Investigations into the material failure of resistance weld spots on ultrahigh-strength steels

In order to investigate the material failure at weld spots on ultrahigh-strength steels, tensile tests were carried out in the surroundings of weld spots with different loads and hardness measurements. The hardness measurements indicated a significant drop in the strength in the heat-affected zone around the weld spot. In the case of uniaxial loads, the tensile tests showed that the failure location was dependent on the strength of the welded base material. In this respect, cracks arose not only at the weld spot but also in the base material. Macro shell elements which portray the smeared properties of the weld spot and of its surroundings were subsequently developed on the basis of the experimentally obtained values. The necessity of resorting to more detailed modelling, which would have implied longer computing times, was thus avoided. In order to validate the procedure, the experimental tensile tests were investigated numerically in the simulation. The comparison between the simulation and the experiment shows a good congruence all in all, not only for the prediction of the failure location but also for the point in time of the failure. Therefore, this macro model for the influence of the weld spot was successfully validated for load cases with membrane loads in sheets. In advanced work, it must be investigated what precise influence the tempering treatment of the base material has on the crack location. In this respect, it should be analysed as from what tempering temperature the crack location changes and what causes this has. In the simulation, the validation must also take place on a component. Moreover, it must be checked whether the decreased strength in the material around the weld spot has an influence on its load-bearing capacity. Subsequently, further materials from bodymaking can be calibrated selectively with regard to the crack initiation at the weld spot. © 2010, TWI Ltd.