循环变形下软质热塑性聚氨酯的应变诱导强化
Strain induced strengthening of soft thermoplastic polyurethanes under cyclic deformation
摘要：
我们研究了三种不同的商业热塑性聚氨酯弹性体（TPU）在单轴拉伸下的循环力学行为，这些弹性体通常被认为是普通填充弹性体的可持续替代品。所有TPU具有相似的硬段含量和线性模量，但如X射线分析所示，大应变性能明显不同。尽管存在这些差异，但我们发现在逐步循环加载中调节后的硬化效应与化学交联填充橡胶中观察到的常见软化（也称为Mullins效应）有很大不同。我们提出，这种自增强与TPU中天然存在的硬域的碎片化有关，这些硬域在较小但数量更多的子单元中可能充当新的物理交联点。所提出的硬化机制与拉伸天然橡胶中观察到的应变诱导结晶没有什么不同，但它具有持久性。特别是，它可能会导致局部增强，其中存在不均匀的应变场，就像循环疲劳中裂纹扩展的情况一样，为TPU的众所周知的韧性和耐磨性提供了潜在的解释
英文摘要：
We investigate the cyclic mechanical behavior in uniaxial tension of three different commercial thermoplastic polyurethane elastomers (TPU) often considered as a sustainable replacement for common filled elastomers. All TPU have similar hard segment contents and linear moduli but sensibly different large strain properties as shown by X-Ray analysis. Despite these differences, we found a stiffening effect after conditioning in step cyclic loading which greatly differs from the common softening (also referred as Mullins effect) observed in chemically crosslinked filled rubbers. We propose that this self-reinforcement is related to the fragmentation of hard domains, naturally present in TPU, in smaller but more numerous sub-units that may act as new physical crosslinking points. The proposed stiffening mechanism is not dissimilar to the strain-induced crystallization observed in stretched natural rubber, but it presents a persistent nature. In particular, it may cause a local reinforcement where an inhomogeneous strain field is present, as is the case of a crack propagating in cyclic fatigue, providing a potential explanation for the well-known toughness and wear resistance of TPU
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