Self-Organization at the Crack Tip of Fatigue-Resistant Thermoplastic Polyurethane Elastomers

Despite their technological relevance, the resistance of soft thermoplastic polyurethanes (TPU) to crack propagation in cyclic fatigue has never been investigated in detail. In particular, a clear shortcoming in the literature for this class of materials is the lack of connection between the cyclic fatigue resistance and the large strain behavior that has a fundamental role in defining the material’s resistance to crack propagation. We demonstrate here for the first time that when the strain-induced stiffening mechanism of TPU (already observed for large deformation) is combined with the presence of the nonhomogeneous strain, as in the case of cyclic fatigue, it produces a selective reinforcement in the crack tip area, which is the key to explain the remarkable cyclic fatigue resistance of TPU. Using commercial TPU with similar modulus (∼8 MPa) but different large strain behavior, we show that the described mechanism stems from the multiphase nature of TPU and it is not necessarily linked to a specific large strain property as the case of TPU, which undergoes strain-induced crystallization.