FLEXURAL STRENGTH AND INTERLAMINAR SHEAR-STRENGTH OF STITCHED GRP LAMINATES FOLLOWING REPEATED IMPACTS

The flexural strength and interlaminar shear strength of stitched and non-stitched glass-reinforced plastic (GRP) laminates were studied und er conditions of increasing impact energy and increasing number of rep eated 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, G(Ic), increased with stitch densi ty whereas the Mode II toughness, G(IIc), was not changed by stitching . The three-point flexural strength and short-beam interlaminar shear strength of the GRP before impact loaning were reduced by stitching as a result of stitching damage. Under short-beam lending, the stitches become sires of stress concentration and this contributed to the reduc tion in interlaminar shear strength. The strengths of the laminates we re reduced slightly with increasing impact energy after one impact. Th e strengths were reduced considerably with increasing number of impact s. The laminates suffered severe microstructural damage under repeated impacts, including shear cracking of the resin, delaminations, and cr ushing and fracture of the glass fibres. The impact damage resistance, the post-impact flexural strength and the interlaminar shear strength of the GRP were nor improved by stitching, and this finding differs f rom other impact studies on stitched composites. (C) 1997 Elsevier Sci ence Limited.