Benchmark analyses of probabilistic fracture mechanics for cast stainless steel pipe

摘要

Performance demonstration certification of non-destructive inspection for cast stainless steel (CASS) has been planned but the target flaw depth to be detected has not been determined yet in Japan. The target flaw size is closely connected to the allowable flaw size which is determined by flaw evaluation of the rules on fitness-for-service. However duplex micro-structure of CASS makes low permeability of ultrasonic wave and large flaw size of UT detectability, which might not be acceptable by flaw evaluation. The current JSME rules for fitness-for-service allow only deterministic procedure. For rational mitigation of the acceptable flaw size, application of probabilistic fracture mechanics (PFM) is one of the useful countermeasures. In the paper, benchmark problems for a CASS pipe were proposed with intention applying and verifying PFM codes to CASS pipe's issue. As the fracture modes, fatigue crack extension, plastic collapse and ductile crack initiation were assumed. The PFM analyses were performed in the condition of the combination of crack extension and plastic collapse or ductile fracture to verify the basic functions of the PFM codes. Six organizations participated in the benchmark analysis and failure probabilities from them were compared. As a result the failure probability of each problem by each code showed good agreement and the code for application of CASS issue has been verified. The sensitivity of the failure criterion on the failure probability was discussed.