ELASTIC-PLASTIC COD ESTIMATION FOR NON-IDEALIZED CIRCUMFERENTIAL THROUGH-WALL CRACKS IN PIPES WITH HIGH STRAIN HARDENING EXPONENTS UNDER GLOBAL BENDING MOMENT

In nuclear power plant design, the application of the leak-before-break (LBB) concept eliminates the risk of instantaneous double-ended pipe rupture. By excluding the risk of double-ended rupture, the installation of structural components, such as pipe whip restraints and fluid impact barriers, can be minimized, leading to a simplification of system design and cost savings. To apply the LBB concept, an elastic-plastic fracture mechanics evaluation must be conducted. This evaluation includes a pipe rupture assessment for critical through-wall cracks and an unstable fracture assessment for leak cracks detectable by the leak monitoring system. Performing the fracture mechanics evaluation requires various fracture mechanics parameters, and crack opening displacement (COD) is necessary for predicting leakage rates. In actual cracks, propagation occurs from the deepest point of the inner surface crack, leading to the development of a through-wall crack. This results in a non-idealized crack geometry where the length of the crack on the inner surface of the pipe is greater than that on the outer surface. Under the same crack size and COD conditions, idealized through-wall cracks in pipes tend to predict larger loads than non-idealized cracks in pipes, resulting in overly conservative assessments. Therefore, an accurate evaluation requires a COD estimation for non-idealized through-wall cracks in pipes. Therefore, in this study, a COD estimation for non-idealized circumferential through-wall cracks in pipes subjected to bending moment was presented. The existing COD estimations for non-idealized circumferential through-wall cracks in pipes were limited to materials with low strain hardening exponents. Thus, improvements have been made to extend applicability to materials with high strain hardening exponents.