Effect of Weld Residual Stress Fitting on Stress Intensity Factor for Circumferential Surface Cracks in Pipe

Abstract

Recent studies have shown that the crack growth of PWSCC is mainly driven by the weld residual stress (WRS) within the dissimilar metal weld. The existing stress intensity factor (K) solutions for surface cracks in pipe typically require a 4th order polynomial stress distribution through the pipe wall thickness. However, it is not always possible to accurately represent the through thickness WRS with a 4th order polynomial fit and it is necessary to investigate the effect of the WRS fitting on the calculated stress intensity factors. In this paper, two different methods were used to calculate the stress intensity factor for a semi-elliptical circumferential surface crack in a pipe under a given set of simulated WRS. The first method is the Universal Weight Function Method (UWFM) where the through thickness WRS distribution can be represented as a piece-wise cubic fit. In the second method, the through thickness WRS profiles are represented as a 4th order polynomial curve fit (both using the entire wall thickness data and only using data up to the crack-tip). In addition, three-dimensional finite element (FE) analyses (using the simulated weld residual stress) were conducted to serve as a reference solution. The results of this study demonstrate the potential sensitivity of stress intensity factors to 4th order polynomial fitting artifacts. The piece-wise WRS representations used in the UWFM was not sensitive to these fitting artifacts and the UWFM solutions were in good agreement with the FE results.