Allowable circumferential flaw sizes based on code given and actual measured flow stresses for high toughness ductile pipes subjected to bending and tensile loads

Negyesi, M.*; Ha, Yoosung  ; Hasegawa, Kunio; Lacroix, V.*

Allowable stresses for pipes are determined by combining failure stresses and safety factors. When predicting the plastic collapse failure stresses for high toughness ductile pipes, flow stresses of the pipe materials are indispensable. The flow stress is usually given by the average of the yield strength and the ultimate tensile strength. Inservice Inspection of the American Society of Mechanical Engineers (ASME) Code Section XI states to use the yield strengths and the ultimate tensile strengths defined by the ASME Code Section II Materials. The ASME Code Section XI also states that actual measured yield strengths and ultimate tensile strengths are possible to be used, alternatively. Values of yield strength and ultimate tensile strength given by the ASME Code Section II are conservative, corresponding to lower bound values. Then, it is easily inferred that the allowable stresses calculated by the code given flow stress are lower than the allowable stresses calculated by the actual measured flow stress. The objective of this paper is to compare the plastic collapse failure and allowable stresses based on both the code and actual flow stresses for pipes with circumferential flaws subjected to bending and tensile loading. In addition, it is demonstrated that allowable flaw sizes based on both flow stresses do not differ much at low allowable stress. However, when the allowable stress is large, the allowable flaw size based on the code flow stress is substantially lower compared to that based on the actual flow stress.