Stress-state-dependence of damage strain rate tensors caused by growth and coalescence of micro-defects

The paper deals with damage and fracture behavior of ductile metals caused by stress-state-dependent growth and coalescence of defects on the micro-level. The continuum damage model has been generalized to take into account the effect of stress state on damage and failure criteria as well as on evolution equations of damage strains. Different branches are considered corresponding to various micro-mechanisms depending on stress state: growth of voids, formation of micro-shear-cracks and their combination. To be able to get more insight in this complex damage and failure behavior series of three-dimensional micro-mechanical numerical analyses of void containing unit cells have been performed. These calculations cover a wide range of stress states in tension, shear and compression domains. The numerical results are used to show general trends caused by growth and coalescence of micro-defects. They are also used to develop equations for stress-state-dependent damage criteria, to propose evolution equations for damage strain rate tensors, and to introduce a coalescence condition and a fracture criterion formulated in terms of damage strain tensors.