The direct employment of multiscale models based on nano- and micro-mechanical approaches is unsuitable for simulations of forming processes due to considerable numerical effort and long computation times. Hence, the work in this project focuses on the development and implementation of material models and finite element simulations on the macro scale for the dual-phase steel DP800 as well as the case-hardening steel 16MnCrS5. In order to model the complex interactions inside these materials, such as the interaction between growth of micro-cracks and pores in conjunction with plastic flow, it is necessary to incorporate both heterogeneous and evolving plastic anisotropy and anisotropic damage degradation in the context of large strains.

The macroscopic plasticity model involves two principles. On the one hand, a Hill-like fourth order tensor, evolving deformation induced, is introduced. On the other hand, weighted stress modes or structural tensors which also evolve deformation-dependent extend the formulation. Naturally, a deformation-induced evolution of anisotropy depending on the direction of plastic flow is chosen. Furthermore, isotropic and kinematic hardening is incorporated since hardening has a significant influence on the properties of semi-finished products and components manufactured by forming processes.

The evolution of elastic anisotropy is directly connected to the evolution of texture and damage. The challenge of this project in the context of material modeling lies in the coupling between the deformation-induced evolution of anisotropy and the evolution of damage. The anisotropic damage model is formulated in terms of a damage tensor in such a way that micro-mechanical damage effects, e.g. the so-called micro-crack-closure-reopening (MCR) effect, can be included.

In order to properly simulate softening and damage phenomena in the context of finite elements, a suitable regularization scheme needs to be implemented. Only a regularized formulation leads to mesh-independent finite element simulations in the case of damage evolution and enables us to simulate new boundary value problems by using material parameters which have been identified earlier on the basis of different boundary value problems. The challenge of this project in the context of numerics is the establishment of a gradient-enhanced tensorial damage model in order to obtain mesh-independent simulation results (i.e. finer meshes yield equal results) by means of the finite element method. Up to now, research on gradient-enhanced damage models focused mostly on isotropic damage formulations.

Consequently, the goal of this project is to establish an anisotropic gradient-enhanced damage model coupled with a deformation-induced evolution of anisotropy, including kinematic as well as isotropic hardening for the simulation of forming processes, e.g. bending, deep-drawing, cold forging, and to calibrate the established model to the dual-phase steel DP800 and the case-hardening steel 16MnCrS5.

This project presents a central interface between the projects of project area B (characterization) and, in cooperation with project S01, project area A (process technology). Future funding periods are intended to focus on the extension of the model by including rate-dependency of plastic deformation and damage evolution as well as full thermomechanical coupling with healing of damaged material.

Prof. Dr.-Ing. habil. Andreas Menzel
Institut für Mechanik (IM), TU Dortmund

Leon Sprave M.Sc.
Institut für Mechanik (IM), TU Dortmund