Determination of allowable axial flaw sizes in stainless steel pipes based on probabilistic fracture mechanics

Abstract

The flaw evaluation is required if a flaw is detected in the pipe during the service life. The critical flaw size is the limit size of flaw that will cause the pipe to fail under service conditions, while the allowable flaw size is defined as the limit flaw size determined by evaluation procedures based on the standards. Therefore, the evaluation procedures will be simplified by comparing the detected flaw size with the allowable flaw size. Existing standards and studies have provided the allowable circumferential surface flaw size for pipes under combined various loading conditions and the allowable axial surface flaw size for pipes under pure internal pressure. Consequently, the pipes with axial surface semi-elliptical cracks under combined loading are considered in this paper. Normalized critical flaw sizes (ratio of the flaw depth to flaw half-length, a/t) are obtained by using the deterministic failure assessment diagram (FAD) method, and normalized allowable flaw sizes (ratio of the flaw depth to flaw half-length, a/t) are evaluated by probabilistic analysis. The methods were applied to calculate the numerical results for DN150 304 stainless steel pipes with axial flaws under various loading conditions. The smaller value of the allowable internal flaw size and external flaw size is determined as the final allowable flaw size. For considering the failure probabilities, the results of the allowable flaw size based on the probabilistic analysis are more conservative than the allowable end-of-evaluation-period flaw depth based on the limit load criteria by using the safety factor as provided in the Acceptance Standards of Section XI of the ASME Code (ASME XI).