RESIDUAL STRESS EFFECTS ON THE CRACK-FRONT CONSRTAINTS FOR SURFACE CRACKS IN PIPES

摘要

This paper investigates the effect of residual stresses on the linear-elastic K_1-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_1-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_1-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_1-values and T-stresscs show pronounced variations with the change in the ratio of the crack depth over the wall thickness of the pipe (alt). The variation in the crack aspect ratio (the crack depth over the crack length, ale) 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.