TECHNICAL BASIS OF FATIGUE CRACK GROWTH RATE CURVE FOR NI-BASE ALLOY WELD METAL IN AIR ENVIRONMENT

摘要

Recently, incidents of SCC in Ni-base alloy weld metal of BWR components have been reported. When the defects are detected by inspection, structural integrity assessment should be performed for the technical judgment on continuous service based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). The structural integrity assessment includes fatigue crack growth analysis. However, fatigue crack growth analysis for Ni-base alloy is impossible since the fatigue crack growth rate curves in air and the BWR environment are not prescribed yet in the JSME FFS Code. The curve in air environment is needed for the structural integrity assessment of the flaw when the embedded flaw repair which is one of the repair techniques to isolate the defect from water environment by seal welding is applied. In this study, fatigue crack growth tests in air environment were performed for Ni-base alloy weld metal. Based on the test data, fatigue crack growth rate curves with ΔK_(th) of Ni-base alloy weld metal were investigated. It is found that fatigue crack growth data in the Paris region hardly depend on the test temperature and the stress ratio, whereas data around ΔK_(th) are dependent on them. Hence, the curve in the Paris region was regarded as the same curve despite the difference of the test temperature and the stress ratio. The minimum ΔK of final crack growth data in the ΔK decreasing test was adopted as ΔK_(th) of the curve.