Effect of Ductile Damage Evolution in Sheet Metal Forming: Experimental and Numerical Investigations

The numerical simulation based on the Finite Element Method (FEM) is widely used in academic institutes and in the industry. It is a useful tool to predict many phenomena present in the classical manufacturing forming processes such as necking, fracture, springback, buckling and wrinkling. But, the results of such numerical model depend strongly on the parameters of the constitutive behavior model. In the first part of this work, we focus on the traditional identification of the constitutive law using oriented tensile tests (0°, 45°, and 90° with respect to the rolling direction). A Digital Image Correlation (DIC) method is used in order to measure the displacements on the surface of the specimen and to analyze the necking evolution and the instability along the shear band. Therefore, bulge tests involving a number of die shapes (circular and elliptic) were developed. In a second step, a mixed numerical–experimental method is used for the identification of the plastic behavior of the stainless steel metal sheet. The initial parameters of the inverse identification were extracted from a uniaxial tensile test. The optimization procedure uses a combination of a Monte-Carlo and a Levenberg-Marquardt algorithm. In the second part of this work, according to some results obtained by SEM (Scaning Electron Microscopy) of the crack zones on the tensile specimens, a Gurson Tvergaard Needleman (GTN) ductile model of damage has been selected for the numerical simulations. This model was introduced in order to give informations concerning crack initiations during hydroforming. At the end of the paper, experimental and numerical comparisons of sheet metal forming applications are presented and validate the proposed approach

Analysis of the thinning phenomenon variations in sheet metal forming process

In many manufacturing areas such as the automotive industry (outer panels, inner panels, stiffeners etc...), the packaging industry (petfood containers, beverage cans etc...) and the household appliances industry (housings etc...), the control of the thinning variations in sheet metal forming process is a major point to study in order to ameliorate the final quality of the produced parts. In this framework, several bulge tests have been developed in order to study the thinning phenomenon during sheet metal forming processes. In this presentation, measurement of the thickness of the deformed specimens has been done using the ImageAnalyser software (based on an image analysis technique) developed by the CMAO research group in the LGP. The AISI 304L stainless steel has been selected as the tested material. Both a cylindrical and an elliptical die allowing the analysis of the thickness variation versus the load ratio and the anisotropy of sheet have been used in this work. In a second part of this communication, we present a numerical model based on the Hill 1948 anisotropic material model. The numerical results are discussed and compared with the experiments

2D and 3D numerical models of metal cutting with damage effects

In this paper a two-dimensional and a three-dimensional finite element models of unsteady-state metal cutting are presented. These models take into account dynamic effects, thermo-mechanical coupling, constitutive damage law and contact with friction. The simulations concern the study of the unsteady-state process of chip formation. The yield stress is taken as a function of the strain, the strain rate and the temperature in order to reflect realistic behavior in metal cutting. Unsteady-state process simulation needs a material separation criterion (chip criterion) and thus, many models in the literature use an arbitrary criterion based on the effective plastic strain, the strain energy density or the distance between nodes of parts and tool edge. The damage constitutive law adopted in models presented here allows defining advanced simulations of tool's penetration in workpiece and chip formation. The originality introduced here is that this damage law has been defined from tensile and torsion tests, and we applied it for machining process. Stresses and temperature distributions, chip formation and tool forces are shown at different stages of the cutting process. Finally, we present a three-dimensional oblique model to simulate the unsteady-state process of chip formation. This model, using the damage law defined before, allows an advanced simulation close to the real cutting process. The final part shows a milling application. An arbitrary Lagrangian Eulerian formulation (ALE) is used for these simulations; this formalism combines both the advantages of Eulerian and Lagrangian representations in a single description, it is exploited to reduce finite element mesh distortions

A numerical simulation of steady state metal cutting

An arbitrary Lagrangian–Eulerian (ALE ) approach is used to model the orthogonal metal cutting in a steady state situation. The thermomechanical model includes the effects of elasticity, plasticity, strain rate, large strains and friction with heat generated between the tool and the chip. The ALE formulation can combine the advantages of both the Eulerian and Lagrangian approaches in a single description. Particularly, problems linked to the free surface in a Eulerian description and those linked to severe mesh distortions in a Lagrangian one can be solved by this formulation. The ALE governing equations are briefly reviewed in this paper; finite element and finite volume methods are used for the discretization of the conservation equations and an explicit time integration is adopted. Only the steady state solution is required; the ALE formulation is exploited to update the free and the contact surfaces. The model predicts the thermomechanical quantities, the chip geometry and the cutting forces from the cutting data and the material and friction parameters. Cutting experiments were performed with 42CD4 steel and comparisons of experimental tool forces and chip geometry with the numerical results are presented

Expropriated from the hereafter: the fate of the landless in the Southern Highlands of Madagascar

During the period following the abolition of slavery by the French colonial government in 1896, the Southern Highlands of Madagascar was settled by ex-slaves. These early settlers constructed a foundation myth of themselves as tompon-tany, or 'masters of the land', a discourse not only equating land with tombs, kinship and ancestors, but also coupled with a skilful deployment of 'Malagasy customs'. In order to exclude later migrants who also wanted to settle, the 'masters of the land' attempted to establish control over holdings in the area. To this end, and to reinforce their own legitimacy as landholders, the tompon-tany labelled subsequent migrants andevo ('lave' or of 'slave descent') who - as a tombless people - have no rights to land. Because they have neither tombs nor ancestors, the landless andevo are socially ostracised and economically marginalised. As an 'impure people', they are not entitled to a place in the hereafter

Caractérisation en dynamique rapide du comportement de matériaux utilisés en aéronautique

L'objectif de cette thèse est d'apporter une contribution aux procédures de modélisation des problèmes en dynamique rapide. La caractérisation du comportement élastoplastique de matériaux métalliques, l'aluminium 2017 et l'acier 42CD4, en dynamique rapide constitue l'application principale des différentes démarches et méthodes mises en œuvre. Sur le plan numérique, le travail réalisé a permis d'étendre la gamme d'utilisation des lois d'écoulement par l'implémentation de nouvelles formes de lois dans un code de calcul explicite. Cette implémentation , basée sur la méthode du retour radial, a été validée au moyen de différents cas tests. La partie expérimentale du travail est relative à l'installation et à l'exploitation d'un nouveau lanceur à gaz au laboratoire. Le dispositif est destiné à réaliser des essais d'impact dans différentes configurations afin de caractériser le comportement dynamique des matériaux et des interfaces. La procédure d'identification, basée sur le programme PILOTE développé en interne, utilise une combinaison d'algorithmes de Monte-Carlo et de Levenberg-Marquard. Les résultats sont présentés pour l'aluminium 2017, l'acier 42CD4 et un interface utilisant ces deux matériaux. La validation est obtenue par comparaison entre expérience et calcul sur des structures "non conventionnelles"TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

An ALE three-dimensional model of orthogonal and oblique metal cutting processes

An Arbitrary Lagrangian Eulerian (ALE) approach is used in this paper to model continuous three-dimensional orthogonal and oblique steady-state metal cutting processes. The thermomechanical coupled model includes the effects of elastoplasticity, high strain and strain rates, heat generation and friction between the chip and the tool. A thermal-viscoplastic constitutive equation associated with the Johnson-Cook flow law is adopted for the workpiece. A classic Coulomb friction law associated with heat generation and heat transfer is used tomodel the toolchip contact. The model is suitable for predicting thermo-mechanical quantities, chip geometry and the cutting forces from a set of cutting data, material and contact parameters. Cutting experiments and numerical simulations were performed on a 42CD4 steel and comparisons show a reasonable level of agreement