The identification of damage types in carbon fiber–reinforced plastic cross-ply laminates using a novel fiber-optic acoustic emission sensor

In this study, a phase-shifted fiber Bragg grating sensor was employed for acoustic emission detection in cross-ply carbon fiber–reinforced plastic laminates. First, we evaluated the response of the phase-shifted fiber Bragg grating sensor to a simulated acoustic emission source using a pencil lead breakage test. The existing modes of Lamb waves were observed, and their relations to the acoustic emission source orientation were investigated using a modal acoustic emission method for discriminating between acoustic emission signals generated by transverse cracking and delamination. As a result, those two types of damage were identified during a tensile test. In the time–frequency analysis of the waveforms detected by the phase-shifted fiber Bragg grating sensor, in addition to the S₀ and A₀ modes that are usually used in modal acoustic emissions, we found an A₁ mode in the high-frequency range of the acoustic emission generated by a transverse crack. Then, we conducted a finite element analysis to examine the inherent mechanism of the A₁ mode excitation. The results of the calculation almost agreed with the experimental results and indicated that a phase-shifted fiber Bragg grating sensor has a high sensitivity response to acoustic emission in the carbon fiber–reinforced plastic laminates over a broad bandwidth. Therefore, this novel fiber-optic sensor has been shown to have high potential for monitoring the ultrasonic structural health of composites.