Effect of Pressure Acting on the Flaw Surface on Plastic Collapse Strength for Pipes With the Circumferential Surface Flaw

Abstract The Rules on Fitness-for-Service of 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 this 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 methods that take into account the effect of the internal pressure acting on the flaw surface are derived, and compared the plastic collapse loads with and without considering the internal pressure acting on the flaw surface. When the flaw is shallow and its angle is small, the both plastic collapse loads are almost the same. However, when the flaw is deep and its angle is large, the plastic collapse load decreases due to the internal pressure acting on the flaw surface. Especially when the internal pressure is high, the impact is 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 degrees. Due to these, the impact of internal pressure acting on the flaw surface on collapse strength is negligible. If the flaw depth limitations are eliminated when the flaw angle exceeds 60 degrees or when accurate fracture strength has to be evaluated such as in reliability evaluation, the plastic collapse evaluation methods that take into account the effect of pressure acting on the flaw surface should be employed.