P 806 – Characterization and simplified modeling of the fracture behavior of spot welds from ultra-high strength steels for crash simulation with consideration of the effects of the joints on component behavior
High and ultra-high strength steels are increasingly used in the automotive industry in thin sheet thicknesses for lightweight constructions, an increase in passive safety and conservation of energy. Hot stamped components made of 22MnB5 with a tensile strength up to 1500 MPa are already established in automobile engineering and the steel development is going towards even higher strength press hardened steels. The lightweight steel construction has a high potential to increase the energy efficiency even more.
At present and also in future the well-established and highly automatable resistance spot welding process is used to join the single components of high strength steels to the load bearing construction. Softened zones occur around the spot welds because of annealing effects in these hardened steels due to the heat input during welding. To ensure the deformation behavior of structures built of these steels, the influence of softened zones must be well known and computable in simulations.
Spot welded joints in press hardened, ultra-high strength steel 22MnB5 have been extensively characterized during the research project. In addition to the measurement of the load dependent bearing capacity using the LWF-KS-2 testing, the surrounding softened zone has been characterized with particularly designed specimens, which contain the relevant gradient of mechanical properties. Thereby a spot weld model has been developed, that describes both the load bearing capacity of the joint and the crack initiation in the softened zone. The validation of this model was done by simulation of a particularly designed component test, which reproduces the crack formation in the softened zone during a overloading situation.
The possible field of application and acceptance of press hardened steels have been considerably extended with the results of this research project, because a comprehensive computing method for spot welds in these steels is now available for simulation. The crashworthiness of spot-welded components of press hardened steels can be established already in the conception stage.
Prof. Dr. P. Gumbsch, Dr.-Ing. S. Sommer, Dipl.-Ing. S. Burget, Prof. Dr.-Ing. habil. E. Roos, Dipl.-Ing. H.-J. Wink, Dipl.-Ing. D. Krätschmer, Prof. Dr.-Ing. O. Hahn, Prof. Dr.-Ing. G. Meschut, Dipl.-Wirt.-Ing. F. Klokkers, Dipl.-Ing. D. Hein