Measurement of meso-scale shear deformations for modelling textile composites

Textile preforms undergo complex in-plane and out-of-plane deformations during forming processes. In-plane shear is the most significant mode of deformation during draping or forming around double-curvature surfaces. This paper reports an improved method for measuring the constitutive shear properties of textile preforms—a wide strip bias extension test has been found to give significantly better results in comparison to conventional narrow strip bias test. Shear angles, computed directly from global strains, were in close agreement to those measured using an optical method. Constitutive shear stress–strain relation, computed from the bias extension test data, was in good agreement with KES (Kawabata Evaluation System) test results. Meso-scale tow deformations have been measured using two techniques: an inexpensive flatbed scanner-based full-field strain measurement technique for measuring in-plane tow deformations and a strain-freezing technique for measuring through-thickness tow deformations. With this data, a 3D Representative Volume Element (RVE) can be constructed for each stage of shear deformation. Beyond the geometric shear limit, tow deformations during bias extension appear to be somewhat different from those normally expected from pure shear—a reduction in tow thickness and tow cross-sectional area and a corresponding increase in packing factor.