Examination of Fracture Toughness of Pipes to Satisfy Leak-Before-Break Conditions

This paper describes examinations of the requirements for fracture toughness of piping materials to establish Leak-Before-Break (LBB) behaviors. One of the important elements in establishing LBB is the fracture toughness of the materials. The fracture toughness might vary greatly due to inappropriate material management. Some representative fracture toughness properties of piping materials are plane strain fracture toughness (K_Ic or J_Ic) and fracture resistance curve (J-R curves). Requirements to establish LBB were examined focusing on these two properties. Although it is easy to obtain plane strain fracture toughness (it can, for example, be estimated using the Charpy impact test, etc.), trying to predict ductile fracture strength based only on plane strain fracture toughness will likely result in underestimating the strength. In the LBB Codes of the Japan Society of Mechanical Engineers (JSME), the elastic-plastic fracture mechanics (EPFM) have been defined for the material that shows ductile fracture behaviors. The LBB Codes set the ratio of the plastic collapse strength and ductile fracture strength (P_b′/P_b-cr) as 1.5 or less. Therefore, requirements for the fracture resistance of the piping materials to have the same fracture strength as the LBB Codes were examined. The examinations have shown that the slope of the fracture resistance curve is as important as the plane strain fracture toughness (J_Ic). In order to achieve the same ductile fracture strength with EPFM defined in the LBB Codes, the fracture resistance of the pipe materials should be controlled plane strain fracture toughness (J_Ic) and fracture resistance curve. Materials with high fracture toughness should be used, especially when employing thin-walled or high yield stress pipes.