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Automated procedures for the computational modeling of 3D woven composites: From generated models to image exploitation

par Wintiba, Badadjida
Président du jury Francois, Bertrand
Promoteur Massart, Thierry,Jacques
Publication Non publié, 2019-12-09

Thèse de doctorat

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Résumé :

Numerical prediction of woven composites mechanical properties has been a challenge since decades. Accurate prediction of these mechanical properties lies in the fidelity of the numerical geometries with respect to the real composite microstructure. There are many methods for woven composite geometry generation but they are mostly limited by the residual interpenetrations between yarns which prevent from a discretization conform to the yarns geometries. In a first step of the present research, an automated method for 3D complex woven composites generator is developed. The generation procedure consists of defining an initial situation of yarns made by discretized straight lines with a polygonal cross section attached to each node. A straightening operation consisting of progressive nodal displacement iteratively with a contact management operation is used until the yarns reach their equilibrium position in the RVE. There could remain some minor residual interpenetrations in the obtained RVEs, so a level set based post-processing tool is used systematically after the generation process to suppress the residual interpenetrations, control the yarns volume fractions and insert a thin gap between the yarns for a further discretization conform to the yarns geometry. The discretization tool used subsequently to the geometry generation process uses the analogy to truss structures to generate a high quality tetrahedral mesh for finite elements simulations. Woven composites geometries can also be obtained by exploiting the 3D images of μCT scans. Voxel-based discretizations obtained from image segmentation are nowadays more and more used straightforwardly for mechanical simulations of the behavior of woven composites. The main advantage of this discretization technique is to generate fast regular hexahedral meshes without interpenetrations, that allows producing accurate homogenized elastic properties with acceptable voxel sizes. However, the use of voxel geometries may be questionable for damage simulations because of the potential spurious stress concentrations associated with the jagged (irregular) shape of the mesh at the surface of yarns. In order to overcome this limitation, an automated methodology is proposed here to transform explicit voxel RVEs geometries into implicit smoothed geometries through a level set-based smoothing processing. The method guarantees the absence of any residual interpenetration, and takes intoaccount the optimal determination of critical parameters such as yarns volume fraction, global fiber content in the RVE, and local intra yarns cross sectional fiber fraction especially in regions where yarn contact occur. This is achieved by using a level set-based post-processing tool of the smoothed raw data that suppresses interpenetrations between yarns and that controls locally the fiber fraction in cross sections by local modifications of the yarns cross section. The method offers the advantage to be fully parameterized, as raw voxel geometry information is used as input of the procedure. After the geometry treatment, a subsequent conforming tetrahedral mesh is generated for finite elements simulations.Combining the level set-based post-processing tools developed in the present work offers the advantage to reproduce RVEs geometries close to the real samples. However, since the procedure is purely geometrical, it fails to reproduce the real compaction aspect compared to the μCT scan views of the real composite samples.Yarns in woven composites are settled in a complex manner and subjected to friction and decohesion. Many configurations of yarns geometries are analyzed to evaluate the tangential stress concentrations and show that the choice of the composite RVE and the yarns configuration have a significant impact on the prediction of the stress fields. A comparative study for the prediction of the damage initiation and its location is done between the voxel-based geometry and its smooth representation. It shows that the damage initiates early in the voxel yarns and is quite random in the RVE. In the smoothed RVEs, however, the damage is located at the yarns surfaces in the contacting zones for a tensile load parallel to the yarns and in the zones of higher fiber cross section fiber fraction for yarns perpendicular to the tensile load.