Safety assessment of the small welded pipe system by the test and FEA

Abstract

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.