Fracture toughness testing of Advanced High Strength Steels for automotive applications

Abstract

Carbon dioxide emissions from the transportation sector account for roughly one-third of world-wide CO2 emissions. Governmental pressure to tackle this problem has forced the industry to adapt with regard to material development. One such adaption is the development and application of advanced high-strength steels (AHSS), offering high specific strength combined with improved ductility. This enables lightweighting of consumer cars, as well as improving the overall occupant safety and crashworthiness. Despite these apparent advantages of AHSS, sheet metal forming operations can cause unexpected edge-cracking. In this research the occurrence of edge-cracking is investigated and rationalized with fracture toughness testing. The Essential Work of Fracture (EWF) methodology is applied on double-edge notched (DENT) specimens, an increasingly applied method to characterize the crack propagation resistance of thin sheet metal. A novel sample preparation method is evaluated here, based on sheared notches instead of fatigue pre-cracked specimens. The obtained fracture toughness parameters are validated against laboratory-scale deep-drawing experiments to estimate its predictive capabilities. Scanning electron microscopy (SEM) was employed to examine the fractured surfaces and to identify the relevant micro-mechanisms of fracture. It was found that the obtained EWF results are in good agreement with the observed cracking behaviour during deep-drawing. Both the established method (fatigue pre-cracking) as well as the novel method of sheared notches were able to reveal the intrinsically low fracture toughness, otherwise undetected with conventional tensile testing. From the fractured surfaces of the EWF samples the relevant micro-mechanisms of failure were identified that govern the fracture toughness properties. Transgranular cleavage fracture was responsible for brittle behaviour while failure ductile failure occurred via microvoid coalescence.
Although the fatigue pre-cracking method for EWF testing seems to show a higher accuracy in predicting the cracking susceptibility, the shearing method can be employed as a rapid routine test to identify deterioration of fracture toughness properties early on.