Non-local ductile damage formulations for sheet bulk metal forming

A ductile damage model for sheet bulk metal forming processes and its efficient and accurate treatment in the context of the Finite Element Method is presented. The damage is introduced as a non-local field to overcome pathological mesh dependency. Since standard elements tend to show volumetric locking in the bulk forming process a mixed formulation is implemented in the commercial software simufact.forming to obtain better results.DFG/SFB/TR 7

Towards the effective behaviour of polycrystalline microstructures at finite strains

It is well known that metals behave anisotropically on their microstructure due to their crystalline nature. FE-simulations in the metal forming field however sometimes lack the right macroscopic anisotropies as their type can be unspecific. In order to find a suitable effective elastoplastic material model, a finite crystal plasticity model is used to model the behaviour of polycrystalline materials in representative volume elements (RVEs) representing the microstructure, taking into account the plastic anisotropy due to dislocations occurring within considered slip systems. A multiplicative decomposition of the deformation gradient into elastic and plastic parts is performed, as well as the split of the elastic free energy into volumetric and deviatoric parts resulting in a compact expression of the resolved Schmid stress depending on the slip system vectors. In order to preserve the plastic incompressibility condition, the elastic deformation gradient is updated via an exponential map scheme. To further circumvent singularities stemming from the linear dependency of the slip system vectors, a viscoplastic power-law is introduced providing the evolution of the plastic slips and slip resistances. The model is validated with experimental microstructural data under deformation. Through homogenisation and optimisation techniques, effective stress-strain curves are determined and can be compared to results from real manufacturing and fabrication processes leading to an effective elastoplastic material model which is suitable for metal forming processes at finite strains

Classification of Multiaxial Behaviour of Fine-Grained Concrete for the Calibration of a Microplane Plasticity Model

Fine-grained high-strength concrete has already been tested extensively regarding its uniaxial strength. However, there is a lack of research on the multiaxial performance. In this contribution, some biaxial tests are investigated in order to compare the multiaxial load-bearing behaviour of fine-grained concretes with that of high-strength concretes with normal aggregate from the literature. The comparison pertains to the general biaxial load-bearing behaviour of concrete, the applicability of already existing fracture criteria and the extrapolation for the numerical investigation. This provides an insight into the applicability of existing data for the material characterisation of this fine-grained concrete and, in particular, to compensate for the lack of investigations on fine-grained concretes in general. It is shown, that the calibration of material models for fine-grained concretes based on literature results or normal-grained concrete with similar strength capacity is possible, as long as the uniaxial strength values and the modulus of elasticity are known. For the numerical simulation, a Microplane Drucker–Prager cap plasticity model is introduced and fitted in the first step to the biaxial compression tests. The model parameters are set into relation with the macroscopic quantities, gained from the observable behaviour of the concrete under uniaxial and biaxial compressive loading. It is shown that the model is able to capture the yielding and hardening effects of fine-grained high-strength concrete in different directions

Dynamic brittle fracture by XFEM and gradient-enhanced damage

International audienceA combined continuous-discontinuous approach to fracture is used to model crack prop- agation under asymmetric dynamic loadings. A gradient-enhanced damage model is utilized to evaluate degradation of the material in the process zone. This type of model avoids mesh dependency and pathological effects of the local damage models. A damage-based criteria is utilized for crack propagation. In this framework, crack propagation occurs as a result of material softening and degradation in the process zone in front of the crack. As damage reaches its critical value, by using the extended finite element method (XFEM) a discon- tinuous crack is introduced within the damage band. An operator-split approach is used to solve the problem. Finally a brittle fracture case is modeled to show the capabilities and robustness of the employed approach

3D Dynamic Crack Propagation by the Extended Finite Element Method and a Gradient-Enhanced Damage Model

International audienceA combined continuous-discontinuous approach to fracture is presented to model crack propagation under dynamic loading. A gradient-enhanced damage model is used to evaluate degradation of the material ahead of the crack. This type of model avoids mesh dependency and pathological effects of local damage models. Discrete cracks are reflected by means of extended finite elements (XFEM) and level sets. For the transition between damage and discrete fracture a damage based criterion is utilized. A discrete crack propagates if a critical damage value at the crack front is reached. The propagation direction is also determined through the damage field. Finally a dynamic mode II crack propagation example is simulated to show the capabilities and robustness of the employed approach

Dynamic brittle fracture by XFEM and gradient-enhanced damage

International audienceA combined continuous-discontinuous approach to fracture is used to model crack prop- agation under asymmetric dynamic loadings. A gradient-enhanced damage model is utilized to evaluate degradation of the material in the process zone. This type of model avoids mesh dependency and pathological effects of the local damage models. A damage-based criteria is utilized for crack propagation. In this framework, crack propagation occurs as a result of material softening and degradation in the process zone in front of the crack. As damage reaches its critical value, by using the extended finite element method (XFEM) a discon- tinuous crack is introduced within the damage band. An operator-split approach is used to solve the problem. Finally a brittle fracture case is modeled to show the capabilities and robustness of the employed approach

3d crack propagation by the extended finite element method and a gradient enhanced damage model

International audienc

3d crack propagation by the extended finite element method and a gradient enhanced damage model

International audienc