Fatigue Crack Initiation Of Al-Zn-Mg Alloy Welded Joint

Fatigue failure is the primary failure form of the A7N01 aluminum alloy belonged to Al-Zn-Mg alloy which is one of the main materials in the structure of high-speed vehicle. The fracture mechanics was applied to the fatigue crack propagation process and it has got an ideal solution. But thus far it couldn't predict the crack initiation life that could hold even more than 50 percent of the total fatigue life.In this paper,the fatigue crack initiation properties in the weld metal, heat affected zone and base metal are detected by the notch fatigue test. Fatigue experiment results show that there is little difference in the fatigue crack initiation life between the base metal and the weld under the high stress amplitude. With the reducing of the stress amplitude, the difference appeared gradually. The fatigue crack initiation life of the heat affected zone doesn't have obvious discipline in contrast with the base metal and weld. The total fatigue life of the base metal is higher than the heat affected zone and the weld that is the lowest. The proportion of the initiation life in the total fatigue life is closely related to micro-area properties while less affected with the changes of stress amplitude levels. In the weld, the initiation lifetime accounts more of the whole life than other zones.The fractographs show that the fatigue crack initiates in different places, which are second phase particles rich in Fe in large size, the free surface along the grain boundaries and the pore, for base metal, weld and heat affected zone, respectively. By microstructure analysis it is found that second phase particles have crashed before loading, Fe and Cr are rich in the free surface. The pore in the heat affected zone locates at the fusion line.Based on micro-plastic hypothesis, a continuum damage mechanics model of high cycle fatigue is presented. In this model, the crack closure isn't considered, the damage conditions is confirmed, the dual variable of the plastic strain X is put forward and X is the internal short-range stress that effects the micro-plastic. The parameters in the model are fitted from the crack initiation life curve of A7N01 aluminum alloy and the model can predict the fatigue crack initiation life in the three regions. It is found that the fatigue crack initiation life under 40MPa predicted by the model has a good accordance with the experiment results.