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Comportement mécanique des mousses d'aluminium : caractérisations expérimentales sous sollicitations complexes et simulations numériques dans le cadre de l'élasto-plasticité compressible

Abstract : An extensive experimental program and detailed mechanical analysis were performed to test and model the statistical response of metallic foams under complex loading conditions. Tensile tests were performed on more than eighty specimens of closedcell aluminium foams with four different specimen sizes. These test results show a large scatter and a significant size effect especially on standard
deviation. The average fracture stress and, more significantly, the corresponding scatter decrease for increasing volume sizes. A Weibull statistical analysis is performed and gives a Weibull modulus close to
8. Compression tests were also carried out. Both mean fracture stress in tension and mean peak stress in
compression and the corresponding dispersions are correctly described by a single set of Weibull parameters. The statistical model is extended to multiaxial loading conditions by introducing an effective stress measure involving both the deviatoric part of the stress tensor and its trace. One additional parameter is identified using the average shear yield stress obtained from pure shear tests and torsion tests on solid bars. The model is then able to predict the dispersion found for the shear strength. Two types of combined tension/compressiontorsion loading conditions were then tested experimentally. The non
proportional loading path consists of a tension test followed by torsion, keeping the axial stress constant.
In the proportional loading path, shear and axial stress follow a straight line in the stress space. The
corresponding surface of average yield/fracture stress is found to be symmetric. The experimental results are in good agreement with the predictions of the statistical model. The model predicts a bellshaped surface for the first loading path and a quasielliptic one for the proportional one. The scatter found in the description of this surface is also accounted for accurately by the model. A brief discussion of an extension of Beremins micromechanical model to the statistical failure of brittle foams is presented. Aluminium foams cannot deform homogeneously under compression or multiaxial loading. Strain localization bands form in compression, that are approximately normal to the load axis. The plateau observed
on the overall load displacement curve is due to the formation and propagation of such bands. Densification starts when all cell rows are crushed. These strain localisation phenomena must be taken into account for the identification of a constitutive model. This requires structural computations of the compressed sample. For that purpose, the softening behaviour observed after the initial peak stress on all compression curves is explicitely incorporated into the constitutive modelling. The proposed continuum compressible plasticity was implemented into a finite element program to simulate the band formation
and propagation. The initial softening effect triggers strain localization in narrow bands. The densifi-
cation taking place inside the band after a critical strain is responsible for the formation of new bands near or far from the first one. A good agreement is obtained with experimental results when the heterogeneous density field, deduced from tomographical analyses, is included in the simulation. In particular this heterogeneity induces a slight hardening instead of the theoretical plateau as observed experimentally
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Submitted on : Monday, April 5, 2004 - 5:57:39 PM
Last modification on : Wednesday, October 14, 2020 - 3:52:50 AM
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  • HAL Id : tel-00005740, version 1

Citation

Jean-Sébastien Blazy. Comportement mécanique des mousses d'aluminium : caractérisations expérimentales sous sollicitations complexes et simulations numériques dans le cadre de l'élasto-plasticité compressible. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2003. Français. ⟨tel-00005740⟩

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