Abstract A critical failure mechanism in long-term applications of plastic structures, such as piping systems, is considered to be crack initiation and subsequent Slow Crack Growth (SCG). Thus, safe installation and operation during service lifetime of such structures are not conceivable without the knowledge of the resistance against SCG for any new material, like Unplasticized PolyAmide 12 (PA-U12). Unfortunately, long-term static tests at elevated temperatures lead to unreasonably long test times and measurements may also be affected by thermal aging and hydrolysis. Against this backdrop, the current study examines two accelerated test methods, namely the cyclic Cracked Round Bar (CRB) test as well as the Strain Hardening (SH) test, in order to characterize the SCG behavior. Both were originally developed for PolyEthylene (PE) and successfully implemented in recent years. While the cyclic CRB test measures SCG directly, accelerated through cyclic loading, the SH test quantifies the molecular disentanglement resistance, which determines craze formation and breakdown. The focus of this work was initially put on the extension of the CRB test towards PA-U12 grades, checking the occurrence of actual crack initiation and propagation. Afterwards a correlation to SH test results was done in terms of SCG resistance. Thereby, adequate test conditions were developed and the influence of the Molecular Weight (MW), expressed by the Viscosity Number (VN) of the PA-U12 grades, was considered. Results confirm the suitability of each method to rank SCG resistance and show a good correlation of the abovementioned SMall scale Accelerated Reliable Test (SMART) methods, highlighting their sensitivity to long-term relevant molecular parameters.

中文翻译：

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通过循环开裂圆棒试验和应变硬化试验测量现代 PA-U12 等级的缓慢裂纹扩展性

摘要 塑料结构（如管道系统）长期应用中的一个关键失效机制被认为是裂纹萌生和随后的缓慢裂纹扩展（SCG）。因此，如果不了解任何新材料（如未增塑聚酰胺 12 (PA-U12)）对 SCG 的抵抗力，就无法想象在此类结构的使用寿命期间安全安装和操作。不幸的是，高温下的长期静态测试会导致不合理的长测试时间，并且测量也可能受到热老化和水解的影响。在此背景下，当前的研究检查了两种加速测试方法，即循环裂纹圆棒 (CRB) 测试以及应变硬化 (SH) 测试，以表征 SCG 行为。两者最初都是为聚乙烯 (PE) 开发的，近年来已成功实施。循环 CRB 测试直接测量 SCG，通过循环加载加速，而 SH 测试量化分子解缠结阻力，这决定了裂纹的形成和破裂。这项工作的重点最初是将 CRB 测试扩展到 PA-U12 等级，检查实际裂纹萌生和扩展的发生。之后，根据 SCG 抗性完成了与 SH 测试结果的相关性。因此，开发了适当的测试条件，并考虑了以 PA-U12 等级的粘度数 (VN) 表示的分子量 (MW) 的影响。