Flexural strength and interlaminar shear strength of stitched GRP laminates following repeated impacts

Abstract

The flexural strength and interlaminar shear strength of stitched and non-stitched glass-reinforced plastic (GRP) laminates were studied under conditions of increasing impact energy and increasing number of repeated impacts. The GRP was stitched through the thickness with Kevlar thread in two orientations with a low or high stitch density. The Mode I interlaminar fracture toughness, GIc, increased with stitch density whereas the Mode II toughness, GIIc, was not changed by stitching. The three-point flexural strength and short-beam interlaminar shear strength of the GRP before impact loading were reduced by stitching as a result of stitching damage. Under short-beam loading, the stitches become sites of stress concentration and this contributed to the reduction in interlaminar shear strength. The strengths of the laminates were reduced slightly with increasing impact energy after one impact. The strengths were reduced considerably with increasing number of impacts. The laminates suffered severe microstructural damage under repeated impacts, including shear cracking of the resin, delaminations, and crushing and fracture of the glass fibres. The impact damage resistance, the post-impact flexural strength and the interlaminar shear strength of the GRP were not improved by stitching, and this finding differs from other impact studies on stitched composites.