Static fatigue of thermoplastic elastomers

Abstract

The failure behaviour in static fatigue of thermoplastic elastomers was estimated on the basis of the stochastic theory proposed for the failure of rubber vulcanizates. A statistical analysis of lifetime distributions for a styrene-butadiene-styrene triblock copolymer (SBS) in creep and stress relaxation experiments was made by using a Weibull distribution, and the failure behaviour of SBS was related to morphological changes of structure in stretched states, observed with a transmission electron microscope. Wear-out failure, in which the failure rate increases with time, occurs in the creep and stress-relaxation processes, and is similar to that in a carbon-reinforced rubber vulcanizate. These results suggest that in SBS, polystyrene (PS) domains dispersed in a continuous polybutadiene matrix serve as physical crosslinks and reinforcing fillers. In the stress relaxation process, however, the increased energy dissipation caused by plastic deformation and disruption of the PS domains leads the material to be more stabilized at a constant stretch ratio. This prolongs the lifetime of the material, due to multi-crack initiation at many portions of a specimen. The differences between the failure behaviour of SBS and that of the rubber vulcanizates are mainly caused by morphological changes of the structure in SBS on deformation.