Assignment of acoustic emission to the failure sequence and damage zone growth in glass fiber strand mat-reinforced structural nylon RIM composites

Abstract

The failure sequence and the damage zone evolution of a reaction injection molded (RIM) ≈ 20 wt.% glass strand mat reinforced nylon block copolymer (NBC) was studied by acoustic emission (AE) and transmitted light microscopy. Simultaneous monitoring of the failure mode by AE and transmitted light microscopy lead to a highly reliable discrimination between the observed failure and the related AE signal characteristics based on their amplitude and energy. The failure sequence consisted of the following steps: a) fiber debonding due to crack tip blunting, b) network-type deformation of the GF mat in the relative “soft” matrix with concomitant fiber debonding and voiding, c) bending of the strands with crack opening encouraged by matrix yielding, d) fracture of the bent strands and their filaments followed by pull-out processes. The evolution of the damage zone was also assessed by AE via localization and mapping of the related AE events. It was concluded that the damage zone reaches its maximum extension into the load direction (≈22 mm) at the maximum load. Further loading of the compact tension (CT) specimen decreases this size of the damage zone but strongly increses its extension perpendicular to it, i.e. along the crack growth direction.