Time-Dependent Fracture Behaviour of Polymers at Impact and Quasi-Static Loading Conditions

Abstract

Based on the crack resistance (R) concept of elastic fracture mechanics the crack propagation kinetics at impact and quasi-static loading conditions was analysed both from the experimental and analytical point of view. For blends composed of polyurethane (TPU) and an acrylonitrile–butadiene–styrene copolymer (ABS), it has been shown that the stop block method in multiple-specimens technique is the most suitable method for determining R-curves at impact loading conditions. As shown for a couple of thermoplastic materials such as polypropylene(PP)/ethylene–propylene rubber (EPR)/polyethylene(PE) copolymers, short-glass fibre-reinforced PP, binary blends of polystyrene(PS)–polybutadiene(PB) block copolymers and PP/EPR blends, the crack-tip-opening displacement (CTOD) rate (limit value) relates exclusively to the deformations that occur in the matrix or the major phase, not, however, to those that occur inside the particles or the minor phase, or near interfaces. That means that the CTOD rate is not affected by the glass fibre fraction or the particle distance. Furthermore the CTOD rate is not influenced by the temperature (polycarbonate) and proportional to the loading speed (PP materials). For most of the polymers investigated, such as PC (> 40 °C), PP/EPR/PE, PP, PP/glass fibre composites, PE, ABS, TPU/ABS blends and SB block copolymer blends, a 3-phase process the crack propagation kinetics could be found, with the CTOD rate as a function of the stable crack growth (or of time) quickly converging against a limit value. Here, this stationary crack propagation (Phase III) is preceded by crack-tip blunting/crack initiation (Phase I) and non-stationary stable crack growth (Phase II).