Investigation on the influence of loading-rate on fracture toughness of AHSS grades

The automotive industry is striving for light body-in-white structures while maintaining or improving passenger safety. The aim of this paper is to investigate the influence of the loading rate on the fracture toughness of thin steel sheet metal of three advanced high strength steels. Although steel is a heavy material it plays a significant role for lightweight solutions in car bodies. Three different advanced high strength steel (AHSS) grades, namely dual-phase (DP), quench-partitioning (Q&P) and TRIP-assisted bainitic-ferritic (TBF), are investigated in the present paper. For crash relevant components it is of importance to know the material response under high loading velocities i.e. high strain rates. A standard tensile test system is used for low loading rates, a high-speed tensile testing setup is used to obtain high loading rates. The fracture toughness of the three AHSS grades is evaluated using the methodology of the Essential Work of Fracture (EWF). The tensile specimen used in the present work is the double edge notched tensile (DENT) geometry with a pre-developed crack. High-speed imaging is applied to verify the validity of the evaluation method Essential Work of Fracture at high rates of loading. Results from this work show that knowledge of fracture toughness would improve the understanding of fracture and crack propagation mechanisms for third generation high strength steels used for automotive components.