13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Modeling the Effect of Residual Stress on the Ductile Fracture Behavior of an Aluminum Alloy 5083-H116 Xiaosheng Gao1,*, Jun Zhou1, Matthew Hayden2 1 Department of Mechanical Engineering, The University of Akron, Akron, OH 44325, USA 2 Alloy Development and Mechanics Branch, Naval Surface Warfare Center, West Bethesda, MD 20817, USA * Corresponding author: xgao@uakron.edu Abstract The influence of residual stress on the ductile fracture behavior of an aluminum alloy 5083-H116 is investigated in this study through a series of experiments and finite element analyses. A recently developed stress state dependent I1-J2-J3 plasticity model, is implemented to describe the plastic response of this material. A ductile failure criterion based on the damage parameter defined in terms of the accumulative plastic strain as a function of the stress triaxiality and Lode angle is established. The calibrated I1-J2-J3 plasticity model and ductile failure model are utilized to study the residual stress effect on ductile fracture resistance. A local out-of-plane compression approach is employed to generate residual stress fields in the compact tension specimens. Fracture tests of C(T) specimens having zero, positive and negative residual stresses are conducted. The numerical results, such as load-displacement curves and crack front profiles, are compared with experimental measurements and good agreements are observed. Both experimental and finite element results show significant effect of residual stress on ductile fracture resistance. Keywords Ductile fracture, Plasticity, Damage accumulation, Stress triaxiality, Lode angle 1. Introduction Residual stresses in the engineering structures are generated from forming, welding, assembling, heat treatment etc., which play an important role in either increasing or decreasing the fracture resistance. The compressive residual stress generally improves the fracture toughness, while the tensile residual stress can detrimentally reduce the loading capacity of the structure. This is usually attribute to the additional crack driving force and change of the crack front constraint [1]. To quantify the effect of residual stress on fracture toughness, it is necessary to introduce well characterized and reproducible residual stress fields into fracture specimens. There are plenty of literatures on residual stress generation techniques, which can either be mechanical or thermal process. Almer et al. [2] deformed large tensile specimens and cut the gauge sections to produce C(T) specimens. Meith et al. [3] applied local compression to the sides of fracture specimens. Because of the strain incompatibility between elastic and plastic region caused by the permanent plastic deformation, the residual stress field can be generated in the specimen. This local out-of-plane compression (LOPC) approach is further explored by Mahmoudi et al. [4], who ran a series experiments and finite element analyses to examine how the position of compression tools influences the residual stress distribution in the specimen. In this study, we employ the LOPC approach and use two pairs of compression punches to generate various residual stress fields in C(T) specimens. To model these specimens, a recently developed I1-J2-J3 dependent plasticity model is used to describe the plastic response of the material, an aluminum alloy 5083-H116. The residual stress field is quantified by conducting finite element simulation of the out-of-plane compression process. After the residual stress field is generated, fracture tests of C(T) specimens having positive and negative residual stresses are conducted and simulated numerically. A damage parameter is defined as a weighted integral with respect to the effective strain, where the integrand is the reciprocal of the effective failure strain as a function of the stress triaxiality and the Lode angle. Fracture is assumed to have initiated at a material point once the failure criterion is reached. A mesh-independent, post-initiation material degradation model based on an effective plastic displacement is adopted before the element is removed. The
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