According to linear elastic fracture mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson’s ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the slow crack growth and the environmental stress cracking resistance of two high-density polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behavior of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behavior exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.

中文翻译：

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粘弹性材料的K $ K $和G $ G $方法之间的比较：HDPE的环境应力开裂情况

根据线性弹性断裂力学，应力强度因子和能量释放速率是由所考虑材料的弹性模量和泊松比链接的两个断裂参数。通过引入随时间变化的量，该概念已扩展到线性粘弹性材料的分析。它也用于非线性粘弹性聚合物，即使在这种情况下其精度仍然是一个尚待解决的问题。在这项工作中，研究了两种高密度聚乙烯等级的慢速裂纹扩展和耐环境应力开裂性，它们的分子量分布和耐断裂性有所不同。事实证明，由应力强度因子或能量释放速率提供的两种材料的断裂行为是等效的，尽管这两种聚合物表现出非线性的机械性能。此外，评估了与两个断裂参数相关的随时间变化的有效模量：其值与通过拉伸试验实验确定的模量非常吻合。尽管测试条件不同（即施加的机械负载，温度和存在活跃环境的情况），但仍发现了恒定的相关有效应变，对于两种考虑的聚乙烯，其等效值均相等。