Fracture behavior evaluations for ferritic steel piping with circumferential double flaws on the inner surface

Methods for assessing the structural integrity of nuclear components having some flaws are provided in the Rules on Fitness-for-Service for Nuclear Power Plants of the JSME code (JSME FFS code). Although the JSME FFS code provides such methods for piping with a single flaw, it does not describe any method for fracture assessment of piping with multiple flaws including flaw coalescence criteria. Some investigations on the fracture behavior of mainly austenitic stainless steel piping with multiple flaws, whose fracture mode is plastic collapse, have recently been reported and fracture assessment methods have been proposed. In the present study, fracture tests and analyses of carbon steel piping with a single and two circumferential flaw(s) on the inner surface were conducted to investigate a method for fracture assessment of ferritic steel piping with multiple flaws. It was found that fracture assessment based on the twice elastic slope method and the plastic collapse mechanism gave inadequate results for a large single flaw. Including this case, two kinds of elastic–plastic fracture assessment method, one using the Z-factor in the JSME FFS code and the other by ductile instability analysis, gave conservative estimates of fracture strength even when the structural factor SF was not considered (i.e. SF = 1).