Micromechanical characterization of ductile damage in DP steel

Weight minimization triggered the automotive industry to introduce new advanced high strength steels that show ductile fracture by microvoid evolution under deformation, resulting in unexpected failure without significant necking. Therefore, an extensive study was initiated to gain insight on the formation and development of microstructural damage in dual phase (DP) steel from a mechanical point of view. Different DP (model) microstructures were created that are very similar except for a single microstructural parameter (ferrite grain size or martensite volume percentage). Using our miniaturized Marciniak setup, these DP microstructures were deformed to fracture for three strain paths (uniaxial tension, plane strain tension, and biaxial tension). Exploiting the in-situ SEM visualization capabilities of the miniaturized Marciniak setup, the relevant damage mechanisms were identified and their evolution studied. Finally, a procedure was developed to quantitatively analyze the evolution of these damage mechanisms (post-mortem) by semi-automatically categorizing each damage site over large SEM viewing areas to obtain statistically relevant trends of each damage mechanism with increasing strain, for the strain paths and microstructures. This procedure yielded rich data on the evolutions of the different damage mechanisms as a function of the DP microstructure and deformation mode. Some preliminary observations and hypotheses were formulated.