Determination of mode II delamination toughness of stitched laminated composites

Abstract

Delamination as a result of low-velocity impact loading is a major cause of fibre-reinforced composite failure. Through-thickness reinforcement in the form of transverse stitching is an effective way to suppress delamination in these composite laminates. In this paper, the effect of stitching on Mode II delamination toughness is analysed by using two specimen geometries, namely, the end-notch flexure (ENF) specimen geometry and the end-notch cantilever (ENC) specimen geometry. The ENC specimen geometry allows larger crack propagation lengths which may be necessary to develop fully the stitch-thread bridging zone in the case of compliant stitch threads. Initially the energy release rate, G II( Δa), is determined by the contour integral method and then the use and equivalence of the stress intensity factor approach is demonstrated. The effect of stitching on both specimen geometries is found to be the same, and identical closed form analytical expressions are obtained for the potential energy release rate, G IIR( Δa), required for crack propagation. The effect of stitching is expressed in terms of the stitching parameters α and β. The influence of friction between the crack surfaces is also included in the analysis. A simple design study for sizing the ENF and ENC specimens to minimise geometric non-linear response is presented. The effects of the stitching parameters and various geometric and material properties are examined.