On the fatigue propagation of multiple cracks in friction stir weldments using linear and non-linear models under cyclic tensile loading

In this work, the propagation life of a friction stir-welded sample made of ductile materials is estimated by employing linear elastic fracture mechanics (LEFM) and small-scale yielding (SSY) conditions. The purpose is to demonstrate that by considering the SSY, the prediction of the propagation life of the welded sample can be improved when compared to the traditional LEFM. The process of friction stir welding (FSW) of an AA2024-T3 butt joint is then simulated by using the finite element method. Hence, a thermal analysis of the numerical model is performed, and the calculated temperature field is subsequently subjected to thermo-mechanical analysis. In the latter model, the two defects located in the most critical position, detected experimentally by performing fatigue tests on the same component, are introduced into the model by using the constrained crack faces technique. Furthermore, to enable the thermo-mechanical simulation of the FSW process, temperature-dependent non-linear material properties, material softening, and isotropic hardening are considered. Concerning fatigue crack growth analysis, three simulations of the fatigue crack propagation are performed by using three different propagation laws. The first is performed by considering linear elastic material properties and Vasudevan's law on fatigue crack propagation; the second is by employing non-linear material properties and Kujawski–Ellyin law; the third takes into account the non-linear material properties and UniGrow law. Thereafter, appropriate constraints and a remote fatigue load are applied to the specimen to allow residual stress redistribution and fatigue crack growth, respectively. The constraint effect is also evaluated by the calculation of the T-stress parameter. Finally, a comparison between the numerical and experimental results is presented; consequently, a better agreement with the case of the non-linear model under the SSY conditions is found.