@article{shin_webb_peters_2016, title={Pulsed phase thermography imaging of fatigue-loaded composite adhesively bonded joints}, volume={79}, ISSN={["1879-1174"]}, DOI={10.1016/j.ndteint.2015.11.008}, abstractNote={We applied pulsed phase thermography to image and size damage in adhesively bonded joints. Specifically, the initiation and propagation of fatigue-induced damage in single lap joints with carbon fiber epoxy adherends was investigated. Lap joint specimens with various levels of manufacturing defects were fabricated and loaded in low-cycle fatigue. A calibration specimen with artificial defects was used to design a threshold algorithm for sizing of the damaged regions. The dominant failure mode in specimens without manufacturing defects was fiber-failure, whereas joints failing prematurely demonstrated adhesive failure. Imaging of the lap joints after regular number of fatigue cycles revealed that manufacturing defects could be detected and the resulting, imminent adhesive failure could be identified prior to joint failure. Additionally, the extent of this damage could be accurately estimated through the sizing algorithm. Due to the brittle nature of fiber-failure, it could not be detected prior to failure of the joint, however this was not critical, as the goal was to identify premature failure of the adhesively bonded joint.}, journal={NDT & E INTERNATIONAL}, author={Shin, Peter H. and Webb, Sean C. and Peters, Kara J.}, year={2016}, month={Apr}, pages={7–16} }
 @inproceedings{peters_webb_2015, title={Rapid processing of full-spectral data from embedded FBG sensors}, DOI={10.12783/shm2015/216}, abstractNote={This paper addresses the inversion of applied non-uniform strain fields from measured FBG reflection spectra. With recent advances in instrumentation that can measure such reflected spectra dynamically, these spectra can now be tied to different failure modes in composite structures. This unique capability is particularly useful when the FBG sensors are embedded in composite materials and therefore subjected to highly non-uniform strain fields. The calculation of an arbitrary applied strain field from a measured reflection spectrum remains a significant computational effort, however, due to the need for an optimization algorithm. Therefore this data processing has not been applied to FBG reflected spectra collected from dynamic events, due to the large amount of spectra that must be processed. In this article we present a linearization method to calculate the applied strain field, provided that the measured reflection spectra are collected at a sufficiently high acquisition rate. doi: 10.12783/SHM2015/216}, booktitle={Structural health monitoring 2015: system reliability for verification and implementation, vols. 1 and 2}, author={Peters, Kara and Webb, S.}, year={2015}, pages={1729–1736} }
 @article{webb_shin_peters_zikry_stan_chadderdon_selfridge_schultz_2014, title={Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors: 1. Experiments}, volume={23}, ISSN={["1361-665X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892404653&partnerID=MN8TOARS}, DOI={10.1088/0964-1726/23/2/025016}, abstractNote={In this study we measure the in situ response of a fiber Bragg grating (FBG) sensor embedded in the adhesive layer of a single composite lap joint, subjected to harmonic excitation after fatigue loading. After a fully reversed cyclic fatigue loading is applied to the composite lap joint, the full-spectral response of the sensor is interrogated at 100 kHz during two loading conditions: with and without an added harmonic excitation. The full-spectral information avoided dynamic measurement errors often experienced using conventional peak wavelength and edge filtering techniques. The short-time Fourier transform (STFT) is computed for the extracted peak wavelength information to reveal time-dependent frequencies and amplitudes of the dynamic FBG sensor response. The dynamic response of the FBG sensor indicated a transition to strong nonlinear dynamic behavior as fatigue-induced damage progressed. The ability to measure the dynamic response of the lap joint through sensors embedded in the adhesive layer can provide in situ monitoring of the lap joint condition.}, number={2}, journal={SMART MATERIALS AND STRUCTURES}, author={Webb, S. and Shin, P. and Peters, K. and Zikry, M. A. and Stan, N. and Chadderdon, S. and Selfridge, R. and Schultz, S.}, year={2014}, month={Feb} }
 @article{webb_shin_peters_zikry_stan_chadderdon_selfridge_schultz_2014, title={Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors: 2. Simulations}, volume={23}, ISSN={["1361-665X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84892411813&partnerID=MN8TOARS}, DOI={10.1088/0964-1726/23/2/025017}, abstractNote={In this paper, we simulate the response of fiber Bragg grating (FBG) sensors embedded in the adhesive layer of a composite lap that is subjected to harmonic excitation. To simulate accumulated fatigue damage at the adhesive layer, two forms of numerical nonlinearities are introduced into the model: (1) progressive plastic deformation of the adhesive and (2) changing the boundary of an interfacial defect at the adhesive layer across the overlap shear area. The simulation results are compared with previous measurements of the dynamic, full-spectral response of such FBG sensors for condition monitoring of the lap joint. Short-time Fourier transforms (STFT) of the locally extracted axial strain time histories reveal a transition to nonlinear behavior of the composite lap joint by means of intermittent frequencies that were observed in the experimental measurements and are not associated with the external excitation. The simulation results verify that the nonlinear changes in measured dynamic FBG responses are due to the progression of damage in the lap joint.}, number={2}, journal={SMART MATERIALS AND STRUCTURES}, author={Webb, S. and Shin, P. and Peters, K. and Zikry, M. A. and Stan, N. and Chadderdon, S. and Selfridge, R. and Schultz, S.}, year={2014}, month={Feb} }
 @article{webb_peters_zikry_stan_chadderdon_selfridge_schultz_2014, title={Fiber Bragg grating spectral features for structural health monitoring of composite structures}, volume={9157}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84903169877&partnerID=MN8TOARS}, DOI={10.1117/12.2059625}, abstractNote={We demonstrate the measurement of and applications for reflected spectral signatures obtained from FBG sen- sors in dynamic environments. Three uses of the spectral distortion measurements for monitoring of airframe structures are presented: the measurement of the dynamic response of a laminated plate to an impact event; the measurement of damage induced spectral distortion in a thin plate during vibration loading; and the measurement of the change in dynamic response of an adhesively bonded joint with the progression of fatigue damage.}, journal={23RD INTERNATIONAL CONFERENCE ON OPTICAL FIBRE SENSORS}, author={Webb, Sean and Peters, Kara and Zikry, Mohammed and Stan, Nikola and Chadderdon, Spencer and Selfridge, Richard and Schultz, Stephen}, year={2014} }
 @article{stan_bailey_chadderdon_webb_zikry_peters_selfridge_schultz_2014, title={Increasing dynamic range of a fibre Bragg grating edge-filtering interrogator with a proportional control loop}, volume={25}, ISSN={["1361-6501"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84899634683&partnerID=MN8TOARS}, DOI={10.1088/0957-0233/25/6/065206}, abstractNote={We present a fibre Bragg grating (FBG) interrogator that uses a microcontroller board and a tunable optical filter in a proportional control loop to increase dynamic range and achieve high strain sensitivity. It is an edge-filtering interrogator with added proportional control loop that locks the operating wavelength to the mid-reflection point on the FBG spectrum. The interrogator separates low-frequency (LF) components of strain and measures them with extended dynamic range, while at the same time measuring high-frequency (HF) strain without loss in strain sensitivity. In this paper, we describe the implementation of the interrogator and analyse the characteristics of individual components, such as the speed and voltage resolution of the microcontroller and the tunable optical filter. We measure the performance of the proportional control loop at frequencies up to 1 kHz and characterize the system using control theory. We illustrate the limitation of the conventional interrogator to measure strains greater than 40 μϵ and demonstrate successful application of the proposed interrogator for simultaneous measurement of 450 μϵ LF strain at 50 Hz superimposed with 32 kHz HF strain.}, number={6}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Stan, Nikola and Bailey, D. C. and Chadderdon, S. L. and Webb, S. and Zikry, M. and Peters, K. J. and Selfridge, R. H. and Schultz, S. M.}, year={2014}, month={Jun} }
 @article{webb_oman_peters_stan_chadderdon_selfridge_schultz_2014, title={Localized measurements of composite dynamic response for health monitoring}, volume={9062}, ISSN={["1996-756X"]}, DOI={10.1117/12.2045255}, abstractNote={We demonstrate the measurement of and applications for full-spectral measurements collected from FBG sensors in dynamic loading environments. The measurement of the dynamic response of a laminated plate to an impact event highlights the information gained during the event as compared to after the event. The measurement of damage induced spectral distortion in a thin plate during vibration loading demonstrates the capability of separating spectral distortion due to multiple effects, including damage and vibration loading. Finally, the measurement of the change in dynamic response of an adhesively bonded joint highlights the capability to measure the progression of fatigue damage. Confirmation that the change in FBG response is due to fatigue damage is performed through independent pulsed phase thermography imaging of the adhesively bonded joint.}, journal={SMART SENSOR PHENOMENA, TECHNOLOGY, NETWORKS, AND SYSTEMS INTEGRATION 2014}, author={Webb, Sean and Oman, Kyle and Peters, Karsa and Stan, Nikola and Chadderdon, Sspencer and Selfridge, Richard and Schultz, Stephen}, year={2014} }
 @inproceedings{webb_shin_peters_zikry_schultz_selfridge_2013, title={Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber bragg grating sensors}, booktitle={Structural Health Monitoring 2013, Vols 1 and 2}, author={Webb, S. and Shin, P. and Peters, K. and Zikry, M. and Schultz, S. and Selfridge, R.}, year={2013}, pages={1953–1960} }
 @article{webb_peters_zikry_chadderdon_nikola_selfridge_schultz_2013, title={Full-Spectral Interrogation of Fiber Bragg Grating Sensors Exposed to Steady-State Vibration}, volume={53}, ISSN={["1741-2765"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84874796178&partnerID=MN8TOARS}, DOI={10.1007/s11340-012-9661-x}, number={4}, journal={EXPERIMENTAL MECHANICS}, author={Webb, S. and Peters, K. and Zikry, M. A. and Chadderdon, S. and Nikola, S. and Selfridge, R. and Schultz, S.}, year={2013}, month={Apr}, pages={513–530} }
 @article{shin_webb_peters_2013, title={Nondestructive inspection in adhesive-bonded joint CFRP using pulsed phase thermography}, volume={8705}, ISSN={["0277-786X"]}, DOI={10.1117/12.2018414}, abstractNote={Many forms of damages in fiber reinforcement polymer (FRP) composites are difficult to detect because they occurs in subsurface layers of the composites. One challenging need for inspection capabilities is in adhesively bonded joints between composite components, a common location of premature failure in aerospace structures. This paper investigates pulsed phase thermography (PPT) imaging of fatigue damage in these adhesively bonded joints. Simulated defects were created to calibrate parameters for fatigue loading conditions, PPT imaging parameters, and a damage sizing algorithm for carbon fiber reinforced polymer (CFRP) single lap joints. Afterwards, lap joint specimens were fabricated with varying quality of manufacturing. PPT imaging of the pristine specimens revealed defects such as air bubbles, adhesive thickness variations, and weak bonding surface between the laminate and adhesive. Next, fatigue testing was performed and acquired PPT imaging data identified fatigue induced damage prior to final failure cycles. After failure of each sample, those images were confirmed by visual inspections of failure surface.}, journal={THERMOSENSE: THERMAL INFRARED APPLICATIONS XXXV}, author={Shin, P. H. and Webb, S. C. and Peters, K. J.}, year={2013} }
 @article{webb_shin_peters_selfridge_schultz_2013, title={Nondestructive inspection of CFRP adhesively bonded joints using embedded FBG sensors}, volume={8722}, ISSN={["1996-756X"]}, DOI={10.1117/12.2018443}, abstractNote={One challenging need for inspection capabilities is in adhesively bonded joints between composite components, a common location of premature failure in aerospace structures. In this work we demonstrate that dynamic, full spectral scanning of FBG sensors embedded in the adhesive bond can identify changes in bond quality through the measurement of non-linear dynamics of the joint. Eighteen lap joint specimens were fabricated with varying manufacturing quality. Ten samples also included fiber Bragg grating (FBG) sensors embedded in the adhesive bond for real-time inspection during a simulated flight condition of these single-lap joints. Prior to testing, pulse phase thermography imaging of the pristine specimens revealed defects such as air bubbles, adhesive thickness variations, and weak bonding surface between the laminate and adhesive. The lap joint specimens were then subjected to fatigue loading, with regular interrogation of the FBG sensors at selected load cycle intervals. The FBG data was collected during vibration loading of the lap joint to represent an in-flight environment. Changes in the lap joint dynamic response, including the transition to non-linear responses, were measured from both the full-spectral and peak wavelength FBG data. These changes were correlated to initial manufacturing defects and the progression of fatigue-induced damage independently measured with pulse phase imaging and visual inspections of the failure surfaces.}, journal={FIBER OPTIC SENSORS AND APPLICATIONS X}, author={Webb, S. and Shin, P. and Peters, K. and Selfridge, R. and Schultz, S.}, year={2013} }
 @article{webb_peters_zikry_chadderdon_nikola_selfridge_schultz_2012, title={Full-spectral interrogation of fiber Bragg grating sensors for measurements of damage during steady-state vibration}, volume={8346}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901266148&partnerID=MN8TOARS}, DOI={10.1117/12.915150}, abstractNote={In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor subjected to a non-uniform static strain state and simultaneously exposed to vibration loading. The full spectral response of the sensor is interrogated in reflection at 100 kHz during two loading cases: with and without an added vibration load spectrum. The static tensile loading is increased between each test, in order to increase the magnitude of the non-uniform strain field applied to the FBG sensor. During steady-state vibration, the behavior of the spectral shape of the FBG reflection varies depending on the extent of non-uniform strain. With high-speed full spectral interrogation, it is potentially possible to separate this vibration-induced spectral change from spectral distortions due to non-uniform strain. Such spectral distortion contains valuable information on the static damage state of the surrounding host material.}, journal={SMART SENSOR PHENOMENA, TECHNOLOGY, NETWORKS, AND SYSTEMS INTEGRATION 2012}, author={Webb, S. and Peters, K. and Zikry, M. A. and Chadderdon, S. and Nikola, S. and Selfridge, R. and Schultz, S.}, year={2012} }
 @article{webb_noevere_peters_zikry_vella_chadderdon_selfridge_schultz_2011, title={Full-spectral interrogation of fiber Bragg grating sensors for damage identification}, volume={7982}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79956068726&partnerID=MN8TOARS}, DOI={10.1117/12.881093}, abstractNote={In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor subjected to a non-uniform static strain state and simultaneously exposed to vibration loading. The full spectral response of the sensor is interrogated in reflection at 100 kHz during two loading cases: with and without an added vibration load spectrum. The static tensile loading is increased between each test, in order to increase the magnitude of the non-uniform strain field applied to the FBG sensor. The spectral distortion due to non-uniform strain is observed to change once the sensor is exposed to a non-transient 150 Hz vibration spectrum. With high-speed full spectral interrogation, it is potentially possible to separate this vibration-induced spectral change from spectral distortions due to non-uniform strain. Such spectral distortion contains valuable information on the static damage state of the surrounding host material.}, journal={SMART SENSOR PHENOMENA, TECHNOLOGY, NETWORKS, AND SYSTEMS 2011}, author={Webb, S. and Noevere, A. and Peters, K. and Zikry, M. A. and Vella, T. and Chadderdon, S. and Selfridge, R. and Schultz, S.}, year={2011} }
 @article{webb_peters_zikry_vella_chadderdon_selfridge_schultz_2011, title={Wavelength hopping due to spectral distortion in dynamic fiber Bragg grating sensor measurements}, volume={22}, ISSN={0957-0233 1361-6501}, url={http://dx.doi.org/10.1088/0957-0233/22/6/065301}, DOI={10.1088/0957-0233/22/6/065301}, abstractNote={We demonstrate the measurement of wavelength hopping in dynamic fiber Bragg grating (FBG) sensor measurements and its effect on the interpretation of the dynamic behavior of a composite laminate. Strain measurements are performed with FBG sensors embedded in laminates, subjected to low-velocity impacts, with data acquired using a commercial peak wavelength following controller and a high-speed full-spectral interrogator recently developed by the authors. The peak follower response is theoretically predicted from the full-spectral interrogator measurements. We demonstrate that dynamic wavelength hopping does occur, that it changes the apparent dynamic behavior of the composite and that it can be directly predicted from the dynamic spectral distortion. We also demonstrate that full-spectral data acquisition at speeds lower than those required to fully resolve the dynamic event creates apparent measurement errors due to wavelength hopping as well.}, number={6}, journal={Measurement Science and Technology}, publisher={IOP Publishing}, author={Webb, S and Peters, K and Zikry, M and Vella, T and Chadderdon, S and Selfridge, R and Schultz, S}, year={2011}, month={May}, pages={065301} }
 @inproceedings{hackney_webb_peters_2010, title={Analysis of fiber Bragg grating spectral features for in-situ assessment of sandwich composites}, DOI={10.1115/smasis2010-3886}, abstractNote={In this paper we summarize recent advances derived from full-spectral interrogation of FBG sensors for structural health monitoring and damage identification in composites. Previous work has correlated signals from FBG reflected spectra to stress concentrations near crack tips, curing conditions during processing of composite laminates and the progression of delamination due to multiple low-velocity impacts in woven composite laminates. In this paper we investigate the information gained from FBG sensors embedded at the facesheet-core interface in foam-core sandwich composites subjected to low-velocity impact loading. We also discuss recent advances in interrogation systems for these sensors which will permit dynamic evaluation of these parameters. These results demonstrate the richness of information that can be obtained from full-spectral interrogation of FBG sensors in a complex, multiple stress component environment.}, booktitle={Proceedings of the ASME Conference on Smart Materials, Adaptive Structures and Intelligent Systems, 2010, vol 2}, author={Hackney, D. and Webb, S. and Peters, Kara}, year={2010}, pages={813–820} }
 @article{vella_chadderdon_selfridge_schultz_webb_park_peters_zikry_2010, title={Full-spectrum interrogation of fiber Bragg gratings at 100 kHz for detection of impact loading}, volume={21}, ISSN={["1361-6501"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77958167493&partnerID=MN8TOARS}, DOI={10.1088/0957-0233/21/9/094009}, abstractNote={This paper explains key innovations that allow monitoring of detailed spectral features of an FBG in response to impact loading. The new system demonstrates capture of FBG spectral data at rates of 100 kHz. Rapid capture of the entire reflection spectrum at such high reading rates shows important features that are missed when using systems that merely track changes in the peak location of the spectrum. The update rate of 100 kHz allows resolution of features that occur on transient time scales as short as 10 µs. This paper gives a detailed description of the unique features of the apparatus and processes used to capture the data at such a rapid rate. Furthermore, we demonstrate this interrogation scheme on a composite laminate system during impact.}, number={9}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Vella, T. and Chadderdon, S. and Selfridge, R. and Schultz, S. and Webb, S. and Park, C. and Peters, K. and Zikry, M.}, year={2010}, month={Sep} }
 @article{webb_peters_zikry_vella_chadderdon_selfridge_schultz_2010, title={Impact induced damage assessment in composite laminates through embedded fiber Bragg gratings}, volume={7648}, ISSN={["1996-756X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77953525959&partnerID=MN8TOARS}, DOI={10.1117/12.847543}, abstractNote={In this study we evaluate the measurements of fiber Bragg sensor spectra from a sensor embedded in a composite laminate subjected to multiple low velocity impacts. The full-spectral response of the sensor is interrogated in reflection at 100 kHz during the impact events. The measurement of the time dependent spectra features are compared with previous results obtained at a 534 Hz interrogation rate. With the increased interrogation rate, we can observe a smooth transition in the full-spectra response of the sensor between strikes and the presence of peak-splitting due to transverse compression from the beginning of the laminate lifetime. Finally, at the 100 kHz acquisition rate, it is possible to determine the maximum wavelength and accurately determine the duration of the impact event for all of the strikes.}, journal={SMART SENSOR PHENOMENA, TECHNOLOGY, NETWORKS, AND SYSTEMS 2010}, author={Webb, S. and Peters, K. and Zikry, M. A. and Vella, T. and Chadderdon, S. and Selfridge, R. and Schultz, S.}, year={2010} }