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.

中文翻译：

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抗疲劳热塑性聚氨酯弹性体裂纹尖端的自组织

尽管它们具有技术相关性，但从未详细研究过软质热塑性聚氨酯 (TPU) 在循环疲劳中对裂纹扩展的抵抗力。特别是，这类材料在文献中的一个明显缺点是循环疲劳抗力与大应变行为之间缺乏联系，而大应变行为在定义材料的抗裂纹扩展能力方面具有根本作用。我们在这里首次证明，当 TPU 的应变诱导硬化机制（已经观察到大变形）与非均匀应变的存在相结合时，如在循环疲劳的情况下，它会在裂纹中产生选择性增强尖端区域，这是解释 TPU 卓越的抗循环疲劳性能的关键。