Residual Stress Effects on the Crack-Front Constraints for Surface Cracks in Pipes

This paper investigates the effect of residual stresses on the linear-elastic K_I-T fields along the front of circumferential surface cracks in pipelines. The numerical procedure simulates three typical patterns of residual stresses through a modified eigenstrain approach, which combines a thermal loading with a mechanical traction imposed on the heat-affected zone. The three residual stress profiles considered correspond to the high-heat input, the medium-heat input and the low heat input welding processes for circumferential butt welds in pipes outlined in BS 7910. The linear-elastic K_I-T stresses, computed from the interaction-integral approach, characterize the constraints along the front of the circumferential flaw. The numerical investigation, covering a comprehensive matrix of geometric parameters, shows that different residual stress fields impose substantial effects on the K_I-T stresses along the front of the surface crack in the wall of a pipeline. The deepest point along the crack-front often experiences low crack-front constraints characterized by the computed negative T-stresses for all three residual stress fields considered. The magnitudes of the K_I-values and T-stresses show pronounced variations with the change in the ratio of the crack depth over the wall thickness of the pipe (a/t). The variation in the crack aspect ratio (the crack depth over the crack length, a/c) introduces marginal variation in the computed T stresses. The ratio of the outer diameter to the wall thickness of pipe imposes very little effect on the linear-elastic crack-front constraints for the geometric parameters considered.