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

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

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

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

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

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)

Potential of the Cross Biaxial Test for Anisotropy Characterization Based on Heterogeneous Strain Field

International audienceThe mechanical behavior in cross biaxial tension was investigated for two metallic sheets, an aluminium alloy and a dual phase steel. The heterogeneous strain field in the central gauge area of a cruciform specimen was analyzed by digital image correlation. Minor and major strains were output along several paths, for a given load level just before necking, showing a wide range of strain states, from uniaxial tension to biaxial state. The applied loads along the two loading directions were also recorded, the gap between the two signals being all the most important that the material anisotropy was significant. Moreover, the strain path ratio, defined as the ratio of the minor strain over the major strain, exhibited a sensible non-monotonic evolution along the transverse direction, compared to the rolling direction. Finally, a material parameter identification process with only biaxial tensile test for Bron and Besson anisotropic yield model was proposed, based on the minimization of experimental and numerical principal strains along a specified path in the gauge area of the cruciform specimen

A cruciform shape to study the influence of strain paths on forming limit curves

International audienceThe optimization of sheet metal forming processes requires an accurate prediction of material behavior and forming abilities, especially for aluminum alloys which exhibit generally a low formability compared with typical mild steels. This study presents an original technique based on the use of a cruciform shape for experimental characterization and numerical prediction of forming limit curves (FLCs). By using a cruciform shape, the whole forming limit diagram is covered with a unique geometry thanks to the control of the displacements in the two main directions of the specimen. The test is frictionless and the influence of linear and non-linear strain paths can be easily studied since the strain path is controlled by the imposed displacements, independently on the specimen geometry. The influence of strain paths is first studied by introducing a linear prestrain (uniaxial, plane strain or equi-biaxial), in rolling direction. Afterwards non linear prestrain paths arealso tested

Experimental and numerical study on effect of forming rate on AA5086 sheet formability

International audienceWith increasing application of aluminum alloys in automotive or aeronautic industries, it is necessary to characterize their deformation behaviors at large strains, high strain rates and elevated temperatures, which is relatively lacking today. The aim of this paper is to experimentally and numerically investigate the influence of forming rate and temperature on formability of an AA5086 sheet. Firstly, tensile tests are carried out at different temperatures (20, 230, 290 and 350 ◦C) and at different forming rates (10, 750 and 1000 mm/s). A technique of digital image correlation (DIC) associated with a high-speed camera is applied to evaluate the surface strains and a complete procedure is built to detect the onset of localized necking during the experiments. The influences of initial testing temperature and forming rate on the sheet formability are analyzed. Then in order to numerically determine the formability of this sheet, a form of Voce's constitutive law taking into account the temperature and strain rate is proposed. An inverse analysis is carried out to identify the material parameters of the law for the tested aluminum alloy. Finally, with the above identified law, tensile tests are simulated. The experimental and numerical results show that the testing temperature and forming rate have a great influence on sheet formability. At high forming rates, the sheet formability of AA5086 is lowered up to a certain temperature, above this temperature, the formability is greatly enhanced. Furthermore, the agreement between experimental and numerical results indicates that the proposed constitutive law and the identified material parameters can be appropriate to model the sensitivity of AA5086 sheet towards strain rate and temperature