Load multiplier (Z-factor) for elastic-plastic fracture mechanics assessment of class 2, 3 pipes

This study was aimed at deriving load multiplier (Z-factor) equations to assess the failure load of cracked pipes of class 2 or 3 according to the elastic-plastic fracture mechanics assessment procedure prescribed in the JSME Rules on Fitness-for-Service for Nuclear Power Plants (Codes for Nuclear Power Generation Facilities). The straight pipes with a circumferential surface crack under bending load and those with an axial surface crack subjected to internal pressure were analyzed. First, in order to conduct elastic-plastic fracture mechanics assessments, J-integral values were calculated for a pipe of Rm/t = 30, where Rm is the mean radius and t is the thickness of the pipe. It was confirmed that the J-values obtained were accurate enough for deriving elastic-plastic failure load. Then, the elastic-plastic failure loads were derived for carbon steels SPTP410 and STPG370 for various pipe and crack geometries. It was shown that the Z-factor of pipes with a circumferential pipe increased as the crack depth and pipe diameter increased. The Z-factors were tended to be small for pipes with an axial crack than those for pipes with a circumferential crack because the stress on the cracked plane was larger for the circumferential crack under the same load. Finally, equations for deriving conservative Z-factor of the cracked pipe were obtained. (author)