Torsion testing of spot welds: influence of the sheet thickness and the test rates

Abstract Torsion testing of spot welds offers ideal prerequisites for evaluating the weld diameter, which is the most important quality criteria in resistance spot welding. However, torsion testing plays until yet a minor role in quality assurance and strength testing of spot welds. The reasons for this lie in the application areas and the resulting stresses on spot welds. In general, spot welds are not subjected to torsional stress, as large torsional moments cannot be transmitted through spot welds, or torsion at all is hindered when more than one spot weld is set into the component. In addition, the relevant standards for torsion testing are outdated. Extensive scientific investigations of torsion testing in resistance spot welding date back more than 50 years to Kunsmann’s dissertation. These investigations are still the basis of ISO 17653:2012 and DVS 2916-1, which provide detailed information on the procedure for torsion testing of spot welds. Nevertheless, torsion testing is used at the Technische Universität Dresden to assess the quality of spot welds. The reason for this lies in the fracture behavior that occurs, which offers enormous advantages in evaluating the weld diameter and the fracture surface. This contribution presents the results of the influence of the test rate during the torsion test on two different steel grades representing higher and lower ductile material behavior. The experiments are conducted with different sheet thicknesses. A special focus is set on the influence of the test rate on the strain rate dependency of the materials. On the basis of these results, suggested test rates are derived and proposed that should be used for the quasi-static torsion test and that should also be considered in applicable standards like ISO 17653.