Machining operations involve extremely high rates of plastic deformation. This study presents some insight to this subject by systematic evaluation of the relative performance of six fracture models to identify the most suitable fracture criterion for chip separation. In addition a new algorithm is developed to calculate constants of the damage models. It is demonstrated that due to different failure mechanism a unique fracture model cannot be the representative of crack generation in all machining zone. It is found that predictions of FE approaches, without implementing damage models, for strains and temperatures within the deformation zones are not satisfactory and the predicted resistance of workpiece material to cutting is unrealistically high. It is demonstrated that the first predominant factor governing the material flow stress is strain hardening and strain rate and temperature do not counterbalance the effects of each other. Therefore suitable mathematical equation for material model in machining should include their effects separately to implement the effect of velocity of process and external heat source (friction). It is demonstrated that for b.c.c. metals loading path (history) plays no significant role in changing the flow stress. In addition, extrapolation of these material model equations is possible. Key Words Cumulative-damage fracture model, Material constitutive model, Chip separation/formation, Finite element simulation
