Multiaxial high-cycle fatigue failure criterion and life prediction is important for structural components under complex low stress. A new multiaxial high-cycle fatigue damage parameter based on the plane of maximum shear stress is proposed, which includes the maximum shear stress amplitude, normal stress amplitude and maximum equivalent normal stress. The maximum equivalent normal stress is defined in the form of SWT stress function considering the influence of mean stress. The feasibility of the proposed failure criterion and life prediction model is verified by test data of different materials. The predicted results show that the life prediction error of the proposed fatigue model is basically within three times by comparing with McDiarmid, Matake, Crossland and Papadopoulos fatigue models.

In order to study on tensile and fatigue fracture mechanism of friction stir welded (FSW) joints, the tensile and fatigue behavior of FSW joints are studied based on the microstructure and strain distribution. The large plastic deformation and fracture occurred in the thermo-mechanically affected zone (TMAZ) on retreating side in tension tests. High contents of shear texture and small angle grain boundary reduce the tensile mechanical property of TMAZ material. The fatigue weak area for FSW joints is affected by the loading condition. The strain concentration in the welded nugget zone (WNZ) and base material makes the fatigue fracture liable to happen in these areas for the FSW joints under the stress ratios of 0.1 and −0.3. When the fracture occurred in WNZ, the crack initiation mainly occurred in clusters of hardened particles, while when the fracture happened in base material, the crack initiation mainly occurred near the pit. The crack in WNZ propagated in an intergranular pattern and the crack in the other areas extended in a transgranular mode, leading to a higher crack growth rate of WNZ than of other regions.

The fatigue weak area of aluminum alloy for a friction stir-welded joint is investigated based on the hardness profile, the residual stress measurement and the simulation analysis of fatigue property. The maximum residual stresses appeared at the heat-affected zone of the joint in the fatigue damage process, which was consistent with the fracture location of the fatigue specimen. The fatigue joint model of continuous performance is established ignoring the original negative residual stress; considering that it will be relaxed soon when the joint is under tension-tension cyclic loading. The fatigue parameters of joint model is based on the static mechanical properties of the joint that obtained from the micro-tensile tests and four-point correlation method. The predicted results for the fatigue weak locations and fatigue lives based on the continuous performance joint model are closer to the fatigue experimental results by comparison with the simulation results of the partitioned performance joint model.