THE MATHEMATICAL FORM OF MATERIAL CONSTITUTIVE LAWS APPLICABLE FOR MODELING OF METAL PLASTIC BEHAVIOR IN SIMULATION OF MACHINING

Among the effects of strain hardening، strain-rate hardening، and temperature softening، it has been long argued about which effect is predominant in governing the material flow stress in machining. In addition، there is a belief that the temperature and strain rate counterbalance the effect of each other; therefore there is no need to consider temperature softening and strain-rate hardening while modeling material flow behavior. It means that similar to metal forming processes، the flow stress will be the function of strain only; whereas، the magnitudes of strain-rates، and temperatures involved in machining are several orders higher than those generated in forming processes. On the other hand، measuring material flow stress under high strain-rates and temperatures in controllable conditions needs especial costly equipments; therefore it is worth to discuss whether the stress-strain data obtained in the simple standard tensile test can be used in machining. This paper discusses some requirements for mathematical form of material models applicable to describe the yield surface in simulation of machining. As a result the first predominant factor governing the material flow stress is strain hardening. It is demonstrated that strain-rate and temperature do not counterbalance the effects of each other in all machining zone. Therefore implemented material model in machining should include both considerable effects in a suitable way. It is demonstrated that friction is an important heat source independent to plastic dissipated energy; therefore its contribution in material flow should be considered in convenient method via implementing temperature parameter in mathematical equation of material model. In addition، the velocity of material deformation is another parameter governing the material flow behavior in machining. It is indicated that the strain rate parameter plays the role of suitable corresponding field variable; therefore it should be appeared in mathematical form of material model to include related effects.