Non-contact detection of delamination in impacted cross-ply CFRP using laser generated lamb waves

Carbon fiber reinforced plastic (CFRP) plate tends to suffer large and complicate internal damages at being impacted. The damage significantly reduces the residual compressive strength, and is needed to be detected by an advanced nondestructive method. Both the real-time source location and non-contact damage inspection systems are important for transportation equipment. We first develop a new source location method of impact position by using the arrival time difference of zero-th order anti-symmetric Lamb waves (A(0)-Lamb waves) at selected frequency. AE monitoring system was utilized for this study. The arrival times were automatically determined from the peak arrivals of time-transient wavelet coefficients of four AE transducer signals. This system enables us to determine the impact location within two times of plate thickness within I second. Next, we propose a non-contact laser ultrasonic system for detecting the damage location and shape in cross-ply CFRP plate impacted by flying steel balls. Delamination shapes were revealed by comparing the cross-correlation and/or amplitude difference of laser Lamb waves over sound and damaged zone. Here, A(0)-Lamb waves were monitored by scanning both a line focused pulse YAG laser and a probe laser of heterodyne-type laser interferometer at 5 to 10mm. step. Distance between the incident and probe laser was changed from 10 to 100 mm. Double-tree shaped delamination with 25 mm long was revealed in ball hit CFRP plate.