Ductile fracture modeling : theory, experimental investigation and numerical verification

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (p. 215-228).The fracture initiation in ductile materials is governed by the damaging process along the plastic loading path. A new damage plasticity model for ductile fracture is proposed. Experimental results show that fracture initiation in uncracked ductile solids is sensitive to the hydrostatic pressure and is dependent on the Lode angle. The damage plasticity model is established on a "cylindrical decomposition" system accounting for the pressure dependence, Lode angle dependence and the non-linear damage rule. Two internal variables are adopted to quantify the evolution of material properties. One is the plastic strain and the other is so-called damage variable. The joint effects of pressure and Lode angle define a fracture envelope in the principal stress space. Plastic deformation induced damage is expressed by an integral of the damage rate measured at current loading and deformation status with respect to the fracture envelope. A non-linear damage rule is proposed to characterize the damage accumulation with respect to the plastic strain. Furthermore, a damage related weakening factor is adopted to describe the material deterioration.(cont.) Aluminum alloy 2024-T351 is selected and a series of experiments have been conducted to determine the necessary material parameters for the description of the mechanical and damage properties. The numerical integration procedure is presented. The proposed model is numerically implemented into an explicit code. Simulations were performed and the results show good agreement with the experimental data. Several representative load conditions are also modeled. These simulations illustrate realistic crack patterns. In addition to the damage plasticity model, the micro void shearing mechanism is also introduced into a Gurson-like material model.by Liang Xue.Ph.D