Notch ductility of steels for automotive components

The aim of this work is to experimentally evaluate the notch ductility of a TWinning Induced Plasticity (TWIP) steel, a Dual Phase (DP) steel and two Quenching and Partitioning (Q&P) steels for automotive components. The notch ductility was investigated by studying the resistance to fracture initiation in tensile specimens with notches of various root radii. Fracture toughness is discussed in terms of the critical J-integral Jc. It was found that Jc progressively increases with the notch tip radius q for all the studied steels, demonstrating a high sensitivity of ductility upon notch severity. The relationship between Jc and q allowed to calculate the maximum strain at fracture initiation emax, that is proposed as a deformation limit parameter useful to rank the forming capacity of the steels. It can be considered as the maximum strain that can be sustained ahead of the notch tip up to the onset of fracture advancement. Afterwards, a metallographic analysis by means of light optical microscope and scanning electron microscope was carried on etched specimens. The fracture mechanisms were also investigated by means of scanning electron microscope.