@article{he_rocha_leser_sava_leser_2019, title={Least-squares reverse time migration (LSRTM) for damage imaging using Lamb waves}, volume={28}, ISSN={["1361-665X"]}, DOI={10.1088/1361-665X/ab14b1}, abstractNote={Large-area monitoring and accurate damage quantification are two primary goals of ultrasonic, guided wave-based structural health monitoring (SHM). Reverse-time migration (RTM) is an effective damage imaging technique for both metallic and composite plates. In geophysics, incorporating least-squares inversion into migration can generate images with higher resolution and suppressed artifacts in comparison with conventional RTM. Development of a least-squares reverse time migration (LSRTM) technique is promising for SHM since it could expand the imaging area for a given sensor array while maintaining a relatively high resolution. An LSRTM technique is introduced in this research for damage imaging in an isotropic plate using A0 mode Lamb waves. A finite difference algorithm based on the Mindlin plate theory was used to simulate the flexural wave propagation. To form the theoretical foundations for guided wave-based LSRTM, a forward modeling operator and its adjoint are defined. The damage images from both numerical simulations and experiments show that LSRTM can enhance imaging resolution and reduce artifacts.}, number={6}, journal={SMART MATERIALS AND STRUCTURES}, author={He, Jiaze and Rocha, Daniel C. and Leser, Patrick E. and Sava, Paul and Leser, William P.}, year={2019}, month={Jun} }
 @article{he_leckey_leser_leser_2019, title={Multi-mode reverse time migration damage imaging using ultrasonic guided waves}, volume={94}, ISSN={["1874-9968"]}, DOI={10.1016/j.ultras.2018.08.005}, abstractNote={The sensitivity of Lamb wave modes to a particular defect or instance of damage is dependent on various factors (e.g., the local strain energy density due to that wave mode). As a result, different modes will be more useful than others for damage detection and quantification, dependent on damage type and location. For example, prior work in the field has shown that out-of-plane modes may have a higher sensitivity than in-plane modes to surface defects in plates. The excitability of a certain data acquisition system and the corresponding resolution for damage imaging also varies with frequency. The aim of the present work was to develop a multi-mode damage imaging technique that enables characterization of damage type and size, general sensitivity to unknown damage types, higher resolution imaging, and detectability regardless of the data acquisition system used. A reverse-time migration (RTM) imaging algorithm was combined with a numerical simulator-the three-dimensional (3D) elastodynamic finite integration technique (EFIT)-to provide multi-mode damage imaging. The approach was applied to two simulated case studies featuring damaged isotropic plates. Sensitivities of damage type to wave mode were investigated by separating the A0 and S0 Lamb wave modes obtained from the resultant RTM wavefields.}, journal={ULTRASONICS}, author={He, Jiaze and Leckey, Cara A. C. and Leser, Patrick E. and Leser, William P.}, year={2019}, month={Apr}, pages={319–331} }
 @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{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{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{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{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} }
 @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} }