Inverse identification of the heterogeneous strain hardening of the friction stir welded aluminum sheets using virtual fields method

In this paper, inverse identification of the local heterogeneous strain hardening in friction stir welded aluminum alloy sheets using a single tensile test is presented. During friction stir welding, the mechanical properties of the jointed aluminum sheets are altered from their original states. The finite element-based virtual fields method (FE-VFM) was applied to measure this change in the mechanical properties during the joining. Special numerical schemes were adopted in the FE-VFM for the accurate and numerically efficient identification of the local properties, including a two-step VFM calculation procedure, a sub-zone division approach in the weld-affected zone, quadratic interpolation of the constitutive parameters, and a special type of virtual fields based on the Gauss distribution function. The newly developed method was applied to measure strain-hardening distributions of the friction stir welded AA6061-T6 sheets. The measured distributions of the local strain hardening well captured decreased strength in the narrow thermo-mechanically affected zone (TMAZ). Finally, inversely measured mechanical properties were validated based on finite element analysis.