Aeronautical and automobile industries are more and more interested in integrating fabric-reinforced composite structures in their designs, because of their high performance/mass ratio. Several processes can be used to manufacture composite parts, such as the Resin Transfer Molding manufacturing process (RTM). During the shape forming step, fibrous reinforcements undergo different mechanical loads such as, for instance, out of plane compression, in plane torsion, bending, shear, tension, etc. These loadings induce strains and sometimes damage to the fabric. Therefore, the mechanical properties of the final parts are drastically impacted by the shaping step. Hence, in order to improve the quality of these composite parts, the mechanical behaviour of the fibrous reinforcements during the forming step must be anticipated and understood.

Fabrics are made up of yarns containing hundreds to thousands of fibers. They might be studied from three different scales: macroscopic scale (the fabric), mesoscopic scale (the yarn) and microscopic one (the fiber). The microscopic scale allows to associate to each fiber a homogeneous isotropic behaviour, however, at this scale, the geometric position and the contact of thousands of fibers must be identified. This modeling, therefore, remains complicated and computationally expensive. On the other hand, the macroscopic scale would not allow predicting the defaults induced in the scales below. In fact, the entanglement and the interlacement of yarns could not be taken into account at this level. Therefore, the mesoscopic scale can be considered as a good compromise between reality and complexity.

The aim of the present study is to formulate a mesoscopic constitutive law from the microscopic structure of a fiber yarn. Consequently, friction between fibers, their rearrangement and their geometric position in the yarn will be taken into account in the mechanical behaviour. A Representative Elementary Volume (REV) of a hundred fibers is submitted to different loadings and the mechanical response is analyzed. The resulting data will allow understanding the deformation mechanisms at the microscopic scale. A constitutive law can consequently be proposed for the mesoscopic scale.

The study was initiated by elementary test cases in order to define an efficient simulation strategy, in terms of finite elements type, finite elements solver (Abaqus/implicit or Abaqus/explicit), contact behaviour and boundary conditions. Concerning the finite elements type, fibers are modeled by 3D beam elements for two main reasons: providing a reasonable simulation time (few hours for a few hundred fibers are targeted) and accounting for the fiber aspect ratio (from 150 to 400). Therefore, more simulations and test cases were analyzed in order to check whether the beam elements are adequate for the main aims of this study. Furthermore, the compaction of a parallel fiber network was simulated, results confirmed that a correct implementation and parameterization of the Abaqus/explicit finite elements solver leads to a reasonable accuracy/calculation time ratio. For this type of problems, and among the tasks performed to validate the implementation, test cases were studied to ensure quasistatic simulations and reduce the response instability. The contact behavior of beam elements has been established based on the Hertz contact theory and the comparison with 3D elements. Finally, the simulation strategy will be validated by means of geometric observations on a real compacted fibrous network.