Study of limit load analysis for non-aligned multiple flaws

Abstract

Stress corrosion cracking (SCC) has been detected in the welded components of nuclear power plants. The components of nuclear power plants consist of highly ductile materials such as austenitic stainless steels, and their failure mode is expected to be plastic collapse due to ductile fracture. Limit load at plastic collapse can be evaluated based on net-section approach. The ASME Boiler and Pressure Vessel Code Section XI (ASME Section XI) and The Japan Society of Mechanical Engineers (JSME) Rules on Fitness-for-Service for Nuclear Power Plants (JSME Code) specify limit load analysis for a single flaw based on a net-section stress approach. When the maximum applied stress is lower than the allowable stress evaluated by limit load analysis considering structural factors, flaws are considered acceptable and the components can be used continuously until the end of the evaluation period, such as the next inspection time. According to flaw characterization rules in the code, if adjacent flaws are close to each other, non-aligned flaws are considered aligned flaws on a collapse cross-section. If not, none of the flaws consider the existence of any others. These code rules were determined based on linear fracture mechanics, such as fatigue crack growth and brittle fracture. Thus, the technical bases for the code rules are unclear in terms of plastic collapse due to ductile fracture. In this study, we performed fracture tests on plates and cylinders with two or three non-aligned through-wall flaws in order to develop the evaluation procedure of a collapse load for non-aligned multiple flaws. The evaluation procedure for non-aligned multiple flaws was developed based on the test results.