Extracting material data for superplastic forming simulations

In subatomic particle physics, unstable particles can be studied with a so-called vertex detector placed inside a particle accelerator. A detecting unit close to the accelerator bunch of charged particles must be separated from the accelerator vacuum. A thin sheet with a complex 3D shape prevents the detector vacuum from polluting the accelerator vacuum. Hence, this sheet should be completely leak tight with respect to gases. To produce such a complex thin sheet, superplastic forming can be very attractive if a small number of products is needed. This is a forming process in which a sheet of superplastic material is pressed into a one-sided die by means of gas pressure.\ud In order to develop a material model which can be used in superplastic forming simulations, uniaxial and biaxial experiments are necessary. The uniaxial, tensile, experiments provide information about the one-dimensional material data, such as the stress as a function of equivalent plastic strain and strain rate. These data are extracted from the experiments by using inverse modeling, i.e. simulation of the tensile experiment. To fit these curves into a general material model, three parts in the uniaxial mechanical behavior are considered: initial flow stress, strain hardening and strain softening caused by void growth. Since failure in superplastic materials is preceded by the nucleation and growth of cavities inside the material, the void volume fractions of the tested specimens were also observed.\ud A very important factor in this research is the study of the permeability of the formed sheet with respect to gas. If internal voids start to coalesce, through-thickness channels will start to form, thereby providing a gas leak path. To study the twodimensional behavior, including the gas leakage, bulge experiments were performed. Within these experiments, circular sheets were pressed into a cylindrically shaped die. From these experiments it followed that the plastic straining is dependent on an applied backpressure during the forming stage. This backpressure can postpone cavity nucleation and growth

Multi-scale friction modeling for manufacturing processes: The boundary layer regime

This paper presents a multi-scale friction model for largescale forming simulations. A friction framework has been developed including the effect of surface changes due to normal loading and straining the underlying bulk material. A fast and efficient translation from micro to macro modeling, based on stochastic methods, is incorporated to reduce the computational effort. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. A discrete model has been adopted which accounts for the formation of contact patches ploughing through the contacting material. To simulate metal forming processes a coupling has been made with an implicit Finite Element code. Simulations on a typical metal formed product shows a distribution of friction values. The modest increase in simulation time, compared to a standard Coulomb-based FE simulation, proves the numerical feasibility of the proposed method

Deep drawing simulations of Tailored Blanks and experimental verification

Tailored Blanks are increasingly used in the automotive industry.\ud A combination of different materials, thickness, and coatings can be welded\ud together to form a blank for stamping car body panels. The main advantage\ud of using Tailored Blanks is to have specific characteristics at particular parts\ud of the blank in order to reduce the material weight and costs.\ud To investigate the behaviour of Tailored Blanks during deep drawing, the\ud finite element code DiekA is used. In this paper, simulations of the deep\ud drawing of two products using Tailored Blanks are discussed. For\ud verification, the two products are stamped to gain experimental information.\ud The correlation between the experimental results and the simulation results\ud appears to be satisfactory

Deep drawing simulation of Tailored Blanks

Tailored blanks are increasingly used in the automotive industry. A tailored blank consists of different metal parts, which are joined by a welding process. These metal parts usually have different material properties. Hence, the main advantage of using a tailored blank is to provide the right material properties at specific parts of the blank. The movement of the weld during forming is extremely important. Unwanted weld displacement can cause damage to both the product and the tool. This depends mainly on the original weld position and the process parameters. However experimental determination of the optimum weld position is quite expensive. Therefore a numerical tool has been developed for simulations of tailored blank forming. The Finite Element Code Dieka is used for the deep drawing simulations of some geometrically simple products. The results have been validated by comparing them with experimental data and show a satisfactory correlation

Numerical and physical modelling in forming

An overview will be presented of recent developments concerning the application\ud and development of computer codes for numerical simulation of forming processes. Special\ud attention will be paid to the mathematical modeling of the material deformation and friction,\ud and the effect of these models on the results of simulation

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