of CFRP Laminates Utilizing Acoustic Emission Technique}, volume={23}, ISSN={["1573-4897"]}, DOI={10.1007/s10443-015-9454-3}, number={2}, journal={APPLIED COMPOSITE MATERIALS}, author={WenQin, Han and Ying, Luo and AiJun, Gu and Yuan, Fuh-Gwo}, year={2016}, month={Apr}, pages={155–178} }
 @article{harb_yuan_2016, title={Damage imaging using non-contact air-coupled transducer/laser Doppler vibrometer system}, volume={15}, ISSN={["1741-3168"]}, DOI={10.1177/1475921716636336}, abstractNote={In this work, a rapid imaging technique is proposed for imaging damage in metallic plates using a zero-lag cross-correlation imaging condition in the frequency domain. A fully non-contact, single-side access, hybrid inspection system employing air-coupled transducer (ACT) for the generation of anti-symmetric Lamb wave mode and laser Doppler vibrometer (LDV) for dynamic visualization of the Lamb wavefield is used to experimentally verify the proposed technique on three identical aluminum plates with same notch geometry but with various orientations at different locations. The notches (10 mm × 5 mm) are grooved in the aluminum plates using electric discharge machining with a maximum depth of 40% of plate’s thickness and with the following orientations 90°, 60°, and 45° with respect to the horizontal axis. A damage image is constructed by cross-correlating the forward and backward propagating wavefields in the aluminum plates which are separated by analyzing the actual laser Doppler vibrometer measured wavefield using a frequency–wavenumber filtering post-processing technique. The experimental results demonstrate a strong capability of guided wave zero-lag cross-correlation imaging condition technique in localizing, sizing, and detecting the orientation of relatively small defects in the isotopic material.}, number={2}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Harb, Mohammad Said and Yuan, Fuh-Gwo}, year={2016}, month={Mar}, pages={193–203} }
 @article{wang_luo_zhao_xu_yuan_2016, title={Design of an Orthotropic Piezoelectric Composite Material Phased Array Transducer for Damage Detection in a Concrete Structure}, volume={27}, ISSN={["1432-2110"]}, DOI={10.1080/09349847.2015.1122129}, abstractNote={ABSTRACT In order to verify the feasibility and effectiveness of the sensor used in complex concrete materials, an innovative application of ultrasonic phased array detection for a concrete structure was investigated. Comparing with the traditional piezoelectric composites, orthotropic piezoelectric composite material (OPCM) can be used as transducers in damage detection show clear advantages because of their high sensitivity and directivity along the polarization direction. A low frequency ultrasonic phased array transducer consisting of 16 OPCM elements is studied. The optimal array parameters, such as the phased array element interval, the array element width, and number of elements, are obtained by studying the total displacement changes as various parameters change at the focus point in the concrete structure. This configuration allowed the variation and control of the wave field directivity in the concrete structure during the measurements. The measurements were taken on concrete specimens using a precise time-delay device. The experimental measurements were compared to theoretical calculations to investigate the influence of different array element parameters. The results show that an OPCM phased array transducer can be used to detect damage in a concrete block.}, number={4}, journal={RESEARCH IN NONDESTRUCTIVE EVALUATION}, author={Wang, Ziping and Luo, Ying and Zhao, Guoqi and Xu, Bai Qiang and Yuan, Fuh-Gwo}, year={2016}, pages={204–215} }
 @article{leser_newman_hochhalter_gupta_yuan_2016, title={Embedded Ni-Ti particles for the detection of fatigue crack growth in AA7050}, volume={39}, ISSN={["1460-2695"]}, DOI={10.1111/ffe.12413}, abstractNote={A multi-functional aluminium alloy 7050 (AA7050) containing embedded Ni–Ti shape memory alloy particles to detect fatigue crack growth is proposed. The regions of intense strain near the tip of a growing fatigue crack cause nearby Ni–Ti particles to undergo a solid-state phase transformation from austenite to martensite, releasing a detectable acoustic emission signal that can be used to locate the crack in the monitored component. The AA7050/Ni–Ti composite was made by vacuum hot pressing Ni–Ti powder between rolled AA7050 plates. The effect of hot pressing temperature and subsequent heat treatments (solutionizing and peak ageing) on the Ni–Ti particles was studied. A successful proof-of-concept was demonstrated for AA7050 with embedded particles that emit a measureable and repeatable acoustic emission signal in the presence of a fatigue crack, allowing for quick diagnosis of fatigue crack damage in this material.}, number={6}, journal={FATIGUE & FRACTURE OF ENGINEERING MATERIALS & STRUCTURES}, author={Leser, W. P. and Newman, J. A. and Hochhalter, J. D. and Gupta, V. K. and Yuan, F. G.}, year={2016}, month={Jun}, pages={686–695} }
 @article{zhou_yuan_shi_2016, title={Guided torsional wave generation of a linear in-plane shear piezoelectric array in metallic pipes}, volume={65}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2015.10.021}, abstractNote={Cylindrical guided waves based techniques are effective and promising tools for damage detection in long pipes. The essential operations are generation and reception of guided waves in the structures utilizing transducers. A novel in-plane shear (d36 type) PMNT wafer is proposed to generate and receive the guided wave, especially the torsional waves, in metallic pipes. In contrast to the traditional wafer, this wafer will directly introduce in-plane shear deformation when electrical field is conveniently applied through its thickness direction. A single square d36 PMNT wafer is bonded on the surface of the pipe positioned collinearly with its axis, when actuated can predominantly generate torsional (T) waves along the axial direction, circumferential shear horizontal (C-SH) waves along circumferential direction, and other complex cylindrical Lamb-like wave modes along other helical directions simultaneously. While a linear array of finite square size d36 PMNT wafers was equally spaced circumferentially, when actuated simultaneously can nearly uniform axisymmetric torsional waves generate in pipes and non-symmetric wave modes can be suppressed greatly if the number of the d36 PMNT wafer is sufficiently large. This paper first presents the working mechanism of the linear d36 PMNT array from finite element analysis (FEA) by examining the constructive and destructive displacement wavefield phenomena in metallic pipes. Furthermore, since the amplitude of the received fundamental torsional wave signal strongly depends on frequency, a series of experiments are conducted to determine the frequency tuning curve for the torsional wave mode. All results indicate the linear d36 PMNT array has potential for efficiently generating uniform torsional wavefield of the fundamental torsional wave mode, which is more effective in monitoring structural health in metallic pipes.}, journal={ULTRASONICS}, author={Zhou, Wensong and Yuan, Fuh-Gwo and Shi, Tonglu}, year={2016}, month={Feb}, pages={69–77} }
 @article{he_yuan_2016, title={Lamb Waves Based Fast Subwavelength Imaging Using a DORT-MUSIC Algorithm}, volume={1706}, ISSN={["0094-243X"]}, DOI={10.1063/1.4940494}, abstractNote={A Lamb wave-based, subwavelength imaging algorithm is developed for damage imaging in large-scale, plate-like structures based on a decomposition of the time-reversal operator (DORT) method combined with the multiple signal classification (MUSIC) algorithm in the space-frequency domain. In this study, a rapid, hybrid non-contact scanning system was proposed to image an aluminum plate using a piezoelectric linear array for actuation and a laser Doppler vibrometer (LDV) line-scan for sensing. The physics of wave propagation, reflection, and scattering that underlies the response matrix in the DORT method is mathematically formulated in the context of guided waves. The singular value decomposition (SVD) and MUSIC-based imaging condition enable quantifying the damage severity by a ‘reflectivity’ parameter and super-resolution imaging. With the flexibility of this scanning system, a considerably large area can be imaged using lower frequency Lamb waves with limited line-scans. The experimental results showed t...}, journal={42ND ANNUAL REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: INCORPORATING THE 6TH EUROPEAN-AMERICAN WORKSHOP ON RELIABILITY OF NDE}, author={He, Jiae and Yuan, Fuh-Gwo}, year={2016} }
 @article{he_yuan_2016, title={Lamb wave-based subwavelength damage imaging using the DORT-MUSIC technique in metallic plates}, volume={15}, ISSN={["1741-3168"]}, DOI={10.1177/1475921715623359}, abstractNote={A Lamb wave-based, subwavelength imaging algorithm is developed for damage imaging in a metallic plate based on a decomposition of the time-reversal operator method together with a multiple signal classification imaging condition in the space-frequency domain. In this study, a hybrid non-contact inspection system was proposed to image damage in an aluminum plate using a piezoelectric linear array for actuation and a laser Doppler vibrometer line-scan perpendicular to the piezoelectric array for sensing. The physics of incident waves, reflection, and reflected waves that underlie the transfer matrix in the decomposition of the time-reversal operator method is mathematically formulated in the context of guided waves based on the first-order Born approximation. Singular value decomposition is then employed to decompose the experimentally measured transfer matrix into three matrices, detailing the incident wave propagation from the linear actuator array, reflection from the damage, and followed by reflected waves toward the linear sensing array for each small damage. The singular value decomposition and multiple signal classification imaging condition enable providing insight on both the damage “reflectivity” and detecting damage smaller than a wavelength (subwavelength). With the flexibility of this inspection system, a considerably large area can be imaged using lower frequency Lamb waves with rapid line-scans. The experimental results showed that the hardware system with a signal processing tools such as the decomposition of the time-reversal operator with multiple signal classification (time-reversal with multiple signal classification) imaging technique can provide robust, highly accurate imaging results as well as providing damage reflectivity estimation with unknown material properties.}, number={1}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={He, Jiaze and Yuan, Fuh-Gwo}, year={2016}, month={Jan}, pages={65–80} }
 @article{harb_yuan_2016, title={Non-contact ultrasonic technique for Lamb wave characterization in composite plates}, volume={64}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2015.08.011}, abstractNote={A fully non-contact single-sided air-coupled and laser ultrasonic non-destructive system based on the generation and detection of Lamb waves is implemented for the characterization of A0 Lamb wave mode dispersion in a composite plate. An air-coupled transducer (ACT) radiates acoustic pressure on the surface of the composite and generates Lamb waves within the structure. The out-of-plane velocity of the propagating wave is measured using a laser Doppler vibrometer (LDV). In this study, the non-contact automated system focuses on measuring A0 mode frequency–wavenumber, phase velocity dispersion curves using Snell’s law and group velocity dispersion curves using Morlet wavelet transform (MWT) based on time-of-flight along different wave propagation directions. It is theoretically demonstrated that Snell’s law represents a direct link between the phase velocity of the generated Lamb wave mode and the coincidence angle of the ACT. Using Snell’s law and MWT, the former three dispersion curves of the A0 mode are easily and promptly generated from a set of measurements obtained from a rapid ACT angle scan experiment. In addition, the phase velocity and group velocity polar characteristic wave curves are also computed to analyze experimentally the angular dependency of Lamb wave propagation. In comparison with the results from the theory, it is confirmed that using the ACT/LDV system and implementing simple Snell’s law method is highly sensitive and effective in characterizing the dispersion curves of Lamb waves in composite structures as well as its angular dependency.}, journal={ULTRASONICS}, author={Harb, M. S. and Yuan, F. G.}, year={2016}, month={Jan}, pages={162–169} }
 @article{kwon_huang_zhang_yuan_jiang_2016, title={Study on a flexoelectric microphone using barium strontium titanate}, volume={26}, ISSN={["1361-6439"]}, DOI={10.1088/0960-1317/26/4/045001}, abstractNote={In this study, a flexoelectric microphone was, for the first time, designed and fabricated in a bridge structure using barium strontium titanate (Ba0.65Sr0.35TiO3) ceramic and tested afterwards. The prototyped flexoelectric microphone consists of a 1.5 mm  ×  768 μm  ×  50 μm BST bridge structure and a silicon substrate with a cavity. The sensitivity and resonance frequency were designed to be 0.92 pC/Pa and 98.67 kHz, respectively. The signal to noise ratio was measured to be 74 dB. The results demonstrate that the flexoelectric microphone possesses high sensitivity and a wide working frequency range simultaneously, suggesting that flexoelectricity could be an excellent alternative sensing mechanism for microphone applications.}, number={4}, journal={JOURNAL OF MICROMECHANICS AND MICROENGINEERING}, author={Kwon, S. R. and Huang, W. B. and Zhang, S. J. and Yuan, F. G. and Jiang, X. N.}, year={2016}, month={Apr} }
 @article{xu_yuan_hu_2016, title={System Efficiency of Miniature Horizontal-Axis Wind Turbines With and Without Gear Transmission}, volume={142}, ISSN={["1943-7897"]}, DOI={10.1061/(asce)ey.1943-7897.0000276}, abstractNote={AbstractIn this paper, two miniature horizontal-axis wind-turbine systems (MHAWTs) with and without gear transmission consisting of commercially available off-the-shelf components were designed for harvesting energy from ambient airflow to power wireless sensors. The basic performance of the MHAWTs was tested at various wind speeds (2–8  m/s) and resistive loads (10–500 Ω). The maximum output powers of the MHAWT with and without gear were 8.6 and 11 mW at the wind speed of 4  m/s, which suggested that both MHAWTs enabled powering low-power wireless sensors in natural airflow environment. To analyze the system efficiency and predict the optimal resistive load of the MHAWTs, an equivalent-circuit model was used. According to this model, the maximum system efficiency of the MHAWT system was predicted to be 14.8%. The actual maximum system efficiency of the MHAWTs with and without gears was tested as 8.4% and 7.5% at the wind speed of 8 and 6.5  m/s. In addition, the predicted optimal resistive loads of the M...}, number={1}, journal={JOURNAL OF ENERGY ENGINEERING}, author={Xu, Fujun and Yuan, Fuh-Gwo and Hu, Jingzhen}, year={2016}, month={Mar} }
 @article{palagummi_zou_yuan_2015, title={A Horizontal Diamagnetic Levitation Based Low Frequency Vibration Energy Harvester}, volume={137}, ISSN={["1528-8927"]}, DOI={10.1115/1.4030665}, abstractNote={This paper investigates a horizontal diamagnetic levitation (HDL) system for vibration energy harvesting in contrast to the vertical diamagnetic levitation (VDL) system recently proposed by Wang et al. (2013, “A Magnetically Levitated Vibration Energy Harvester,” Smart Mater. Struct., 22(5), p. 055016). In this configuration, two large magnets, alias lifting magnets (LMs), are arranged co-axially at a distance such that in between them a magnet, alias floating magnet (FM), is passively levitated at a laterally offset equilibrium position. The levitation is stabilized in the horizontal direction by two diamagnetic plates (DPs) made of pyrolytic graphite placed on each side of the FM. This HDL configuration mitigates the limitation on the amplitude of the FM imposed in the VDL configuration and exploits the ability to tailor the geometry to meet specific applications due to its frequency tuning capability. A simple circular coil geometry is designed to replace a portion of the pyrolytic graphite plate without sacrificing the stability of the levitation for transduction. An experimental setup exhibits a weak softening frequency response and validates the theoretical findings; at an input root mean square (RMS) acceleration of 0.0434 m/s2 and at a resonant frequency of 1.2 Hz, the prototype generated a RMS power of 3.6 μW with an average system efficiency of 1.93%.}, number={6}, journal={JOURNAL OF VIBRATION AND ACOUSTICS-TRANSACTIONS OF THE ASME}, author={Palagummi, S. and Zou, J. and Yuan, F. G.}, year={2015}, month={Dec} }
 @article{palagummi_zou_yuan_2015, title={A diamagnetically stabilized horizontally levitated electromagnetic vibration energy harvester}, volume={9431}, ISSN={["1996-756X"]}, DOI={10.1117/12.2077630}, abstractNote={This article investigates a horizontal diamagnetic levitation (HDL) system for vibration energy harvesting. In this configuration, two large magnets, alias lifting magnets, are arranged co-axially at a distance such that in between them a magnet, alias floating magnet, is passively levitated at a laterally offset equilibrium position. The levitation is stabilized in the horizontal direction by two diamagnetic plates made of pyrolytic graphite placed on each side of the floating magnet. This HDL configuration permits large amplitude vibration of the floating magnet and exploits the ability to tailor the geometry to meet specific applications due to its frequency tuning capability. Theoretical modeling techniques are discussed followed by an experimental setup to validate it. At an input root mean square (RMS) acceleration of 0.0434 m/s2 (0.0044 grms) and at a resonant frequency of 1.2 Hz, the prototype generated a RMS power of 3.6 μW with an average system efficiency of 1.93%. Followed by the validation, parametric studies on the geometry of the components are undertaken to show that with the optimized parameters the efficiency can be further enhanced.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2015}, author={Palagummi, S. and Zou, J. and Yuan, F. G.}, year={2015} }
 @article{sui_yan_huang_xu_yuan_jing_2015, title={A lightweight yet sound-proof honeycomb acoustic metamaterial}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4919235}, abstractNote={In this letter, a class of honeycomb acoustic metamaterial possessing lightweight and yet sound-proof properties is designed, theoretically proven, and then experimentally verified. It is here reported that the proposed metamaterial having a remarkably small mass per unit area at 1.3 kg/m2 can achieve low frequency (<500 Hz) sound transmission loss (STL) consistently greater than 45 dB. Furthermore, the sandwich panel which incorporates the honeycomb metamaterial as the core material yields a STL that is consistently greater than 50 dB at low frequencies. The proposed metamaterial is promising for constructing structures that are simultaneously strong, lightweight, and sound-proof.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Sui, Ni and Yan, Xiang and Huang, Tai-Yun and Xu, Jun and Yuan, Fuh-Gwo and Jing, Yun}, year={2015}, month={Apr} }
 @article{harb_yuan_2015, title={A rapid, fully non-contact, hybrid system for generating Lamb wave dispersion curves}, volume={61}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2015.03.006}, abstractNote={A rapid, fully non-contact, hybrid system which encompasses an air-coupled transducer (ACT) and a laser Doppler vibrometer (LDV) is presented for profiling A0 Lamb wave dispersion of an isotropic aluminum plate. The ACT generates ultrasonic pressure incident upon the surface of the plate. The pressure waves are partially refracted into the plate. The LDV is employed to measure the out-of-plane velocity of the excited Lamb wave mode at some distances where the Lamb waves are formed in the plate. The influence of the ACT angle of incidence on Lamb wave excitation is investigated and Snell's law is used to directly compute Lamb wave dispersion curves including phase and group velocity dispersion curves in aluminum plates from incident angles found to generate optimal A0 Lamb wave mode. The measured curves are compared to results obtained from a two-dimensional (2-D) Fast Fourier transform (FFT), Morlet wavelet transform (MWT) and theoretical predictions. It was concluded that the experimental results obtained using Snell's law concept are well in accordance with the theoretical solutions. The high degree of accuracy in the measured data with the theoretical results proved a high sensitivity of the air-coupled and laser ultrasound in characterizing Lamb wave dispersion in plate-like structures. The proposed non-contact hybrid system can effectively characterize the dispersive relation without knowledge of neither the materials characteristics nor the mathematical model.}, journal={ULTRASONICS}, author={Harb, M. S. and Yuan, F. G.}, year={2015}, month={Aug}, pages={62–70} }
 @article{palagummi_yuan_2015, title={An optimal design of a mono-stable vertical diamagnetic levitation based electromagnetic vibration energy harvester}, volume={342}, ISSN={["1095-8568"]}, DOI={10.1016/j.jsv.2014.12.034}, abstractNote={A detailed analysis of a mono-stable vertical diamagnetic levitation (VDL) system for optimal vibration energy harvesting is presented. Initial studies showed that simple analytical techniques such as the dipole model and the image method provide useful guideline for understanding the potential of a diamagnetic levitation system, however, it is discussed here that the more accurate semi-analytical techniques such as the thin coil model and the discrete volume method are needed for quantitative optimization and design of the VDL system. With the semi-analytical techniques, the influence of the cylindrical geometry of the floating magnet, the lifting magnet and the diamagnetic plate are parametrically studied to assess their effects on the levitation gap, size of the system and the natural frequency. For efficient vibration energy harvesting using the VDL system, ways to mitigate eddy current damping and a coil geometry for transduction are critically discussed. With the optimized parameters, an experimental system is realized which showed a hardening type nonlinearity. The results show an overall efficiency of 1.54 percent, a root mean square (rms) power output of 1.72 µW when excited at a peak acceleration of 0.081 m/s2 and at a frequency of 2.1 Hz.}, journal={JOURNAL OF SOUND AND VIBRATION}, author={Palagummi, S. and Yuan, F. G.}, year={2015}, month={Apr}, pages={330–345} }
 @article{girolamo_girolamo_yuan_2015, title={Automated Laser-based Barely Visible Impact Damage Detection in Honeycomb Sandwich Composite Structures}, volume={1650}, ISSN={["0094-243X"]}, DOI={10.1063/1.4914754}, abstractNote={Nondestructive evaluation (NDE) for detection and quantification of damage in composite materials is fundamental in the assessment of the overall structural integrity of modern aerospace systems. Conventional NDE systems have been extensively used to detect the location and size of damages by propagating ultrasonic waves normal to the surface. However they usually require physical contact with the structure and are time consuming and labor intensive. An automated, contactless laser ultrasonic imaging system for barely visible impact damage (BVID) detection in advanced composite structures has been developed to overcome these limitations. Lamb waves are generated by a Q-switched Nd:YAG laser, raster scanned by a set of galvano-mirrors over the damaged area. The out-of-plane vibrations are measured through a laser Doppler Vibrometer (LDV) that is stationary at a point on the corner of the grid. The ultrasonic wave field of the scanned area is reconstructed in polar coordinates and analyzed for high resolution characterization of impact damage in the composite honeycomb panel. Two methodologies are used for ultrasonic wave-field analysis: scattered wave field analysis (SWA) and standing wave energy analysis (SWEA) in the frequency domain. The SWA is employed for processing the wave field and estimate spatially dependent wavenumber values, related to discontinuities in the structural domain. The SWEA algorithm extracts standing waves trapped within damaged areas and, by studying the spectrum of the standing wave field, returns high fidelity damage imaging. While the SWA can be used to locate the impact damage in the honeycomb panel, the SWEA produces damage images in good agreement with X-ray computed tomographic (X-ray CT) scans. The results obtained prove that the laser-based nondestructive system is an effective alternative to overcome limitations of conventional NDI technologies.Nondestructive evaluation (NDE) for detection and quantification of damage in composite materials is fundamental in the assessment of the overall structural integrity of modern aerospace systems. Conventional NDE systems have been extensively used to detect the location and size of damages by propagating ultrasonic waves normal to the surface. However they usually require physical contact with the structure and are time consuming and labor intensive. An automated, contactless laser ultrasonic imaging system for barely visible impact damage (BVID) detection in advanced composite structures has been developed to overcome these limitations. Lamb waves are generated by a Q-switched Nd:YAG laser, raster scanned by a set of galvano-mirrors over the damaged area. The out-of-plane vibrations are measured through a laser Doppler Vibrometer (LDV) that is stationary at a point on the corner of the grid. The ultrasonic wave field of the scanned area is reconstructed in polar coordinates and analyzed for high resolut...}, journal={41ST ANNUAL REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION, VOL 34}, author={Girolamo, D. and Girolamo, L. and Yuan, F. G.}, year={2015}, pages={1392–1400} }
 @article{yan_yuan_2015, title={Conversion of evanescent Lamb waves into propagating waves via a narrow aperture edge}, volume={137}, ISSN={["1520-8524"]}, DOI={10.1121/1.4921599}, abstractNote={This paper presents a quantitative study of conversion of evanescent Lamb waves into propagating in isotropic plates. The conversion is substantiated by prescribing time-harmonic Lamb displacements/tractions through a narrow aperture at an edge of a semi-infinite plate. Complex-valued dispersion and group velocity curves are employed to characterize the conversion process. The amplitude coefficient of the propagating Lamb modes converted from evanescent is quantified based on the complex reciprocity theorem via a finite element analysis. The power flow generated into the plate can be separated into radiative and reactive parts made on the basis of propagating and evanescent Lamb waves, where propagating Lamb waves are theoretically proved to radiate pure real power flow, and evanescent Lamb waves carry reactive pure imaginary power flow. The propagating power conversion efficiency is then defined to quantitatively describe the conversion. The conversion efficiency is strongly frequency dependent and can be significant. With the converted propagating waves from evanescent, sensors at far-field can recapture some localized damage information that is generally possessed in evanescent waves and may have potential application in structural health monitoring.}, number={6}, journal={JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA}, author={Yan, Xiang and Yuan, Fuh-Gwo}, year={2015}, month={Jun}, pages={3523–3533} }
 @article{he_yuan_2015, title={Damage identification for composite structures using a cross-correlation reverse-time migration technique}, volume={14}, ISSN={["1741-3168"]}, DOI={10.1177/1475921715602546}, abstractNote={This article presents a reverse-time migration technique to image damage by cross-correlating forward and backward propagating wavefields in composite structures using flexural wave signals. First, theory and procedures are presented for damage imaging for composite plates using a zero-lag cross-correlation imaging condition for reverse-time migration, briefly called cross-correlation-based reverse-time migration. The zero-lag cross-correlation was calculated between the forward wavefield and the backward wavefield. The forward wavefield is formed by the excitation from the actuator using a finite difference method, and the backward wavefield is generated by back-propagating the time-reversed scattered wavefield using the same finite difference method. Simulation studies were first examined to verify the capability of using the proposed zero-lag cross-correlation imaging condition to image single and multiple sites of damage. Two experiments were conducted where either the surface-mounted piezoelectric wafers or non-contact laser Doppler vibrometer was used for receiving the scattered wave signals along a linear array. The scattered wave signals were extrapolated in reverse-time to generate backward propagating wavefields. The experimental studies demonstrated that the cross-correlation-based reverse-time migration can accurately locate and image multiple sites of damage with improved resolution and higher efficiency in comparison with classical pre-stack reverse-time migration.}, number={6}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={He, Jiaze and Yuan, Fuh-Gwo}, year={2015}, month={Nov}, pages={558–570} }
 @article{huang_yang_zhang_yuan_jiang_2015, title={Direct Measurement of Opening Mode Stress Intensity Factors Using Flexoelectric Strain Gradient Sensors}, volume={55}, ISSN={["1741-2765"]}, DOI={10.1007/s11340-014-9914-y}, number={2}, journal={EXPERIMENTAL MECHANICS}, author={Huang, Wenbin and Yang, Shaorui and Zhang, Ningyi and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2015}, month={Feb}, pages={313–320} }
 @article{zhou_li_yuan_2015, title={Fundamental understanding of wave generation and reception using d(36) type piezoelectric transducers}, volume={57}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2014.11.003}, abstractNote={A new piezoelectric wafer made from a PMN-PT single crystal with dominant piezoelectric coefficient d36 is proposed to generate and detect guided waves on isotropic plates. The in-plane shear coupled with electric field arising from the piezoelectric coefficient is not usually present for conventional piezoelectric wafers, such as lead zirconate titanate (PZT). The direct piezoelectric effect of coefficient d36 indicates that under external in-plane shear stress the charge is induced on a face perpendicular to the poled z-direction. On thin plates, this type of piezoelectric wafer will generate shear horizontal (SH) waves in two orthogonal wave propagation directions as well as two Lamb wave modes in other wave propagation directions. Finite element analyses are employed to explore the wave disturbance in terms of time-varying displacements excited by the d36 wafer in different directions of wave propagation to understand all the guided wave modes accurately. Experiments are conducted to examine the voltage responses received by this type of wafer, and also investigate results of tuning frequency and effects of d31 piezoelectric coefficient, which is intentionally ignored in the finite element analysis. All results demonstrate the main features and utility of proposed d36 piezoelectric wafer for guided wave generation and detection in structural health monitoring.}, journal={ULTRASONICS}, author={Zhou, Wensong and Li, Hui and Yuan, Fuh-Gwo}, year={2015}, month={Mar}, pages={135–143} }
 @article{li_yuan_2015, title={Gradient enhanced damage sizing for structural health management}, volume={24}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/24/2/025036}, abstractNote={A gradient enhanced method is proposed to extract a probability distribution of damage size based on damage images from structural health monitoring. The method provides comprehensive information about damage size and enables prediction of remaining useful life (RUL) of aircraft plate-like structures. A three-step procedure is designed to construct a likelihood function about damage size from intensity image, and a gradient function is employed a priori to obtain a narrow distribution of damage size in the Bayesian framework, providing the empirical probability density function of damage size, from which the probability of damage size larger than critical crack size can be calculated. RUL of plate-like structures can be obtained by calculating the cycles after which the crack size would reach a critical value by a damage growth model. The proposed method converts an ultrasonic damage imaging result to probability density function of damage size, with potential to provide accurate and precise estimation of RUL.}, number={2}, journal={SMART MATERIALS AND STRUCTURES}, author={Li, Gang and Yuan, Fuh-Gwo}, year={2015}, month={Feb} }
 @article{harb_yuan_2015, title={Lamb Wave Dispersion and Anisotropy Profiling of Composite Plates via Non-Contact Air-Coupled and Laser Ultrasound}, volume={1650}, ISSN={["0094-243X"]}, DOI={10.1063/1.4914734}, abstractNote={Conventional ultrasound inspection has been a standard non-destructive testing method for providing an in-service evaluation and noninvasive means of probing the interior of a structure. In particular, measurement of the propagation characteristics of Lamb waves allows inspection of plates that are typical components in aerospace industry. A rapid, complete non-contact hybrid approach for excitation and detection of Lamb waves is presented and applied for non-destructive evaluation of composites. An air-coupled transducer (ACT) excites ultrasonic waves on the surface of a composite plate, generating different propagating Lamb wave modes and a laser Doppler vibrometer (LDV) is used to measure the out-of-plane velocity of the plate. This technology, based on direct waveform imaging, focuses on measuring dispersive curves for A0 mode in a composite laminate and its anisotropy. A two-dimensional fast Fourier transform (2D-FFT) is applied to out-of-plane velocity data captured experimentally using LDV to go from the time-spatial domain to frequency-wavenumber domain. The result is a 2D array of amplitudes at discrete frequencies and wavenumbers for A0 mode in a given propagation direction along the composite. The peak values of the curve are then used to construct frequency wavenumber and phase velocity dispersion curves, which are also obtained directly using Snell’s law and the incident angle of the excited ultrasonic waves. A high resolution and strong correlation between numerical and experimental results are observed for dispersive curves with Snell’s law method in comparison to 2D-FFT method. Dispersion curves as well as velocity curves for the composite plate along different directions of wave propagation are measured. The visual read-out of the dispersion curves at different propagation directions as well as the phase velocity curves provide profiling and measurements of the composite anisotropy. The results proved a high sensitivity of the air-coupled and laser ultrasound technique in non-contact characterization of Lamb wave dispersion and material anisotropy of composite plates using simple Snell’s law method.Conventional ultrasound inspection has been a standard non-destructive testing method for providing an in-service evaluation and noninvasive means of probing the interior of a structure. In particular, measurement of the propagation characteristics of Lamb waves allows inspection of plates that are typical components in aerospace industry. A rapid, complete non-contact hybrid approach for excitation and detection of Lamb waves is presented and applied for non-destructive evaluation of composites. An air-coupled transducer (ACT) excites ultrasonic waves on the surface of a composite plate, generating different propagating Lamb wave modes and a laser Doppler vibrometer (LDV) is used to measure the out-of-plane velocity of the plate. This technology, based on direct waveform imaging, focuses on measuring dispersive curves for A0 mode in a composite laminate and its anisotropy. A two-dimensional fast Fourier transform (2D-FFT) is applied to out-of-plane velocity data captured experimentally using LDV to go fr...}, journal={41ST ANNUAL REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION, VOL 34}, author={Harb, M. S. and Yuan, F. G.}, year={2015}, pages={1229–1238} }
 @misc{zhu_liu_hu_yuan_huang_2015, title={Microstructural designs of plate-type elastic metamaterial and their potential applications: a review}, volume={6}, ISSN={["1947-542X"]}, DOI={10.1080/19475411.2015.1025249}, abstractNote={Elastic metamaterials are of growing interest due to their unique effective properties and wave manipulation abilities. Unlike phononic crystals based on the Bragg scattering mechanism, elastic metamaterials (EMMs) are based on the locally resonant (LR) mechanism and can fully control elastic waves at a subwavelength scale. Microstructural designs of EMMs in plate-like structures have attracted a great deal of attention. In this paper, the recent advances in the microstructural designs of LR-based EMM plates are reviewed. Their potential applications in the fields of low frequency guided wave attenuation, wave manipulation and energy trapping at a subwavelength scale, and structural health monitoring are discussed.}, number={1}, journal={INTERNATIONAL JOURNAL OF SMART AND NANO MATERIALS}, author={Zhu, R. and Liu, X. N. and Hu, G. K. and Yuan, F. G. and Huang, G. L.}, year={2015}, pages={14–40} }
 @article{zhang_xu_xu_xu_luo_yuan_2015, title={Numerical modeling of guided ultrasonic waves generated and received by PZT wafer in a beam}, volume={9543}, ISBN={["978-1-62841-709-8"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2182259}, abstractNote={The spectral finite element method (SFEM) is developed to predict guided ultrasonic waves in the surface-bonded piezoelectric wafer and beam structure. The Timoshenko beam theory, the Euler-Bernoulli beam theory and linear piezoelectricity are used to model the base beam and electric-mechanical behavior of the piezoelectric wafer respectively. Using Hamilton’s principle, the governing equations are obtained in the time domain, and then the SFEM are formulated from coupled differential equations of motion transformed into the frequency domain via the discrete Fourier transform. The SFEM is used to analyze the dispersion characteristics, mode conversion of guided waves and the interaction of waves and notch. The high accuracy of the present SFEM is verified by comparing with the finite element method results.}, journal={THIRD INTERNATIONAL SYMPOSIUM ON LASER INTERACTION WITH MATTER}, author={Zhang, J. X. and Xu, B. Q. and Xu, G. D. and Xu, C. G. and Luo, Y. and Yuan, F. G.}, year={2015} }
 @inproceedings{leser_hochhalter_newman_leser_warner_wawrzynek_yuan_2015, title={Probabilistic fatigue damage prognosis using a surrogate model trained via 3D finite element analysis}, DOI={10.12783/shm2015/299}, abstractNote={Utilizing inverse uncertainty quantification techniques, structural health monitoring data can be integrated with damage progression models to form probabilistic predictions of a structure’s remaining useful life. However, damage evolution in realistic structures is physically complex. Accurately representing this behavior requires high-fidelity models which are typically computationally prohibitive. In this paper, high-fidelity fatigue crack growth simulation times are significantly reduced using a surrogate model trained via finite element analysis. The new approach is applied to experimental damage diagnosis data to form a probabilistic prediction of remaining useful life for a test specimen under mixed-mode conditions. doi: 10.12783/SHM2015/299}, booktitle={Structural health monitoring 2015: system reliability for verification and implementation, vols. 1 and 2}, author={Leser, P. E. and Hochhalter, J. D. and Newman, J. A. and Leser, W. P. and Warner, J. E. and Wawrzynek, P. A. and Yuan, F. G.}, year={2015}, pages={2407–2414} }
 @article{sui_yan_huang_xu_yuan_jing_2015, title={Response to "Comment on 'A lightweight yet sound-proof honeycomb acoustic metamaterial'" [Appl. Phys. Lett. 107, 216101 (2015)]}, volume={107}, ISSN={["1077-3118"]}, DOI={10.1063/1.4936238}, abstractNote={First Page}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Sui, Ni and Yan, Xiang and Huang, Tai-Yun and Xu, Jun and Yuan, Fuh-Gwo and Jing, Yun}, year={2015}, month={Nov} }
 @article{mcelroy_leone_ratcliffe_czabaj_yuan_2015, title={Simulation of delamination-migration and core crushing in a CFRP sandwich structure}, volume={79}, ISSN={["1878-5840"]}, DOI={10.1016/j.compositesa.2015.08.026}, abstractNote={Following the onset of damage caused by an impact load on a composite laminate structure, delaminations often form propagating outwards from the point of impact and in some cases can migrate via matrix cracks between plies as they grow. The goal of the present study is to develop an accurate finite element modeling technique for simulation of the delamination–migration phenomena in laminate impact damage processes. An experiment was devised where, under a quasi-static indentation load, an embedded delamination in the facesheet of a laminate sandwich specimen migrates via a transverse matrix crack and then continues to grow on a new ply interface. Using data from this test for validation purposes, several finite element damage simulation methods were investigated. Comparing the experimental results with those of the different models reveals certain modeling features that are important to include in a numerical simulation of delamination–migration and some that may be neglected.}, journal={COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING}, author={McElroy, M. and Leone, F. and Ratcliffe, J. and Czabaj, M. and Yuan, F. G.}, year={2015}, month={Dec}, pages={192–202} }
 @article{xu_xu_luo_xu_yuan_2015, title={Ultrasonic Guided Wave Based Horizontal Crack Imaging in Metal Plate by Local Wavenumber Analysis}, volume={9543}, ISBN={["978-1-62841-709-8"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2182199}, abstractNote={Ultrasonic guided waves are one of the most prominent tools for SHM in plate-like structure. However, complex propagation characteristics of guided waves as well as traditional contact ultrasonic transducers limit its application in the practical damage detection. Scanning Laser Doppler vibrometer (SLDV) technology is an effective non-contact method to obtain ultrasonic guided wavefield with ultra-high spatial resolution. Based on abundant wavefield data, wavenumber imaging algorithms are capable of not only damage location, but also assessment of damage characteristics such as size and shape. In this work, we adopt local wavenumber analysis method for horizontal crack detection in platelike structure. Instead of using SLDV in experiment, 3D finite element numerical method is adopted to obtain full ultrasonic guided wavefield data. Since the horizontal cracks result in decrease of local thickness, the wavenumber in corresponding area shows significant increase, which is used as indicators for crack imaging. The effects of different damage shapes, depths and spatial window sizes on imaging are also discussed. Numerical simulation results and imaging algorithm laid the foundation for the method applied in experiment and practice.}, journal={THIRD INTERNATIONAL SYMPOSIUM ON LASER INTERACTION WITH MATTER}, author={Xu, C. G. and Xu, B. Q. and Luo, Y. and Xu, G. D. and Yuan, F. G.}, year={2015} }
 @article{kwon_huang_zhang_yuan_jiang_2014, title={A new type of microphone using flexoelectric barium strontium titnate}, volume={9062}, ISSN={["1996-756X"]}, DOI={10.1117/12.2045072}, abstractNote={A flexoelectric bridge-structured microphone using bulk barium strontium titanate (Ba0.65Sr0.35TiO3 or BST) ceramic was investigated in this study. The flexoelectric microphone was installed in an anechoic box and exposed to the sound pressure emitted from a loud speaker. Charge sensitivity of the flexoelectric microphone was measured and calibrated using a reference microphone. The 1.5 mm×768 μm×50 μm micro-machined bridge-structured flexoelectric microphone has a sensitivity of 0.92 pC/Pa, while its resonance frequency was calculated to be 98.67 kHz. The analytical and experimental results show that the flexoelectric microphone has both high sensitivity and broad bandwidth, indicating that flexoelectric microphones are potential candidates for many applications.}, journal={SMART SENSOR PHENOMENA, TECHNOLOGY, NETWORKS, AND SYSTEMS INTEGRATION 2014}, author={Kwon, Seol Ryung and Huang, Wenbin and Zhang, Shujun and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2014} }
 @article{cao_yuan_li_2014, title={A super-compact metamaterial absorber cell in L-band}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4875835}, abstractNote={A super-compact metamaterial absorber (SMA) unit cell in L band (1–2 GHz) is proposed, which is composed of a pair of electric ring resonator (ERR) and its complementary ERR (CERR) with a high dielectric substrate sandwiched in between. The CERR has a cross sectional area where approximately the etched copper foil in ERR is now retained, and the remaining region is now etched. In contract with quarter-wavelength (λ/4) thickness which is required for conventional absorbers, the largest in-plane dimension and thickness of the SMA cell are only λ/33 and λ/250, respectively, leading to a super compact cell volume. While traditionally the long straight wire is introduced in electric-LC resonators for increasing inductance, the CERR achieves similar inductance with much shorter lengths. Thus, its cell dimensions can be drastically reduced without compromising its performance. In addition, simulations together with an automated phase correction algorithm show that the SMA is a type of metamaterial possessing sim...}, number={18}, journal={JOURNAL OF APPLIED PHYSICS}, author={Cao, Z. X. and Yuan, F. G. and Li, L. H.}, year={2014}, month={May} }
 @article{huang_kwon_zhang_yuan_jiang_2014, title={A trapezoidal flexoelectric accelerometer}, volume={25}, ISSN={["1530-8138"]}, DOI={10.1177/1045389x13491021}, abstractNote={In this article a new acceleration sensor using flexoelectric barium strontium titanate cantilever was designed, fabricated, and tested for vibration monitoring. The flexoelectric sensors were configured as a trapezoidal unimorph with a barium strontium titanate layer bonded onto a steel substrate. Seismic mass was attached to the unimorph tip to amplify the transverse flexoelectric response of the barium strontium titanate layer. The theoretical model was developed and validated by vibration tests using the prototyped flexoelectric unimorph. The prototyped accelerometer with thickness of 0.1 mm and length and width in millimeters showed a stable sensitivity of 0.84 pC/g over the frequency range of 100 Hz–1.6 kHz. The aging property of the flexoelectric material was demonstrated to be much better than that of the reported piezoelectric materials right after poling. Scaling effect analysis was also performed for flexoelectric unimorphs. The test results and initial scaling effect analysis indicate that micro/nano flexoelectric sensing holds promise for a broad range of applications.}, number={3}, journal={JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES}, author={Huang, Wenbin and Kwon, Seol-Ryung and Zhang, Shujun and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2014}, month={Feb}, pages={271–277} }
 @article{cao_yuan_li_2014, title={An automated phase correction algorithm for retrieving permittivity and permeability of electromagnetic metamaterials}, volume={4}, ISSN={["2158-3226"]}, DOI={10.1063/1.4882155}, abstractNote={To retrieve complex-valued effective permittivity and permeability of electromagnetic metamaterials (EMMs) based on resonant effect from scattering parameters using a complex logarithmic function is not inevitable. When complex values are expressed in terms of magnitude and phase, an infinite number of permissible phase angles is permissible due to the multi-valued property of complex logarithmic functions. Special attention needs to be paid to ensure continuity of the effective permittivity and permeability of lossy metamaterials as frequency sweeps. In this paper, an automated phase correction (APC) algorithm is proposed to properly trace and compensate phase angles of the complex logarithmic function which may experience abrupt phase jumps near the resonant frequency region of the concerned EMMs, and hence the continuity of the effective optical properties of lossy metamaterials is ensured. The algorithm is then verified to extract effective optical properties from the simulated scattering parameters o...}, number={6}, journal={AIP ADVANCES}, author={Cao, Z. X. and Yuan, F. G. and Li, L. H.}, year={2014}, month={Jun} }
 @article{huang_yang_zhang_yuan_jiang_2014, title={Cracks Monitoring and Characterization Using Ba0.64Sr0.36TiO3 Flexoelectric Strain Gradient Sensors}, volume={9061}, ISSN={["0277-786X"]}, DOI={10.1117/12.2045166}, abstractNote={This paper presents a new method for monitoring and characterizing cracks using Ba0.64Sr0.36TiO3 flexoelectric strain gradient sensors. Firstly, strain gradient field around the mixed mode asymptotic crack tip was analyzed, followed by the derivation of induced flexoelectric polarization in the strain gradient sensors attached in the vicinity of a crack tip. It was found that the flexoelectric polarization of the sensor can be expressed as a function of the stress intensity factors of crack and relative coordinates between the sensor and crack. Given the information of the crack size, further analysis demonstrates that the location of the crack can be traced through the calculation based on flexoelectric outputs of the distributed sensors. A specimen with Mode-I crack was then prepared with two strain gradient sensors (4.7 mm × 0.9 mm × 0.3 mm) attached close to the crack tip to verify the analytical model for detection of cracks. The experimental results yield accurate location of the crack, confirming that flexoelectric strain gradient sensing can be a good avenue for monitoring cracks.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2014}, author={Huang, Wenbin and Yang, Shaorui and Zhang, Ningyi and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2014} }
 @article{huang_shu_kwon_zhang_yuan_jiang_2014, title={Fabrication and measurement of a flexoelectric micro-pyramid composite}, volume={4}, ISSN={["2158-3226"]}, DOI={10.1063/1.4904024}, abstractNote={A fabrication method by combining precision mechanical dicing and wet etching was developed to prepare micro-pyramid structures based on (Ba0.67Sr0.33)TiO3 ceramics. The effective piezoelectric properties of flexoelectric pyramid structures in ten micrometers scale were investigated and measured through converse flexoelectric effect. The scaling effect of the flexoelectric response was demonstrated as the structure size shrinks down. The results do suggest the great potential of flexoelectric micro pyramids as an alternative to lead-free piezoelectric material.}, number={12}, journal={AIP ADVANCES}, author={Huang, Wenbin and Shu, Longlong and Kwon, Seol Ryung and Zhang, Shujun and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2014}, month={Dec} }
 @article{kwon_huang_shu_yuan_maria_jiang_2014, title={Flexoelectricity in barium strontium titanate thin film}, volume={105}, ISSN={["1077-3118"]}, DOI={10.1063/1.4898139}, abstractNote={Flexoelectricity, the linear coupling between the strain gradient and the induced electric polarization, has been intensively studied as an alternative to piezoelectricity. Especially, it is of interest to develop flexoelectric devices on micro/nano scales due to the inherent scaling effect of flexoelectric effect. Ba0.7Sr0.3TiO3 thin film with a thickness of 130 nm was fabricated on a silicon wafer using a RF magnetron sputtering process. The flexoelectric coefficients of the prepared thin films were determined experimentally. It was revealed that the thin films possessed a transverse flexoelectric coefficient of 24.5 μC/m at Curie temperature (∼28 °C) and 17.44 μC/m at 41 °C. The measured flexoelectric coefficients are comparable to that of bulk BST ceramics, which are reported to be 10–100 μC/m. This result suggests that the flexoelectric thin film structures can be effectively used for micro/nano-sensing devices.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Kwon, Seol Ryung and Huang, Wenbin and Shu, Longlong and Yuan, Fuh-Gwo and Maria, Jon-Paul and Jiang, Xiaoning}, year={2014}, month={Oct} }
 @article{zhou_li_yuan_2014, title={Guided wave generation, sensing and damage detection using in-plane shear piezoelectric wafers}, volume={23}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/23/1/015014}, abstractNote={This work presents guided wave generation, sensing, and damage detection in metallic plates using in-plane shear (d36 type) piezoelectric wafers as actuators and sensors. The conventional lead zirconate titanate (PZT) based on induced in-plane normal strain (d31 type) has been widely used to excite and receive guided waves in plates, pipes or thin-walled structures. The d36 type of piezoelectric wafer, however, induces in-plane (or called face) shear deformation in the plane normal to its polarization direction. This form of electromechanical coupling generates more significant shear horizontal (SH) waves in certain wave propagation directions, whose amplitudes are much greater than those of Lamb waves. In this paper, an analysis of SH waves generated using in-plane shear electromechanical coupling is firstly presented, followed by a multiphysics finite element analysis for comparison purposes. Voltage responses of both the conventional d31 and the new d36 sensors are obtained for comparison purposes. Results indicate that this type of wafer has the potential to provide a simple quantitative estimation of damage in structural health monitoring.}, number={1}, journal={SMART MATERIALS AND STRUCTURES}, author={Zhou, Wensong and Li, Hui and Yuan, Fuh-Gwo}, year={2014}, month={Jan} }
 @article{xu_yuan_hu_qiu_2014, title={Miniature horizontal axis wind turbine system for multipurpose application}, volume={75}, ISSN={["1873-6785"]}, DOI={10.1016/j.energy.2014.07.046}, abstractNote={A MWT (miniature wind turbine) has received great attention recently for powering WISP (Wireless Intelligent Sensor Platform). In this study, two MHAWTs (miniature horizontal axis wind turbines) with and without gear transmission were designed and fabricated. A physics-based model was proposed and the optimal load resistances of the MHAWTs were predicted. The open circuit voltages, output powers and net efficiencies were measured under various ambient winds and load resistances. The experimental results showed the optimal load resistances matched well with the predicted results; the MHAWT without gear obtained higher output power at the wind speed of 2 m/s to 6 m/s, while the geared MHAWT exhibited better performance at the wind speed higher than 6 m/s. In addition, a DCM (discontinuous conduction mode) buck-boost converter was adopted as an interface circuit to maximize the charging power from MHAWTs to rechargeable batteries, exhibiting maximum efficiencies above 85%. The charging power reached about 8 mW and 36 mW at the wind speeds of 4 m/s and 6 m/s respectively, which indicated that the MHAWTs were capable for sufficient energy harvesting for powering low-power electronics continuously.}, journal={ENERGY}, author={Xu, F. J. and Yuan, F. G. and Hu, J. Z. and Qiu, Y. P.}, year={2014}, month={Oct}, pages={216–224} }
 @article{thiagarajan_wang_bradford_zhu_yuan_2014, title={Stabilizing carbon nanotube yarns using chemical vapor infiltration}, volume={90}, ISSN={["1879-1050"]}, DOI={10.1016/j.compscitech.2013.10.008}, abstractNote={Carbon nanotube (CNT) yarns exhibit high strength, low density, and relatively good conductivity and piezoresistivity, which makes them an ideal candidate for many advanced applications such as reinforcements for multifunctional composites. However, CNT yarns usually lack the required property stability under load. In this paper a method for stabilizing CNT yarn using chemical vapor infiltration (CVI) to infiltrate and deposit pyrocarbon into CNT yarns is reported. The deposited pyrocarbon effectively binds neighboring CNTs to inhibit inter-nanotube sliding under load, which consequently stabilize the CNT yarns. Relaxation tests showed that compared to pristine CNT yarns, the stabilized yarns have higher electrical stability as well as load retention (∼93% versus ∼61%). There was also a concomitant increase in density along with improved electrical conductivity, mechanical strength and stiffness. Furthermore, under sonication the CVI treated yarns resisted disintegration, making them suitable for electrochemical applications.}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Thiagarajan, V. and Wang, X. and Bradford, P. D. and Zhu, Y. T. and Yuan, F. G.}, year={2014}, month={Jan}, pages={82–87} }
 @article{wang_palagummi_liu_yuan_2013, title={A magnetically levitated vibration energy harvester}, volume={22}, ISSN={["0964-1726"]}, DOI={10.1088/0964-1726/22/5/055016}, abstractNote={In this paper a novel electromagnetic vibration type energy harvester that uses a diamagnetic levitation system is conceptualized, designed, fabricated, and tested. The harvester uses two diamagnetic plates made of pyrolytic graphite between which a cylindrical magnet levitates passively. Two thick cylindrical coils, placed in grooves which are engraved in the pyrolytic graphite plates, are used to convert the mechanical energy into electrical energy efficiently. The geometric configurations of the coils are selected based on the field distribution of the magnet to enhance the efficiency of the harvester. A thorough theoretical analysis is carried out to compare with experimental results. At an input power of 103.45 μW and at a frequency of 2.7 Hz, the harvester generated a power of 0.74 μW with a system efficiency of 0.72%. Both theoretical and experimental results show that this new energy harvesting system can capture low frequency broadband spectra.}, number={5}, journal={SMART MATERIALS AND STRUCTURES}, author={Wang, X. Y. and Palagummi, S. and Liu, L. and Yuan, F. G.}, year={2013}, month={May} }
 @article{yan_huang_kwon_yang_jiang_yuan_2013, title={A sensor for the direct measurement of curvature based on flexoelectricity}, volume={22}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/22/8/085016}, abstractNote={A direct curvature sensing measurement based on the flexoelectricity of Ba0.64Sr0.36TiO3 (BST) material through electromechanical coupling is proposed and developed in this paper. The curvature sensing was demonstrated in four point bending tests of a beam with bonded BST curvature sensors under different applied loads with low time-harmonic frequencies from 0.5 to 3 Hz. A shear lag concept which describes the efficiency of the loading transfer from the epoxy bonding layer was taken into account in extracting the actual curvature from the sensor measurement. A finite element analysis has been performed to estimate the curvature transfer efficiency and the bonding layer thickness is found to be a critical parameter in determining the curvature transfer. Experimental results showed a good linearity of charge output dependence on curvature inputs in a limited frequency range and showed a curvature sensitivity of 30.78 pC m, in comparison with 32.48 pC m from theoretical predictions. Using the measured curvature, the bending stiffness of the beam was then obtained from the experimentally obtained moment–curvature curve. This work demonstrated that the flexoelectric BST sensor provides a direct curvature measurement instead of using a traditional strain gage sensor through interpolation, and thus offers an important avenue for on-line and in situ structural health monitoring.}, number={8}, journal={SMART MATERIALS AND STRUCTURES}, author={Yan, Xiang and Huang, Wenbin and Kwon, Seol Ryung and Yang, Shaorui and Jiang, Xiaoning and Yuan, Fuh-Gwo}, year={2013}, month={Aug} }
 @article{palagummi_yuan_2013, title={A vibration energy harvester using diamagnetic levitation}, volume={8688}, ISSN={["1996-756X"]}, DOI={10.1117/12.2009657}, abstractNote={In this paper a novel electromagnetic vibration type energy harvester which uses a diamagnetic levitation system is conceptualized, designed, fabricated, and tested. The harvester uses two diamagnetic plates made of pyrolytic graphite between which a cylindrical magnet levitates passively. Two archimedean spiral coils are placed in grooves which are engraved in the pyrolytic graphite plates, used to convert the mechanical energy into electrical energy efficiently. The geometric configurations of coils are selected based on the field distribution of the magnet to enhance the efficiency of the harvester. A thorough theoretical analysis is done to compare with the experiment results. At an input power of 103.45 μW and at a frequency of 2.7 Hz, the harvester generated a power of 0.744 μW at an efficiency of 0.72 %. Both theoretical and experimental results show that this new energy harvesting system is efficient and can capture low frequency broadband spectra.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2013}, author={Palagummi, S. and Yuan, F. G.}, year={2013} }
 @inproceedings{he_yuan_2013, title={Damage identification for composite structures using a cross-correlation reverse-time migration technique}, booktitle={Structural Health Monitoring 2013, Vols 1 and 2}, author={He, J. and Yuan, F. G.}, year={2013}, pages={2185–2193} }
 @article{yan_huang_kwon_yang_jiang_yuan_2013, title={Design of a curvature sensor using a flexoelectric material}, volume={8692}, ISSN={["1996-756X"]}, DOI={10.1117/12.2009941}, abstractNote={A curvature sensor based on flexoelectricity using Ba0.64Sr0.36TiO3 (BST) material is proposed and developed in this paper. The working principle of the sensor is based on the flexoelectricity, exhibiting coupling between mechanical strain gradient and electric polarization. A BST curvature sensor is lab prepared using a conventional solid state processing method. The curvature sensing is demonstrated in four point bending tests of the beam under harmonic loads. BST sensors are attached on both side surfaces of an aluminum beam, located symmetrically with respect to its neutral axis. Analyses have shown that the epoxy bonding layer plays a critical role for curvature transfer. Consequently a shear lag effect is taken into account for extracting actual curvature from the sensor measurement. Experimental results demonstrated good linearity from the charge outputs under the frequencies tests and showed a sensor sensitivity of 30.78pC•m in comparison with 32.48pC•m from theoretical prediction. The BST sensor provides a direct curvature measure instead of using traditional strain gage through interpolation and may offer an optional avenue for on-line and in-situ structural health monitoring.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2013}, author={Yan, X. and Huang, W. B. and Kwon, S. R. and Yang, S. R. and Jiang, X. N. and Yuan, F. G.}, year={2013} }
 @article{chen_li_yuan_2013, title={Development of time-reversal method for impact source identification on plate structures}, volume={20}, ISSN={["1875-9203"]}, DOI={10.1155/2013/312169}, abstractNote={This paper presents a detailed study on the impact source identification of a plate structure using time-reversal (T-R) method. Prior to impact monitoring, the plate is calibrated (or characterized) by transfer functions at discrete locations on the plate surface. Both impact location and impact loading time-history are identified using T-R technique and associated signal processing algorithms. Numerical verification for finite-size isotropic plates under low velocity impacts is performed to demonstrate the versatility of T-R method for impact source identification. The tradeoff between accuracy of the impact location detection and calibration spacing is studied in detail. In particular, the effect of plate thickness on calibration spacing has been examined. A number of parameters selected for determining the impact location, approximated transfer functions and steps taken for reconstructing the impact loading time-history are also examined. Two types of noise with various intensities contaminated in strain response and/or transfer functions are investigated for demonstrating the stability and reliability of the T-R method. The results show that T-R method is robust against noise in impact location detection and force reconstruction in circumventing the inherent ill-conditioned inverse problem. Only transfer functions are needed to be calibrated and four sensors are requested in T-R method for impact identification.}, number={3}, journal={SHOCK AND VIBRATION}, author={Chen, Chunlin and Li, Yulong and Yuan, Fuh-Gwo}, year={2013}, pages={561–573} }
 @article{liu_yuan_2013, title={Diamagnetic levitation for nonlinear vibration energy harvesting: Theoretical modeling and analysis}, volume={332}, ISSN={["1095-8568"]}, DOI={10.1016/j.jsv.2012.08.004}, abstractNote={This paper provides theoretical modeling and analysis of applying diamagnetic levitation for nonlinear vibration energy harvesting in detail by first identifying potential merits as well as limitations. Based on a magnetic dipole model, analytical analysis is conducted by providing simplified analytical expressions of restoring forces and electromagnetic damping which are then transformed into a hardening spring model and results in a Duffing equation with strong nonlinearity. In addition, constraints on physical geometry are discussed and derived in the view of practical energy harvester design. More importantly, the derivation and discussion extended to multi-well potential suggest that diamagnetic levitation may enable designing an energy harvester that subject to cross-well chaos with a compact volume and wideband responses. Without mechanical damping in any form, diamagnetic levitation may be considered as a promising mechanism for developing vibration energy harvesters with great performance, and the paper provides a technology push on the possibility.}, number={2}, journal={JOURNAL OF SOUND AND VIBRATION}, author={Liu, Lei and Yuan, F. G.}, year={2013}, month={Jan}, pages={455–464} }
 @article{xu_lu_shi_yuan_2013, title={Experimental and numerical studies on vertical properties of a new multi-dimensional earthquake isolation and mitigation device}, volume={20}, ISSN={["1875-9203"]}, DOI={10.1155/2013/325387}, abstractNote={When designing critical structures such as long-span structures and high-rise buildings, earthquake excitation in the vertical direction, in addition to the horizontal direction, should also be taken into consideration. Study on new devices that can mitigate and isolate multi-dimensional (including both horizontal and vertical) earthquake actions has a remarkable significance. A new kind of multi-dimensional earthquake isolation and mitigation device was recently developed, and experimental study on vertical performances of the device under different excitation frequencies and amplitudes has been carried out in this paper. The characteristics of the vertical properties including the initial stiffness, the energy dissipation stiffness, the energy dissipation per cycle and the vertical damping ratio changing with excitation frequency and amplitude were studied, and the formulas describing the characteristics were proposed. It can be concluded that the initial stiffness and the energy dissipation stiffness increase slightly with increasing frequency, while the energy dissipation per cycle and the damping ratio decrease slightly with increasing frequency, the initial stiffness, the energy dissipation stiffness and the damping ratio will decrease slightly with increasing excitation amplitudes, and the proposed formulas can describe the vertical properties of the multi-dimensional earthquake isolation and mitigation device changing with excitation frequency and amplitude.}, number={3}, journal={SHOCK AND VIBRATION}, author={Xu, Zhaodong and Lu, Liheng and Shi, Benqiang and Yuan, Fuhgwo}, year={2013}, pages={401–410} }
 @article{zhu_huang_yuan_2013, title={Fast damage imaging using the time-reversal technique in the frequency-wavenumber domain}, volume={22}, ISSN={["0964-1726"]}, DOI={10.1088/0964-1726/22/7/075028}, abstractNote={The time-reversal technique has been successfully used in structural health monitoring (SHM) for quantitative imaging of damage. However, the technique is very time-consuming when it is implemented in the time domain. In this paper, we study the technique in the frequency–wavenumber (f–k) domain for fast real-time imaging of multiple damage sites in plates using scattered flexural plate waves. Based on Mindlin plate theory, the time reversibility of dispersive flexural waves in an isotropic plate is theoretically investigated in the f–k domain. A fast damage imaging technique is developed by using the cross-correlation between the back-propagated scattered wavefield and the incident wavefield in the frequency domain. Numerical simulations demonstrate that the proposed technique cannot only localize multiple damage sites but also potentially identify their sizes. Moreover, the time-reversal technique in the f–k domain is about two orders of magnitude faster than the method in the time domain. Finally, experimental testing of an on-line SHM system with a sparse piezoelectric sensor array is conducted for fast multiple damage identification using the proposed technique.}, number={7}, journal={SMART MATERIALS AND STRUCTURES}, author={Zhu, R. and Huang, G. L. and Yuan, F. G.}, year={2013}, month={Jul} }
 @article{kwon_huang_zhang_yuan_jiang_2013, title={Flexoelectric sensing using a multilayered barium strontium titanate structure}, volume={22}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/22/11/115017}, abstractNote={The flexoelectric effect has been recently explored for its promise in electromechanical sensing. However, the relatively low flexoelectric coefficients of ferroelectrics inhibit the potential to develop flexoelectric sensing devices. In this paper, a multilayered structure using flexoelectric barium strontium titanate (Ba0.65Sr0.35TiO3 or BST) ceramic was fabricated in an attempt to enhance the effective flexoelectric coefficients using its inherent scale effect, and hence to improve the flexoelectric sensitivity. The performances of piezoelectric and flexoelectric cantilevers with the same dimensions and under the same conditions were compared. Owing to the flexoelectric scaling effect, under the same force input, the BST flexoelectric structure generated a higher charge output than its piezoelectric P(VDF-TrFE) and PMN-30PT counterparts when its thickness was less than 73.1 μm and 1.43 μm, respectively. Also, amplification of the charge output using the multilayered structure was then experimentally verified. The prototyped structure consisted of three layers of 350 μm-thick BST plates with a parallel electric connection. The charge output was approximately 287% of that obtained using a single-layer structure with the same total thickness of the multilayered structure under the same end deflection input, which suggests high sensitivity sensing can be achieved using multilayer flexoelectric structures.}, number={11}, journal={SMART MATERIALS AND STRUCTURES}, author={Kwon, S. R. and Huang, W. B. and Zhang, S. J. and Yuan, F. G. and Jiang, X. N.}, year={2013}, month={Nov} }
 @article{yan_zhu_huang_yuan_2013, title={Focusing flexural Lamb waves by designing elastic metamaterials bonded on a plate}, volume={8695}, ISSN={["1996-756X"]}, DOI={10.1117/12.2009506}, abstractNote={In this paper, a method to focus flexural Lamb waves to a local area by mounting elastic metamaterials (EMMs) on the surface of the plate is proposed. The EMM consists of silicon rubber and lead connected in series bonded vertically on an aluminum plate. A simplified effective mass-“spring”-mass model is used to study the EMM plate. The frequency-dependent effective mass density of the EMM plate is determined with the aid of the numerically based effective medium method. By making use of the low locally resonant frequency of the EMM plate, the EMM plate is carefully designed with different dimensions to attain high effective mass densities. The effective mass density can be assumed to dominate the change of wave velocity and propagation direction in the EMM plate. An effective mass density profile is then employed along the transverse direction of wave propagation to achieve focusing. Finally, numerical simulation with finite element method (FEM) is utilized to investigate the focusing phenomenon of the A0 mode Lamb waves at 30 kHz and the out-of-plane displacement response beyond the EMM region. Numerical simulation results have shown that focusing the low frequency A0 mode Lamb waves using EMMs is feasible. The focusing may have potential applications in structural health monitoring by manipulating Lamb waves through controlling and focusing Lamb waves to any arbitrary location of the plate with amplified displacement and yet largely retained five-peaked toneburst waveform.}, journal={HEALTH MONITORING OF STRUCTURAL AND BIOLOGICAL SYSTEMS 2013}, author={Yan, X. and Zhu, R. and Huang, G. L. and Yuan, F. G.}, year={2013} }
 @article{yan_zhu_huang_yuan_2013, title={Focusing guided waves using surface bonded elastic metamaterials}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4821258}, abstractNote={Bonding a two-dimensional planar array of small lead discs on an aluminum plate with silicone rubber is shown numerically to focus low-frequency flexural guided waves. The “effective mass density profile” of this type of elastic metamaterials (EMMs), perpendicular to wave propagation direction, is carefully tailored and designed, which allows rays of flexural A0 mode Lamb waves to bend in succession and then focus through a 7 × 9 planar array. Numerical simulations show that Lamb waves can be focused beyond EMMs region with amplified displacement and yet largely retained narrow banded waveform, which may have potential application in structural health monitoring.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Yan, Xiang and Zhu, Rui and Huang, Guoliang and Yuan, Fuh-Gwo}, year={2013}, month={Sep} }
 @article{xu_yuan_liu_hu_qiu_2013, title={Performance Prediction and Demonstration of a Miniature Horizontal Axis Wind Turbine}, volume={139}, ISSN={["1943-7897"]}, DOI={10.1061/(asce)ey.1943-7897.0000125}, abstractNote={A miniature wind turbine (MWT) has received great attention recently for powering low-power devices. In this paper, a physics-based comprehensive model for predicting the performance of a miniature horizontal axis wind turbine (MHAWT) was established. The turbine rotor performance was investigated and an approximation of the power coefficient of the turbine rotor was made. Incorporation of the approximation with the equivalent circuit model, which was proposed in accordance with the principles of the MHAWT, in addition to its overall system performance versus the resistive load and ambient wind speed, was predicted. To demonstrate predictive modeling capability, the MHAWT system comprised of commercially available off-the-shelf components was designed and its performance was experimentally tested. The results matched well with those by prediction modeling, which implies that the proposed model holds promise in estimating and optimizing the performance of the MWT.}, number={3}, journal={JOURNAL OF ENERGY ENGINEERING}, author={Xu, Fujun and Yuan, Fuh-Gwo and Liu, Lei and Hu, Jingzhen and Qiu, Yiping}, year={2013}, month={Sep}, pages={143–152} }
 @article{liu_liu_yuan_2012, title={Damage localization using a power-efficient distributed on-board signal processing algorithm in a wireless sensor network}, volume={21}, DOI={10.1088/0964-1726/21/2/025005}, abstractNote={A distributed on-board algorithm that is embedded and executed within a group of wireless sensors to locate structural damages in isotropic plates is presented. The algorithm is based on an energy-decay model of Lamb waves and singular value decomposition (SVD) to determine damage locations. A sensor group consists of a small number of sensors, each of which independently collects wave signals and evaluates wave energy upon an external triggering signal sent from a base station. The energy values, usually a few bytes in length, are then sent to the base station to determine the presence and location of damages. In comparison with traditional centralized approaches in which whole datasets are required to be transmitted, the proposed algorithm yields much less wireless communication traffic, yet with a modest amount of computation required within sensors. Experiments have shown that the algorithm is robust to locate damage for isotropic plate structures and is very power efficient, with more than an order-of-magnitude power saving.}, number={2}, journal={Smart Materials & Structures}, author={Liu, L. and Liu, S. T. and Yuan, F. G.}, year={2012} }
 @article{zhou_yang_yuan_2012, title={Design of a magnetostrictive sensor for structural health monitoring of non-ferromagnetic plates}, volume={14}, number={1}, journal={Journal of Vibroengineering}, author={Zhou, L. and Yang, Y. J. and Yuan, F. G.}, year={2012}, pages={280–291} }
 @article{an_haftka_kim_yuan_kwak_sohn_yeum_2012, title={Experimental study on identifying cracks of increasing size using ultrasonic excitation}, volume={11}, ISSN={["1741-3168"]}, DOI={10.1177/1475921711406581}, abstractNote={In structural health monitoring, crack identification using scattered ultrasonic waves from a crack is one of the most active research areas. Crack size estimation is important for judging the severity of the damage. If measurements are frequently performed as the crack grows, then a better estimation of crack size may be possible by analyzing sensor signals for the same crack location with different sizes. The objective of this article is to explore the relationship between the sensor signal amplitude and crack size through experiments and simulation for estimating the size. Cracks are machined into an aluminum plate and measurements are carried out with ultrasound excitation using piezoelectric transducer arrays that alternate their role as actuators or sensors. Initially, a hole of 2.5 mm diameter is drilled in the plate, and it is gradually machined to a crack with a size up to 50 mm. Signal amplitude is measured from the sensor arrays. The migration technique is used to image the crack and to find the crack location. The maximum received signal amplitude is found to vary linearly with size from simulation and this agrees with measurements with crack size up to 30 mm. The deviation between the simulation and experiment increases as the crack grows.}, number={1}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={An, Jungeun and Haftka, Raphael T. and Kim, Nam H. and Yuan, Fuh-Gwo and Kwak, Byung Man and Sohn, Hoon and Yeum, Chul Min}, year={2012}, month={Jan}, pages={95–108} }
 @inproceedings{huang_kim_zhang_yuan_jiang_2012, title={Flexoelectric materials and structures for M/NEMS}, DOI={10.1115/imece2011-64520}, abstractNote={Recent research progress on flexoelectricity suggests that dramatic enhancement of effective piezoelectric properties desirable for advanced M/NEMS, in principle, is attainable through flexoelectric (FE) effect and scale effect. In this paper, the transverse flexoelectric coefficient μ12 of barium strontium titanate (BST) microcantilevers with thicknesses ranging from 1.4 mm down to 30 μm was measured at room temperature. It was found that μ12 remains to be constant (8.5 μC/m) for all fabricated microcantilevers. Effective piezoelectric coefficients of these microcantilevers were also calculated, indicating that significantly increased effective piezoelectric coefficients can be obtained from microcantilevers with thickness of microns and nanometers, which is promising for micro/nano electromechanical systems (M/NEMS).Copyright © 2011 by ASME}, booktitle={Proceedings of the ASME International Mechanical Engineering Congress and Exposition, 2011, vol 11}, author={Huang, W. B. and Kim, K. and Zhang, S. J. and Yuan, F. G. and Jiang, X. N.}, year={2012}, pages={761–766} }
 @article{huang_yan_kwon_zhang_yuan_jiang_2012, title={Flexoelectric strain gradient detection using Ba0.64Sr0.36TiO3 for sensing}, volume={101}, ISSN={["0003-6951"]}, DOI={10.1063/1.4772803}, abstractNote={Strain gradient sensing offers an alternative avenue for in-situ monitoring of onset and growth of cracks in structural health monitoring, where the strain gradient is the most sensitive measurand. In this study, flexoelectric strain gradient sensing structures using Ba0.64Sr0.36TiO3 (BST) were attached on the proximity of an open hole in an aluminum plate, to monitor strain gradient variations of the specimen under a uniaxial dynamic load. Charge outputs of the BST micro-bars showed good linearity with the average strain gradients, with a sensitivity of 88 pC m, which is in good agreement with the theoretical estimation by assuming a 30 μm bonding thickness.}, number={25}, journal={APPLIED PHYSICS LETTERS}, author={Huang, Wenbin and Yan, Xiang and Kwon, Seol Ryung and Zhang, Shujun and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2012}, month={Dec} }
 @article{chen_li_yuan_2012, title={Impact source identification in finite isotropic plates using a time-reversal method: experimental study}, volume={21}, ISSN={["0964-1726"]}, DOI={10.1088/0964-1726/21/10/105025}, abstractNote={An impact source identification technique for finite isotropic plates using a time-reversal method with associated signal processing algorithms is investigated experimentally. A series of low velocity impact experiments on an aluminum plate is performed to verify this impact identification method using merely four piezoelectric sensors. Validation of the experimental system for impact source identification using the time-reversal method is first examined. Critical issues concerning the spacing of calibrated points, and associated problems, have been studied in detail. The appropriate spacing of the transfer function is determined and both the impact location and impact loading history are well estimated. The factors which affect the accuracy of impact location detection and force reconstruction are analyzed. The results of the impact identification experiments are repeatable and stable. The time-reversal method is suited to identifying impact events in plate-like structures.}, number={10}, journal={SMART MATERIALS AND STRUCTURES}, author={Chen, Chunlin and Li, Yulong and Yuan, Fuh-Gwo}, year={2012}, month={Oct} }
 @article{wang_yong_li_bradford_liu_tucker_cai_wang_yuan_zhu_2012, title={Ultrastrong, Stiff and Multifunctional Carbon Nanotube Composites}, volume={1}, ISSN={2166-3831}, url={http://dx.doi.org/10.1080/21663831.2012.686586}, DOI={10.1080/21663831.2012.686586}, abstractNote={Carbon nanotubes (CNTs) are an order of magnitude stronger than any other current engineering fiber. However, for the past two decades, it has been a challenge to utilize their reinforcement potential in composites. Here, we report CNT composites with unprecedented multifunctionalities, including record high strength (3.8 GPa), high Young's modulus (293 GPa), electrical conductivity (1230 S·cm −1), and thermal conductivity (41 W m −1 K −1). These superior properties are derived from the long length, high volume fraction, good alignment and reduced waviness of the CNTs, which were produced by a novel-processing approach that can be easily scaled up for industrial production.}, number={1}, journal={Materials Research Letters}, publisher={Informa UK Limited}, author={Wang, X. and Yong, Z.Z. and Li, Q.W. and Bradford, P.D. and Liu, W. and Tucker, D.S. and Cai, W. and Wang, H. and Yuan, F.G. and Zhu, Y.T.}, year={2012}, month={Oct}, pages={19–25} }
 @article{zhao_yuan_2011, title={Carbon nanotube yarn sensors for structural health monitoring of composites}, volume={7983}, ISSN={["0277-786X"]}, DOI={10.1117/12.880938}, abstractNote={With increasing application of composite materials, real time monitoring of composite structures becomes vital for maintenance purpose as well as prevention of catastrophic failure. It has been reported that carbon nanotubes (CNTs) have excellent piezoresistive properties, which may enable a new generation of sensors in nano or micro scales. We report here a novel prototype of carbon nanotube yarn sensors with excellent repeatability and stability for in-situ structural health monitoring. The CNT yarn is spun directly from CNT arrays, and its electrical resistance increases linearly with tensile strain, which makes it an ideal strain sensor. Importantly, it shows repeatable piezoresistive behavior under repetitive straining and unloading. Yarn sensors show stable resistances at temperatures ranging from -196° to 110°. Neat yarn sensors are also embedded into resin to monitor the loading conditions of the composites. With multiple yarn sensor elements aligned in the composite, the crack initiation and propagation could be monitored. Yarn sensors could be easily incorporated into composite structures with minimal invasiveness and weight penalty to enable the structure has self-sensing capabilities.}, journal={NONDESTRUCTIVE CHARACTERIZATION FOR COMPOSITE MATERIALS, AEROSPACE ENGINEERING, CIVIL INFRASTRUCTURE, AND HOMELAND SECURITY 2011}, author={Zhao, Haibo and Yuan, Fuh-Gwo}, year={2011} }
 @article{wang_bradford_liu_zhao_inoue_maria_li_yuan_zhu_2011, title={Mechanical and electrical property improvement in CNT/Nylon composites through drawing and stretching}, volume={71}, ISSN={["1879-1050"]}, DOI={10.1016/j.compscitech.2011.07.023}, abstractNote={The excellent mechanical properties of carbon nanotubes (CNTs) make them the ideal reinforcements for high performance composites. The misalignment and waviness of CNTs within composites are two major issues that limit the reinforcing efficiency. We report an effective method to increase the strength and stiffness of high volume fraction, aligned CNT composites by reducing CNT waviness using a drawing and stretching approach. Stretching the composites after fabrication improved the ultimate strength by 50%, 150%, and 190% corresponding to stretch ratios of 2%, 4% and 7%, respectively. Improvement of the electrical conductivities exhibited a similar trend. These results demonstrate the importance of straightening and aligning CNTs in improving the composite strength and electrical conductivity.}, number={14}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Wang, Xin and Bradford, Philip D. and Liu, Wei and Zhao, Haibo and Inoue, Yoku and Maria, Jon-Paul and Li, Qingwen and Yuan, Fuh-Gwo and Zhu, Yuntian}, year={2011}, month={Sep}, pages={1677–1683} }
 @article{liu_yuan_2011, title={Nonlinear vibration energy harvester using diamagnetic levitation}, volume={98}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3583675}, DOI={10.1063/1.3583675}, abstractNote={This letter proposes a nonlinear vibration energy harvester based on stabilized magnetic levitation using diamagnetic. Restoring forces induced by the magnetic field in harvesting vibration energy is employed instead of the forces introduced by conventional mechanical suspensions; therefore dissipation of vibration energy into heat through mechanical suspensions is eliminated. The core of the design consists of two spiral coils made of diamagnetic materials, which serve dual purposes: providing nonlinear restoring force and harnessing eddy current to power external circuits. From the theoretical analysis presented, the proposed harvester has the potential to provide wideband power outputs in low frequency range.}, number={20}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Liu, L. and Yuan, F. G.}, year={2011}, month={May}, pages={203507} }
 @article{liu_zhang_xu_bradford_wang_zhao_zhang_jia_yuan_li_et al._2011, title={Producing superior composites by winding carbon nanotubes onto a mandrel under a poly(vinyl alcohol) spray}, volume={49}, ISSN={["1873-3891"]}, DOI={10.1016/j.carbon.2011.06.089}, abstractNote={A simple method for processing high-performance carbon nanotube (CNT)/poly(vinyl alcohol) (PVA) composites by coupling the spraying of a PVA solution with the continuous winding of CNT sheets from an array onto a rotating mandrel is reported. This method allows the CNT composites to have a high CNT volume fraction, while having a high degree of alignment, long CNTs, and good integration with the matrix, which are extremely difficult to realize simultaneously by other processes. As a result, the composites have a toughness, strength and electrical conductivity up to 100 J/g, 1.8 GPa and 780 S/cm, respectively. Such a one-step synthesis process is promising for industrial productions and also works for different types of polymers.}, number={14}, journal={CARBON}, author={Liu, Wei and Zhang, Xiaohua and Xu, Geng and Bradford, Philip D. and Wang, Xin and Zhao, Haibo and Zhang, Yingying and Jia, Quanxi and Yuan, Fuh-Gwo and Li, Qingwen and et al.}, year={2011}, month={Nov}, pages={4786–4791} }
 @article{huang_kim_zhang_yuan_jiang_2011, title={Scaling effect of flexoelectric (Ba,Sr)TiO3 microcantilevers}, volume={5}, ISSN={["1862-6254"]}, DOI={10.1002/pssr.201105326}, abstractNote={The flexoelectric microcantilever offers an alternative approach for the development of micro/nano‐sensors. The transverse flexoelectric coefficients µ12 of barium strontium titanate microcantilevers were measured at room temperature, and found to keep the same value of 8.5 µC/m for microcantilevers with thickness ranging from 30 µm to 1.4 mm. The calculated effective piezoelectric coefficient and electrical energy density of flexoelectric cantilevers are superior to those of their piezoelectric counterparts, suggesting that the flexoelectricity‐induced polarization can be significantly increased as structures are scaled down due to the scaling effect of strain gradient, holding promise for flexoelectric micro/nano cantilever sensing applications. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={9}, journal={PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS}, author={Huang, Wenbin and Kim, Kyungrim and Zhang, Shujun and Yuan, Fuh-Gwo and Jiang, Xiaoning}, year={2011}, month={Sep}, pages={350–352} }
 @article{luo_wang_xu_yuan_2011, title={The pre-stack migration imaging technique for damages identification in concrete structures}, volume={1}, ISSN={2095-0349}, url={http://dx.doi.org/10.1063/2.1105104}, DOI={10.1063/2.1105104}, abstractNote={Pre-stack migration imaging (PMI) method, which is used in geophysical exploration by the performance of single side detection and visually display, can be used to identify the location, orientation, and severity of damages in concrete structure. In particular, this letter focuses on the experimental study by using a finite number of sensors for further practical applications. A concrete structure with a surface-mounted linear PZT transducers array is illustrated. Three types of damages, horizontal, dipping and V-shaped crack damage, have been studied. A pre-stack reverse time migration technique is used to back-propagate the scattering waves and to image damages in concrete structure. The migration results from the scattering waves of an artificial damage are presented. It is shown that the existence of the damage in concrete structure is correctly revealed through migration process.}, number={5}, journal={Theoretical and Applied Mechanics Letters}, publisher={Elsevier BV}, author={Luo, Ying and Wang, Ziping and Xu, Baiqiang and Yuan, Fuhgwo}, year={2011}, pages={051004} }
 @article{liu_yuan_2010, title={A Linear Mapping Technique for Dispersion Removal of Lamb Waves}, volume={9}, ISSN={["1741-3168"]}, DOI={10.1177/1475921709341012}, abstractNote={A robust signal processing technique using linear mapping for removing dispersion of Lamb waves is presented in this article. Based on the assumption that the dispersion relation characteristic can be adequately approximated by a finite polynomial in the region close to the high wave energy intensity, the dispersion effect begins to reveal in the second-order term of the polynomial. The linear mapping performed in the finite usable frequency domain is to transform the original in priori known dispersion relation into the linear dispersion relation, i.e., truncated the polynomial up to the linear term which is nondispersive. The linear mapping technique does not require the propagation-path lengths and can be applied to the signals consisting of multiple arrivals with the same wave mode or dispersion characteristic. Synthetic and experimental data for isotropic plates with finite in-plane dimensions excited by the fundamental flexural wave mode are shown to demonstrate the robustness of the proposed dispersion removal technique.}, number={1}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Liu, L. and Yuan, F. G.}, year={2010}, month={Jan}, pages={75–86} }
 @article{zhao_bradford_wang_liu_luo_jia_zhu_yuan_2010, title={An intermetallic Fe-Zr catalyst used for growing long carbon nanotube arrays}, volume={64}, ISSN={["1873-4979"]}, DOI={10.1016/j.matlet.2010.05.045}, abstractNote={Metallic nanoparticles containing single and binary components have been known for their catalytic properties to grow carbon nanotube (CNT) arrays. In this paper, an intermetallic catalyst consisting of iron and zirconium was used to grow millimeter long, well aligned arrays. The Fe–Zr catalysts enabled the growth of 1.7 mm-long carbon nanotube arrays in 45 min. A comparison with pure iron catalyst indicated that adding Zr to iron can stabilize the Fe catalyst at the CNT growth temperature and moderate its reactivity. SEM images showed the different growth behaviors for Fe–Zr and Fe catalysts. The long, uniform CNT arrays grown here have potential applications in many advanced composites.}, number={18}, journal={MATERIALS LETTERS}, author={Zhao, Haibo and Bradford, Philip D. and Wang, Xin and Liu, Wei and Luo, Tzy Jiun Mark and Jia, Quanxi and Zhu, Yuntian and Yuan, Fuh-Gwo}, year={2010}, month={Sep}, pages={1947–1950} }
 @article{zhao_zhang_bradford_zhou_jia_yuan_zhu_2010, title={Carbon nanotube yarn strain sensors}, volume={21}, ISSN={["1361-6528"]}, DOI={10.1088/0957-4484/21/30/305502}, abstractNote={Carbon nanotube (CNT) based sensors are often fabricated by dispersing CNTs into different types of polymer. In this paper, a prototype carbon nanotube (CNT) yarn strain sensor with excellent repeatability and stability for in situ structural health monitoring was developed. The CNT yarn was spun directly from CNT arrays, and its electrical resistance increased linearly with tensile strain, making it an ideal strain sensor. It showed consistent piezoresistive behavior under repetitive straining and unloading, and good resistance stability at temperatures ranging from 77 to 373 K. The sensors can be easily embedded into composite structures with minimal invasiveness and weight penalty. We have also demonstrated their ability to monitor crack initiation and propagation.}, number={30}, journal={NANOTECHNOLOGY}, author={Zhao, Haibo and Zhang, Yingying and Bradford, Philip D. and Zhou, Qian and Jia, Quanxi and Yuan, Fuh-Gwo and Zhu, Yuntian}, year={2010}, month={Jul} }
 @article{chen_yuan_2010, title={Impact source identification in finite isotropic plates using a time-reversal method: theoretical study}, volume={19}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/19/10/105028}, abstractNote={This paper aims to identify impact sources on plate-like structures based on the synthetic time-reversal (T-R) concept using an array of sensors. The impact source characteristics, namely, impact location and impact loading time history, are reconstructed using the invariance of time-reversal concept, reciprocal theory, and signal processing algorithms. Numerical verification for two finite isotropic plates under low and high velocity impacts is performed to demonstrate the versatility of the synthetic T-R method for impact source identification. The results show that the impact location and time history of the impact force with various shapes and frequency bands can be readily obtained with only four sensors distributed around the impact location. The effects of time duration and the inaccuracy in the estimated impact location on the accuracy of the time history of the impact force using the T-R method are investigated. Since the T-R technique retraces all the multi-paths of reflected waves from the geometrical boundaries back to the impact location, it is well suited for quantifying the impact characteristics for complex structures. In addition, this method is robust against noise and it is suggested that a small number of sensors is sufficient to quantify the impact source characteristics through simple computation; thus it holds promise for the development of passive structural health monitoring (SHM) systems for impact monitoring in near real-time.}, number={10}, journal={SMART MATERIALS AND STRUCTURES}, author={Chen, Chunlin and Yuan, Fuh-Gwo}, year={2010}, month={Oct} }
 @article{hu_xu_huang_yuan_2010, title={Optimal design of a vibration-based energy harvester using magnetostrictive material (MsM)}, volume={20}, ISSN={0964-1726 1361-665X}, url={http://dx.doi.org/10.1088/0964-1726/20/1/015021}, DOI={10.1088/0964-1726/20/1/015021}, abstractNote={In this study, an optimal vibration-based energy harvesting system using magnetostrictive material (MsM) was designed and tested to enable the powering of a wireless sensor. In particular, the conversion efficiency, converting from magnetic to electric energy, is approximately modeled from the magnetic field induced by the beam vibration. A number of factors that affect the output power such as the number of MsM layers, coil design and load matching are analyzed and explored in the design optimization. From the measurements, the open-circuit voltage can reach 1.5 V when the MsM cantilever beam operates at the second natural frequency 324 Hz. The AC output power is 970 µW, giving a power density of 279 µW cm − 3. The attempt to use electrical reactive components (either inductors or capacitors) to resonate the system at any frequency has also been analyzed and tested experimentally. The results showed that this approach is not feasible to optimize the power. Since the MsM device has low output voltage characteristics, a full-wave quadrupler has been designed to boost the rectified output voltage. To deliver the maximum output power to the load, a complex conjugate impedance matching between the load and the MsM device is implemented using a discontinuous conduction mode (DCM) buck-boost converter. The DC output power after the voltage quadrupler reaches 705 µW and the corresponding power density is 202 µW cm − 3. The output power delivered to a lithium rechargeable battery is around 630 µW, independent of the load resistance.}, number={1}, journal={Smart Materials and Structures}, publisher={IOP Publishing}, author={Hu, J and Xu, F and Huang, A Q and Yuan, F G}, year={2010}, month={Dec}, pages={015021} }
 @article{coppe_haftka_kim_yuan_2010, title={Uncertainty Reduction of Damage Growth Properties Using Structural Health Monitoring}, volume={47}, ISSN={["1533-3868"]}, DOI={10.2514/1.c000279}, abstractNote={Structural health monitoring provides sensor data that can monitor fatigue-induced damage in service. This information may in turn be used to improve the characterization of material properties that govern damage growth for the structure beingmonitored. These properties are oftenwidely distributed amongnominally identicalmaterials because of differences in manufacturing processes and due to aging effects. Improved accuracy in damage growth characteristics would allowmore accurate prediction of the remaining useful life of the structural component. In this paper, a probabilistic approach using Bayesian inference is employed to progressively reduce the uncertainty in structure-specific damage growth parameters in spite of noise and bias in sensor measurements. Starting from an initial wide distribution of damage growth parameters that are obtained from coupon tests, the distribution is progressively narrowed using damage growth data between consecutivemeasurements. Detailed discussions on how to construct the likelihood function under the given noise of sensor data and how to update the distribution are presented. The approach is applied to simulated damage growth in fuselage panels due to cycles of pressurization. It is shown that the proposed method rapidly converges to the accurate damage growth parameters when the initial damage size is relatively large: e.g., 20 mm. Fairly accurate damage growth parameters are obtained even with measurement errors of 5mm. Using the identified damage growth parameters, it is shown that the 95% conservative remaining useful life converges to the true remaining useful life from the conservative side. The proposed approach may have the potential of turning aircraft into flying fatigue laboratories.}, number={6}, journal={JOURNAL OF AIRCRAFT}, author={Coppe, Alexandra and Haftka, Raphael T. and Kim, Nam H. and Yuan, Fuh-Gwo}, year={2010}, pages={2030–2038} }
 @article{nojavan_yuan_2009, title={Damage identification using electromagnetic waves based on born imaging algorithm}, volume={135}, DOI={10.1061/(ASCE)0733-9399(2009)135:7(717)}, abstractNote={Reconstructing damage geometry with computationally efficient and effective algorithms is of primary importance in establishing a robust structural health monitoring (SHM) system. In this paper, Born imaging algorithm is proposed for three-dimensional (3D) damage imaging of reinforced concrete structures using electromagnetic waves. This algorithm is derived in time domain for inhomogeneous isotropic and lossy structures. In order to reduce the computational cost of the algorithm, different imaging conditions are introduced. Numerical simulations in a 2D transverse magnetic case for a reinforced concrete slab with multiple damages are performed to test the effectiveness of the algorithm. In this simulated study, sensor data, incident field, and back-propagated field are computed via a finite difference time-domain method. It is concluded that the proposed imaging algorithm is capable of efficiently identifying the damages’ geometries and may be employed in a SHM system.}, number={7}, journal={Journal of Engineering Mechanics}, author={Nojavan, S. and Yuan, F. G.}, year={2009}, pages={717–728} }
 @article{liu_yuan_2008, title={Active damage localization for plate-like structures using wireless sensors and a distributed algorithm}, volume={17}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/17/5/055022}, abstractNote={Wireless structural health monitoring (SHM) systems have emerged as a promising technology for robust and cost-effective structural monitoring. However, the applications of wireless sensors on active diagnosis for structural health monitoring (SHM) have not been extensively investigated. Due to limited energy sources, battery-powered wireless sensors can only perform limited functions and are expected to operate at a low duty cycle. Conventional designs are not suitable for sensing high frequency signals, e.g. in the ultrasonic frequency range. More importantly, algorithms to detect structural damage with a vast amount of data usually require considerable processing and communication time and result in unaffordable power consumption for wireless sensors. In this study, an energy-efficient wireless sensor for supporting high frequency signals and a distributed damage localization algorithm for plate-like structures are proposed, discussed and validated to supplement recent advances made for active sensing-based SHM. First, the power consumption of a wireless sensor is discussed and identified. Then the design of a wireless sensor for active diagnosis using piezoelectric sensors is introduced. The newly developed wireless sensor utilizes an optimized combination of field programmable gate array (FPGA) and conventional microcontroller to address the tradeoff between power consumption and speed requirement. The proposed damage localization algorithm, based on an energy decay model, enables wireless sensors to be practically used in active diagnosis. The power consumption for data communication can be minimized while the power budget for data processing can still be affordable for a battery-powered wireless sensor. The Levenberg–Marquardt method is employed in a mains-powered sensor node or PC to locate damage. Experimental results and discussion on the improvement of power efficiency are given.}, number={5}, journal={SMART MATERIALS AND STRUCTURES}, author={Liu, L. and Yuan, F. G.}, year={2008}, month={Oct} }
 @article{wang_yuan_2008, title={Vibration energy harvesting by magnetostrictive material}, volume={17}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/17/4/045009}, abstractNote={A new class of vibration energy harvester based on magnetostrictive material (MsM), Metglas 2605SC, is designed, developed and tested. It contains two submodules: an MsM harvesting device and an energy harvesting circuit. Compared to piezoelectric materials, the Metglas 2605SC offers advantages including higher energy conversion efficiency, longer life cycles, lack of depolarization and higher flexibility to survive in strong ambient vibrations. To enhance the energy conversion efficiency and alleviate the need of a bias magnetic field, Metglas ribbons are transversely annealed by a strong magnetic field along their width direction. To analyze the MsM harvesting device a generalized electromechanical circuit model is derived from Hamilton’s principle in conjunction with the normal mode superposition method based on Euler–Bernoulli beam theory. The MsM harvesting device is equivalent to an electromechanical gyrator in series with an inductor. In addition, the proposed model can be readily extended to a more practical case of a cantilever beam element with a tip mass. The energy harvesting circuit, which interfaces with a wireless sensor and accumulates the harvested energy into an ultracapacitor, is designed on a printed circuit board (PCB) with plane dimension 25 mm × 35 mm. It mainly consists of a voltage quadrupler, a 3 F ultracapacitor and a smart regulator. The output DC voltage from the PCB can be adjusted within 2.0–5.5 V. In experiments, the maximum output power and power density on the resistor can reach 200 µW and 900 µW cm−3, respectively, at a low frequency of 58 Hz. For a working prototype under a vibration with resonance frequency of 1.1 kHz and peak acceleration of 8.06 m s−2 (0.82 g), the average power and power density during charging the ultracapacitor can achieve 576 µW and 606 µW cm−3, respectively, which compete favorably with piezoelectric vibration energy harvesters.}, number={4}, journal={SMART MATERIALS AND STRUCTURES}, author={Wang, Lei and Yuan, F. G.}, year={2008}, month={Aug} }
 @article{yuan_zhou_yuan_2008, title={Wave reflection and transmission in composite beams containing semi-infinite delamination}, volume={313}, ISSN={["0022-460X"]}, DOI={10.1016/j.jsv.2007.12.012}, abstractNote={Wave reflection and transmission in composite beams containing a semi-infinite delamination is studied analytically based on Timoshenko beam theory. Two extreme cases of delaminated surface conditions: non-contact (open) and fully contact (closed) delaminations, are examined, respectively, for a unidirectional composite beam. Analytical solutions of reflection and transmission coefficients for time harmonic flexural waves in a semi-infinite delaminated beam are obtained. The portion of reflected and transmitted power (energy) depends strongly on the frequency of the incident flexural waves as well as the delamination position. The power reflection and transmission ratios are also calculated and verified through energy conservation. The transmitted energy among various wave modes is also investigated. The interaction of narrowband incident wave with delamination at different positions through the thickness of a composite beam is then studied by analytical analysis and verified by finite element analysis.}, number={3-5}, journal={JOURNAL OF SOUND AND VIBRATION}, author={Yuan, Wan-Chun and Zhou, Li and Yuan, Fuh-Gwo}, year={2008}, month={Jun}, pages={676–695} }
 @article{liu_yuan_2008, title={Wireless sensors with dual-controller architecture for active diagnosis in structural health monitoring}, volume={17}, ISSN={["0964-1726"]}, DOI={10.1088/0964-1726/17/2/025016}, abstractNote={Wireless sensor technology, which integrates transducers with microcontrollers and wireless communication, has become increasingly vital in structural health monitoring (SHM) applications. However, the low I/O (input/output) throughput of conventional wireless sensors impedes their usage in applications using high-frequency signals, such as active diagnosis and passive acoustic emission (AE). In this paper, the limitations of extending conventional wireless sensors to handle high-speed acquisition are first identified and discussed. Based on the efforts made in improving wireless sensors with centralized system architecture, a novel dual-controller based architecture is proposed to facilitate high-speed data acquisition and improve power efficiency. Then, a wireless sensor platform, specifically designed for active diagnosis employing stress waves to localize damages, is presented. The newly developed wireless sensor with dimensions of 30 mm × 30 mm × 35 mm utilizes a field programmable gate array (FPGA) as a secondary controller and can support a sampling rate up to 20 million samples per second (Msps). Laboratory experiments for verification show that the wireless sensor can explore new applications at the opposite end of the spectrum from conventional applications: those involving high fidelity and high-speed data acquisition.}, number={2}, journal={SMART MATERIALS & STRUCTURES}, author={Liu, L. and Yuan, F. G.}, year={2008}, month={Apr} }
 @article{zhou_yuan_meng_2007, title={A pre-stack migration method for damage identification in composite structures}, volume={3}, ISSN={["1738-1584"]}, DOI={10.12989/sss.2007.3.4.439}, abstractNote={In this paper a damage imaging technique using pre-stack migration is developed using Lamb (guided) wave propagation in composite structures for imaging multi damages by both numerical simulations and experimental studies. In particular, the paper focuses on the experimental study using a finite number of sensors for future practical applications. A composite laminate with a surface-mounted linear piezoelectric ceramic (PZT) disk array is illustrated as an example. Two types of damages, one straight-crack damage and two simulated circular-shaped delamination damage, have been studied. First, Mindlin plate theory is used to model Lamb waves propagating in laminates. The group velocities of flexural waves in the composite laminate are also derived from dispersion relations and validated by experiments. Then the pre-stack migration technique is performed by using a two-dimensional explicit finite difference algorithm to back-propagate the scattered energy to the damages and damages are imaged together with the excitation-time imaging conditions. Stacking these images together deduces the resulting image of damages. Both simulations and experimental results show that the pre-stack migration method is a promising method for damage identification in composite structures.}, number={4}, journal={SMART STRUCTURES AND SYSTEMS}, author={Zhou, L. and Yuan, F. G. and Meng, W. J.}, year={2007}, month={Oct}, pages={439–454} }
 @article{wang_yuan_2007, title={Active damage localization technique based on energy propagation of Lamb waves}, volume={3}, ISSN={["1738-1584"]}, DOI={10.12989/sss.2007.3.2.201}, abstractNote={An active damage detection technique is introduced to locate damage in an isotropic plate using Lamb waves. This technique uses a time-domain energy model of Lamb waves in plates that the wave amplitude inversely decays with the propagation distance along a ray direction. Accordingly the damage localization is formulated as a least-squares problem to minimize an error function between the model and the measured data. An active sensing system with integrated actuators/sensors is controlled to excite/receive mode of Lamb waves in the plate. Scattered wave signals from the damage can be obtained by subtracting the baseline signal of the undamaged plate from the recorded signal of the damaged plate. In the experimental study, after collecting the scattered wave signals, a discrete wavelet transform (DWT) is employed to extract the first scattered wave pack from the damage, then an iterative method is derived to solve the least-squares problem for locating the damage. Since this method does not rely on time-of-flight but wave energy measurement, it is more robust, reliable, and noise-tolerant. Both numerical and experimental examples are performed to verify the efficiency and accuracy of the method, and the results demonstrate that the estimated damage position stably converges to the targeted damage.}, number={2}, journal={SMART STRUCTURES AND SYSTEMS}, author={Wang, Lei and Yuan, F. G.}, year={2007}, month={Apr}, pages={201–217} }
 @article{wang_yuan_2007, title={Group velocity and characteristic wave curves of Lamb waves in composites: Modeling and experiments}, volume={67}, ISSN={["0266-3538"]}, DOI={10.1016/j.compscitech.2006.09.023}, abstractNote={The propagation characteristics of Lamb waves in composites, with emphasis on group velocity and characteristic wave curves, are investigated theoretically and experimentally. In particular, the experimental study focuses on the existence of multiple higher-order Lamb wave modes that can be observed from piezoelectric sensors by the excitation of ultrasonic frequencies. Using three-dimensional (3-D) elasticity theory, the exact dispersion relations governed by transcendental equations are numerically solved for an infinite number of possible wave modes. For symmetric laminates, a robust method by imposing boundary conditions on the mid-plane and top surface is proposed to separate symmetric and anti-symmetric wave modes. A new semi-exact method is developed to calculate group velocities of Lamb waves in composites. Meanwhile, three characteristic wave curves: velocity, slowness, and wave curves are adopted to analyze the angular dependency of Lamb wave propagation. The dispersive and anisotropic behavior of Lamb waves in a two different types of symmetric laminates is studied in detail theoretically. In the experimental study, two surface-mounted piezoelectric actuators are excited either symmetric or anti-symmetric wave modes with narrowband signals, and a Gabor wavelet transform is used to extract group velocities from arrival times of Lamb wave received by a piezoelectric sensor. In comparison with the results from the theory and experiment, it is confirmed that multiple higher-order Lamb waves can be excited from piezoelectric actuators and the measured group velocities agree well with those from 3-D elasticity theory.}, number={7-8}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Wang, Lei and Yuan, F. G.}, year={2007}, month={Jun}, pages={1370–1384} }
 @article{nojavan_yuan_2006, title={Damage imaging of reinforced concrete structures using electromagnetic migration algorithm}, volume={43}, ISSN={["0020-7683"]}, DOI={10.1016/j.ijsolstr.2005.08.017}, abstractNote={Reconstructing damage geometry with computationally efficient algorithms is of primary importance in establishing a robust structural health monitoring system (SHMS). In this paper electromagnetic migration, a linearized imaging algorithm, is adopted to image the damages in reinforced concrete structures. This algorithm is formulated in time-domain for 3-D inhomogeneous isotropic and lossy structures. In order to reduce the computational cost and to examine the damage resolution of this imaging algorithm, different imaging conditions are introduced. Numerical simulations in 2-D transverse magnetic (TM) wave for a reinforced concrete slab with multiple damages are performed to test the effectiveness of the algorithm. All synthetic sensor data, incident field, and migration field are computed via a finite difference time-domain (FDTD) method. It is concluded that the proposed imaging algorithm is capable of efficiently identifying the damages geometries, is robust against measurement noise, and may be employed in a SHMS.}, number={18-19}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Nojavan, Saeed and Yuan, Fuh-Gwo}, year={2006}, month={Sep}, pages={5886–5908} }
 @article{jin_yuan_2005, title={Atomistic simulations of J-integral in 2D graphene nanosystems}, volume={5}, ISSN={["1533-4880"]}, DOI={10.1166/jnn.2005.414}, abstractNote={The J-integral is investigated in discrete atomic systems using molecular mechanics simulations. A method of calculating J-integral in specified atomic domains is developed. Two cases, a semiinfinite crack in an infinite domain under the remote K-field deformation and a finite crack length in a finite geometry under the tensile and shear deformation prescribed on the boundary, are studied in the two-dimensional graphene sheets and the values of J-integral are obtained under small-strain deformation. The comparison with energy release rates in Mode I and Mode II based on continuum theory of linear elastic fracture mechanics show good agreements. Meanwhile, the nonlinear strain and stress relation of a 2D graphene sheet is evaluated and is fitted with a power law curve. With necessary modifications on the Tersoff-Brenner potential, the critical values of J-integral of 2D graphene systems, which denoted as Jc, are eventually obtained. The results are then compared with those from the relevant references.}, number={12}, journal={JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY}, author={Jin, Y and Yuan, FG}, year={2005}, month={Dec}, pages={2099–2107} }
 @article{wang_yuan_2005, title={Damage identification in a composite plate using prestack reverse-time migration technique}, volume={4}, ISSN={["1741-3168"]}, DOI={10.1177/1475921705055233}, abstractNote={Migration technique, which is normally used in geophysical prospecting, is proposed to locate and image multiple delamination damages in a laminated composite plate. In this simulation study, an active diagnostic system with a linear array of actuators/sensors is used to excite/receive the lowest mode of flexural waves in the laminate. The wavefield scattered from the damages and sensor array data are synthesized using a two-dimensional explicit finite difference scheme to model wave propagation in the laminate based on the Mindlin plate theory. A prestack reverse-time migration technique is then adopted to interpret the synthetic sensor array data and to visualize the damages. The phase and group velocities of flexural waves in the composite plate are derived from the dispersion relations, and subsequently an excitation-time imaging condition specifically for migration of waves in the plate is introduced based on ray tracing and group velocity. Then the prestack reverse-time migration is performed using the same finite difference scheme to back-propagate the scattered energy to the damages. During the migration process, the laminate is imaged in terms of velocity of the transverse deformation. The locations and dimensions of the damages can be visually displayed. Simulated results demonstrate that multiple delamination damages can be successfully identified and the resulting image correlates well with the target damages.}, number={3}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Wang, L and Yuan, FG}, year={2005}, month={Sep}, pages={195–211} }
 @article{lin_yuan_2005, title={Experimental study applying a migration technique in structural health monitoring}, volume={4}, ISSN={["1741-3168"]}, DOI={10.1177/1475921705057973}, abstractNote={This article presents the experimental results of adopting a geophysical migration technique to interpret the ultrasonic flexural wave signals for the purpose of realizing quantitative damage identification in structures. In this study, a homogeneous isotropic plate is examined with a surface-mounted linear array of piezoelectric ceramic (PZT) disk. The piezoelectric disks function as actuators to excite flexural waves and also as sensors to receive the waves scattered from the structural damage in the plate. A prestack reverse-time migration technique, which is an advanced technique in geophysics to reverse the reflection wavefield and to image the Earth’s interior, is then used to back-propagate the scattering waves and to image damage in the plate. The configuration of the experimental setup is presented and its capability of accurately generating and receiving flexural waves is validated by comparing the collected signals with an analytical solution of transient response of a narrowband signal in a piezoelectric sensor/actuator integrated plate using Mindlin plate theory. Finally, the migration results from the scattering waves of an artificial damage are presented. It is shown that the existence of the damage is correctly revealed through migration process in the experiment as it has been shown using synthetic data.}, number={4}, journal={STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL}, author={Lin, X and Yuan, FG}, year={2005}, month={Dec}, pages={341–353} }
 @article{jin_yuan_2005, title={Nanoscopic modeling of fracture of 2D graphene systems}, volume={5}, ISSN={["1533-4880"]}, DOI={10.1166/jnn.2005.071}, abstractNote={Macroscopic fracture parameters are investigated on 2D graphene systems containing atomic-scale cracks. In the discrete atomistic simulations the interatomic forces are described by the Tersoff-Brenner potential. Two methods to calculate the elastic energy release rates in atomic systems, the global energy method and the local force method, are developed. The values of energy release rates of several graphene systems in symmetric (mode I) and antisymmetric (mode II) small deformation are obtained from atomistic simulations and then compared with the results obtained through homogenized material properties based on linear elastic fracture mechanics. The results show good agreement between discrete atomistic and continuum mechanics modeling for fracture. Meanwhile, atomic stress fields in front of crack tips are investigated through molecular mechanics simulation by applying remote K-field deformation. The atomic stress distributions match very well with those of linear elastic solutions. These establish connections of fracture parameters between microscopic and macroscopic description of fracture in covalently bonded solids.}, number={4}, journal={JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY}, author={Jin, Y and Yuan, FG}, year={2005}, month={Apr}, pages={601–608} }
 @article{yang_yuan_2005, title={Transient wave propagation of isotropic plates using a higher-order plate theory}, volume={42}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2004.12.014}, abstractNote={Transient wave propagation of isotropic thin plates using a higher-order plate theory is presented in this paper. The aim of this investigation is to assess the applicability of the higher-order plate theory in describing wave behavior of isotropic plates at higher frequencies. Both extensional and flexural waves are considered. A complete discussion of dispersion of isotropic plates is first investigated. All the wave modes and wave behavior for each mode in the low and high-frequency ranges are provided in detail. Using the dispersion relation and integral transforms, exact integral solutions for an isotropic plate subjected to pure impulse load and a number of wave excitations based on the higher-order theory are obtained and asymptotic solutions which highlight the physics of waves are also presented. The axisymmetric three-dimensional analytical solutions of linear wave equations are also presented for comparison. Results show that the higher-order theory can predict the wave behavior closely with exact linear wave solutions at higher frequencies.}, number={14}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Yang, S and Yuan, FG}, year={2005}, month={Jul}, pages={4115–4153} }
 @article{hutapea_yuan_pagano_2003, title={Micro-stress prediction in composite laminates with high stress gradients}, volume={40}, ISSN={["0020-7683"]}, DOI={10.1016/S0020-7683(03)00018-0}, abstractNote={The objective of this research is to develop a macroscopic theory, which can provide the connection between macro-mechanics and micro-mechanics in characterizing the micro-stress of composite laminates in regions of high macroscopic stress gradients. The micro-polar theory, a class of higher-order elasticity theory, of composite laminate mechanics is implemented in a well-known Pipes–Pagano free edge boundary problem. The micro-polar homogenization method to determine the micro-polar anisotropic effective elastic moduli is presented. A displacement-based finite element method based on micro-polar theory in anisotropic solids is developed in analyzing composite laminates. The effects of fiber volume fraction and cell size on the normal stress along the artificial interface resulting from ply homogenization of the composite laminate are also investigated. The stress response based on micro-polar theory is compared with those deduced from the micro-mechanics and classical elasticity theory. Special attention of the investigation focuses on the stress fields near the free edge where the high macro-stress gradient occurs. The normal stresses along the artificial interface and especially, the micro-stress along the fiber/matrix interface on the critical cell near the free edge where the high macro-stress gradient detected are the focus of this investigation. These micro-stresses are expected to dominate the failure initiation process in composite laminate. A micro-stress recovery scheme based on micro-polar analysis for the prediction of interface micro-stresses in the critical cell near the free edge is found to be in very good agreement with “exact” micro-stress solutions. It is demonstrated that the micro-polar theory is able to capture the micro-stress accurately from the homogenized solutions.}, number={9}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Hutapea, P and Yuan, FG and Pagano, NJ}, year={2003}, month={May}, pages={2215–2248} }
 @article{jin_yuan_2003, title={Simulation of elastic properties of single-walled carbon nanotubes}, volume={63}, ISSN={["1879-1050"]}, DOI={10.1016/S0266-3538(03)00074-5}, abstractNote={In this paper, selected effective elastic moduli of single-walled carbon nanotubes are simulated numerically. This effective macroscopic behavior is studied using molecular dynamics (MD) simulations in which the dynamic response and mutual force interaction among atoms of the nanostructures are obtained when subjected to small-strain deformation. Both force and energy approaches that link the behavior at the atomic and macroscopic scales of the nanotubes are used to predict the elastic moduli under different deformation modes. A comparison of the elastic constants obtained from MD simulation with available experimental data is made.}, number={11}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Jin, Y and Yuan, FG}, year={2003}, month={Aug}, pages={1507–1515} }
 @article{yuan_pagano_2003, title={Size scales for accurate homogenization in the presence of severe stress gradients}, volume={10}, DOI={10.1080/15376490390231836}, number={4}, journal={Mechanics of Advanced Materials and Structures}, author={Yuan, F. G. and Pagano, N. J.}, year={2003}, pages={353–365} }
 @article{yuan_yang_yang_2003, title={Three-dimensional Green's functions for composite laminates}, volume={40}, ISSN={["1879-2146"]}, DOI={10.1016/S0020-7683(02)00545-0}, abstractNote={The three-dimensional Green’s functions due to a point force in composite laminates are solved by using generalized Stroh formalism and two-dimensional Fourier transforms. Each layer of the composite is generally anisotropic and linearly elastic. The interfaces between different layers are parallel to the top and bottom surfaces of the composite and are perfectly bonded. The Green’s functions of point forces applied at the free surface, interface, and in the interior of a layer are derived in the Fourier transformed domain respectively. The surfaces are imposed by a proportional spring-type boundary condition. The spring-type condition may be reduced to traction-free, displacement-fixed, and mirror-symmetric conditions. Numerical examples are given to demonstrate the validity and elegance of the present formulation of three-dimensional point-force Green’s functions for composite laminates.}, number={2}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Yuan, FG and Yang, S and Yang, B}, year={2003}, month={Jan}, pages={331–342} }
 @article{yang_pan_yuan_2003, title={Three-dimensional stress analyses in composite laminates with an elastically pinned hole}, volume={40}, ISSN={["0020-7683"]}, DOI={10.1016/S0020-7683(02)00523-1}, abstractNote={We present a three-dimensional (3-D) stress analysis for composite laminates with an elastically pinned circular hole. The effects of friction, bearing force and bypass loading on the stress redistribution are studied in detail. The numerical approach is based on a multilayer boundary element method (MLBEM), a non-traditional BEM particularly designed for anisotropic composite laminates, coupled with the traditional BEM for the pin filling the hole. The unique characteristic of the MLBEM is that the fundamental solution employs Green’s functions that satisfy the interfacial continuity conditions and top- and bottom-surface traction-free and symmetry conditions. This fundamental solution allows us to design a BE scheme without involving discretization on the interfaces and surfaces unless the laminates are imposed by different boundary conditions. Consequently, in this case of pinned joint, only the hole surface among the composite boundary and interfaces needs to be discretized. A Coulomb-type friction law is used to simulate the frictional contact interaction between the composite and pin. To solve the frictional contact problem, an iterative scheme of successive over-relaxation has been proposed where the contact location and frictional contact condition are determined at the same time in the iteration solution. By applying the MLBEM, stress analyses are performed for a laminate plate with the stacking sequence (0/∓45/90)s. The issues of engineering interests, such as the loading-sequence and cycling dependencies of stress state due to the presence of friction, are addressed. The solutions, shown by complicated contact maps and stress states around the hole, suggested that a 3-D approach to pinned composite joints is necessary for the interpretation of the underlying physics.}, number={8}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Yang, B and Pan, E and Yuan, FG}, year={2003}, month={Apr}, pages={2017–2035} }
 @article{yuan_yang_2002, title={Crack-tip fields in anisotropic shells}, volume={113}, ISSN={["0376-9429"]}, DOI={10.1023/A:1014245311814}, number={4}, journal={INTERNATIONAL JOURNAL OF FRACTURE}, author={Yuan, FG and Yang, S}, year={2002}, month={Feb}, pages={309–326} }
 @article{lin_yuan_2001, title={Damage detection of a plate using migration technique}, volume={12}, DOI={10.1106/L388-OYY3-VDQC-4V5C}, number={7}, journal={Journal of Intelligent Material Systems and Structures}, author={Lin, X. and Yuan, F. G.}, year={2001}, pages={469–482} }
 @article{lin_yuan_2001, title={Detection of multiple damages by prestack reverse-time migration}, volume={39}, ISSN={["0001-1452"]}, DOI={10.2514/2.1220}, abstractNote={An approach to detect and image multiple damages in a platelike structure is presented. A structural health monitoring system (SHMS) with a linear array of actuators/sensors is proposed. The integrated actuators/sensors are used to activate/receive lowest-order antisymmetric A0 mode Lamb waves. A migration technique used in geophysical exploration and seismic prospecting is adopted to interpret the backscattering wave e eld and to image thee awsinthestructure.Theproposed approachismodeledbya two-dimensional explicite nitedifferencemethod both in simulating the ree ection waves and in implementing the prestack migration. An analytical solution based on Mindlin plate theory (Mindlin, R. D., “ Ine uence of Rotary Inertia and Shear on Flexural Motions of Isotropic, Elastic Plates,” Journal of Applied Mechanics , Vol. 18, No. 1, 1951, pp. 31 ‐38) is derived to verify the accuracy of the numerical algorithm. An excitation-time imaging condition specie cally for the migration of waves in a plate is introduced based on a ray tracing concept, and prestack reverse-time migration is proceeded to propagate the ree ection energy back to the damages. The plate is imaged in terms of the velocity of transverse deformation after migration,thus, thelocations, dimensions, and seriousness of thee awscan bevisually displayed. Numerical results show that multiple damages can be successfully detected and the image of the damages correlate well with the target damages. The conclusion is that prestack migration can be a prospective technique in SHMS applications.}, number={11}, journal={AIAA JOURNAL}, author={Lin, X and Yuan, FG}, year={2001}, month={Nov}, pages={2206–2215} }
 @article{lin_yuan_2001, title={Diagnostic Lamb waves in an integrated piezoelectric sensor/actuator plate: analytical and experimental studies}, volume={10}, ISSN={["0964-1726"]}, DOI={10.1088/0964-1726/10/5/307}, abstractNote={The objective of this study is to model the diagnostic transient waves in an integrated piezoelectric sensor/actuator plate with a view to using it as a first step towards establishing an entire structural health monitoring system and to provide experimental verification of the proposed models. PZT ceramic disks are surface mounted on an aluminum plate acting as both actuators and sensors to generate and collect A0 mode Lamb waves. Mindlin plate theory is adopted to model the propagating waves by taking both transverse shear and rotary inertia effects into account. Actuator and sensor models are both proposed. The interaction between an actuator and the host plate is modeled based on classical lamination theory. The converse piezoelectric effect of the actuator is treated as an equivalent bending moment applied to the host plate. The sensor acts as a capacitor that converts the sensed strain change into a voltage response. An analytical expression for the sensor output voltage in terms of the given input excitation signal is derived, and then experimental work is performed to verify the accuracy of the analytical model. Experimental results show that single-mode Lamb waves in the plate can be successfully generated and collected through the integrated PZT disks. The experiment also shows that the predicted sensor output for both amplitude and phase agrees well with experimentally collected data.}, number={5}, journal={SMART MATERIALS & STRUCTURES}, author={Lin, X and Yuan, FG}, year={2001}, month={Oct}, pages={907–913} }
 @article{yuan_yang_2001, title={Fracture behavior of stitched warp-knit fabric composites}, volume={108}, ISSN={["0376-9429"]}, DOI={10.1023/a:1007610908503}, number={1}, journal={INTERNATIONAL JOURNAL OF FRACTURE}, author={Yuan, FG and Yang, S}, year={2001}, month={Mar}, pages={73–94} }
 @article{pan_yang_cai_yuan_2001, title={Stress analyses around holes in composite laminates using boundary element method}, volume={25}, ISSN={["0955-7997"]}, DOI={10.1016/S0955-7997(00)00066-7}, abstractNote={A three-dimensional (3D) boundary element method (BEM) is developed for the analysis of composite laminates with holes. Instead of using Kelvin-type Green's functions of anisotropic infinite space, 3D layered Green's functions with the materials of each layer being generally anisotropic, derived recently in the Fourier transform domain, are implemented into a 3D BEM formulation. A novel numerical algorithm is designed to calculate layered Green's functions efficiently. It should be noted that since layered Green's functions satisfy exactly the continuity conditions along the interfaces and top and bottom free surfaces a priori, the model becomes truly 2D and discretization is only needed along the hole surface and prescribed traction and/or displacement boundaries. To test the validity and accuracy of the proposed method, the present layered BEM formulation is applied to the problem of an infinite anisotropic plate with a circular hole where the analytical solution is available. It is found that even with a very coarse mesh, the present BEM can predict the hoop stress very accurately along the hole surface. The BEM formulation is then applied to analyze two composite laminates (90/0)s and (−45/45)s, under a remote in-plane strain, that have been studied previously with different approaches. For the (90/0)s case, the hoop stresses along the hole surface predicted by the present layered BEM formulation are in very close agreement with the previous results. For the (−45/45)s case, however, it is found that a nearly converged solution (less than 5% convergence by doubling the mesh) by the present method is at significant variance with the previous ones that are lack-of-convergence checks. It can be expected that for designing the bolted joints of composites with many layers, a computational tool developed based on the present techniques would be robust and offer a much better solution with regard to accuracy, versatility and design cycle time.}, number={1}, journal={ENGINEERING ANALYSIS WITH BOUNDARY ELEMENTS}, author={Pan, E and Yang, B and Cai, G and Yuan, FG}, year={2001}, month={Jan}, pages={31–40} }
 @article{wang_yuan_takao_2000, title={A unified loading parameter for creep-crack growth}, volume={456}, ISSN={["1471-2946"]}, DOI={10.1098/rspa.2000.0512}, abstractNote={This paper presents a unified loading parameter JT for creep–crack growth. The parameter JT is a path–independent integral and is derived through an energy analysis based on an assumption that a small creep deformation dominant region exists around the crack tip. The relationship between JT and existing path–independent or path–dependent integrals for special creep–crack growth problems is examined. Existing path–independent or path–dependent integrals for special cases of short or long time are included in JT. The present path–independent integral JT is not limited to any specified constitutive relations except for a small region close to the crack tip, and then it is valid for general creep materials. Energy analysis shows that JT possesses a definite physical meaning and can be used as a loading parameter to characterize the crack growth in time–dependent materials, such as metal under various high–temperature environments and other rate–sensitive materials. In fact, the JT is a kind of J–type integral for creep materials and it also has an intimate relation with existing C(t) and C* integrals. An experimental procedure for evaluating JTas well as C* is discussed.}, number={1993}, journal={PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES}, author={Wang, WX and Yuan, FG and Takao, Y}, year={2000}, month={Jan}, pages={163–183} }
 @article{yuan_yang_kim_2000, title={Analysis of axisymmetrically-loaded filament wound composite cylindrical shells}, volume={50}, ISSN={["1879-1085"]}, DOI={10.1016/S0263-8223(00)00061-1}, abstractNote={Exact elasticity solutions for laminated composite cylindrical shells under axisymmetric loading are presented. The material of the shell is assumed to be cylindrically anisotropic. Based on the three-dimensional anisotropic elasticity, three coupled governing partial differential equations in terms of displacements are developed. The general expressions for the stresses and displacements by using the Frobenius method in the radial coordinate are discussed. The closed form solutions based on Flügge thin shell theory are also derived for comparison purposes. Detailed stress and displacement distributions for long laminated shells with layups [90/0/45/−45]s and [45/−45]s under periodic band loads are investigated.}, number={2}, journal={COMPOSITE STRUCTURES}, author={Yuan, FG and Yang, W and Kim, H}, year={2000}, month={Oct}, pages={115–130} }
 @article{yuan_yang_2000, title={Asymptotic crack-tip fields in an anisotropic plate subjected to bending, twisting moments and transverse shear loads}, volume={60}, ISSN={["1879-1050"]}, DOI={10.1016/S0266-3538(00)00043-9}, abstractNote={Asymptotic crack-tip fields, including the effect of transverse shear deformation, in an anisotropic plate under bending, twisting moments and transverse shear loads are presented. By utilizing the Hellinger–Reissner variational principle, the equilibrium equations and generalized strain/stress relations for anisotropic Reissner plate theory are obtained. Assuming the displacement and stress resultant are in a separation-of-variable form, it is found that, for the first two order terms of the asymptotic solution, the equations governing crack-tip fields of anisotropic plate bending are analogous to those governing plane stress and anti-plane deformation of anisotropic elasticity. Thus the Stroh formalism can be used to characterize the crack-tip fields of the anisotropic plate up to the second-order term and the energy release rate can be expressed in a very compact form in terms of stress intensity factors and the Barnett–Lothe tensor L. The first three order terms of the crack-tip displacement and stress fields including the “T-stresses” in bending and transverse shear are presented. The displacement and stress fields near crack tips in isotropic plates up to the second order are also provided. The energy release rate expression for orthotropic and isotropic plates is also derived. The solutions of anisotropic plates by a combination of in-plane and bending loads is also investigated. The expression of the path-independent integral, J, in terms of the generalized stress and strain is derived which is useful to calculate the value of stress intensity factors. Finally, on the basis of the asymptotic solutions, methods of determining stress intensity factors and T-stresses are proposed.}, number={12-13}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Yuan, FG and Yang, S}, year={2000}, pages={2489–2502} }
 @article{pan_yuan_2000, title={Boundary element analysis of three-dimensional cracks in anisotropic solids}, volume={48}, ISSN={["0029-5981"]}, DOI={10.1002/(SICI)1097-0207(20000520)48:2<211::AID-NME875>3.0.CO;2-A}, abstractNote={This paper presents a boundary element analysis of linear elastic fracture mechanics in three-dimensional cracks of anisotropic solids. The method is a single-domain based, thus it can model the solids with multiple interacting cracks or damage. In addition, the method can apply the fracture analysis in both bounded and unbounded anisotropic media and the stress intensity factors (SIFs) can be deduced directly from the boundary element solutions. 
 
 
 
The present boundary element formulation is based on a pair of boundary integral equations, namely, the displacement and traction boundary integral equations. While the former is collocated exclusively on the uncracked boundary, the latter is discretized only on one side of the crack surface. The displacement and/or traction are used as unknown variables on the uncracked boundary and the relative crack opening displacement (COD) (i.e. displacement discontinuity, or dislocation) is treated as a unknown quantity on the crack surface. This formulation possesses the advantages of both the traditional displacement boundary element method (BEM) and the displacement discontinuity (or dislocation) method, and thus eliminates the deficiency associated with the BEMs in modelling fracture behaviour of the solids. Special crack-front elements are introduced to capture the crack-tip behaviour. Numerical examples of stress intensity factors (SIFs) calculation are given for transversely isotropic orthotropic and anisotropic solids. For a penny-shaped or a square-shaped crack located in the plane of isotropy, the SIFs obtained with the present formulation are in very good agreement with existing closed-form solutions and numerical results. For the crack not aligned with the plane of isotropy or in an anisotropic solid under remote pure tension, mixed mode fracture behavior occurs due to the material anisotropy and SIFs strongly depend on material anisotropy. Copyright © 2000 John Wiley & Sons, Ltd.}, number={2}, journal={INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING}, author={Pan, E and Yuan, FG}, year={2000}, month={May}, pages={211–237} }
 @article{yuan_yang_2000, title={Crack-tip fields for matrix cracks between dissimilar elastic and creeping materials}, volume={103}, DOI={10.1023/A:1007620114187}, number={4}, journal={International Journal of Fracture}, author={Yuan, F. G. and Yang, S.}, year={2000}, pages={327–360} }
 @article{yang_yuan_2000, title={Determination and representation of the stress coefficient terms by path-independent integrals in anisotropic cracked solids}, volume={101}, ISSN={["0376-9429"]}, DOI={10.1023/A:1007639126484}, number={4}, journal={INTERNATIONAL JOURNAL OF FRACTURE}, author={Yang, S and Yuan, FG}, year={2000}, month={Feb}, pages={291–319} }
 @article{yang_yuan_2000, title={Kinked crack in anisotropic bodies}, volume={37}, ISSN={["1879-2146"]}, DOI={10.1016/S0020-7683(99)00222-X}, abstractNote={Solutions are presented for a crack kinking out of the crack plane in a generally anisotropic elastic body under two-dimensional deformation. Based on Stroh formalism, a system of singular integral equations governing the kinking crack with small kink length is given in a simple, straightforward form. The explicit expressions of the stress intensity factors, T-stresses, and energy release rates at the kinked crack tip are presented in terms of some nondimensional coefficients together with the stress intensity factors, T-stresses, and the coefficients of the third term acting on the main crack tip prior to crack kinking. The nondimensional coefficients depend on kink angle and material constants, but not on kink length. The energy release rate ratio which may characterize the competition along different crack growth directions is provided. The role of T-stresses and the third-term applied at the main crack field are determined which can be significant in the kinking and the stability of the kinked crack. Based on the energy release rate fracture criterion, the stability condition of the kinked crack is derived. The influences of anisotropy and loading mixity on the implications of crack kinking behavior is also given. The results for monoclinic materials with symmetry plane at x3=0 are derived from general results. Numerical results for the stress intensity factors, T-stresses at the kinked acrack tip and the energy release rate ratio for some special cases are provided. The dimensionless coefficients for crack kinking of orthotropic materials at the right angle to the main crack plane are tabulated.}, number={45}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Yang, S and Yuan, FG}, year={2000}, month={Nov}, pages={6635–6682} }
 @article{pagano_yuan_2000, title={The significance of effective modulus theory (homogenization) in composite laminate mechanics}, volume={60}, ISSN={["0266-3538"]}, DOI={10.1016/S0266-3538(00)00042-7}, abstractNote={In this work, we consider the issues regarding the homogenization of fiber-reinforced layers in a laminate in the presence of macroscopic (ply-level) stress gradients. This is accomplished by considering a series of (free edge) boundary value problems similar to those treated by Pagano and Rybicki in 1974. Despite our inability to provide such a homogenization theory, if one truly exists, we can devise approaches to predict the fiber/matrix interfacial stresses in an arbitrary cell by applying certain displacements and/or tractions on the cell boundaries. These boundary conditions are those derived by representing each layer in the laminate by conventional effective modulus theory. It is shown that these approximations can lead to reasonably accurate interfacial stresses and offer great promise as a means of solving practical laminate problems reinforced by fibers of moderate diameter.}, number={12-13}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Pagano, NJ and Yuan, FG}, year={2000}, pages={2471–2488} }
 @article{pan_yuan_2000, title={Three-dimensional Green's functions in anisotropic bimaterials}, volume={37}, DOI={10.1016/S0020-7683(99)00216-4}, abstractNote={In this paper, three-dimensional Green’s functions for anisotropic bimaterials are studied based on Stroh formalism and two-dimensional Fourier transforms. Although the Green’s functions can be expressed exactly in the Fourier transform domain, it is difficult to obtain the explicit expressions of the Green’s functions in the physical domain due to the general anisotropy of the material and a geometry plane involved. Utilizing Fourier inverse transform in the polar coordinate and combining with Mindlin’s superposition method, the physical-domain bimaterial Green’s functions are derived as a sum of a full-space Green’s function and a complementary part. While the full-space Green’s function is in an explicit form, the complementary part is expressed in terms of simple regular line-integrals over [0, 2π] that are suitable for standard numerical integration. Furthermore, the present bimaterial Green’s functions can be reduced to the special cases such as half-space, surface, interfacial, and full-space Green’s functions. Numerical examples are given for both half-space and bimaterial cases with isotropic, transversely isotropic, and anisotropic material properties to verify the applicability of the technique. For the half-space case with isotropic or transversely isotropic material properties, the Green’s function solutions are in excellent agreement with the existing analytical solutions. For anisotropic half-space and bimaterial cases, numerical results show the strong dependence of the Green’s functions on the material properties.}, number={38}, journal={International Journal of Solids and Structures}, author={Pan, E. and Yuan, F. G.}, year={2000}, pages={5329–5351} }
 @article{pan_yuan_2000, title={Three-dimensional Green's functions in anisotropic piezoelectric bimaterials}, volume={38}, ISSN={["0020-7225"]}, DOI={10.1016/S0020-7225(00)00017-3}, abstractNote={In this paper, a recently proposed method by E. Pan and F.G. Yuan (Int. J. Solids Struct., 2000) for the calculation of the elastic bimaterial Green’s functions is extended to the analysis of three-dimensional Green’s functions for anisotropic piezoelectric bimaterials. The method is based on the Stroh formalism and two-dimensional Fourier transforms in combination with Mindlin’s superposition method. We first derive Green’s functions in exact form in the Fourier transform domain. When inverting the Fourier transform, a polar coordinate transform is introduced so that the radial integral from 0 to +∞ can be carried out exactly. Therefore, the bimaterial Green’s functions in the physical domain are derived as a sum of a full-space Green’s function and a complementary part. While the full-space Green’s function is in an explicit form, as derived recently by E. Pan and F. Tonon (Int. J. Solids Struct., 37 (2000): 943–958), the complementary part is expressed in terms of simple regular line integrals over [0, 2π] that are suitable for standard numerical integration. Furthermore, the present bimaterial Green’s functions can be reduced to the special cases such as half-space, surface, interfacial, and full-space Green’s functions. Uncoupled solutions for the purely elastic and purely electric case can also be simply obtained by setting the piezoelectric coefficients equal to zero. Numerical examples for Green’s functions are given for both half-space and bimaterial cases with transversely isotropic and anisotropic material properties to verify the applicability of the technique. Certain interesting features associated with these Green’s functions are observed and discussed, as related to the selected material properties.}, number={17}, journal={INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE}, author={Pan, E and Yuan, FG}, year={2000}, month={Nov}, pages={1939–1960} }
 @article{cai_yuan_1999, title={Electric current-induced stresses at the crack tip in conductors}, volume={96}, ISSN={["0376-9429"]}, DOI={10.1023/A:1018670829822}, number={3}, journal={INTERNATIONAL JOURNAL OF FRACTURE}, author={Cai, GX and Yuan, FG}, year={1999}, pages={279–301} }
 @article{hutapea_yuan_1999, title={The effect of thermal aging on the Mode-I interlaminar fracture behavior of a high-temperature IM7/LaRC-RP46 composite}, volume={59}, ISSN={["0266-3538"]}, DOI={10.1016/S0266-3538(98)00164-X}, abstractNote={High-temperature Mode-I interlaminar fracture tests have been conducted to evaluate the effect of thermal aging at various temperatures on the fracture behavior of IM7/LaRC-RP46 composites. Double cantilever beam specimens made of unidirectional laminates were used in the test program. The specimens were aged at high temperatures for times up to and including 6000 h. Changes in fracture behavior that occurred over periods of aging time and aging temperature have been measured and determined for variations of test temperature and loading rate. As expected, the crack growth at high temperature showed non-linear load–displacement response as a result of the rate-dependent nature of behaviour of the matrix. Present loading rates had no measurable effect on GIC values for unaged and aged specimens. High temperature increased GIC values of aged and unaged specimens at temperature near Tg and decreased at temperatures above Tg. Thermal aging for longer times also significantly reduced GIC initiation values for aged specimens. In contrast, thermal aging increased GIC propagation values as a result of the occurrence of increased fiber bridging, fiber breakage and microcracks, as deduced from SEM studies. Results also indicated that high temperatures slowed crack-growth rates. However, thermal aging and higher loading rates accelerated crack-growth rates. The effect of thermal aging and temperature on the crack growth rate, da/dt, can be correlated with the C* parameter in the power-law relationship.}, number={8}, journal={COMPOSITES SCIENCE AND TECHNOLOGY}, author={Hutapea, P and Yuan, FG}, year={1999}, pages={1271–1286} }
 @article{cai_yuan_1998, title={Stresses around the crack tip due to electric current and self-induced magnetic field}, volume={29}, ISSN={["0965-9978"]}, DOI={10.1016/S0965-9978(97)00078-1}, abstractNote={Mode-I stress distributions around the crack tip in a thin conducting plate under a uniform electric current flow are investigated. The stresses are generated by the electromagnetic forces, which in turn are caused by the interplay between electric current and the self-induced magnetic field. The current flow is disturbed by the presence of the crack and so is the magnetic field, which is induced by the electric current. A numerical analysis procedure is developed to analyze the coupled problem of magnetic field and stress distributions, and an algorithm based on the Biot–Savart integral is proposed to reduce the calculation of magnetic field in the thin plate to a two-dimensional analysis problem. The electric current, magnetic flux, and stress fields show the −1/2 singularity at the crack tip and the normalized mode-I stress intensity factors are determined numerically.}, number={3-6}, journal={ADVANCES IN ENGINEERING SOFTWARE}, author={Cai, GX and Yuan, FG}, year={1998}, pages={297–306} }
 @article{wang_takao_yuan_potter_pater_1998, title={The interlaminar Mode I fracture of IM7/LaRC-RP46 composites at high temperatures}, volume={32}, ISSN={["0021-9983"]}, DOI={10.1177/002199839803201603}, abstractNote={In this paper, the interlaminar Mode I fracture behavior of a IM7/LaRC-RP46 composite laminate at elevated temperatures was investigated by DCB (double cantilever beam) static fracture testing. Straight-sided DCB specimens were made from unidirectional laminates and tested at three high temperature levels of 232°C (450°F), 288°C (550°F) and 343°C (650°F) at five crosshead rates from 0.51 mm/min. (0.00033 in./sec) to 6.1 mm/min. (0.004 in./sec). Unlike the linear load-displacement response observed at room temperature, the load-displacement response at high temperatures behaves nonlinearly before reaching the maximum load. Two crack growth regions of slow crack extension and relatively fast crack propagation were observed and the slow crack extension caused the nonlinear load-displacement response. The initial fracture toughness, G IC decreased slightly, while the propagation value increased by nearly 30% to 100% with increased temperature. The materials exhibited the highest fracture toughness at 288°C among the three different tested temperatures. The complex combination effects of fiber bridging and the change of matrix properties and fiber/matrix interfacial adhesion at elevated temperatures are found to be the major reason resulting in the variation of G IC with temperature. After the temperature increased beyond T g (glass transition temperature), which is approximately 310°C, the propagation fracture toughness decreased drastically and aging effects of the high temperature on the matrix are observed. No significant effects of crosshead rate on the G IC were observed in the range of the present rates. However, the crosshead rate is found to significantly influenced the crack growth rate.}, number={16}, journal={JOURNAL OF COMPOSITE MATERIALS}, author={Wang, WX and Takao, Y and Yuan, FG and Potter, BD and Pater, RH}, year={1998}, pages={1508–1526} }
 @article{yuan_hsieh_1998, title={Three-dimensional wave propagation in composite cylindrical shells}, volume={42}, ISSN={["0263-8223"]}, DOI={10.1016/S0263-8223(98)00063-4}, abstractNote={An exact solution of free harmonic wave propagation in a composite laminated cylindrical shell is presented within the framework of the theory of three-dimensional cylindrically anisotropic elasticity. The dispersion equation governing these motions is established through a helical solution form in conjunction with a Frobenius power series method. Degenerate cases of infinite wavelength, axisymmetric waves, and of orthotropic shells are derived and discussed. Analytical results are presented for a variety of cases and are compared with those obtained from a Flügge shell theory.}, number={2}, journal={COMPOSITE STRUCTURES}, author={Yuan, FG and Hsieh, CC}, year={1998}, month={Jun}, pages={153–167} }
 @article{yuan_yang_1997, title={Crack-tip fields in elastic-plastic material under plane stress mode I loading}, volume={85}, ISSN={["0376-9429"]}, DOI={10.1023/A:1007361116709}, number={2}, journal={INTERNATIONAL JOURNAL OF FRACTURE}, author={Yuan, FG and Yang, S}, year={1997}, pages={131–155} }
 @article{yuan_pagano_cai_1997, title={Elastic moduli of brittle matrix composites with interfacial debonding}, volume={34}, ISSN={["1879-2146"]}, DOI={10.1016/S0020-7683(96)00003-0}, abstractNote={Elastic moduli of brittle matrix composites with uni-symmetric and doubly-symmetric interfacial debonding are studied.† Traction continuity and displacement continuity conditions are imposed along the boundary of adjacent representative elements. With careful consideration of symmetric and skew-symmetric conditions in the cell, the uni-symmetric and doubly-symmetric debonding cases can be modelled by a half cell or a quarter cell problem, respectively. The RVE boundaries, in general, do not remain straight for the composite under loading. Parametric studies assessing the effect of the debonding angle, the shear moduli ratios in the constituents and the fiber volume fractions on the composite shear moduli are also presented through finite element analyses. Comparison of the shear moduli between the current compatible displacement field and the linear straight boundary displacements of the RVE is addressed.}, number={2}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Yuan, FG and Pagano, NJ and Cai, X}, year={1997}, month={Jan}, pages={177–201} }
 @article{yuan_yang_1997, title={The curved interfacial crack between dissimilar isotropic solids}, volume={34}, ISSN={["0020-7683"]}, DOI={10.1016/S0020-7683(96)00049-2}, abstractNote={The paper examines analytically the role of curvature on the stress distribution of a curved interfacial crack between dissimilar isotropic solids. The crack-tip fields under in-plane and antiplane shear loading are studied, respectively. Using an asymptotic expansion of the circular interface geometry, the asymptotic solutions of the stress and displacement fields in the vicinity of the curved crack tip derived from modified stress functions is obtained. The eigenfunctions associated with the eigenvalues λ for the curved crack consist of not only rλ terms, but also rλ+1, rλ+2, … terms. In some cases, the terms rλ+1(ln r), rλ+2(ln r), etc. may also exist. Two examples, frictionless contact near the circular crack-tip under in-plane loading and circular interfacial crack subject to antiplane shear loading, are derived in a closed-form asymptotic solution to elucidate the curvature effect. The case of fully open interfacial crack is also briefly described. Comparing the eigenfunction solutions of straight interfaces, the curvature effect enters the stress fields from the third-order term of the asymptotic solution for both cases. The condition for the existence of the r½(lnr) term in the circular interfacial crack with frictionless contact is presented explicitly.}, number={6}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Yuan, FG and Yang, S}, year={1997}, month={Feb}, pages={641–660} }