Ductile dynamic fracture modeling using embedded discontinuities in CGI machining simulations

A major driving force for the industry to simulate various manufacturing processes is the incorporation of new design materials e.g. in order to promote lightweight design often leading to significant changes in manufacturing conditions, which can be assessed in an efficient way by simulation rather than more expensive testing. In the current contribution we are concerned with the constitutive modeling of Compacted Graphite Iron (CGI) with respect to orthogonal machining simulations. Although CGI consists in general of pearlite, graphite and ferrite, focus is placed on the constitutive modeling of the pearlitic phase since this is the dominating constituent with respect to the machinability issues. In this study, the continuum hardening response is modeled using the Johnson-Cook (JC) plasticity model; a model that has been extensively used in the literature for the modeling effects of large strains, high strain rates and high temperatures related to machining. In earlier works, the JC plasticity model has been used along with ductile fracture response that has been described with the element deletion based on Johnson-Cook dynamic failure criterion. In the current development we it is proposed to use a continuum damage approach for the continuous behavior up to the critical stress-strain states where discontinuous bifurcation occurs. Whenever a critical state has been diagnosed, a Cohesive Zone (CZ) is established so that the actual critical stress state is located right at the onset of stress degradation in the CZ. Both pre-peak continuum damage and post-peak CZ damage, representing distributed and localized damage evolution, respectively, are considered in the formulation. Both the pre- and post-peak damage evolutions are defined as a post-processing of the effective stress response. The localized cohesive zone damage is kinematically realized as an element embedded discontinuity which is introduced elementwise, thereby facilitating the model developments in standard FE-packages. The orthogonal machining simulations show that the new continuum damage model appears to be sensitive to element locking as well as significant element size dependence without the cohesive zone enhancement of the model. In order to investigate the extent of locking behavior in simulations and also the possible pathological mesh dependency the series of 2D shear test simulations with both triangular and rectangular elements with different sizes have been conducted and results are compared