An Anisotropic Behaviour Analysis of AA2024 Aluminium Alloy Undergoing Large Plastic Deformations

International audienceThe mechanical behaviour of metals during real forming processes must be related to their anisotropic properties. Concerning the analysis of the anisotropic behaviour of aluminium alloys this one has been the subject of various studies, generally in the field of sheet forming processes ((Malo et al., 1998), (Lademo et al., 1999)). In the last decades the majority of the fundamentals works search to define the mathematical description of the anisotropy starting from modified quadratic Hill criteria, non-quadratic ones such as Cazacu-Barlat (Cazacu & Barlat, 2001), Banabic (Banabic, 2002) or other sophisticated models synthesized in (Khalfallah, 2004). On the experimental point of view the many researches describe the rolled sheet properties of aluminium alloys (Choi & Barlat, 1999), (Li et al., 2004), (Park, 1999)) and report the anisotropic response during the mechanical deformation from uniaxial tensile tests, wire drawing or simple shear ones ((Fjeldl & Roven , 1996), (Hu et al., 1998), (Lloyd & Kenny, 1980), (Yonn, 2005)). Until now relatively few studies concerns the use of the channel die compression test, where deformation history is close of the principal sheet forming process such as the cold rolling one (Francillette et al., 1998). In this study an aluminium alloy (AA2024) is analyzed in order to characterize its anisotropy and its mechanical behaviour with this latter mechanical test. In a first part, the microstructure of the material is defined through optical and SEM microscopy, EBSD and X ray measurements. Micro-macro approaches will be used to valid the experimental measurements. Next, mechanical tests mainly the channel die compression one (see Francillette et al, 2003) and the tensile one are used in order to determine the macroscopic anisotropic behaviour of the material. A rigorous analytical model, able to describe the large plastic deformation of the material specimen which occurs during these experimental tests, will be developed. The main idea consist to define analytical equations which permits to compute the stress, the plastic strain rate and the cumulated plastic strain corresponding to a parallelepiped material undergoing a channel die upsetting loading. Final formula will be established to compute all the coefficients corresponding to a quadratic Hill criterion. A comparison with the well known computation model corresponding to the tensile test will be made. Next, a more general Hill criterion, taking into account variation of its coefficients with the plastic strain, will be analyzed. Starting from the previous mathematical description, a general methodology, able to identify rigorously all the parameters defining Aluminium Alloys, Theory and Applications. The laws of variation of the computed quantities with the plastic strain, will be presented. Finally an application for a plan and normal anisotropic formulation, corresponding to AA2024 aluminium alloy rolled plate, will be detailed. A new approach will be then proposed in order to predict the Lankford coefficient values and a validation will be made by a comparison of these ones with those obtained from classical tensile tests

Principes pédagogiques d'enseignement et d'évaluation dans le domaine de la Mécanique Appliquée. Cohérence COURS-TD-TP-PROJET

Depuis la réforme pédagogique menée en 2006 à l'INSA de RENNES, l'enseignement du 1ère Cycle Ingénieur à chercher à introduire des modules d'enseignement type sensibilisation et pré-spécialisation, le principal objectif étant l'introduction des connaissances fondamentales dans le domaine de sciences appliquées, notamment dans le domaine de la Mécanique Appliquée, en forte corrélation avec les modules d’enseignement des filières de spécialisation du 2ème Cycle. Ceci a conduit tout d'abord à la conception des modules d’enseignement axés sur une imbrication forte entre les notions théoriques et les applications. Des exercices académiques, avec des degrés de complexité croissants, on été introduites, en aboutissant toujours sur des applications de synthèses. Il a été aussi nécessaire de faire évoluer l'enseignement de ces disciplines en 2ème Cycle, sous forme de module scientifiques en utilisant une cohérence globale, partant des notions acquises de mathématiques et physique. En tenant compte de cette évolution, il est connu que pour améliorer la qualité de l’enseignement, l’utilisation d’une pédagogie active joue un rôle fondamental. Elle doit être centrée sur la motivation continue des élevés en présentant d’une manière pragmatique les besoins des notions qui vont été introduites, le rôle de la capacité cognitive, du « stockage » et de la « compréhension » des informations. Il a été aussi important d’assurer la cohérence et la logique entre la théorie et la pratique, en introduisant pendant le cursus d'enseignement des évaluations périodiques et continues, voir des Mini-Projets. Une telle méthodologie a été employée pour l'enseignement des Cours, TD et TP du 2ème Cycle du Département GMA dans le domaine de la Mécanique. L’idée centrale a été toujours de s’appuyer fortement sur l’utilisation des exemples concrets et bien identifiés dès le départ, constituant le fil rouge du contenu de l’enseignement. Cet article détaille la méthode employée et les principes d’enseignement et d'évaluation utilisés

Conception d'un nouveau test de frottement pour le forgeage à froifd par extrusion et études tribologiques d'un alliage d'aluminium

Pour mieux caractériser le frottement lors des procédés de mise en forme, il faut utiliser des tests tribologiques capables de reproduire les conditions réelles de contact. Les auteurs proposent ainsi la conception optimale d'une filière d'extrusion afin de pouvoir mieux identifier le frottement à partir d'une technique d'Analyse Inverse. Des tests sur des tribomètres classiques seront aussi utilisés, ainsi que des analyses expérimentales a posteriori de la surface de l'échantillon afin de mieux expliquer les phénomènes de contact

Finite element method coupled with a numerical cellular automaton model to simulate the residual stress of dual phase DP600 steel Nd:YAG laser welding

A sequentially coupled thermo-mechanical model has been developed to investigate the residual stresses of DP600 dual phase steel welded by a Nd:YAG laser process. A numerical cellular automaton (CA) model has been firstly built to simulate the microstructure evolution during a laser welding. The obtained CA microstructure orientation results are input into a finite element model (FEM) as material anisotropic parameters together with a corresponding constitutive law characterizing the material behavior taking into account the dual phase ferrite-martensite hardening, the temperature sensitivity and the strain rate influence. The numerical residual stresses predicted by the proposed coupled CA-FEM model have variations close to those of experimental observation. The anisotropy of dual phase DP600 steel has been analyzed both from the solidification process using the numerical CA model and from the base metal thermo-mechanical behavior influence using a sequentially coupled thermal-elastic-plastic model and a plane anisotropy theory. It is observed that the residual stress longitudinal to welding direction is more influenced by the material anisotropy and the temperature sensitivity. The analysis of DP600 coupled anisotropy model consists of two parts: I. Anisotropy of weld: The developed numerical CA model take into account welding temperature field history, the nucleus density, nucleus position and diffusion behavior. The obtained dendrites solidification directions are applied to analyze the microstructure evolution during the welding process and to compute the orthotropic elastic constants of DP600 weld. The martensitic phase properties and the orientations of the weld material are input into a FEM taking into account the corresponding anisotropy.  II The anisotropy of base metal: A lot of specimens undergoing tensile tests along three different orientations corresponding to 0°, 45°and 90° with the plate rolling direction have been performed. In a classical way the plane anisotropy of steels uses the Hill-48 theory and the corresponding parameters are estimated according to the Lankford computation method. A specific thermo-mechanical rheological law is proposed for formulation of the equivalent stress starting from constitutive models proposed in previous works by A. Gavrus. The experimental method used to estimate the residual stresses by X-Ray diffraction uses the estimation of the distance between crystallographic planes as an internal strain gauge. The diffractometer used for these studies was equipped with a 4 circle goniometer - (Seifert MZ VI - TS) and a PSD detector. All the obtained results during this study lead to a better understanding of the relationship between the laser welding process and the welding microstructure, residual stresses and global weld quality. From analysis of the proposed simulation models, several conclusions can be drawn:   Hardening: the classical Voce or Ludwick model are found unable to perfectly describe the observed material behavior of DP600 dual phase steel in the range of plastic strain from 0 to 0.2. A proposed synthesis rheological model is used to describe in a physical sense the obtained experimental stress-strain curves. Temperature sensitivity: the proposed formulation shows a good accordance with the obtained experimental data. The temperature sensitivity is found to have an important influence on residual stresses simulation. Strain rate: a strain rate sensitivity model is generated based on experimental data obtained at room temperature. As a first approximation, the strain rate sensitivities for high temperatures are supposed to be the same as that for room temperature. The obtained strain rate sensitivity model is input into FEM model and numerical simulation results of residual stress are compared with the FEM model supposing the unit strain rate sensitivity. Anisotropy: the weld anisotropy has little influence while the base metal anisotropy has an important influence on numerical residual stress distribution

Experimental and numerical study concerning the anisotropic behavior of an AA2024-T351 thick sheet using the unified formalism, a multi-mechanism model and a polycrystalline approach

The purpose of this work is to evaluate the anisotropic mechanical behavior of an AA2024-T351 thick plate using three multi-scales approaches: a Macroscopic Unified formalism (HILL 48), a Multi-Mechanisms Model and a Polycrystalline plasticity model. The experimental results obtained from uniaxial tensile tests are first described. Thereafter, the theoretical formulations and numerical identification of studied elastoplastic approaches are presented. Finally, finite element simulations of tensile tests and analysis of the thick sheet stretching forming process are performed

Some aspects regarding the influence of the anisotropy of an AA2021-T351 rolled thick plate on its tribological behaviour

Some tribological phenomena concerning the friction behavior of metallic or non-metallic materials at lower values of sliding speeds are essentially different from those corresponding to high speeds. Based on previous experimental and numerical studies developed in the GCGM laboratory of INSA Rennes concerning bulk and surface anisotropy of metallic thick plates together with their mechanical behavior evolution under important thermo-mechanical loading gradients, this scientific paper proposes to study the influence of a rolled thick sheet aluminum anisotropy on the stick-slip phenomenon concerning a range of low and very low sliding speeds (0.2-200 mm/min). It is known that the stick-slip phenomenon occurs if the static friction coefficient has a larger value as compared to the kinetic one, especially in the case of friction couplers with a dry or a limited friction regime (when the sliding speed is in the range of 0.01-3 mm/s or when the angular speed is somewhere in the range of 1-25 rad/s). For this study it was used an UMT Micro-Scratch Equipment of the Machine Elements and Tribology Department (OMTR)- University Politehnica of Bucharest (UPB). The equipment can provide rotational, translational or reciprocating motions with speeds starting from 0.1?m/s up to 10m/s. A constant or a progressive normal load between 0.05 N and 1000 N can be applied on the material sample. Experimental friction tests were made on an anisotropic rolled aluminum alloy AA2024-T351 using an ultra-high-molecular-weight polyethylene (UHMWPE) cylindrical pion. The AA2024-T351 is one of the most popular high-strength aluminum alloys, having a low corrosion resistance and good mechanical properties at high temperatures: ultimate tensile strength > 420 N/mm2, yield strength at 0.2% > 260 N/mm2, HD harshness around 120 Kgf/mm2, LF 108 cycles of 125 MPa, Young modulus 71000-74000 MPa and Poisson coefficient of 0.33. The ultra-high-molecular-weight polyethylene is characterized by an excellent resistance to abrasion and its main mechanical properties are defined by a ultimate tensile strength around 231.1 MPa, a yield strength of 25.6±3.3 MPa, a Young modulus 915±42.3 MPa and a Poisson coefficient of 0.46. All the tests have been performed by moving the polyethylene pion along a specified trajectory under a constant normal force and at a uniform sliding speed. In order to analyze the influence of the aluminum alloy anisotropy, the tests were made along the three directions: a longitudinal one, corresponding to the rolling direction of the sample (0°), a transverse one, which represents the direction perpendicular to the rolling direction (90°), and a median direction (45°). Three different normal forces (3N, 5N, 7N) have been applied corresponding to three different sliding speeds (0.005 mm/s, 0.05 mm/s, 0.5 mm/s) along a specified trajectory of 3 mm length. It was observed that for a speed of 0.005 mm/s the friction coefficient decreases if the force increases for all the three principal anisotropic directions. When the sliding speed is increased up to 0.5 mm/s, the friction coefficient remains constant. It can be seen that the friction coefficient has different values along the three loading directions (longitudinal, transversal or median): the highest friction coefficient occurs for the direction perpendicular to the rolling direction and the lowest for the rolling direction. However concerning the median direction, the corresponding friction coefficient has little change with the sliding speeds or the normal forces. Specific theoretical and numerical analysis will be performed concerning the bulk and surface anisotropy of studied aluminum plate, especially regarding the influence on the tribologic properties and corresponding friction model formulations

Etude du comportement mécanique du Polycarbonate par indentation conique

International audienceLe Polycarbonate est un polymère amorphe beaucoup utilisé dans le domaine industriel du fait à la fois de sa transparence et de sa résistance aux chocs. Malgré cela, les modèles de comportement proposés dans la bibliographie [1,2 et 3] ne sont souvent applicables que pour des essais uni axiaux simples. Ainsi, nous avons utilisé le modèle de comportement de G'sell beaucoup cité dans la bibliographie pour simuler l'indentation conique avec un indenteur de demi-angle respectif θ = 80° et θ = 70,3°. La loi de G'sell ne prenant pas en compte la partie élastique ; nous l'avons modifiée en ajoutant un terme élastique linéaire qui influe beaucoup sur la réponse en indentation des matériaux. La courbe d'indentation obtenue par simulation numérique de l'essai d'indentation par la méthode des éléments finis en utilisant la loi de G'sell donne une courbe un peu plus raide que celle obtenue expérimentalement, dans les mêmes conditions. Cependant, si la loi de G'sell modifiée semble très bien modéliser le comportement en indentation dans la partie chargement, elle conduit à une courbe de déchargement bien différente de celle obtenue expérimentalement. Certains phénomènes, non pris en compte dans la loi de G'sell, semble avoir une grande influence sur le retour élastique après indentation. Dans ce travail, plusieurs modèles de comportement sont utilisées pour simuler l'essai d'indentation et les résultats seront discutés. Figure 1: Comparaison entre la courbe contrainte-déformation obtenue par essai de compression et celle obtenue par différents modèle

Contribution to the formability improvement in sheet metal stamping by a novel technique to control press kinetics

The thickness variation during sheet deep drawing process has an impact on the quality of the final product and can lead to local material fractures. To minimize this phenomenon caused by varying degrees of stretching in different areas of the sheet, various methods can be employed. This research paper introduces a novel technique for metal sheet deep drawing, specifically designed for parts with complex geometric shapes that exhibit significant variations in deformation levels across different areas. The objective of this method is to enhance the quality of the deep drawing process by reducing thickness variations in the final part. In this proposed approach, the vertical movement of the punch is accomplished through two vertical rotational movements. This increases the flexibility of the deformation process, ensuring that the active tools elements occupy the most advantageous positions determined by the material flow in the die. As a result, the material's deformability is improved, allowing for a higher degree of deformation. Additionally, this new method offers a relatively simple design and kinematics solutions for the press and eliminates the need for lengthy assembly-disassembly times

A rheological analysis of solid polymers using an inverse method applied to a finite element model of the torsion and tensile tests

International audienceThe torsion and traction mechanical tests are generally used in order to analyses the rheology of solid polymers. This paper deals with an identification of the rheological constitutive equation for a polycarbonate and a polyethylene material. To take into account strain localization, caused by increasing values of the plastic strain and of the temperature gradients, a finite element simulation of the experimental tests is used. Then, the values of the constitutive parameters are computed from an Inverse Analysis method. Accuracy of the material behavior identification results obtained by the Inverse Finite Element Method will be demonstrated

An Inverse Analysis Method Applied to Optimization of Specimen’s Shape for Performing Hot Rapid Crushing Tests from Homogeneous Initial Temperature Field

Specific experimental tests with loadings conditions close to those of industrial fast forming processes as rapid forging, rapid stamping or high speed machining, characterized by large plastic strains, localized deformations and important gradients of strain rates, strain and temperature, requires to analyses material flow behavior at different initial temperatures. One of the more important conditions to obtain intrinsic rheological constitutive equations is to have a quasi-homogenous initial temperature distribution and especially to keep constant the material microstructure during the specimens heating. The rapid induction heating seems to be one of the most reliable processes. This scientific study proposes an inverse analysis technique based on numerical finite element modelling to define on the thermal point of view, optimal specimen shapes for performing hot rapid crushing tests from homogenous initial temperature field