Multiscale investigation of ductile damage in dual phase steels

Comprehending the damage mechanisms in dual phase steels holds paramount significance due to their extensive application in structural and safety components within the mobility sector. Beyond the influence of stress states on damage, apprehending anisotropic damage behaviour is pivotal for optimising manufacturing processes and reducing material waste. This study investigates the anisotropic damage behaviour of DP980 and DP600 first at the macroscale, employing a hybrid experimental-numerical methodology, facilitated by a novel extension to the Modified Bai-Wierzbicki (MBW) damage model. The findings reveal that accounting solely for anisotropic plastic deformation fails to adequately explain the anisotropic damage observed in experiments. This points to the crucial role of microstructural features in dictating anisotropic damage phenomena. Subsequently, by employing uncoupled damage indicators within a crystal plasticity finite element (CPFEM) framework, the deformation in statistically equivalent microstructure models was examined under uniaxial tension along various loading directions with respect to the rolling direction. This exploration was enabled by the newly developed stress-controlled proportional periodic boundary condition. The study determined that martensite volume fraction and the variations in its morphology contribute to the anisotropic damage observed in DP980, with the presence of discontinuous martensite bands having an adverse impact. The study also delved into the influence of stress states on distinct damage mechanisms, namely martensite/ferrite (M/F) decohesion, ferrite/ferrite (F/F) decohesion, and martensite cracking, using the newly developed coupled crystal plasticity-continuum damage model with implicit gradient regularisation. The results highlighted that the intensity of damage is indeed affected by the stress state, with prominent active damage mechanisms identified. Finally, a computational homogenisation framework to predict localisation using bifurcation theory from 3D microstructure has been introduced and demonstrated. The potential of using the localisation strains, obtained from the bifurcation analysis, as scale bridging constituents has been contemplated upon. The developed methodologies and material models in this research facilitate the incorporation of damage into computation-driven material design.