Fracture of Automotive High Strength Steels

This research is focused on the study of local deformation damage initiation and propagation in DP1000 steels which are good candidate for future generation of cars. The potential of DP1000 for applications in next generation of cars relies on a better understanding of the relationship between its overall mechanical properties and the deformation and damage of its microstructure. Such understanding will in turn favours the advancement in the development of future steels. Damage development and plastic deformation have been studied in a statistically meaningful way by performing a DIC procedure conducted at two different scales simultaneously. Plastic deformation in both ferrite and martensite phase analysed over a large representative microstructure are statistically measured up to the UTS point revealing that the martensite phase in the DP1000 is deforms plastically at very large strain values and showing a very similar strain heterogeneity as observed in the ferrite. A new experimental procedure to study crack propagation in DP1000 steel has been designed for the development of a laboratory scale punch test that generate loading conditions representative of industrial forming operations for the study of damage. Cracks were observed to form from the top outer surface and propagating towards the mid thickness. Void formation is found to take place near the ferrite-martensite boundaries in the ferrite phase. Crack paths are observed to propagate only in the ferrite phase and preferably goes around the martensite phase without crossing or breaking the martensite island. Effect of processing conditions on the macroscopic mechanical properties of DP1000 will also be investigated using the newly developed experimental procedures.