A new method for fatigue life evaluation under out-of-phase variable amplitude loadings and its application to thin-walled magnesium welds

摘要

Depending on material ductility out-of-phase loads can result in fatigue life increase or fatigue life reduction compared to in-phase loading. Especially the latter case is crucial and must be taken into account as fatigue life evaluation is performed. Area covered by rotating shear stress vectors in section planes is used in order to provide a numerical out-of-phase measure of the load. Fatigue life evaluation for the variable amplitudes loadings is performed using the Palmgren-Miner linear damage accumulation, whereas the total damage of every cycle is split up into the stress amplitude component and the out-of-phase component. In order to compute the two components a modification of the Rainflow counting method, which keeps track of the time intervals, where the cycles occur, must be used. The method takes into account different slopes of the pure axial and the pure torsional Woehler curves using a Woehler curve interpolation. Fatigue life tests under constant and variable amplitude (Gauß-distributed spectrum with L_s = 5 • 10~4 cycles) loadings were performed on the tube-tube thin-walled welded specimens made of magnesium (AZ31 and AZ61) alloys. The tests included pure axial, pure torsional and combined in-phase and out-of-phase loadings with the load ratio R, R = -1. Magnesium welds show a fatigue life reduction under out-of-phase loads. FE-models of the specimens according to the notch stress approach were built in order to obtain local linear elastic stresses in the weld. The fatigue life assessment under multiaxial variable amplitude in-phase and out-of-phase loading with the developed method, showed a good agreement with the experimental results.