Ductile damage modeling of the machining process

Abstract: Machining processes are among the most common manufacturing processes for producing components used on a daily basis. It is a complex material removal process. Today, the research and development within the manufacturing industry is addressed by cost efficient numerical simulation strategies rather than by costly experimental procedures. The numerical simulation tools must then be able to model material subjected to inelastic deformations at high strain-rates and elevated temperatures and use reliable and well defined models for ductile material behavior and fracture.   The development of the modeling strategy, for the ductile material and fracture response herein, is embedded in a continuum thermodynamics framework. The Johnson-Cook constitutive model is applied for the effective visco-plastic material response. The ductile fracture behavior, is modelled by a continuum damage enhanced material formulation where an inelastic damage threshold is followed by a damage evolution law. A set of smeared damage evolution models are proposed, which are shown to give mesh independent results for quasi-static and isothermal conditions.   In addition, for more general situations, an alternative continuum progressive ductile damage model coupled to thermodynamics is formulated, where the damage induced fracture area production is based on a progression speed and a length-scale parameter. The damage evolution is then governed by a damage driving energy which, defined from the dissipation rate, consists of both elastic and inelastic contributions. In this way, the model is able to represent the ductile fracture process in a thermodynamically consistent way at high strain-rates and elevated temperatures, while partly preserving the mesh independent response. Moreover, the ability of the model to capture the material and fracture response at various states of stress triaxiality is investigated and results are compared with experiments. The damage model is shown to be able to capture the fracture response with the appropriate formulation of the damage driving energy including at least the inelastic part.   Finally, the continuum progressive ductile damage model is applied in a rigid visco-plastic context for the simulation of the orthogonal machining process. Here, simulation results for the difficult-to-cut material Alloy 718 are compared with experimentally determined forces, tool-chip contact-lengths and chip shapes at varying cutting speeds. Even though the proposed damage model consists of few parameters it is able to represent the cutting parameters considered in good agreement with experiments.