Thermal degradation of the mode I interlaminar fracture properties of stitched glass fibre vinyl ester composites

Changes to the Mode I interlaminar fracture toughness, G(Ic) and fracture m echanisms of stitched and unstitched fibreglass/vinyl ester composites were investigated after exposure to elevated temperatures. The fibreglass was s titched th rough the thickness with Kevlar(R)-49 thread in two orientations with two stitch densities, and then resin transfer moulded with a cold-cur ing vinyl ester resin. After curing at room temperature (similar to 20 degr ees C) for several weeks, the composites were heated to between 100 and 300 degrees C for 1 h or at 175 degrees C for times ranging from 0.25-100 h. T he G(Ic) values, which were measured using the double cantilever beam metho d, of stitched composites in the cold-cured condition were between 1.5 and 2.3 times higher than the unstitched composite. It was observed with scanni ng electron microscopy that th is toughening occurred by deflection of the crack tip at the stitches, by the ability of the stitches to remain intact for a short distance (7-15 mm) behind the crack front, a nd by partial pull -out of broken stitches. The interlaminar fracture toughness of the unstitc hed composite increased slightly following heating, despite a possible brea kdown of the chem ica I structure of the vinyl ester between 150 and 300 de grees C. In contrast, the interlaminar toughness of the stitched composites was degraded significantly by heating, and this was probably caused by the rmal deterioration of the Kevlar(R) stitches. This study reveals that the e levated-temperature post-curing of stitched composites will reduce the effe ctiveness of Kevlar(R) stitching in raising the Mode I interlaminar fractur e toughness. (C) 1998 Chapman & Hall.