New engineering J-estimation schemes for complex cracked pipes based on energy equivalence concept

A complex crack has 360° internal surface crack in the circumferential section of the pipe and through-wall crack in the same section. Previous studies have shown that the fracture behavior of complex crack is different from those of through-wall crack or surface crack. Therefore, it is necessary to develop the new engineering scheme to accurately predict the fracture characteristics of a complex cracked pipe. This study proposes the new approximate J-estimation scheme for complex cracked pipe subject to bending moment. The method is based on the concept of an energy equivalence between cracked pipe and reduced thickness pipe without a crack. Elastic-plastic J-integral solution is provided in closed-form. To derive the solution, both the energy release in the radial direction of the surface crack and the circumferential direction of the through-wall crack were considered. To verify the proposed solution, J-integral was calculated for various geometric variables and material properties, and compared with the calculation results using the GE/EPRI based J-estimation scheme of the complex crack. Also, the effect of consideration of crack propagation direction on J-integral was systematically analyzed.