@article{chang_yuan_2020, title={Damage imaging in a stiffened curved composite sandwich panel with wavenumber index via Riesz transform}, volume={19}, url={https://doi.org/10.1177/1475921719858432}, DOI={10.1177/1475921719858432}, abstractNote={ Imaging a damage using the phase of wavefield “video” in the physical domain is developed and applied to a stiffened curved composite sandwich panel for visualization of a barely visible impact damage. The ultrasonic guided waves are generated by the thermoelastic effect induced by a Nd:YAG Q-switched pulse laser and then are captured point-by-point by a laser Doppler vibrometer. The wavefield “video” is reconstructed by measuring out-of-plane velocity on the smooth outside surface of the panel. Newly generated wavenumbers from the geometry/material discontinuities caused by either the stiffener or the impact damage can be detected by observing the change of wavenumber from the reconstructed wavefield “video.” The instantaneous wavenumber (i.e. the magnitude of instantaneous wavevector) derived via Riesz transform and its difference can be shown and highlighted using a proposed imaging condition, named as wavenumber index. The wavenumber index sums the wavenumber values followed by a wave energy threshold filter which is performed in the time domain. This is in contrast to other imaging conditions implemented in the frequency-wavenumber domain by the use of complex wavenumber filtering and wave mode decomposition. Since wavenumber index is the phase-based imaging technique instead of conventional intensity-based technique for the wavefield “video,” this technique is robust in that the impact damages located in the vicinity of geometry/material discontinuities can yield consistent damage image resolution with high sensitivity even for wave propagating from the direction across the stiffener. The barely visible impact damage of the composite structure becomes therefore “visible” with the proposed imaging technique. }, number={3}, journal={Structural Health Monitoring}, publisher={SAGE Publications}, author={Chang, Huan-Yu and Yuan, Fuh-Gwo}, year={2020}, month={May}, pages={902–916} }
 @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{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} }