Detection and imaging of local ply angle in carbon fiber reinforced plastics using laser ultrasound and tilt filter processing

Wrinkles in carbon-fiber reinforced plastics (CFRP) are extremely dangerous, dramatically changing the performance of a composite structure. However, wrinkles are one of the most difficult defects to detect and characterize with ultrasound since they do not produce large echoes like other flaws. Recent developments in laser-ultrasound (LU) have enabled non-contact, high-resolution imaging of CFRPs, providing sub-ply resolution and detailed visualization of CFRP structures. In this paper, we demonstrate that the superior resolution of LU systems can be used to help track ply orientation variations in composites. To improve the signal-to-noise ratio of LU generated signals, we propose a local cross-correlation based tilt-filtration procedure for coherent spatial signal averaging that accounts for the shape of subsequent layers. In addition, local ply angle is a straightforward output of the proposed processing, which can be used directly as an indicator of wrinkle severity.