CAE modelling of spot welds

The aim of this study is the development of a macro modelling FEM approach for the characterization of the mechanical behaviour and failure of the spot welds under different loading conditions. In a vehicle, the spot welds are widely used to join the different frame components. The vehicle capability to provide adequate safety in the case of crash situations is affected also by the mechanical properties of the spot welds. It is therefore necessary to take into account the spot weld behaviour in a structural crashworthiness analysis in order to accurately predict the vehicle response. Since the FEM model of the spot weld will be implemented in larger FEM models (vehicle frame), a single solid element was used to model the spot weld and the mechanical properties of both the weld nugget and heat affected zone of the welded area. The focus of this work is the study of the spot weld behaviour at the macro-scale level. The FEM models were developed by taking as reference the force-displacement data obtained with the experimental tests performed on the specimens with a single spot weld. Five different loading conditions were considered to measure the mechanical properties of the spot weld: the tensile load (90° test), the shear load, the combined tensile-shear load (30° and 60° test) and the bending moment load (peel test). Two different materials (CR340 and FeP04 steel) and different thicknesses for the specimens were tested. In the first part of the study, the grips used to clamp the specimens to the testing machine were not taken into account. A material with elastic and plastic properties was defined for the solid element of the spot weld: the parameters of the connection material card were evaluated through an optimization algorithm. A damage and failure criteria was implemented to characterize the spot weld rupture. In order to validate the numerical results obtained, a comparison between the experimental and simulated force-displacement curves was done. Since the curve fitting procedure does not reach a good result, it was necessary to model also the grips used to clamp the metal sheets in the experimental tests. This second modelling approach allows for improvements of the obtained numerical results.