H.H. Wisselink

Failure Predictions for DP Steel Cross-die Test Using Anisotropic Damage

The Lemaitres continuum damage model is well known in the field of damage mechanics. The anisotropic damage model given by Lemaitre is relatively simple, applicable to nonproportional loads and uses only four damage parameters. The hypothesis of strain equivalence is used to map the effective stress to the nominal stress. Both the isotropic and anisotropic damage models from Lemaitre are implemented in an in-house implicit finite element code. The damage model is coupled with an elasto-plastic material model using anisotropic plasticity (Hill-48 yield criterion) and strain-rate dependent isotropic hardening. The Lemaitre continuum damage model is based on the small strain assumption; therefore, the model is implemented in an incremental co-rotational framework to make it applicable for large strains. The damage dissipation potential was slightly adapted to incorporate a different damage evolution behavior under compression and tension. A tensile test and a low-cycle fatigue test were used to determine the damage parameters. The damage evolution was modified to incorporate strain rate sensitivity by making two of the damage parameters a function of strain rate. The model is applied to predict failure in a cross-die deep drawing process, which is well known for having a wide variety of strains and strain path changes. The failure predictions obtained from the anisotropic damage models are in good agreement with the experimental results, whereas the predictions obtained from the isotropic damage model are slightly conservative. The anisotropic damage model predicts the crack direction more accurately compared to the predictions based on principal stress directions using the isotropic damage model. The set of damage parameters, determined in a uniaxial condition, gives a good failure prediction under other triaxiality conditions.

Finite Element Simulation of the Stretch‐Forming of Aircraft Skins

In the aerospace industry stretch forming is often used to produce skin parts. During stretch forming a sheet is clamped at two sides and stretched over a die, such that the sheet gets the shape of the die. However for complex shapes it is necessary to use expensive intermediate heat-treatments in order to avoid Liiders lines and still achieve large deformations. To optimize this process FEM simulations of this process are performed. A leading edge skin part, made of aluminium AA2024, has been chosen for a preliminary study. The material is modelled with the Vegter yield function, to account for the anisotropic behaviour of the aluminium sheet. Each annealing step is considered to reduce the work hardening completely. The strains in the part have been measured and are used for validation of the simulations. The used FEM model and the experimental results will be presented and conclusions and recommendations for future research will be given.

Material-induced anisotropic damage in DP600

Plasticity-induced damage development in metals is anisotropic by nature. The anisotropy in damage is driven by two different phenomena: anisotropic deformation state i.e. load-induced anisotropic damage (LIAD) and anisotropic microstructure i.e. material-induced anisotropic damage (MIAD). The contribution of second-phase particles can be anisotropic in terms of shape as well as distribution. Most of the continuum anisotropic damage models mimic the phenomenon of LIAD only. Not much attention has been paid to MIAD. This work shows the existence of MIAD in a (pre-production) grade of dual-phase steel (DP600). The aim is to see the influence of MIAD on post-localization deformation behavior and final failure mode. The deformation in this material is almost isotropic up to localization but the post-localization deformation and final failure mode is different when loaded in 0° and 90° to rolling direction. Tensile specimens were deformed up to final failure. A few specimens were stopped just before the final failure. Scanning electron microscopic analysis was carried out to study martensite morphology and damage in these specimens. The martensite morphology showed anisotropy in shape and orientation in the undeformed specimens. Significant MIAD was observed in the deformed tensile specimens due to the anisotropic martensite morphology. MIAD explains direction-dependent post-localization deformation, final failure mode, and formability of this material. Lemaitres anisotropic damage model is modified to account for MIAD in a phenomenological manner. The MIAD parameters were determined from tensile tests carried out in 0°, 45°, and 90° to the rolling direction.

Viscoplastic Regularization of Local Damage Models: A Latent Solution

Local damage models are known to produce pathological mesh dependence in finite element simulations. The solution is to either use a regularization technique or to adopt a non-local damage model. Viscoplasticity is one technique which can regularize the mesh dependence of local damage model by incorporating a physical phenomenon in the constitutive model i.e. rate effects. A detailed numerical study of viscoplastic regularization is carried out in this work. Two case studies were considered i.e. a bar with shear loading and a sheet metal under tensile loading. The influence of hardening / softening parameters, prescribed deformation rate and mesh size on the regularization was studied. It was found that the primary viscoplastic length scale is a function of hardening and softening parameters but does not depend upon the deformation rate. Mesh dependency appeared at higher damage values. This mesh dependence can be reduced by mesh refinement in the localized region and also by increasing the deformation rates. The viscoplastic regularization was successfully used with a local anisotropic damage model to predict failure in a cross die drawing process with the actual physical process parameters.

A Plasticity Induced Anisotropic Damage Model for Sheet Forming Processes

The global fuel crisis and increasing public safety concerns are driving the automotive industry to design high strength and low weight vehicles. The development of Dual Phase (DP) steels has been a big step forward in achieving this goal. DP steels are used in many automotive body-in-white structural components such as A and B pillar reinforcements, longitudinal members and crash structure parts. DP steels are also used in other industrial sectors such as precision tubes, train seats and Liquid Petroleum Gas (LPG) cylinders. Although the ductility of DP steel is higher than classical high strength steels, it is lower than that of classical deep drawing steels it has to replace. The low ductility of DP steels is attributed to damage development. Damage not only weakens the material but also reduces the ductility by formation of meso-cracks due to interacting micro defects. Damage in a material usually refers to presence of micro defects in the material. It is a known fact that plastic deformation induces damage in DP steels. Therefore damage development in these steels have to be included in the simulation of the forming process. In ductile metals, damage leads to crack initiation. A crack is anisotropic which makes damage anisotropic in nature. However, most researchers assume damage to be an isotropic phenomenon. For correct and accurate simulation results, damage shall be considered as anisotropic, especially if the results are used to determine the crack propagation direction. This paper presents an efficient plasticity induced anisotropic damage model to simulate complex failure mechanisms and accurately predict failure in macro-scale sheet forming processes. Anisotropy in damage can be categorized based on the cause which induces the anisotropy, i.e. the loading state and the material microstructure. According to the Load Induced Anisotropic Damage (LIAD) model, if the material is deformed in one direction then damage will be higher in this direction compared to the other two orthogonal directions, irrespective of the microstructure of the material. According to Material Induced Anisotropic Damage (MIAD) model, if there is an anisotropy in shape or distribution of the particles responsible for damage (hard second phase particles, inclusions or impurities) then the material will have different damage characteristics for different orientations in the sheet material. The LIAD part of the damage model is a modification of Lemaitre’s (ML) anisotropic damage model. Modifications are made for damage development under compression state and influence of strain rate on damage, and are presented in this paper. Viscoplastic regularization is used to avoid pathological mesh dependency. The MIAD part of the model is an extension of the LIAD model. Experimental evidence is given of the MIAD phenomenon in DP600 steel. The experimental analysis is carried out using tensile tests, optical strain measurement system (ARAMIS) and scanning electron microscopy. The extension to incorporate MIAD in the ML anisotropic damage model is presented in this paper as well. The paper concludes with a validation of the anisotropic damage model for different applications. The MIAD part of the model is validated by experimental cylindrical cup drawing wheras the LIAD part of the model is validated by the cross die drawing process.

Validation of Modified Lemaitre’s Anisotropic Damage Model with the Cross Die Drawing Test

Dual Phase (DP) steels are widely replacing the traditional forming steels in automotive industry. Advanced damage models are required to accurately predict the formability of DP steels. In this work, Lemaitre’s anisotropic damage model has been slightly modified for sheet metal forming applications and for strain rate dependent materials. The damage evolution law is adapted to take into account the strain rate dependency and negative triaxialities. The damage parameters for pre-production DP600 steel were determined. The modified damage models (isotropic and anisotropic) were validated using the cross die drawing test. The anisotropic damage model predicts the crack direction more accurately.

Do Advanced Material Models Contribute to Accuracy in Industrial Sheet Forming Simulations

The accuracy of material models can have a large impact on the overall accuracy of material forming simulations in general and sheet forming simulations in particular. For large strain plastic deformations, the material model usually consists of a yield function and a hardening relation, optionally including the influence of temperature and strain rate. In large-scale simulations it is favourable to keep the model as simple as possible. The ‘allowable’ error in a material model should be in balance with other errors, like the discretisation error and errors in contact and friction modelling. The required accuracy depends on the application and the goal of the analysis. In many occasions, strain rate and temperature dependency can be ignored, but for warm forming this is clearly not the case. Furthermore, numerical simulation of the onset of necking requires a much better material model than needed for the calculation of the global deformation field before necking.