Computational modelling of machining - Mesh objective ductile damage modelling

Licentiate thesis, 2016

Abstract To strengthen the competitiveness the manufacturing industry strives for a continuous development of cost efficient manufacturing processes and improved product quality. These research and development issues are addressed by increasing the implementation of simulation tools based on finite element method (FEM). To represent the material response during the machining process, reliable and well-defined constitutive and fracture models are required. In the current work the well-established and widely used visco-plastic Johnson-Cook (JC) constitutive model is utilized for the stress response in the material. To account for the ductile damage in the material the JC-fracture model is combined with the JC-constitutive model. However, the major drawback with the JC- fracture model is that it exhibits a pathological mesh dependence. Therefore, relating the local continuum damage theory with principles of maximum dissipation, combined with concepts from the phase field modeling, two different mesh objective damage models were derived. Numerical examples, re ecting the highly localized plastic shear deformations that occur in the vicinity of the cutting edge during machining, were utilized to validate and verify the mesh objective damage models. The general example, verifying the mesh objective strategy, considers the shearing of a pearlitic plate with structured mesh. The results indicate that the mesh dependence is removed when strain-rate and temperature dependence is excluded from the model. Additionally an investigation regarding the in uence of element distortion was conducted. For this emphasis a hat specimen with unstructured mesh was subjected to severe shear deformation while neglecting the temperature and strain-rate dependence. The results show that realistic damage path is captured, and also quantitatively, a good agreement is obtained for the effective stress and plastic strain levels compared to literature. Extending the JC-constitutive model by incorporating visco-plasticity (and excluding the mesh objective enhancement), still results in a pathological mesh dependence which is contrary to what has been argued in literature. The perforation of a Weldox 460 E steel plate by a blunt-nosed projectile was used to validate the modeling. The numerical simulations were compared with results from literature and experimental findings and were found to be in a good agreement. Based on the numerical examples conducted the models are able to predict damage evolution at ductile fracture in a reliable manner. This enables a possibility to accurately evaluate the machining process with respect to operating parameters e.g. cutting force, temperature distribution, chip morphology and residual stresses. Hence, improving the understanding of the complex phenomena occurring during the machining process and the product quality while increasing the cost efficiency of the process.