| dc.description.abstract | Ductile fracture involving nucleation, growth and coalescence of microscale voids limits the manufacturability and mechanical performance of a variety of structural materials. This phenomenon is affected by length-scales arising from the material microstructure, the geometry of deformation and the loading condition. These length-scales in turn interact and evolve during the deformation process, resulting in often unknown and counterintuitive subsequent fracture processes. The aim of this dissertation is to understand how the nucleation and growth of macroscopic cracks in ductile materials depend on these evolving length-scales. Such an understanding enables microstructure informed prediction of ductile fracture and design of fracture resistant material microstructures. The microstructure of a variety of structural metals and alloys can be idealized as ductile matrix with randomly distributed inclusions. The size, spacing and volume fraction of these inclusions introduce microstructure-based length-scales. To investigate the micromechanism(s) of inclusion driven ductile fracture and its implications on fracture toughness of the material, a series of microstructure-based finite element calculations are carried out. Several features of crack growth behavior and dependence of fracture toughness on microstructural and material parameters observed in experiments, naturally emerge in these calculations. The results of these calculations also provide guidelines for microstructural engineering to increase fracture toughness. For example, the results show that for a material with small inclusions, increasing the mean inclusion spacing has a greater effect on fracture toughness than for a material with large inclusions. The pressing need of our time to decrease anthropogenic emissions of greenhouse gasses requires the use of high strength, fracture resistant structural materials such as advanced dual-phase steels to reduce vehicle weight and emissions. However, as the strength of dual-phase steels increases, the steel becomes more prone to ductile fracture under bending dominated manufacturing processes. Thus, the effect of length-scales induced by bending, intended dual-phase (ferrite and martensite) microstructure, and size and location of unintended inclusions on the bendability of dual-phase steels are quantified through microstructure-based finite element calculations. Here as well, several features of ductile fracture of dual-phase steels under bending observed in experiments, naturally emerge in these calculations. The results of these calculations show that efforts to improve the bendability of advanced dual-phase steels must focus on improving the properties of the softer ferrite phase. Furthermore, supervised machine learning is utilized to understand the effects of uncertainty associated with both, the intended and the unintended microstructural features, on the bendability of the dual-phase steels. Another set of calculations aim at investigating the potential of low-density micro-architectured metallic materials to outperform the high fracture toughness of natural materials with density less than water. The results show that it is possible to design micro-architectured metallic materials that possess an exceptional combination of high strength and fracture toughness at low densities that no other existing lightweight materials can offer. | en |
