Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress–stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some noncovalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to fatigue fracture. We study a hydrogel containing both covalently cross-linked polyacrylamide and un-cross-linked poly(vinyl alcohol). For a sample without precut crack, the stress–stretch curve recovers after thousands of loading cycles. For a sample with a precut crack, however, the crack extends cycle by cycle. The threshold for fatigue fracture depends on the covalent network but negligibly on noncovalent interactions. Above the threshold, the noncovalent interactions slow down the extension of the crack under cyclic loads.

中文翻译：

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自恢复水凝胶的疲劳断裂

具有优异机械性能的水凝胶正在为许多应用而紧张开发。其中一些水凝胶可以在多次加载循环后恢复其应力-拉伸曲线。这些水凝胶被称为自恢复水凝胶，甚至是无疲劳水凝胶。这种水凝胶通常包含共价聚合物网络，以及一些非共价、可逆的相互作用。在这里，我们表明自恢复水凝胶仍然容易发生疲劳断裂。我们研究了一种含有共价交联聚丙烯酰胺和非交联聚乙烯醇的水凝胶。对于没有预切裂纹的样品，应力-拉伸曲线在数千次加载循环后恢复。然而，对于带有预切裂纹的样品，裂纹会逐个周期地扩展。疲劳断裂的阈值取决于共价网络，但可忽略不计非共价相互作用。在阈值之上，非共价相互作用减缓了循环载荷下裂纹的扩展。