Failures of small pipe systems continue to occur frequently at nuclear power plants and aerospace equipment resulting in degraded systems and unscheduled downtime. Currently, there is a lack of detailed technical requirements and evaluation criteria for the small pipe in the existing design codes. In the design code, if referring to the general fatigue evaluation method for metal parts, the results may be too conservative. It has been shown that fatigue tests based on structural parts can significantly extend the component life by recently studies. At the same time, for the small pipe, the influence of the crack propagation stage on the fatigue life (S–N curve) is usually ignored in the existing evaluation process. On the other hand, fatigue-related failures are generally detected as small cracks or leaks before major pressure boundary ruptures occur, so the “Leak-Before-Break” (LBB) can be also applied for the small pipe. This study is undertaken to improve the understanding of the fatigue life of the small pipe (comparing the pipe test results with the data in the code) and the characteristics of fatigue crack propagation. Also, the applicability of the LBB concept is discussed. The results show that the S–N curve obtained by the pipe test is higher than that gained by the standard specimen. The crack propagation life accounts about 3% of the fatigue life in the study case. The critical failure mode is dominated by the plastic collapse; it is therefore shown that the plastic collapse load or critical crack size can be increased by increasing the tensile strength of the materials. A safety factor of 9.25 for the LBB assessment is contained in the case; therefore, it is also useful to warn the final fracture of the pipe by installing a leak detection system.

核电站和航空航天设备的小管道系统故障继续频繁发生，导致系统性能下降和计划外停机。目前，现有的设计规范中缺乏对小管的详细技术要求和评价标准。在设计规范中，如果参考金属零件的一般疲劳评价方法，结果可能过于保守。最近的研究表明，基于结构件的疲劳试验可以显着延长部件寿命。同时，对于小管子，现有的评价过程通常忽略裂纹扩展阶段对疲劳寿命（S-N曲线）的影响。另一方面，与疲劳相关的故障通常在主要压力边界破裂发生之前检测为小裂纹或泄漏，因此“泄漏前断裂”（LBB）也可以应用于小管道。本研究旨在提高对小管子疲劳寿命（将管子试验结果与规范中的数据进行比较）和疲劳裂纹扩展特性的理解。此外，还讨论了 LBB 概念的适用性。结果表明，管道试验得到的S-N曲线高于标准试件得到的S-N曲线。在研究案例中，裂纹扩展寿命约占疲劳寿命的 3%。临界失效模式以塑性坍塌为主；因此表明，可以通过增加材料的抗拉强度来增加塑性坍塌载荷或临界裂纹尺寸。案例中包含用于 LBB 评估的安全系数 9.25；因此，通过安装泄漏检测系统来警告管道的最终断裂也很有用。