On the correlation between fracture toughness and crash resistance of advanced high strength steels

Automotive industry players have devoted large efforts to identify the material parameters governing the crash resistance of Advanced High Strength Steels (AHSS). Such knowledge is essential to improve impact performance prediction and optimize new steel development Nevertheless, there is still an open discussion about which are the most relevant properties on AHSS crashworthiness. In this work, the authors investigate the correlation between the fracture toughness of different AHSS and their crash failure behaviour. Fracture toughness is measured in the frame of fracture mechanics, through the essential work of fracture methodology. Two fracture resistance parameters are characterized: the fracture toughness at cracking initiation, w(e)(i), and the essential work of fracture, w(e). Toughness values are compared with the results of axial impact tests, which are evaluated according to the energy absorbed and the cracking behaviour observed in crash boxes. Results show that fracture toughness permits to describe different crash events in terms of crack initiation and crack propagation and allows ranking AHSS impact resistance; steels with higher w(e) present better crash performance. Therefore, fracture toughness is proposed as a key material property to predict the crash resistance of AHSS and as a relevant design parameter for crash resistant parts.