Reverberation-based high-speed guided-wave ultrasonic propagation imager for structural inspection of thick composites

Abstract

Increasing use of thick composite structures in many fields, including the aerospace and shipbuilding industries, necessitates appropriate inspection techniques to ensure structural integrity. Of many nondestructive testing techniques, laser ultrasonic testing is a non-contact method that yields accurate inspection results with high spatial resolution; this method was adopted in this study for the inspection of thick composite structures. We propose a novel guided-wave ultrasonic propagation imaging (UPI) method, called the reverberation wave UPI (R-wave UPI), which is capable of faster target scanning and clearer highlighting of confining waves. To prove the merits of the R-wave UPI, a relatively thin carbon/epoxy composite (thickness, 12 mm) was first tested using the proposed method and the conventional through-transmission UPI. The R-wave UPI modifies the guided-wave UPI with a faster pulse repetition frequency (PRF) laser and ultrasonic energy mapping with straightforward parameter determination. The through-transmission UPI consisting of a raster mechanical scanner, a Q-switched laser, and a laser Doppler vibrometer, was used as a reference, whereas the R-wave UPI consisting of an angular laser mirror scanner, a Q-switched laser, and a PZT sensor was operated at a high PRF in the kilohertz regime to induce reverberation. The reverberation phenomenon was used in the post-image processing, called ultrasonic energy mapping (UEM). UEM with a high PRF was then compared to the through-transmission inspection reference to verify its accuracy and reliability for mapping of structural anomalies. Finally, a thick and curved composite blade with a rough and curved surface that could not be inspected using the conventional through-transmission UPI was tested using 7 kHz PRF and its delamination was successfully visualized using the UEM.