@article{wang_wee_peters_2022, title={Amplification of Lamb-Wave Detection via Fiber Bragg Gratings Using Ultrasonic Horns}, volume={5}, ISSN={["2572-3898"]}, DOI={10.1115/1.4053582}, abstractNote={
 Fiber Bragg Grating (FBG) sensors are often applied as Lamb wave detectors for structural health monitoring systems. Analyzing the measured signal for the identification of structural damage requires a high signal-to-noise ratio because of the low amplitude Lamb waves. This paper applies a two-dimensional ultrasonic horn between the structure and a remotely bonded FBG sensor to increase the amplitudes of the measured signal. Experimentally we test a variety of ultrasonic geometries and demonstrate a 100% increase in the measured ultrasonic signal amplitude using a metallic ultrasonic horn with step-down geometry. A bonding procedure for the combined ultrasonic horn and optical fiber is also developed that produces repeatable signal measurements. For some horn geometries, an additional vibration signal at the Lamb wave excitation frequency is observed in the measurements. Laser Doppler vibrometry measurements and finite element analysis demonstrate that the signal is due to the natural vibration of the horn. The experimental results demonstrate that using an aluminum ultrasonic horn to focus wave is an excellent method to increase the sensitivity of the FBG to the small amplitude Lamb wave, provided the horn vibration characteristics are taken account in the design of the measurement system.}, number={3}, journal={JOURNAL OF NONDESTRUCTIVE EVALUATION, DIAGNOSTICS AND PROGNOSTICS OF ENGINEERING SYSTEMS}, author={Wang, Chia-Fu and Wee, Junghyun and Peters, Kara}, year={2022}, month={Aug} }
 @article{wang_wee_peters_2022, title={Amplifying Lamb Wave Detection for Fiber Bragg Grating with a Phononic Crystal GRIN Lens Waveguide}, volume={22}, ISSN={["1424-8220"]}, url={https://www.mdpi.com/1424-8220/22/21/8426}, DOI={10.3390/s22218426}, abstractNote={This paper demonstrates that a graded-index (GRIN) phononic lens, combined with a channel waveguide, can focus anti-symmetric Lamb waves for extraction by a detector with strong directional sensitivity. Guided ultrasonic wave inspection is commonly applied for structural health monitoring applications; however, obtaining sufficient signal amplitude is a challenge. In addition, fiber Bragg grating (FBG) sensors have strong directional sensitivity. We fabricate the GRIN structure, followed by a channel waveguide starting at the focal point, using a commercial 3D printer and mount it on a thin aluminum plate. We characterize the focusing of the A0 mode Lamb wave in the plate, traveling across the GRIN lens using 3D laser Doppler vibrometry. We also measure the extraction of focused energy using an FBG sensor, examining the optimal sensor bond location and bond length in the channel of the waveguide for maximum signal extraction. The measured amplification of the ultrasound signal is compared to theoretical predictions. The results demonstrate that significant amplification of the waveform is achieved and that selecting the location of the FBG sensor in the channel is critical to optimizing the amplification.}, number={21}, journal={SENSORS}, author={Wang, Chia-Fu and Wee, Junghyun and Peters, Kara}, year={2022}, month={Nov} }
 @article{kim_wee_peters_2022, title={Demonstration of Coherent Interference between Acoustic Waves Using a Fiber Ring Resonator}, volume={22}, ISSN={["1424-8220"]}, url={https://www.mdpi.com/1424-8220/22/11/4163}, DOI={10.3390/s22114163}, abstractNote={Optical fibers were previously demonstrated to propagate and detect acoustic modes that were converted from Lamb waves for structural health-monitoring applications; typically, a fiber Bragg grating sensor in the optical fiber is used to detect acoustic modes. Acoustic modes can transfer from one fiber to another through a simple adhesive bond coupler, preserving the waveform of the acoustic mode. This paper experimentally investigates the coherence of acoustic waves through the adhesive coupler, using a fiber ring resonator (FRR) configuration. This configuration was chosen because the wave coupled to the second fiber interferes with the original wave after it encircles the fiber ring. We performed this experiment using different geometries of optical fibers in the ring, including a standard single-mode optical fiber, a hollow silica capillary tube, and a large-diameter multi-mode fiber. The results demonstrate that the acoustic wave, when transferring through an adhesive coupler, interferes coherently even when the main and ring fibers are of different types. Finally, we demonstrate that the FRR can be applied for sensing applications by measuring the mode attenuations in the ring due to a changing external environment (water-level sensing) and measuring the optical-path length change in the ring (temperature sensing).}, number={11}, journal={SENSORS}, author={Kim, Jee Myung and Wee, Junghyun and Peters, Kara}, year={2022}, month={Jun} }
 @article{navratil_wee_peters_2022, title={Ultrasonic frequency response of fiber Bragg grating under direct and remote adhesive bonding configurations}, volume={33}, ISSN={["1361-6501"]}, DOI={10.1088/1361-6501/ac2fea}, abstractNote={Ultrasonic inspection based structural health monitoring is a powerful technique for damage detection in a structure. Ultrasonic waves are often excited at different frequencies to detect damages of different sizes, therefore understanding the frequency response of the sensor can be used to optimize the sensor performance. Fiber Bragg gratings (FBGs) are widely used for this ultrasound collection. The sensitivity of FBGs to a particular ultrasonic frequency is a function of the FBG length and the ultrasonic wavelength. Recently the authors demonstrated that its ultrasound sensitivity is improved for some conditions when the FBG is bonded at a distance away from the adhesive bond, referred to as remote bonding. However, the frequency response of this configuration has not been studied. Therefore, in this paper we measure and compare the ultrasonic frequency responses between a conventional directly bonded FBG and remotely bonded FBG. In theory, the FBG sensitivity varies as a function of ultrasound wavelength-to-grating length (λ/L) ratio. Therefore, for this experimental study, we maintain L constant and vary λ by changing the frequency of the input ultrasonic waves. We demonstrate that there is a region, below a cut-off values of λ/L, for which the remotely bonded FBG output has a higher sensitivity to the Lamb wave amplitude than the directly bonded FBG. The exact value of this λ/L cut-off depends on the mechanical properties of the structure, the windowing of the input Lamb wave, and the FBG properties. We also demonstrate that windowing the Lamb wave excitation signal has a similar affect to apodizing the FBG sensor in modifying the sensitivity response curve.}, number={1}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Navratil, Andrew and Wee, Junghyun and Peters, Kara}, year={2022}, month={Jan} }
 @article{kim_marashi_wee_peters_2021, title={Acoustic wave coupling between optical fibers of different geometries}, volume={60}, ISSN={["2155-3165"]}, DOI={10.1364/AO.441494}, abstractNote={In this study, we investigate coupling of acoustic guided waves from different types of input fibers, through a bonded coupler, to an optical fiber. These acoustic waves can then be detected with conventional fiber Bragg gratings (FBGs). The input waves are measured using a high-resolution 3D laser Doppler vibrometer, and the output waves in the optical fiber are measured using an FBG. We demonstrate that the wave coupling between two waveguides varies with the cross-sectional area and the modulus of elasticity of the fibers.}, number={36}, journal={APPLIED OPTICS}, author={Kim, Jee Myung and Marashi, Cameron and Wee, Junghyun and Peters, Kara}, year={2021}, month={Dec}, pages={11042–11049} }
 @article{navratil_wee_peters_2021, title={Fiber Bragg grating sensor response to ultrasonic Lamb waves with varying frequency}, volume={11591}, ISSN={["1996-756X"]}, DOI={10.1117/12.2584277}, abstractNote={In structural health monitoring (SHM) applications, fiber Bragg grating (FBG) sensors are typically bonded directly to the surface of a structure to detect ultrasonic waves for damage identification. The sensitivity of the bonded FBGs to guided waves varies significantly with input ultrasound wavelength (λ)-to-FBG grating length (L) ratio, i.e., λ/L. Recently, the authors have demonstrated that the detection sensitivity of an FBG can be potentially increased by remotely bonding the optical fiber at a distance away from the FBG, however its response as a function of λ/L has not been studied in detail. In this work, we investigate the ultrasound detection of directly bonded FBG and remotely bonded FBG with varying λ/L. Specifically, we maintain L constant and change λ by varying the ultrasound excitation frequency. Using a 3D laser Doppler vibrometer (LDV) we first characterize input Lamb waves as a function of frequency, which are excited in a thin plate using a broadband transducer. Next, we measure the output response of directly bonded FBG and remotely bonded FBG to the same input Lamb waves. Finally, we examine the output FBG responses normalized with the LDV measurements of input waves, investigating the FBG sensitivity as a function of λ/L ratio. Understanding this sensitivity is important because many guided wave signals, for example generated by acoustic emission, are broadband. Additionally, multiple frequencies are often used for guided wave imaging of structures.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, author={Navratil, Andrew and Wee, Junghyun and Peters, Kara}, year={2021} }
 @article{kim_marashi_wee_peters_2021, title={Investigation on acoustic wave transfer variation between fibers of different diameters and types using acoustic coupler}, volume={11591}, ISSN={["1996-756X"]}, DOI={10.1117/12.2584249}, abstractNote={When using fiber Bragg grating (FBG) sensors in structural health monitoring (SHM) applications, one of the drawbacks is that the sensor location is fixed once it is installed and it is difficult to extend an already existing system. The use of an acoustic coupler to transfer fiber guided traveling waves from one fiber to another could resolve this issue as the system could be modified for extension. In this study, we investigate the coupling of optical fiber guided waves between two different types of fibers through an acoustic coupler. Specifically, input waves are launched into an input fiber and coupled to an output fiber through the acoustic coupler. The input waves are measured using a high-resolution 3D laser Doppler Vibrometer (LDV) and the output waves from the output fiber are measured using an FBG. We demonstrate that the wave coupling between two fibers varies with the cross-sectional area of the input fiber.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2021}, author={Kim, Jee Myung and Marashi, Cameron and Wee, Junghyun and Peters, Kara}, year={2021} }
 @misc{soman_wee_peters_2021, title={Optical Fiber Sensors for Ultrasonic Structural Health Monitoring: A Review}, volume={21}, ISSN={["1424-8220"]}, url={https://www.mdpi.com/1424-8220/21/21/7345}, DOI={10.3390/s21217345}, abstractNote={Guided waves (GW) and acoustic emission (AE) -based structural health monitoring (SHM) have widespread applications in structures, as the monitoring of an entire structure is possible with a limited number of sensors. Optical fiber-based sensors offer several advantages, such as their low weight, small size, ability to be embedded, and immunity to electro-magnetic interference. Therefore, they have long been regarded as an ideal sensing solution for SHM. In this review, the different optical fiber technologies used for ultrasonic sensing are discussed in detail. Special attention has been given to the new developments in the use of FBG sensors for ultrasonic measurements, as they are the most promising and widely used of the sensors. The paper highlights the physics of the wave coupling to the optical fiber and explains the different phenomena such as directional sensitivity and directional coupling of the wave. Applications of the different sensors in real SHM applications have also been discussed. Finally, the review identifies the encouraging trends and future areas where the field is expected to develop.}, number={21}, journal={SENSORS}, author={Soman, Rohan and Wee, Junghyun and Peters, Kara}, year={2021}, month={Nov} }
 @article{wee_alexander_peters_2021, title={Self-referencing ultrasound detection of fiber Bragg grating sensor with two adhesive bonds}, volume={32}, ISSN={["1361-6501"]}, DOI={10.1088/1361-6501/ac065c}, abstractNote={In practical structural health monitoring applications, the environmental conditions typically vary over time. This variation can potentially affect the ultrasound detection of a sensor for damage detection, because ultrasound propagation as well as its coupling through adhesive bond layer to the sensor can be altered. In this case, a reference signal measured without structural defect under some initial condition may not be valid for comparison with the distorted signal (due to defect) that is measured under a different condition. In order to extract reference and distorted signals under the same condition at the same time, in this work we investigate a self-referencing ultrasound detection of fiber Bragg grating (FBG) by bonding an optical fiber at two different locations away from the FBG. We demonstrate that ultrasounds in a thin structure can couple to optical fiber guided ultrasounds through two adhesive bonds. We examine the ultrasounds trapped within the optical fiber, which are producing signal interference and beating behavior of the output FBG response. Based on understanding the ultrasound coupling mechanism through two adhesive bonds, finally we test the self-referencing ultrasound detection case, investigating the output FBG response that contains a combined signal of distorted and reference signals extracted through each bond.}, number={10}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Wee, Junghyun and Alexander, Kevin and Peters, Kara}, year={2021}, month={Oct} }
 @article{soman_wee_peters_ostachowicz_2020, title={Optimization of sensor placement for guided waves based SHM using fiber Bragg grating sensors}, volume={11379}, ISSN={["1996-756X"]}, DOI={10.1117/12.2558393}, abstractNote={Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantage but their use has been limited for the GW sensing due to their limited sensitivity. But the use of the edge-filtering approach in the remote bonded configuration has enhanced their sensitivity and allows use of these sensors for GW measurement. Although these sensors are light in weight and need no additional wiring there is still significant motivation to reduce the number of sensors while maintaining the quality and reliability of the assessment due to their high cost. In addition, for the safety and reliability of the structures it is of utmost importance that the entire structure can be investigated. Hence it is necessary to optimize the locations of the sensors in order to maximize the coverage while limiting the number of sensors used. The problem posed for the optimization of the FBG sensors for GW is different from any other work in this area due to the directional sensitivity shown by these sensors. Hence a novel optimization problem is developed for the application of FBG sensors for the GW based SHM. This paper develops a genetic algorithm based optimization methodology to incorporate the directional sensitivity. The methodology is shown analytically, using inputs from experimental investigations.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2020}, author={Soman, Rohan and Wee, Junghyun and Peters, Kara and Ostachowicz, Wieslaw}, year={2020} }
 @article{wee_alexander_peters_2020, title={Self-referencing ultrasound detection of fiber Bragg grating sensor remotely bonded at two locations}, volume={11379}, ISSN={["1996-756X"]}, DOI={10.1117/12.2559252}, abstractNote={Ultrasound measurement for damage detection in practical structural health monitoring (SHM) applications is often affected by varying environmental condition. In this case, a baseline reference signal measured under initial conditions may not be valid for comparison with the distorted signal (due to structural damage) that is measured under different conditions. In this study, we investigate a self-referencing ultrasound detection of fiber Bragg grating (FBG) by bonding an optical fiber at two different locations away from the FBG. We first investigate the extraction of ultrasonic waves from two different adhesive bond locations, which are measured with a single FBG sensor located between the two bonds. Based on understanding the ultrasound coupling mechanism through two adhesive bonds, we test the self-referencing ultrasound with the presence of a damage in a structure, examining the output FBG response that contains a combined signal of distorted and reference signals extracted through each bond.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2020}, author={Wee, Junghyun and Alexander, Kevin and Peters, Kara}, year={2020} }
 @article{wee_hackney_wells_bradford_peters_2020, title={Ultrasonic Lamb wave measurement sensitivity of aligned carbon nanotube coated fiber Bragg grating}, volume={2}, ISSN={["2515-7647"]}, DOI={10.1088/2515-7647/ab525e}, abstractNote={Fiber Bragg grating (FBG) sensors are typically bonded on the surface of a structure using an adhesive to collect ultrasonic waves for damage detection in structural health monitoring applications. However, the ultrasonic wave transfer from structure to optical fiber suffers signal attenuation due to the adhesive bond layer, which has a significantly different acoustic impedance than the optical fiber. Therefore, this paper develops a systematic procedure to fabricate an aligned carbon nanotube (CNT)-wrapped FBGs for acoustic impedance matching. Specifically, we first develop an automated CNT winding system to fabricate CNT-wrapped FBGs with varying CNT layer thickness, which are bonded to an aluminum plate for ultrasonic sensitivity testing. We demonstrate that CNT wrapped FBGs do not necessarily produce an increased sensitivity as compared to a reference polyimide-coated FBG, however some outliers are observed with a significant improvement. Using a scanning electron microscopy we examine the cross-section of CNT/adhesive layers, identifying a unique CNT/adhesive bonding morphology with a stiff exterior shell and a relatively compliant inner layer. Finite element simulation validates that this two-layered bonding geometry is most likely the source of the increased FBG ultrasonic sensitivity for the outliers.}, number={1}, journal={JOURNAL OF PHYSICS-PHOTONICS}, author={Wee, Junghyun and Hackney, Drew and Wells, Brian and Bradford, Philip D. and Peters, Kara}, year={2020}, month={Jan} }
 @article{wee_hackney_peters_2019, title={Preferential directional coupling to ultrasonic sensor using adhesive tape}, volume={58}, ISSN={["1560-2303"]}, DOI={10.1117/1.OE.58.7.072003}, abstractNote={Abstract. For structural health monitoring applications, recent studies have demonstrated an increased ultrasonic detection sensitivity of fiber Bragg grating (FBG) sensors through remote bonding of the FBG to a structure. In this case, the S0 or A0 Lamb waves in a structure are coupled to guided L01 waves in an optical fiber at the adhesive bond location, resulting in L01 modes of equal amplitudes propagating in both directions along the optical fiber. We demonstrate that when the adhesive bond is replaced with adhesive tape the S0 mode couples to the same L01 modes, however, with a preferential direction. Therefore, our study investigates the mechanism causing the preferential direction. We first identify different ultrasonic coupling pathways from a plate to an FBG through adhesive tape, demonstrating that both direct and indirect pathways are active, but that the indirect pathway produces the directional coupling. Then we test different adhesive tape parameters through experiments and simulations, demonstrating that flexural rigidity and bond length are parameters that can be used to control the directionality. The results of this work could be applied to design multiplexed FBG sensor arrays with specified signal pathways through the optical fiber networks.}, number={7}, journal={OPTICAL ENGINEERING}, author={Wee, Junghyun and Hackney, Drew and Peters, Kara}, year={2019}, month={Jul} }
 @article{wee_hackney_peters_2019, title={Ultrasonic Lamb wave mode conversion to optical fiber guided mode with varying input conditions}, volume={10970}, ISSN={["1996-756X"]}, DOI={10.1117/12.2514057}, abstractNote={In structural health monitoring (SHM) applications, one of the advantages of utilizing a surface bonded fiber Bragg grating (FBG) sensor for damage detection is its increased sensitivity in collecting ultrasonic waves. Recent studies have demonstrated that for a certain bonding condition the output FBG response can be increased by bonding the optical fiber at a distance away from the FBG to collect optical fiber guided wave (L01 mode) that is converted from S0 Lamb wave, referred as remote bonding. However in order to apply the remote bonding configuration in practical situations, the S0 mode conversion to L01 mode through an adhesive bond under various conditions must be characterized. This work investigates how the coupled L01 mode changes with varying input S0 mode frequency and angle of incidence through an adhesive bond. The goal of this work is to better understand the S0 mode conversion to the L01 mode in order to implement the remote bonding configuration for an improved SHM of a structure.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2019}, author={Wee, Junghyun and Hackney, Drew and Peters, Kara}, year={2019} }
 @article{wee_hackney_bradford_peters_2018, title={Effect of continuous optical fiber bonding on ultrasonic detection using fiber Bragg grating}, volume={10598}, ISSN={["1996-756X"]}, DOI={10.1117/12.2295833}, abstractNote={For laboratory demonstrations, Lamb wave detection using fiber Bragg grating (FBG) sensors is typically performed with only the grating location spot bonded and with the fiber axis aligned with the ultrasonic propagation direction. However, in reality, the entire length of fiber is often bonded to protect the fiber from any environmental damage, referred to here as continuous bonding. Theoretically, the Lamb wave signal can couple to the guided traveling wave in the optical fiber at any adhered location, which could potentially produce output signal distortion. In this paper, we investigate the impact of continuously bonding a long length of optical fiber on the measured Lamb wave signal detected by an FBG. Therefore, an experiment is performed to measure the Lamb wave signals excited from a PZT actuator using a surface bonded FBG with varying optical fiber bond length, indicating that the output FBG response remains constant with changing length. The second experiment investigates the FBG angular response to the traveling wave in the optical fiber, and compares to the conventional case where FBG directly measures the Lamb waves with varying angle. Specifically, the optical fiber is bonded to the plate at a distance away from the FBG. The Lamb wave is launched to the bond location with varying angles, which is coupled to traveling wave, then measured with FBG. The results indicate that the mechanism of the Lamb wave transfer to the directly bonded FBG is through displacement matching, whereas that of the traveling wave is through a forced excitation.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2018}, author={Wee, Junghyun and Hackney, Drew and Bradford, Philip and Peters, Kara}, year={2018} }
 @article{wee_hackney_bradford_peters_2018, title={Experimental Study on Directionality of Ultrasonic Wave Coupling Using Surface-Bonded Fiber Bragg Grating Sensors}, volume={36}, ISSN={["1558-2213"]}, DOI={10.1109/jlt.2017.2769960}, abstractNote={Recent studies demonstrated the potential of increasing the Lamb wave detection sensitivity of fiber Bragg grating (FBG) sensors by bonding the optical fiber away from the grating location, instead of the conventional method of bonding the FBG directly. The FBG located at a remote location further along the optical fiber collects the guided traveling wave in the optical fiber generated from the Lamb wave signal. This remote bonding method could potentially be extended to a series of multiplexed FBGs. However, previous experiments also detected coupling to guided traveling waves in both directions in the optical fiber, which could have significant effects on multiplexed signals. In this paper, we measure the coupled signal amplitudes in both forward and backward directions, when ultrasonic waves couple from a thin plate to an optical fiber and from an optical fiber to a thin plate. The forward- and backward-induced modes are measured in both the optical fiber and the plate. The same experiment is then performed for the case when ultrasonic signal is coupled from the optical fiber to the plate. In addition, two different types of bonding, cyanoacrylate adhesive and frictional bond, are explored to investigate how the signal conversion depends on the bonding method. The results demonstrate that the coupling of ultrasonic waves from a thin structure to an optical fiber and from an optical fiber to the structure is complex. The coupling does not only occur in the direction of the wave propagation, but can be coupled into both forward and backward modes, depending on the bonding configuration used.}, number={4}, journal={JOURNAL OF LIGHTWAVE TECHNOLOGY}, author={Wee, Junghyun and Hackney, Drew and Bradford, Philip and Peters, Kara}, year={2018}, month={Feb}, pages={932–938} }
 @article{wee_hackney_bradford_peters_2017, title={Bi-directional ultrasonic wave coupling to FBGs in continuously bonded optical fiber sensing}, volume={56}, ISSN={["2155-3165"]}, DOI={10.1364/ao.56.007262}, abstractNote={Fiber Bragg grating (FBG) sensors are typically spot-bonded onto the surface of a structure to detect ultrasonic waves in laboratory demonstrations. However, to protect the rest of the optical fiber from any environmental damage during real applications, bonding the entire length of fiber, called continuous bonding, is commonly done. In this paper, we investigate the impact of continuously bonding FBGs on the measured Lamb wave signal. In theory, the ultrasonic wave signal can bi-directionally transfer between the optical fiber and the plate at any adhered location, which could potentially produce output signal distortion for the continuous bonding case. Therefore, an experiment is performed to investigate the plate-to-fiber and fiber-to-plate signal transfer, from which the signal coupling coefficient of each case is theoretically estimated based on the experimental data. We demonstrate that the two coupling coefficients are comparable, with the plate-to-fiber case approximately 19% larger than the fiber-to-plate case. Finally, the signal waveform and arrival time of the output FBG responses are compared between the continuous and spot bonding cases. The results indicate that the resulting Lamb wave signal output is only that directly detected at the FBG location; however, a slight difference in signal waveform is observed between the two bonding configurations. This paper demonstrates the practicality of using continuously bonded FBGs for ultrasonic wave detection in structural health monitoring (SHM) applications.}, number={25}, journal={APPLIED OPTICS}, author={Wee, Junghyun and Hackney, Drew and Bradford, Philip and Peters, Kara}, year={2017}, month={Sep}, pages={7262–7268} }
 @article{wee_hackney_bradford_peters_2017, title={Mechanisms of signal coupling to optical fiber for FBG sensor detection of Lamb waves}, volume={10323}, ISSN={["1996-756X"]}, DOI={10.1117/12.2263278}, abstractNote={One of the major challenges when using fiber Bragg grating sensors (FBGs) to detect Lamb wave or acoustic emission signals in structures is the low sensitivity of these sensors to surface waves propagating in the structure. The authors have previously demonstrated that remote bonding of the optical fiber away from the FBG can increase the measured signal amplitude. In this paper we investigate the potential mechanisms for this increase through finite element simulations and demonstrate that the shear lag effect through the adhesive is the major source of the signal amplitude difference between the direct and remote bonding cases.}, journal={2017 25TH INTERNATIONAL CONFERENCE ON OPTICAL FIBER SENSORS (OFS)}, author={Wee, Junghyun and Hackney, Drew and Bradford, Philip and Peters, Kara}, year={2017} }
 @article{wee_hackney_bradford_peters_2017, title={Simulating increased Lamb wave detection sensitivity of surface bonded fiber Bragg grating}, volume={26}, ISSN={["1361-665X"]}, DOI={10.1088/1361-665x/aa646b}, abstractNote={Fiber Bragg grating (FBG) sensors are excellent transducers for collecting ultrasonic wave signals for structural health monitoring (SHM). Typically, FBG sensors are directly bonded to the surface of a structure to detect signals. Unfortunately, demodulating relevant information from the collected signal demands a high signal-to-noise ratio because the structural ultrasonic waves have low amplitudes. Our previous experimental work demonstrated that the optical fiber could be bonded at a distance away from the FBG location, referred to here as remote bonding. This remote bonding technique increased the output signal amplitude compared to the direct bonding case, however the mechanism causing the increase was not explored. In this work, we simulate the previous experimental work through transient analysis based on the finite element method, and the output FBG response is calculated through the transfer matrix method. The model is first constructed without an adhesive to assume an ideal bonding condition, investigating the difference in excitation signal coherence along the FBG length between the two bonding configurations. A second model is constructed with an adhesive to investigate the effect of the presence of the adhesive around the FBG. The results demonstrate that the amplitude increase is originated not from the excitation signal coherence, but from the shear lag effect which causes immature signal amplitude development in the direct bonding case compared to the remote bonding case. The results also indicate that depending on the adhesive properties the surface-bonded optical fiber manifests varying resonant frequency, therefore resulting in a peak amplitude response when the input excitation frequency is matched. This work provides beneficial reference for selecting adhesive and calibrating sensing system for maximum ultrasonic detection sensitivity using the FBG sensor.}, number={4}, journal={SMART MATERIALS AND STRUCTURES}, author={Wee, J. and Hackney, D. A. and Bradford, P. D. and Peters, K. J.}, year={2017}, month={Apr} }
 @inproceedings{wee_hackney_bradford_peters_2017, title={The adhesive effect on ultrasonic Lamb wave detection sensitivity of remotely-bonded fiber Bragg grating sensors}, volume={10168}, DOI={10.1117/12.2260009}, abstractNote={Fiber Bragg grating (FBG) sensors are excellent transducers for ultrasonic signal detection in structural health monitoring (SHM) application. While the FBG sensors are typically bonded directly on the surface of a structure to collect signals, one of the major challenges arises from demodulating relevant information from the low amplitude signal. The authors have experimentally demonstrated that the ultrasonic wave detection sensitivity of FBG sensors can be increased by bonding optical fiber away from the FBG location. This configuration is referred to here as remote bonding. However the mechanism causing this phenomenon has not been explored. In this work, we simulate the previous experimental work through a transient analysis based on the finite element method, and the output FBG response is calculated through the transfer matrix method. We first model an optical fiber bonded on the surface of an aluminum plate with an adhesive. The consistent input signal is excited to the plate, which is detected by the directly and remotely-bonded FBGs. The effect of the presence of the adhesive around the FBG is investigated by analyzing strain and displacement along the length of the FBGs at the locations of direct and remote bonding cases, and the consequent output FBG responses. The result demonstrates that the sensitivity difference between the direct and remote bonding cases is originated from shear lag effect due to adhesive.}, booktitle={Sensors and smart structures technologies for civil, mechanical, and aerospace systems 2017}, author={Wee, J. and Hackney, D. A. and Bradford, P. D. and Peters, Kara}, year={2017} }
 @article{wee_wells_hackney_bradford_peters_2016, title={Increasing signal amplitude in fiber Bragg grating detection of Lamb waves using remote bonding}, volume={55}, ISSN={["2155-3165"]}, DOI={10.1364/ao.55.005564}, abstractNote={Networks of fiber Bragg grating (FBG) sensors can serve as structural health monitoring systems for large-scale structures based on the collection of ultrasonic waves. The demodulation of structural Lamb waves using FBG sensors requires a high signal-to-noise ratio because the Lamb waves are of low amplitudes. This paper compares the signal transfer amplitudes between two adhesive mounting configurations for an FBG to detect Lamb waves propagating in an aluminum plate: a directly bonded FBG and a remotely bonded FBG. In the directly bonded FBG case, the Lamb waves create in-plane and out-of-plane displacements, which are transferred through the adhesive bond and detected by the FBG sensor. In the remotely bonded FBG case, the Lamb waves are converted into longitudinal and flexural traveling waves in the optical fiber at the adhesive bond, which propagate through the optical fiber and are detected by the FBG sensor. A theoretical prediction of overall signal attenuation also is performed, which is the combination of material attenuation in the plate and optical fiber and attenuation due to wave spreading in the plate. The experimental results demonstrate that remote bonding of the FBG significantly increases the signal amplitude measured by the FBG.}, number={21}, journal={APPLIED OPTICS}, author={Wee, Junghyun and Wells, Brian and Hackney, Drew and Bradford, Philip and Peters, Kara}, year={2016}, month={Jul}, pages={5564–5569} }
 @article{wee_hackney_peters_wells_bradford_2016, title={Sensitivity of contact-free fiber Bragg grating sensors to ultrasonic Lamb waves}, volume={9803}, ISSN={["1996-756X"]}, DOI={10.1117/12.2218924}, abstractNote={Networks of fiber Bragg grating (FBG) sensors can serve as structural health monitoring (SHM) systems for large-scale structures based on the collection of ultrasonic waves. The demodulation of structural Lamb waves requires a high signal-to-noise ratio because Lamb waves have a low amplitude. This paper investigates the signal transfer between Lamb waves propagating in an aluminum plate collected by an optical fiber containing a FBG. The fiber is bonded to the plate at locations away from the FBG. The Lamb waves are converted into longitudinal and flexural traveling waves propagating along the optical fiber, which are then transmitted to the Bragg grating. The signal wave amplitude is measured for different distances between the bond location and the Bragg grating. Bonding the optical fiber away from the FBG location and closer to the signal source produces a significant increase in signal amplitude, here measured to be 5.1 times that of bonding the Bragg grating itself. The arrival time of the different measured wave coupling paths are also calculated theoretically, verifying the source of the measured signals. The effect of the bond length to Lamb wavelength ratio is investigated, showing a peak response as the bond length is reduced compared to the wavelength. This study demonstrates that coupling Lamb waves into guided traveling waves in an optical fiber away from the FBG increases the signal-to-noise ratio of Lamb wave detection, as compared to direct transfer of the Lamb wave to the optical fiber at the location of the FBG.}, journal={SENSORS AND SMART STRUCTURES TECHNOLOGIES FOR CIVIL, MECHANICAL, AND AEROSPACE SYSTEMS 2016}, author={Wee, Junghyun and Hackney, Drew and Peters, Kara and Wells, Brian and Bradford, Philip}, year={2016} }