Remote defect visualization of standard composite coupons using a mobile pulse-echo ultrasonic propagation imager

Abstract

A new laser-ultrasonic-based defect monitoring system, a mobile pulse-echo ultrasonic propagation imager, has been developed to evaluate damage to composite and metallic structures. The system integrates a Q-switched Nd:YAG laser for the generation of thermoelastic waves, a laser Doppler vibrometry sensor for sensing ultrasound and a two-axis translation stage for raster scanning of the combined generation and sensing laser beams. The system allows scanning of both ultrasound generation and sensing laser beams simultaneously. Thus, a full-field pulse-echo ultrasound, as large as the scan area, can be generated that visualizes the longitudinal wave propagation through the thickness. The full-field pulse-echo ultrasonic wave propagation imaging algorithm is used for real-time structural damage evaluation. Four F-16 Lockheed Martin aircraft reference standard composite specimens with different shapes, sizes, thicknesses, compositions, and aspect ratio of damages were tested: the stepped graphite-to-aluminum, the graphite-to-graphite, and the steel-to-graphite composite panel with flat bottom holes and the graphite-to-aluminum core sandwich structure with core missing. Moreover, the effects of laser pulse energy on damage visualization using the system has been presented and tested on the 4-mm-thick aluminum plate with 2-mm-thick wall-thinning. The proposed system successfully detected and visualized all the artificially introduced defects in the standard coupons.