Computational modelling of machining - Mesh objective ductile damage modelling

Sammanfattning: AbstractTo strengthen the competitiveness the manufacturing industry strives for a continuousdevelopment of cost efficient manufacturing processes and improved product quality.These research and development issues are addressed by increasing the implementationof simulation tools based on finite element method (FEM). To represent the materialresponse during the machining process, reliable and well-defined constitutive and fracturemodels are required. In the current work the well-established and widely used visco-plasticJohnson-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 withthe JC-constitutive model. However, the major drawback with the JC- fracture model isthat it exhibits a pathological mesh dependence. Therefore, relating the local continuumdamage theory with principles of maximum dissipation, combined with concepts from thephase field modeling, two different mesh objective damage models were derived. Numericalexamples, reecting the highly localized plastic shear deformations that occur in thevicinity of the cutting edge during machining, were utilized to validate and verify the meshobjective damage models. The general example, verifying the mesh objective strategy,considers the shearing of a pearlitic plate with structured mesh. The results indicate thatthe mesh dependence is removed when strain-rate and temperature dependence is excludedfrom the model. Additionally an investigation regarding the inuence of element distortionwas conducted. For this emphasis a hat specimen with unstructured mesh was subjectedto severe shear deformation while neglecting the temperature and strain-rate dependence.The results show that realistic damage path is captured, and also quantitatively, a goodagreement is obtained for the effective stress and plastic strain levels compared to literature.Extending the JC-constitutive model by incorporating visco-plasticity (and excludingthe mesh objective enhancement), still results in a pathological mesh dependence whichis contrary to what has been argued in literature. The perforation of a Weldox 460 Esteel plate by a blunt-nosed projectile was used to validate the modeling. The numericalsimulations were compared with results from literature and experimental findings andwere found to be in a good agreement. Based on the numerical examples conducted themodels are able to predict damage evolution at ductile fracture in a reliable manner.This enables a possibility to accurately evaluate the machining process with respectto operating parameters e.g. cutting force, temperature distribution, chip morphologyand residual stresses. Hence, improving the understanding of the complex phenomenaoccurring during the machining process and the product quality while increasing the costefficiency of the process.