Fracture behaviour of glass-fibre mat-reinforced structural nylon RIM composites studied by microscopic and acoustic emission techniques

The fracture and failure behaviour of continuous glass-fibre mat-reinforced nylon block copolymer were studied at monotonic increased loading at different temperatures (T=−40 to 80 °C and deformation rates (v=1 and 1000 mm min−1). The fracture toughness,K c, was determined on compact tension specimens of different size in order to elucidate size effects.K c increased with increasing glass-fibre mat content and with deformation rate, whereas increasing temperature resulted in lowerK c values.K c was unaffected by the free ligament width of the compact tension specimens used. The failure manner was studied by acoustic emission and microscopic techniques (transmitted light and scanning electron microscopy). Simultaneous monitoring of the failure mode by acoustic emission and transmitted light microscopy allowed the failure sequence to be deduced and led to a reliable discrimination between the observed failure events based on their acoustic emission signal characteristics (e.g. amplitude, energy). For this composite with a very ductile thermoplastic matrix the following failure steps were concluded: (a) fibre debonding due to crack-tip blunting, (b) network-type deformation of the glass-fibre mat with concomitant fibre debonding and voiding of the matrix, (c) formation of kinked strands with crack opening due to matrix yielding, (d) fracture of the bent filaments within the strands followed by pull-out processes (fibre-fibre, fibre-matrix). The development of the damage zone was also assessed by acoustic emission via localization of the related events. It was established that the damage zone reaches its maximum dimension at the maximum load and only its shape changes along the crack-growth direction upon further loading.