Impact of Unstitched and Stitched Laminates by Line Loading

Abstract

Results of impact responses for both unstitched and stitched laminates to line-loading at low striking velocities are presented in this paper. These laminates, made of E-glass dry preform and having a (0°₂/90°₂)_s stacking sequence, were fabricated by resin transfer molding. For the stitched laminates, Kevlar-29 untwisted roving of 3000 denier was used for the through-the-thickness reinforcement. Impact tests were conducted on a specially designed falling tower with a line-nosed impactor. It was found that the number of matrix cracks, the splitting crack lengths, the delamination areas, and the laminate residual deflections were all much reduced due to the stitching reinforcement. The major impact-induced damage mechanism gradually changed from delamination for the unstitched laminates to the plastic-hinge type of localized deformation at the impact location for the stitched specimens as the stitching density increased. Further, delamination damage in an unstitched laminate could be detected from the recorded strain and acceleration histories.