Effect of welding residual stresses on fatigue crack growth thresholds

Fatigue crack growth thresholds ΔK_th were determined for friction stir welded butt joints made from aluminium alloys AA2024 and AA6013. Plotting the thresholds as a function of load ratio R showed distinctly higher amounts for welded joints as compared to those for parent material at small load ratios, but differences became smaller with increasing load ratio, until thresholds became finally identical for the highest R values. Applying Döker’s concept of two controlling parameters, namely ΔK and K_max [1], and plotting ΔK_th versus K_max, however, revealed that the effective threshold ΔK_th,eff determined at very high R ratios was nearly independent on the alloy and, simultaneously, was identical for parent material and respective welded joints. Thus, differences in threshold behaviour were only caused by the second threshold K_max,th, which was significantly higher for welded joints as compared to parent material. Differences in K_max,th coincided with compressive residual stresses determined by cut-compliance measurements in terms of stress intensity factors K_rs acting at the crack tip. Based on the analytical approach described by Döker [1], only one characteristic K_rs value was needed to calculate all thresholds of welded joints for 0 ⩽ R ⩽ 1 provided a base material master curve is available.