Effect of pressure acting on the flaw surface on plastic collapse strength for pipes with circumferential surface flaws

The Rules on Fitness-for-Service of the Japan Society of Mechanical Engineers (the rules on FFS) stipulates the limit load evaluation method based on the plastic collapse strength as one of the fracture evaluation methods for pipes with circumferential flaws. However, since the limit load evaluation method treats flaws as a decrease in thickness regardless of their location, the internal pressure acting on the flaw surface is not taken into consideration even when the flaws are on the inner surface of the pipe. The limit load evaluation method that takes into account the effect of the internal pressure acting on the flaw surface was derived, and the results were compared for plastic collapse loads with and without considering the internal pressure acting on the flaw surface. When the flaw was shallow and its angle was small, both plastic collapse loads were almost the same. However, when the flaw was deep and its angle was large, the plastic collapse load decreased due to the internal pressure acting on the flaw surface. Especially when the internal pressure was high, the impact was large. In the rules on FFS, the safety factor is taken into consideration in the evaluation of allowable stress, and the flaw depth is limited by a regulatory requirement when the flaw angle exceeds 60°. Due to this consideration and requirement, the impact of internal pressure acting on the flaw surface on collapse strength is negligible. If the flaw depth limitation is eliminated when the flaw angle exceeds 60° or when accurate fracture strength has to be evaluated such as in reliability evaluation, the plastic collapse evaluation formula that take into account the effect of pressure acting on the flaw surface should be employed.