A study of ductile fracture prediction in microforming process : constitutive modeling, numerical simulation and experimental verification

viii, 148 leaves : illustrations (some color) ; 30 cmPolyU Library Call No.: [THS] LG51 .H577P ME 2014 RanIn macro-scaled plastic deformation, or macroforming, the so-called ductile fracture has been studied from the perspectives of physics, deformation mechanism, affecting factor and prediction criterion. In micro-scaled plastic deformation, or microforming, all of these are relatively new and have not yet been extensively investigated. In tandem with this, an exploration on the applicability of the traditional fracture criteria in micro-scaled plastic deformation and the study of how size effect affects the deformation and fracture behaviors in the process is critical. Using micro flanged upsetting as a case study process, the fracture in microforming process is studied via experimental and finite element (FE) simulation. The FE simulation is conducted using established model based on the widely accepted surface layer model in microforming arena. Both physical experiments and simulations show that the size effect has a significant influence over fracture formation in micro-scaled plastic deformation. It is found that the ductile fracture affected by size effect is difficult to occur in microforming process under the same deformation conditions at which the fracture happens in macroforming scenario. The research thus provides an in-depth understanding of ductile fracture in micro-scaled plastic deformation. In the first stage of this research, a general constitutive model is established and Freudenthal fracture criterion is used as it is a classical damage accumulation criterion. The influence of grain itself to fracture initiation in the entire deformation process is studied. The primary fracture prediction is conducted with this model. In the second stage, dislocation density is implemented into the hybrid constitutive model to distinguish the stress contribution of body-centered cubic (BCC) and face-centered cubic (FCC) structures in multiphase materials. Freudenthal fracture criterion is used as the fracture criterion for the compression-dominative deformation process. The stress-induced fracture map is first proposed to evaluate the effects of grain and feature sizes on the fracture behavior in microforming. Then, six uncoupled fracture criteria are studied and discussed in order to determine their applicability in micro-scaled forming process. The fracture strain via simulation is compared with the actual experimental fracture strain and a generalized form of ductile fracture criteria is provided for a better explanation and understanding of the criteria. In the last part of this research, the flow stress model for sheet metal deformation process is presented. Using micro-scaled deep drawing process for experiment verification, the validity of the surface layer model is discussed and the explanation for the difference of simulation and experimental results is presented.Department of Mechanical EngineeringPh.D., Dept. of Mechanical Engineering, The Hong Kong Polytechnic University, 201