Effect of strain rate on the forming behaviour of sheet metals

https://hal.archives-ouvertes.fr/hal-01005604/document

Calibration of anisotropic yield criterion with conventional tests or biaxial test

Development of a biaxial tensile machine for characterization of sheet metals

https://hal.archives-ouvertes.fr/hal-00806279/document

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

https://hal.archives-ouvertes.fr/hal-01157311/document

Investigations of the effect of strain path changes on forming limit curves using an in-plane biaxial tensile test

The 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).

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

Lamellar cast irons are known as heterogeneous materials. Their overall hardness is determined by the microhardness of their microstructure contents. This study pertains to extend the indentation size effect (ISE) analyses to these heterogeneous materials. Additionally to macrohardness tests, Vickers microindentation tests were carried out on two cast irons with fully pearlitic matrix. Tensile models for pearlitic steel with both Tabor's law and Taylor's model were introduced in the ISE analyses. Microhardness expressions of the pearlitic phase within these irons were thus obtained. The obtained expressions indicate that the tensile model and the concept of geometrically necessary dislocations (GNDs) can be used to predict the ISE of the pearlitic matrix. For this pearlitic phase, it is shown that the summation of the stresses, associated with the GNDs and SSDs, is more adequate than to consider only one work-hardening stress generated by the two types of dislocations density in the ISE study.

Microhardness model based on geometrically necessary dislocations for heterogeneous material

L’utilisation de procedes de mise en forme de toles minces (emboutissage, hydroformage, formage incremental, …) est tres repandue dans l’industrie. L’amelioration de la prediction des outils de simulation numerique passe par une identification precise de la loi de comportement et des limites de formabilite du materiau constituant la tole. Pour cela, il est souhaitable d’etudier le comportement des toles minces sous des etats de chargements proches de ceux rencontres dans le procede de mise en forme. A cet effet, l’essai de traction biaxial dans le plan de la tole a ete retenu parmi d’autres techniques de sollicitations biaxiales. Dans cette etude, un banc d’essais de traction biaxiale est developpe avec pour objectif d’identifier, a partir d’un meme essai rheologique, la loi de comportement et les limites de formabilite du materiau etudie sous sollicitations statiques ou dynamiques. Ce dispositif experimental est compose de quatre verins hydrauliques disposes horizontalement. La capacite de chaque verin est de 50kN, la vitesse de deplacement sur chaque axe peut atteindre 2m/s. Le pilotage permet d’imposer des rapports de biaxialite lineaires et non-lineaires. Ces essais biaxiaux sont realises sur une eprouvette plane dite « eprouvette cruciforme ». La geometrie optimale de celle-ci est definie a partir d’une etude parametrique realisee sur differentes formes d’eprouvettes cruciformes issues de la litterature. La difficulte principale dans la definition de cette geometrie est de s’assurer de l’apparition de la striction localisee au centre de l’eprouvette afin de maitriser le chemin de deformation impose par le deplacement des verins. Au cours de l’essai, la zone centrale de l’eprouvette est filmee et une technique de correlation d’image est ensuite utilisee en post-traitement pour obtenir le champ de deformation dans l’eprouvette. Par la suite, la procedure experimentale et la geometrie de l’eprouvette sont validees a partir d’une campagne d’essais realisee sur un alliage d’aluminium AA2017. Une procedure specifique de detection de l’apparition de la striction localisee pour cette forme d’eprouvette est definie. Une campagne experimentale est menee sur une tole en alliage d’aluminium AA5086. La courbe limite de formage (CLF) experimentale de cette tole est etablie a partir de la procedure definie precedemment. Enfin, les courbes forces/deplacements experimentales sont utilisees afin d’identifier une loi de comportement par methode inverse a partir d’un modele elements finis (EF) de l’essai biaxial. L’interet d’integrer une grandeur locale dans la procedure d’identification de la loi rheologique est presente. La loi de comportement identifiee est ensuite implementee dans le modele EF de l’essai de traction biaxial afin de determiner la CLF numerique du materiau etudie. Les CLF experimentale et numerique definies a partir de l’essai de traction biaxial sont comparees a celle obtenue sur le meme materiau par un dispositif d’essais de type Marciniak.

Développement d'un banc d'essai de traction biaxale pour la caracterisation de la formabilité et du comportement élastoplastique de tôles métalliques

The piston material choice is a major factor in the design stages of internal combustion engines for its importance to improving the durability and the operation reliability during the piston life cycle. Indeed, even as many researches have been conducted for a long time to improve the piston performances in the diesel engine, considerable numbers of piston damages still significantly occur. This research work is an assessment by the finite element method (FEM) of a set of piston materials for the purpose of being used in a direct injection diesel engine. The main objective is to predict stresses concentrations and the clearance between the piston and the cylinder (to avoid metal-metal adhesion) from thermomechanical solicitations. The stresses and deformations are evaluated in a 3D piston model by using ABAQUS software. A first assessment step is performed in heat convection/conduction modes to determine the temperature distribution. Then, this last one is coupled to the pressure field resulting from the gas combustion in order to compute stresses and the magnitude displacement. The obtained results show that the austenitic steel AS12UNG with fibre-reinforced possess low thermomechanical stress values compared to other material types. This material allows a minimum failure risk and therefore contributes to the enhancement of the piston design.

/pdf/study-of-piston-thermo-elastic-behaviour-under-38yq7wgu6s.pdf

Ibrahim Zidane

Papers

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

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

Microhardness model based on geometrically necessary dislocations for heterogeneous material

Développement d'un banc d'essai de traction biaxale pour la caracterisation de la formabilité et du comportement élastoplastique de tôles métalliques

Study of Piston Thermo-Elastic Behaviour under Thermomechanical Solicitations