Tow-Scale Mechanics for Composite Forming Simulations

Abstract

Abstract. Composites are processed by a variety of forming techniques at both preforming and consolidation stages; ranging from hand draping, diaphragm forming, vacuum infusion to Resin Transfer Molding. During these processes, individual fabric or prepreg layers are subjected to inplane tension and shear, inter-ply shear, transverse compression and out-of-plane bending forces. These forming forces are translated into individual tow-level forces leading to tow deformations. Each tow is subjected to tension, transverse compaction (in the plane of the fabric due to shear and normal to the fabric plane due to consolidation force), bending and torsion. The resulting tow geometry and local fibre volume fractions (within a tow) would have a significant impact on resin flow as well as mechanical properties of the composite. In this paper, we present computational as well as experimental approaches to predicting tow deformations, when subjected to various loading conditions. The test rigs, shown in figure 1, can measure stress-strain behaviour of a tow in bending, torsion and transverse compression respectively. Figure shows buckling of carbon tow – bending stiffness can be computed from the post-buckling behavior. Torsional moments at monotonically increased twist angle were measured using a very sensitive torque sensor. An anvil, nearly same size as a tow, is used to compress a tow (under controlled axial tension) and the cross-sectional shape is computed from the flattened image (recorded using a high resolution camera). A mechanics-based model has been developed to predict tow-scale deformations under transverse compression, tension, bending and torsion modes of deformation. Individual fibres in a tow are modeled as ‘3D elastica’ and a simple inter-fibre friction model has been incorporated. Initially developed for twisted fibre bundles, the elastic-based model works reasonably well for untwisted fibre tows (by assuming an extremely small twist level for convergence). Full paper will present comparison between experimental and theoretical results.