A Novel Finite Element Approach to Modeling Hard Turning in Due Consideration of the Viscoplastic Asymmetry Effect

A novel material model for strain rate and temperature dependent asymmetric viscoplastic deformation behavior considering transformation induced plasticity (TRIP) as a crucial phenomenon influencing the hard turning process-oriented ductility was developed. Within the framework of viscoplasticity and continuum damage mechanics, the well-established Johnson-Cook flow stress model has been upgraded by the concept of weighting functions accounting for the asymmetric viscoplastic material behavior under different stress conditions during hard machining of chrome bearing steel AISI 52100. Moreover, the extended Johnson-Cook model incorporates the ductility alteration caused by transformation induced plasticity by applying the Leblond-approach. Based on the theoretical proceeding, a material routine for flow stress computation considering the viscoplastic asymmetry has been developed and applied within hard turning simulations using the commercial Finite-Element-Method (FEM) software DEFORM. In addition, the hard turning simulation model accounts for the phase transitions between martensite and austenite during the process-related material heating as well as austenite and white layer as a consequence of the so-called reverse martensite transformation. A decisive actuating variable concerning the feasibility and accuracy of the performed hard turning modelling is the austenite start temperature, which has been determined in consideration of the externally applied stress. Within the scope of the material modelling, a novel quantity referred to as stress mode factor has been introduced in order to enable the identification of areas in the cutting zone subjected to heterogeneous load conditions. The stress mode factor appraisal within the aforementioned bearing steel material routine discloses a new path to establish the influence of dissimilar stress states on the work piece heat balance as well as the impact of transformation induced plasticity on the stress distribution in the cutting zone.