MODE-II DELAMINATION TOUGHNESS OF STITCHED GRAPHITE EPOXY TEXTILE COMPOSITES/

Effects of through-thickness stitching on Mode II delamination toughne ss (G(IIc)) of graphite/epoxy uniweave textile laminates have been inv estigated by performing end-notched flerure tests. Different stitch ya ms and stitch densities were considered. The crack propagation in stit ched laminates was observed to be gradual and steady, unlike in the un stitched laminates where it was sudden. New methods of evaluating the effective or apparent critical strain-energy release rate for stitched laminates are presented. Stitching results in an excellent improvemen t in G(IIc) The apparent G(IIc) was 5 to 15 times that of the unstitch ed laminates, depending on the stitching parameters. There appears to be an optimum stitch density at which the toughness will be a maximum. The critical strain-energy release rate increases with increase in cr ack length as more and more stitches bridge the delamination. (C) 1997 Elsevier Science Limited.