Caractérisation et modélisation de comportements mécaniques limites de matériaux

Dans un contexte d'exigences environnementales et économiques croissantes, notamment dans le domaine des transports (objectifs de réduction des émissions de CO2), l'optimisation de toutes les étapes du cycle de vie d'un produit est devenue une obsession pour tous les acteurs du monde industriel. Lors de la conception d'un nouveau système mécanique, chaque élément peut être optimisé grâce à une intégration très en amont des contraintes fonctionnelles, des contraintes de service et des contraintes des procédés de fabrication. Cette vision très intégrée de la conception des systèmes autorise un choix optimal des matériaux constitutifs et des procédés de fabrication associés. L'élaboration de nouvelles nuances de matériaux et la mise au point de nouveaux procédés de fabrication contribuent à alimenter cette phase d'optimisation pour le développement de systèmes aux performances mécaniques réellement innovantes. L'allégement des structures constitue le principal objectif de cette démarche d'optimisation. Pour atteindre cet objectif, tout le potentiel des matériaux doit être exploité, jusqu'à leurs limites d'utilisation. La connaissance précise des comportements limites des matériaux pressentis conditionne donc le succès de la démarche. Pour y parvenir, il est indispensable de développer des approches couplées expérimentation-modélisation-identification. La phase de caractérisation expérimentale nécessite la mise en œuvre de dispositifs spécifiques, susceptibles de se rapprocher des conditions d'utilisation réelles du matériau, que ce soit en service ou lors d'une opération de mise en forme

An in-plane tensile test for rheological and formability identification : comparison between experimental and numerical FLC

International audienceBoth accurate constitutive laws and formability limits of materials are essential for a numerical optimization of sheet forming processes. To identify these behaviors, experimental databases are needed. In this work, experiments are performed from a biaxial device able to give for a unique in-plane specimen a good prediction of rheological parameters and formability. The proposed device is a servo-hydraulic testing machine provided with four independent dynamic actuators. By localizing necking in the central zone of the specimen, the strain path in this zone is controlled by the speed ratio between the two axes and the whole forming limit diagram can be covered. The experimental forming limit curve for the aluminium alloy AA5086 is determined thanks to a rigorous procedure for detecting the onset of necking in the specimen. Material parameters (constants of both hardening law and anisotropic yield criterion) are identified from the global measurement of force versus displacement curves by means of an inverse analysis procedure. Comparison between experimental and numerical forming limit curves are presented. For the numerical FLCs, two sets of material parameters are compared, the former is identified through the classical uniaxial test and the latter thanks to the dedicated cruciform specimen

Influence of temperature and strain rate on the formability of aluminium alloys: Comparison between experimental and predictive results

International audienceThe use of sheet metal forming processes can be limited by the formability of materials, especially in the case of aluminium alloys. To improve the formability, warm forming processes can be considered. In this work, the effects of temperature and strain rate on the formability of a given aluminium alloy (AA5086) have been studied by means of both experimental and predictive approaches. Experimental tests have been carried out with a Marciniak stamping experimental device. Forming limit curves (FLCs) have been established on a temperature range going from ambient temperature to 200°C and on a strain rate range going from quasi-static up to 2s-1. In order to predict the experimental temperature and strain rate sensitivities, a predictive model based on the finite element simulation of the classical Marciniak and Kuczynski (M-K) geometrical model is proposed. The limit strains obtained with this model are very sensitive to the thermo-viscoplastic behaviour modeling and to the calibration of the initial geometrical imperfection controlling the onset of necking

Cruciform shape benefits for experimental and numerical evaluation of sheet metal formability

International audienceThe optimization of sheet metal forming processes requires accurate evaluations of material forming abilities. This paper presents an original technique based on the use of a cruciform shape for experimental characterization and numerical prediction of forming limit curves. The whole forming limit diagram is covered with a unique geometry by controlling displacements in the two main directions of the cruciform shape. The test is frictionless and the influence of linear and non-linear strain paths can be easily studied. The modelling of the cruciform shape with the finite element method permits to plot forming limit curves without any calibration step, essential for the classical Marciniak-Kuczynski (M-K) model. Experimental and numerical results are presented for an aluminium alloy 5086. These results are respectively compared with the ones from classical techniques: Marciniak test and numerical M-K model

Identification of Anisotropic Yield Criterion Parameters from a Single Biaxial Tensile Test

International audienceThe present work deals with the calibration strategy of yield functions used to describe the plastic anisotropic behavior of metallic sheets. In this paper, Bron and Besson yield criterion is used to model the plastic anisotropic behavior of AA5086 sheets. This yield model is flexible enough since the anisotropy is represented by 12 parameters (4 isotropic parameters and 8 anisotropic parameters in plane stress condition) in the form of two linear fourth order transformation tensors. The parameters of this anisotropic yield model have been identified from a single dedicated cross biaxial tensile test. It is shown, from finite element simulations, that the strain distribution in the center of the cruciform specimen is significantly dependent on the yield criterion. Moreover, this cross biaxial test involves a large range of strain paths in the center of the specimen. The calibration stage is performed by means of an optimization procedure minimizing the gap between experimental and numerical values of the principal strains along a specified path in the gauge area of the cruciform specimen. It is shown that the material parameters of Bron and Besson anisotropic yield model can be determined accurately by a unique biaxial tensile test

Development of an in-plane biaxial test for forming limit curve (FLC) characterization of metallic sheets

International audienceThe main objective of this work is to propose a new experimental device able to give for a single specimen a good prediction of rheological parameters and formability under static and dynamic conditions (for intermediate strain rates). In this paper, we focus on the characterization of sheet metal forming. The proposed device is a servo-hydraulic testing machine provided with four independent dynamic actuators allowing biaxial tensile tests on cruciform specimens. The formability is evaluated thanks to the classical forming limit diagram (FLD), and one of the difficulties of this study was the design of a dedicated specimen for which the necking phenomenon appears in its central zone. If necking is located in the central zone of the specimen, then the speed ratio between the two axes controls the strain path in this zone and a whole forming limit curve can be covered. Such a specimen is proposed through a numerical and experimental validation procedure. A rigorous procedure for the detection of numerical and experimental forming strains is also presented. Finally, an experimental forming limit curve is determined and validated for an aluminium alloy dedicated to the sheet forming processes (AA5086)

Theoretical and numerical study of strain rate sensitivity on formability of sheet metal

International audienceIn the present work, the formability of an aluminium alloy (AA5083) sheet at elevated temperature (240±C and 300±C) is investigated by theoretical and numerical approaches, using the Swift hardening law. For the theoretical one, an algorithm based on Newton-Raphson method is developed to calculate the limit strains in the frame of the M-K (Marciniak-Kuczynski) model. Numerically, the M-K model is simulated with the commercially available finite-element code ABAQUS. The comparison between the theoretical and numerical evaluation of FLCs shows a good agreement between two approaches. Finally, the effect of strain rate sensitivity index (m) and forming speed on formability is analyzed. Results reveal that the formability is an increasing function of m while there is no significant influence of forming speed on the sheet formability

Identification of sheet metal hardening for large strains with an in-plane biaxial tensile test and a dedicated cross specimen

International audienceIn this work, an in-plane biaxial tensile test of cruciform specimen is performed to identify the hardening behaviour of metallic sheets under large strains. Firstly, an optimal shape of the specimen is suggested. Then, a biaxial tensile test is carried out for an aluminium alloy AA5086. Experimental forces on the two axes of the specimen are measured during the test and strains in the central area of the specimen are post-treated by means of Digital Image Correlation (DIC) technique. Finally, by considering different yield criteria, the associated hardening laws are identified thanks to an inverse procedure based on a Finite Element (FE) modeling of the biaxial tensile test and on the experimental data mentioned above. The identified biaxial flow curves are then compared with the ones from the classical uniaxial tensile test

Theoretical and numerical study of strain rate influence on AA5083 formability

International audienceWith the application of new forming techniques (hydroforming, incremental forming), it is necessary to improve the characterization of the formability of materials and in particular the influence of strain rate. This paper begins with the characterization of material behavior of an aluminum alloy 5083 at high temperatures. To describe its visco-plastic behavior, Swift's hardening law is used and the corresponding parameter values are identified. Then, two different approaches are introduced to construct FLDs (forming limit diagrams) of this alloy sheet and evaluate the effect of the rate sensitivity index on its formability. The first one is theoretical (the M-K model), and an algorithm is developed to calculate the limit strains by this model. In the second approach, the Marciniak test is simulated with the commercially available finite-element program ABAQUS. Based on FEM results, different failure criteria are discussed and an appropriate one is chosen to determine the onset of localized necking. With the material behavior data corresponding to AA5083 at 150 ◦C, parametric studies are carried out to evaluate the effect of the strain rate sensitivity index. The comparison of results by these two approaches shows the same tendency that an improvement of the formability with increasing strain rate sensitivity is observed. Finally, by consideration of the compensating effects of the strain hardening and rate sensitivity indices, the FLDs of this sheet at 150, 240 and 300 ◦C are determined and compared. Results showthat the formability of AA5083 seems not to be improved up to a certain temperature (between 240 and 300 ◦C), above this temperature, the formability is greatly enhanced

New analytical method to evaluate the powerplant and chassis coupling in the improvement vehicle NVH

International audienceThe design of an automotive powerplant mounting system is an essential part in vehicle safety and improving the vehicle noise, vibration and harshness (NVH) characteristics. One of the main problems encountered in the automotive design is isolating low frequency vibrations of the powerplant from the rest of the vehicle. The significant powerplant mass makes the choice of frequency and mode arrangements a critical design decision. Several powerplant mounting schemes have been developed to improve NVH properties concentrating on the positioning and design of resilient supports. However these methods are based on decoupling rigid body modes from a grounded powerplant model which ignores chassis and suspension system interactions. But it cannot be stated that decoupling the grounded rigid body modes of the powerplant will systematically reduce chassis vibrations. In this paper, a new analytical method is proposed to examine the mechanisms of coupling between the powerplant and the vehicle chassis and subsystems. The analytical procedure expands the equation of motion of the vehicle components to such that a domain of boundary conditions used in the 6 degrees-of-freedom powerplant mounting model can be defined. An example of this new procedure is given for improving NVH chassis response at idle speed using the torque roll axis decoupling strategy