Evaluation of Residual Stress Fields in Friction Stir Welded Zone Based on the Plastic Strain Increment and Mises Yield Criterion

The residual stress generated during the welding process affects the mechanical properties of the material. In this study, a measurement method of residual stress fields in the friction stir welded zone is developed based on the plastic strain increment and Mises yield criterion. First, the relationship among the residual stress, the measured local yield strength, and the plastic strain increment is derived according to the Mises yield criterion. Then, through uniaxial tensile tests, the stress–strain curve of any point in the friction stir welded zone is obtained using the digital image correlation method. Finally, the distribution of residual stress around the friction stir welded zone is characterized by the derived relationship on the basis of accurately obtaining the local yield strength. The feasibility of the method developed in this paper for measuring residual stress is verified by comparison with the results of the mechanical method combined with the charge coupled device Moiré method. Additionally, other material parameters such as modulus and yield strength can also be measured at the same time. The proposed method is applied to a mechanical properties evaluation of the friction stir welded joint of an aluminum alloy 6061T6 thin plate. The results show that the residual stress around the weld is tensile, showing a bimodal distribution, and the peak value of the residual stress is located in the thermomechanical affected zone. The strength has a double-valley distribution, and the local yield strength of the thermomechanical affected zone is the lowest. The mechanical properties of the thermomechanical affected zone are relatively poor, and it is the location that is prone to fracture.