Damage does not cut it – Saturated damage in FEM modelling of metal cutting breaks the simulation but not the chip

Abstract

Damage models are used in metal cutting simulations to adjust the flow stress and to enable the serrated chip formation and chip breakage. The models describe the rupture strain of the material that is a function of temperature, stress, strain and strain rate. When a damage parameter in the model reaches a critical value, the flow stress of the material decreases to a predetermined fraction. Damage models show good results in predicting the cutting forces, chip thickness and serration frequency, but using them has serious disadvantages. First, damage models describe rupture strain, that is valid in metal cutting for chip breakage, but not for damage softening or chip serration, since those are based on adiabatic shear banding. Second point, shown in this paper, when material has reached the critical damage, the saturated model doesn’t perform as intended. The damage model is saturated in simulations with multiple cutting passes. The initial cut deforms the layer under the tool. The same layer is then cut during the next cutting pass. During this cut, the damage model is already saturated. The damage model issues are relevant to all machining simulations because all machining processes include the multiple cutting passes.