| A dislocation-density based multiple-slip crystal plasticity formulation that is based on accounting for microstructural heterogeneities, dislocation-density interactions, and dislocation-density grain boundary (GB) interactions, such as dislocation-density transmission and blockage, has been developed to investigate microstructural quasi-static and dynamic failure modes in martensitic steel alloys. The balance between the generation and annihilation of dislocations, through glissile and forest interactions at the slip system level is taken as the basis for the evolution of mobile and immobile dislocation densities, and the evolution equations are then coupled to a framework that relates it to a general class of crystallographies and deformation modes. The formulation accounts for variant morphologies and orientation relationships (ORs) that are uniquely inherent to lath martensitic microstructures. Dislocation-density GB interactions, which are based on dislocation-density accumulation and transmission at variant boundaries, is used to predict stress accumulation or relaxations. A microstructural failure criterion based on resolving these stresses onto martensitic variant cleavage planes, and specialized finite-element (FE) methodologies using overlapping elements to represent evolving fracture surfaces is used for a detailed analysis of fracture nucleation and intergranular and transgranular crack growth in martensitic steels.;The interrelated effects of microstructural characteristics, such as variant morphology, variant distribution, ORs, and relative block and packet sizes, on the dominant dislocation-density mechanisms for the localization of plastic strains, void interactions, and the initiation and propagation of intergranular and transgranular fracture modes in martensitic microstructures that are representative of SEM/EBSD mapped images of martensitic steels, subjected to a broad spectrum of loading conditions ranging from quasi-static to high strain-rates, is analyzed.;The results indicate that the local dislocation-density behavior at the variant boundaries and the interiors are the dominant microstructural factors that influence the failure initiation and growth, which are consistent with experimental observations. The relative effects of the block and packet boundaries is investigated, and the orientation of the cleavage planes in relation to the slip planes and the lath morphology can be used to determine the dominant intergranular and transgranular failure modes. Block sizes along the lath long and lateral directions are identified as the key microstructural characteristics for toughening mechanisms, such as crack arrest and deflection. This indicates that optimal distributions and sizes of blocks and packets in martensitic steels can be determined for desired ductility, delayed crack nucleation and greater fracture toughness. This framework can then be used to obtain validated design guidelines for a new generation of high strength and high toughness steels. |
