Modeling of cast iron materials related to machining

Doktorsavhandling, 2013

A microstructure level simulation model of cast iron machining based on micrograph image analysis has been developed. The simulation tool has been developed for the orthogonal machining process involving 2D representations of a range of cast iron microstructures. The benefits achieved from this approach are: a better understanding of the machinability of the work-piece material related to its composition and microstructures, and qualitative predictions of e.g. the cutting force connected to a variation of the microstructural properties. Compacted graphite iron was taken as a prototype material due to unsolved issues regarding the machining of this type of cast iron in industry. As to the modeling, focus is placed on the pearlitic phase since it is the dominating constituent with respect to strength, and the continuous deformation behavior is described using the Johnson-Cook (JC) visco-plasticity model. Various formats of this model, using both hyperelastic-inelastic and hypoelastic-inelastic formulations, have been used to investigate possible differences in response and computational efficiency. In order to further describe the material degradation during machining, a continuum damage evolution is proposed as an enhancement of the JC model. Finite element results obtained from many simulations have been compared to machining tests with promising results. However, a severe mesh dependence has been observed in the simulations caused by the local damage modeling. A special investigation of this mesh dependence has been undertaken based on the resulting behavior of the Johnson Cook (JC) plasticity model combined with two different types of damage formulations. The results show a similar extent of the mesh dependence for both damage models, and that the viscous regularization efects, due to rate dependence of the model, are absent. As a remedy to the observed mesh dependence, the final contribution is concerned with ductile dynamic fracture modeling using FE-element embedded discontinuities. To characterize the homogenized continuous/discontinuous macro-behavior, a discontinuous enhancement is proposed at a sub-scale based on homogenization theory. In the corresponding FE-application, localized cohesive zone damage is kinematically realized as an element embedded discontinuity, which is introduced elementwise, thereby facilitating the model implementation in standard FE packages. In the considered numerical examples the proposed continuous/discontinuous ductile fracture modeling exhibits no significant element size dependence.