Studying the Effect of Residual Stresses on Fatigue Cracks through Full-field Methods

In this effort, a corner crack was grown from a notch in a nickel-based superalloy specimen with a shot-peened surface treatment to induce residual stresses. The crack length was less than 200 microns, and the full displacement field near the crack was determined using advanced digital image correlation. The specimen was then annealed at elevated temperatures to reduce or eliminate the residual stresses, and the full displacement field near the crack was again determined. The displacements after annealing were indeed significantly larger than those previous to annealing, demonstrating the reduction in residual stresses. Modeling has been done to determine the approximate level of residual stresses induced and then reduced through annealing. Through this method, the effect of residual stresses on short fatigue cracks can be directly studied. Further work will be discussed on the effect of temperature on residual stresses, an area of great concern in predicting fatigue lives of components. The degree to which these residual stresses are reduced under service conditions is not well understood, and the approach described here is expected to be extremely useful in determining and predicting this residual-stress reduction, leading to greatly enhanced life prediction where residual stresses are involved.