DETERMINATION OF MODE-II DELAMINATION TOUGHNESS OF STITCHED LAMINATEDCOMPOSITES

Delamination as a result of low-velocity impact loading is a major cau se of fibre-reinforced composite failure. Through-thickness reinforcem ent in the form of transverse stitching is an effective way to suppres s delamination in these composite laminates. In this paper, the effect of stitching on Mode II delamination toughness is analysed by using t wo specimen geometries, namely, the end-notch flexure (ENF) specimen g eometry 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 cas e of compliant stitch threads. Initially the energy release rate, G(II )(Delta a), is determined by the contour integral method and then the use and equivalence of the stress intensity factor approach is demonst rated The effect of stitching on both specimen geometries is found to be the same, and identical closed form analytical expressions are obta ined for the potential energy release rate, G(IIR)(Delta a), required for crack propagation. The effect of stitching is expressed in terms o f the stitching parameters alpha and beta. The influence of friction b etween the crack surfaces is also included in the analysis. A simple d esign study for sizing the ENF and ENC specimens to minimise geometric non-linear response is presented. The effects of the stitching parame ters and various geometric and material properties are examined.