近接した軸方向き裂に対する破壊評価のための合体基準

概要

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In a structural integrity assessment of cracked pipes according to the fitness-for-service code, the influence of mechanical interaction between multiple cracks is considered by assuming coalescence of the interacting cracks. In this study, criteria of the coalescence were examined by finite element analyses for axial surface cracks located inside a pipe. Longitudinal semi-elliptical surface cracks were assumed to be on a straight pipe which was subjected to internal pressure. The changes in the stress intensity factor, J-integral value and limit load were investigated under various conditions. The intensity of the interaction depended not only on the relative position of the cracks but also on the pipe and crack geometries. Based on the analysis results, the coalescence criteria for assessing the failure strength of the cracked pipe were discussed. It was concluded that, for the stress intensity factor and J-integral value evaluations, the cracks should be treated as a combined crack when the offset distance H is less than 0.6a, where a is the crack depth. As for the limit load analyses, the criteria of S ≦ 3a and H ≦ 3a/2 were obtained. Furthermore, for limit load analyses, it was found that the surface length of the combined cracks could be assumed to be equal to the sum of their lengths. The ligament length (distance between the cracks) was not necessary to be included in the surface length of combined crack.