Influence of Imperfections on the Performance of Resistance Spot Welded Joints Made of AHSS

In the highly automated auto body production, resistance spot welding is the most important joining technology for steel components. This is as well given for Advanced High Strength Steels (AHSS), which are interesting for weight reduction and improving the safety of the passenger cell. But due to the specific microstructure and mechanical properties of AHSS and the rough conditions in automotive production lines, weld discontinuities such as cracks may arise. Furthermore, the manufacturing processes in automobile industry are generally associated with form, position and dimensional deviations of the components. As a consequence, production-related gaps between sheet metal components are existent. These two types of imperfection could generally influence the stress conditions at the spot weld and must therefore be taken into account regarding the performance of spot welded joints.

As a contribution to a manufacture-oriented analysis of the fatigue life of spot welded joints, shear tension specimens were produced with gaps between the sheet metal parts as well as reproducible surface cracks in the weld area. The fatigue behaviour of the test specimens was analysed in terms of cycles to failure and the specimen stiffness. The failure criterion was a loss of 30% of the specimen stiffness relative to the initial stiffness at the beginning of the test. The results were compared to those of equally produced gap-free and crack-free specimens, tested in the same manner.

Additionally, the influence of gaps on the shear tension strength as well as the local surface strain at the spot weld, given by 3D-strain field measurement, were determined under static loading.

To explain the impact of the produced imperfections on the stress conditions at the spot weld 3-dimensional FE-simulations where performed.