The Prediction of Fatigue Crack Propagation in a Friction Stir Welded Specimen Containing a Residual Stress Distribution

An on-line compliance-based method for the measurement of the component of residual stress normal to a growing fatigue crack has been evaluated.  Results from this crack-compliance method for specimens containing a friction stir weld are presented, and found to be in excellent agreement with residual stress data obtained using the cut-compliance method.  Variable stress-intensity factor tests were designed to demonstrate that a simple superposition model, summing the applied stress-intensity factor with the residual stress contribution, can be used to determine the local crack-tip stress-intensity factor.  Finite element and J-integral analyses have been developed to predict weld-induced residual stress using thermal expansion/contraction and an equivalent ΔT for the welding process.  An equivalent ΔT was established and applied to an analysis for all specimen geometries investigated to yield predicted residual stress distributions in very good agreement with experimental results obtained using the crack and cut-compliance methods.  Results from 3D finite element analyses for the propagation of a crack in components containing friction stir welds will be presented and compared to experimental observations.