Visualization of hidden damage from scattered wavefield reconstructed using an integrated high-speed camera system

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 S₀and fundamental shear horizontal wave mode SH₀(induced due to reflection/scattering of the incident S₀wave 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 SH₀and S₀wave 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 SH₀mode in particular, as the SH₀mode 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.