Fundamental Consideration of Maximum Allowable Flaw Lengths for Limit Load Evaluation Based on Flat Plates for the ASME Code Section XI

Abstract When flaws are detected in power plants, they are evaluated to determine their impact on component integrity. Three conditions are imposed on the components in question to ensure they can operate safely. First, the applied stress must be less than the allowable stress. The second is that the maximum allowable flaw depth should be set to prevent coolant leakage from the pressurized pipes. According to the ASME Code Section XI, the allowable flaw depth should be less than 75% of the pipe wall thickness, even if the first condition is met. The third condition is the maximum allowable flaw length, which is intended to prevent a guillotine break in the case of a circumferential flaw or a split fracture in the case of an axial flaw. The current maximum allowable flaw length is defined as the length at which through-wall flawed piping fails due to applied stress. Therefore, current maximum allowable lengths are irrespective of flaw depth. However, the failure stress for a shallow flaw is higher than for a through-wall flaw, and the elongation for a shallow flaw is greater than for a through-wall flaw in a plate subjected to tensile loading. Furthermore, if the length of a shallow flaw exceeds the maximum allowable length determined by a through-wall flaw, the shallow flaw is not acceptable. This paper uses a flat plate model with surface flaws to examine the characteristics of flaw lengths and proposes a new methodology for determining the maximum allowable flaw lengths.