Cyclic fatigue failure of TPU using a crack propagation approach

Abstract

Thermoplastic polyurethane elastomers (TPU) are stretchable, tough, wear resistant and easily processable soft materials. Especially because of their recyclability, TPUs can be suitable candidates to replace rubbers in several applications such as damping, footwear and cable coatings. However, their capacity to operate under cyclic loads over many cycles was rarely investigated, mainly due to their complex strain-dependent morphology and viscoplastic character. Additionally, the absence of chemical crosslinks results in a certain degree of creep and plastic deformation when TPUs are cyclically strained, questioning how to unambiguously define fracture mechanics variables such as the energy release rate G, typically used to evaluate fatigue crack growth in chemically crosslinked elastomers. We show that, when TPUs are cyclically loaded up to the same value of maximum stretch, their stress-stretch curve changes with the number of applied cycles, but eventually achieves a steady-state. We propose a suitable methodology to evaluate the cyclic fatigue resistance in TPUs, based on a fracture mechanics approach with some additional treatments to account for the higher tendency to creep of TPUs than thermoset rubbers. Comparing the obtained results of TPU with those for classical filled rubbers with a similar small strain modulus, we underline the excellent toughness and cyclic fatigue resistance of TPUs, opening new opportunities in their use for applications requiring to resist to crack propagation under cyclic loading at large strains.