A material model for warm forming of aluminium sheet

A material model has been developed to simulate the warm forming of Al–Mg\ud sheet. Both the hardening behaviour, including temperature and strain rate effects, and the\ud biaxial stress–strain response of the sheet are considered. A physically-based hardening model\ud according to Bergström is used. This model incorporates the influence of the temperature\ud and strain rate on the flow stress and on the hardening rate based on dynamic recovery. For\ud deformations at constant temperature and strain rate, the Bergström model reduces to the well\ud known Voce hardening model. The Bergström/Voce models can be fitted quite well to the results\ud of monotonic tensile tests of an AA 5754-O alloy.\ud The biaxial stress–strain response of the material is experimentally determined by uniaxial,\ud plane strain, simple shear and equi-biaxial stress tests. It is demonstrated that the widely used\ud Hill ’48 yield locus is inappropriate for simulation of deformation of aluminium. The low Rvalues\ud for aluminium lead to a significant underestimation of the equi-biaxial yield stress. In\ud the simulation of the deep drawing of a cylindrical cup this results in a much too thin bottom of\ud the cup. The Vegter yield criterion is sufficiently flexible to accurately represent the shape of the\ud yield locus and the anisotropy.\ud

Aluminium sheet forming at elevated temperatures

The formability of aluminum sheet depends on the temperature of the material and the strain\ud rate. E.g. the limiting drawing ratio can be improved by increasing the temperature uniformly, but even more by\ud heating the flange and cooling the punch. To accurately simulate the deep drawing or stretching of aluminum\ud sheet at elevated temperatures, a material model is required that incorporates the temperature and strain-rate dependency.\ud In this paper simulations are presented of the deep drawing of a cylindrical cup, using axi-symmetric\ud elements. Two material models are compared. First a phenomenological material model is used, in which\ud the parameters of a Ludwik–Nadai hardening curve are made temperature and strain-rate dependent. Then a\ud physically-based model, according to Bergstr¨om is used. The model incorporates the influence of the temperature\ud on the flow stress and on the hardening rate and includes dynamic recovery aspect

A material model for aluminium sheet forming at elevated temperatures

In order to accurately simulate the deep drawing or stretching of aluminum sheet at elevated\ud temperatures, a model is required that incorporates the temperature and strain-rate dependency of the material.\ud In this paper two models are compared: a phenomenological material model in which the parameters of a\ud Ludwik–Nadai hardening curve and a power law strain-rate influence are made temperature dependent and a\ud physically-based model according to Bergstr¨om. The model incorporates the influence of the temperature on\ud the flow stress and on the hardening rate and includes dynamic recovery aspects. Although both models can be\ud fitted quite well to monotonic tensile tests, large differences appear if strain rate jumps are applied

Warm Deep Drawing of Aluminium Sheet

Aluminium sheet drawing processes can be improved by manipulating local flow behaviour\ud by means of elevated temperatures and temperature gradients in the tooling. Forming tests\ud showed that a substantial improvement is possible not only for 5xxx but also for 6xxx series\ud alloys. Finite element method simulations can be a powerful tool for the design of warm\ud forming processes and tooling. Their accuracy will depend on the availability of materials\ud models that are capable of describing the influence of temperature and strain rate on the flow\ud stresses. Two models, an adapted Nadai power law and a dislocation based Bergström type\ud model, are compared by means of simulations of a cup drawing process. Experimental\ud drawing test data are used to validate the modelling approaches, whereas the model parameters\ud follow from tensile tests

Aluminium sheet forming simulations: influence of the yield surface

The accuracy of simulations of the plastic deformation of sheet metal depend to a large extend on\ud the description of the yield surface, the hardening and the friction. In this paper simulations of deep drawing of\ud an AlMg alloy with a shell model are presented. The yield surface is described by a Von Mises, a Hill ’48 and a\ud Vegter yield function. The parameters for the model are based on biaxial experiments. It is concluded that the\ud shape of the yield locus has a minor influence on the prediction of the punch force–displacement diagram and a\ud large influence on the prediction of the thickness strains. The Vegter model performs much better than the Hill\ud ’48 model, based on the same R-values

Blanking by means of the finite element method

This paper summarizes the results of simulating the blanking process by means\ud of the Finite Element Method. Unlike most of the research in this eld, the focus is not on the blanking process itself but on the deformed shape of a product after blanking. Two ways of determining the shape of a product after blanking are investigated. One way is to calculate the internal stresses caused by the blanking process and relax these stresses to calculate the new shape. The internal stresses can be transfered into an equivalent load model that can characterize the blanking process. With this equivalent load model the deformed shape of a product after blanking can be determined in a very fast and easy way.\ud Experiments are done to verify the results. Both introduced methods give qualitatively good results. Also some suggestions for improvements are made

Scan-rescan reproducibility of segmental aortic wall shear stress as assessed by phase-specific segmentation with 4D flow MRI in healthy volunteers

Cardiovascular Aspects of Radiolog

Scan-rescan reproducibility of diastolic left ventricular kinetic energy, viscous energy loss and vorticity assessment using 4D flow MRI: analysis in healthy subjects

Cardiovascular Aspects of Radiolog