The damage to stitched GRP laminates by underwater explosion shock loading

This paper examines the effect of impulse loading from an underwater explosive shock wave on the damage to glass-reinforced polymer (GRP) laminates stitched through-the-thickness with Kevlar thread. Glass-fibre preforms were stitched in the parallel or transverse directions with a density of 3 or 6 stitches/cm(2), and then fabricated by resin-transfer moulding. The response of the stitched GRP laminates to explosion loading at two shock levels was compared to a non-stitched laminate. The damage to the non-stitched and stitched laminates appeared to be similar, with both the polymer matrix and glass fibres experiencing cracking while some of the polymer/glass interfaces were delaminated. The damage occurred by the bending of the laminates under the pressure exerted by the shock wave. At a relatively low shock level, no significant damage to the Kevlar threads occurred, but at a higher shock level many of the threads were broken. The stitching was effective in reducing the spread of delamination damage, particularly when the stitch orientation was in the parallel direction and/or the stitch density was high. However, the stitches acted as stress concentrators under shock loading, resulting in a large amount of damage localised around the stitches. Measurements of the residual tensile strengths of the damaged laminates are presented.