Allowable Axial Flaw Sizes Based on Code Given and Actual Measured Flow Stresses for High Toughness Ductile Pipes Subjected to Internal Pressure

Abstract When evaluating allowable flaw sizes for high toughness ductile pipes, failure stresses are necessary for the flawed pipes. The failure stresses are estimated by flow stresses for the pipe materials and the flow stress is usually given by the average of the yield strength and the ultimate tensile strength. 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 allowable stresses and allowable flaw lengths based on both the code and actual measured flow stresses for pipes with axial flaws subjected to internal pressure. It is further demonstrated that allowable flaw lengths based on the code flow stress are substantially shorter than those calculated based on the actual measured flow stress.