Development of accelerated damage detection techniques using noncontact laser ultrasonic scanning

Abstract

This dissertation aims to accelerate laser based nondestructive testing with the developed techniques inspired by binary search. Two accelerated damage detection techniques are developed in this dissertation. Inspired by binary search, damage is localized and visualized with reduced scanning points and scanning time. The number of scanning points that is necessary for damage localization and visualization is dramatically reduced from NM to $4log_2N \cdot log_2M$ even for the worst case scenario. N and M represent the number of equally spaced scanning points in the $x$ and $y$ directions, respectively, which are required to obtain full-field wave propagation images of the target inspection region. The binary search algorithm is based on examining the interactions between the ultrasonic waves and damage, such as reflections and transmissions. A time-domain ultrasonic response is transformed into a spatial ultrasonic domain to better identify these interactions. Instead of a traditional matching pursuit approach which tries to solve an $\ell_0$ minimization problem using a greedy sequential algorithm, basis pursuit approach solves an $\ell_1$ minimization problem to transform a time-domain response with a better resolution. The feasibility of the developed damage detection techniques is validated in both numerical and experimental ways. The developed techniques can reduce the number of scanning points and scanning time over 90%.