Non-proportional deformation paths for sheet metal: experiments and models

For mild steel, after significant plastic deformation in one direction, a subsequent deformation in an orthogonal direction shows a typical stress overshoot compared to monotonic deformation. This phenomenon is investigated experimentally and numerically on a DC06 material. Two models that incorporate the observed overshoot are compared. In the Teodosiu-Hu model, pre-strain influences the rate of kinematic hardening by a rather complex set of evolution equations. The shape of the elastic domain is not changed. Another way to describe the observed overshoot is by distortional hardening, like in the model by Levkovitch et al. In this model, a deformation in one direction directly influences the shape of the yield locus, which is apparent even without additional plastic deformation in another direction. Both models can represent the experimental results well, but in the original implementations, the Teodosiu model performs better.\ud \ud KEYWORDS: non-proportional loading, plasticity, material model, distortional hardenin

Consistent plane stress-3D conversion of hardening models and yield criteria

Material models in FE-simulations are used both in 3D and plane stress situations. In this\ud paper it is shown that for models that include kinematic hardening, the conversion from a 3D to a plane\ud stress algorithm needs more adaptations than only eliminating the thickness components. An example\ud and a consistent conversion are presented. Secondly it is discussed how to convert a 2D plane stress\ud yield criterion into a full 3D yield criterion. This is essential for using solid-shell elements with 2D yield\ud criteria, typically used in sheet metal formin

Evaluation of stresses in a combined plane strain-simple shear test

A biaxial testing device for sheet metal has been developed that can impose a combination of\ud plane strain and simple shear deformation. The specimen has a large width to height ratio and a small height\ud to thickness ratio. The forces in tensile and shear direction are easily measured and the tensile stress and shear\ud stress can easily be derived. For a full description of stresses, however, the stress in lateral direction should also\ud be known. This stress is a result of the constraint, imposed by the large width to height ratio and cannot be\ud measured directly. The strain in the specimen is measured on the surface. By imposing the Drucker normality\ud principle, the direction of the tangent to the yield surface is known and the unknown stress increment in lateral\ud direction can be obtained. Computer simulations are performed to test whether the intended approach can\ud recalculate all stress components from measurement of 3 in-plane strains and just 2 stresses. Without hardening,\ud good results are obtained for a complete interval between the pure shear point up to a point between uniaxial\ud stress and the plane strain point on the yield locus. With hardening, the algorithm requires a lot of data points\ud to avoid drifting from the exact solution. It is noted that, although the normality rule is used, it is not necessary\ud to have an a-priori knowledge of the yield functio

A Dislocation based Constitutive Model for Warm Forming of Aluminum Sheet

The formability of aluminum sheet can be improved considerably by increasing the temperature.\ud At elevated temperatures, the mechanical response of the material becomes strain rate dependent.\ud To accurately simulate warm forming of aluminum sheet, a material model is required that\ud incorporates the temperature and strain rate dependency. In this paper, the dislocation based\ud Alflow hardening model is used. The model incorporates the influence of the temperature and\ud strain rate effect on the flow stress by means of the storage and dynamic recovery of dislocations.\ud It also includes the effects of solute level, particle fraction and grain size. Cylindrical cup deep\ud drawing simulations are presented using shell elements. The anisotropic behavior of the sheet is\ud described by using the Vegter yield locus. Experimental drawing test data is used to validate the\ud modeling approach, where the model parameters follow from tensile tests

Thermo-mechanical Forming of Al–Mg–Si Alloys: Modeling and Experiments

In an ongoing quest to realize lighter vehicles with improved fuel efficiency, deformation characteristics of the material AA 6016 is investigated. In the first part of this study, material behavior of Al–Mg–Si sheet alloy is investigated under different process (temperature and strain rate) and loading (uniaxial and biaxial) conditions experimentally. Later, warm cylindrical cup deep drawing experiments were performed to study the effect of various parameters on warm forming processes, such as the effect of punch velocity, holding time, temper and temperature on force-displacement response. The plastic anisotropy of the material which can be directly reflected by the earing behavior of the drawn cups has also been studied. Finite element simulations can be a powerful tool for the design of warm forming processes and tooling. Their accuracy will depend on the availability of material models that are capable of describing the influence of temperature and strain rate on the flow stresses. The physically based Nes model is used to describe the influence of temperature and strain rate and the Vegter yield criterion is used to describe the plastic anisotropy of the sheet. Experimental drawing test data are used to validate the modeling approaches

A numerical approach to robust in-line control of roll forming processes

The quality of roll formed products is known to be highly sensitive and dependent on the process parameters and thus the unavoidable variations of these parameters during mass production. To maintain a constant high product quality, a new roll former with an adjustable final roll forming stand is developed at Deakin University enabling the continuous compensation for possible shape defects. In this work, a numerical approach to robust in-line control of the roll forming of a V-section profile is presented, combining the aspects of robust process design and in-line compensation methods. A numerical study is performed to determine the relationship between controllable process settings and uncontrollable variation of incoming material properties with respect to the common product defects longitudinal bow and springback. The computationally expensive non-linear FE simulations used in this study are subsequently replaced by metamod-els based on efficient Single Response Surfaces. Using these metamodels, the optimal setting for the adjustable stand is determined with robust optimization techniques and the effect on product quality analyzed. It is shown that the subsequent adjustment of the final roll stand position leads to a significantly improved product quality by preventing product defects and minimizing the deteriorating effects of scattering variables

Effect of temperature on anisotropy in forming simulations of aluminum alloys

A combined experimental and numerical study of the effect of temperature on anisotropy in warm forming of AA 6016-T4 aluminum was performed. The anisotropy coefficients of the Vegter yield function were calculated from crystal plasticity models with an adequate combination of extra slip systems. Curve fitting was used to fit the anisotropy coefficients calculated at discrete temperatures. This temperature dependent constitutive model was successfully applied to the coupled thermo-mechanical analysis of deep drawing of aluminum sheet and results were compared with experiments