Transverse compaction of twisted carbon yarns: Experiment and elasto-plastic Mohr–Coulomb modeling

The mechanical characterization of the carbon yarns, the major component of woven textiles for composite materials, is essential for optimizing the fabrication and the final properties of such materials. This work focuses on carbon yarns formed by twisting several elementary tows, each composed of thousands of carbon fibers, whose weak cohesion is ensured by a sizing agent. Different twist magnitudes are studied in dry and wet conditions. Transverse compaction tests are performed with loading-unloading cycles of increasing amplitudes. They have shown that the yarn cross section approaches a critical state where the fiber volume fraction remains constant, irrespective of twist level and humidity. The experimental results evidence an elasto-plastic behavior, analogous to that of a 2D granular medium. A simple model is proposed with a cohesive non-associated Mohr-Coulomb plasticity, having very few constitutive parameters. The effect of twist, water, and sizing can be accounted for.