Jean-Claude Gelin

An inverse method for determining viscoplastic properties of aluminium alloys

Abstract Parameter identification plays a major role in modelling finite strain viscoplasticity. The tests used to assess the values of the constitutive parameters must reproduce faithfully the deformation state. For such tests, the deformation state is usually non-homogeneous and the only information available is the load-displacement data actually measured during the test, whereas the constitutive model expresses the flow stress as a function of the viscoplastic strain, the viscoplastic strain rate, the temperature and some internal variable. This constitutes a typical inverse problem. A numerical procedure which integrates optimization and finite element analysis is developed for solving a three dimensional parameter estimation problem in elasto-viscoplasticity. The optimization is performed using a modified Levenberg-Marquardt method to take into account the constraints on the parameters. The direct problem is solved using a finite element method based on an implicit prediction-correction algorithm in finite transformation. The method is applied to the plane strain compression test to determine viscoplastic material parameters of aluminium alloys.

Manufacture of thin composite structures by the RTM process : Numerical simulation of the shaping operation

The glass-fibre preform shaping process is the first stage of the RTM manufacturing process for the production of thin fabric-reinforced composite structures. A numerical tool for the simulation of this shaping process is proposed. The aim of the software developed is to be able to determine if a drawing operation is possible before the manufacturing of expensive tools. The approach used is a mechanical one. The behaviour of the flat fabrics used in the shaping process is obtained from the tensile curve of a single yarn and the current position of yarns during the shaping operation. Discrete finite elements made of yarns are used, which lead to a good numerical efficiency, especially in the case of elements directed by the yarns. A set of comparisons between shaping simulations and experimental results proves the validity of the proposed formulation.

Determination of the optimal process parameters in metal injection molding from experiments and numerical modeling

Abstract The determination of optimal process parameters to produce parts by metal injection molding without defects and with required mechanical properties is discussed in the paper, based on experiments and numerical modeling. The experiments are carried out with a multi-cavity mould specially designed and equipped to measure and record different parameters during the injection stage. The debinding and sintering cycles are optimized to get the components free of defects, too. Based on modeling techniques using a biphasic flow formulation and a newly developed explicit algorithm, numerical simulations are realized to predict the segregation effects in injection. This novel algorithm in fractional steps and the related finite element software lead efficiently to accurate correlations between experiments and simulations.

An Improved Finite Element Method for the Analysis of Damage and Ductile Fracture in Cold Forming Processes

Summary The damage in metal forming processes such as cold forward extrusion or wire drawing depends on the state of stress and strain involved by the processes in conjonction with the material structure and level of second phase particles. In order to predict fracture occurence, the first possibility is to use a local fracture criterion; the major problem in this approach is to identify the adequate parameters which correspond to the real strain path involved in the process. A new approach consists in using constitutive equations related to the material damage. The authors have used this approach to predict the fracture occurence: a modified form of the Berg-Gurson plastic potential in conjonction with an improved finite element analysis in large strain range have been implemented. The parameters of the above described model are determined by means of experiments and finite element calculations in the case of upset cylinders and of tensile notched cylindrical specimens. An application to forward extrusion of cylinders is then presented: the range for the internal cracking is predicted by the proposed finite element calculations. This example illustrates well the newness of the analysis.

An inverse solution procedure for material parameters identification in large plastic deformations

The identification of materials rheological behaviour in the non-linear range is based on experimental tests. When using direct identification methods, one faces the problem of the interpretation of the experimental tests, which requires the assumption of deformation homogeneity and therefore the use of approximation methods. Since this assumption is often not satisfied in the case of non-linear behaviour, material parameters are not assessed precisely. In the paper, an inverse identification method is proposed to avoid the problems raised by interpretation of the experimental tests and to determine material parameters more accurately. The algorithm developed consists of both an optimization method and a finite element method. This method is applied to the inverse identification of viscoplastic parameters of an aluminium alloy, with an investigation on the effect of the initial guess and errors in experimental data on the identified values.

Identification of material parameters directly from metal forming processes

Abstract An identification scheme for the determination of material parameters directly from forming processes is presented. This scheme is based on the coupling of the finite element method (FEM) and an optimisation technique. The FEM is used to evaluate the behaviour of the material during the forming process, whereas the optimisation technique allows for the adjustment of material parameters so that the calculated response matches the measured one. This identification scheme is applied for illustration, first to the determination of aluminium alloy behaviour from a tensile test, then to a 3D cross deep drawing test.

A finite element-based identification method for complex metallic material behaviours

The paper addresses the identfication of material parameters for complex behaviours using the finite element method (FEM). This method associated to an optimization algorithm allows an accurate identification as it requires few assumptions and provides an exact representation of the experimental test used. Different examples are presented proving the efficiency of the proposed method.

Computational prediction of the localized necking in sheet forming based on microstructural material aspects

The paper deals with the influence of microstructural material aspects on the localized necking in sheet metal forming. The approach developed accounts for various microstructural material aspects as crystallographic slip, work hardening at microstructural level and lattice rotation which induces the texture evolution. The analyses are performed using the linear stability analysis combined with a polycrystalline model to build Forming Limit Diagrams and to study their evolution with the initial and induced anisotropy.

Force, torque and plastic flow analysis in rotary upsetting of ring shaped billets

The orbital upsetting of rings has been analysed for a Mises material by using an upper bound approach. Forging forces, rocking die torques and ring profiles are calculated at each step of the process. Experiments are described in which rings made of 1045 mild steel, 52100 chromium alloyed steel and 316 stainless steel are upset at room temperature on a 1.6 MN rotary press. The main parameters are: orbital oscillation angle, 2°, upper die oscillations, 200 min−1 and lower die speed, 4.10 mm s−1. An experimental rocking die placed on a conventional testing machine has been used for the rotary upsetting of rings made of Plasticine as model material. The simulation parameters are: oscillation, 2°, upper die oscillations, 40 min−1, speed, 98.4 mm min−1. The theoretical values of forging forces, rocking torques, and ring profiles are in keeping with the experiments. So the proposed upper bound approach may be considered as a good model for rotary upsetting.

Modelling and simulation of the aquadraw deep drawing process

Summary The aquadraw deep drawing operation with an hydraulic counter-pressure where the die cavity is filled of an hydraulic hid is analysed taking into account as well as the hydraulic How between the die and the sheet metal as the deformation of the sheet material. The analysis developed in the paper focuses on the main parameters that controls the process. An hydrodynamic study of the pressure in the die cavity and an analysis of the flow between the die cavity and the sheet metal are presented and qualitative results are given that agree experimental ones. The deformation of the sheet metal is analysed using a finite element approach raking into account the fluid pressure distribution. The results of the numerical simulations in the case of axisymmetric parts are compared with experimental ones obtained on an instrumented aquadraw press and the agreement is good. The results obtained can be used for a better control of the die cavity pressure to optimize the thickness strain in the formed shape for given process parameters.