Identification of mechanical behavior of twisted carbon yarns from in-situ and ex-situ tests

In the aviation industry, the use of composites such as OMC (Organic Matrix Composites) has strongly grown in recent years.This has led to the development and optimization of the main elements that make such parts.In particular, 3D woven composites have the capability to adapt to complex three-dimensional shapes.Therefore, the study of such materials is essential to capture all the mechanical transformations that occurred internally in the course of its fabrication.This work focuses on the mechanical characterization of carbon yarns forming the fibrous structure (ie, the reinforcement) of the composite material.The yarn consists of a number of twisted tows, each composed of thousands of carbon fibers, the cohesion of which is provided by a sizing agent.In the first part of this work, the spinning process, which creates the yarn by twisting the tows is described and analyzed.This process rearranges the fibers in the yarn cross section and creates a balanced configuration so that the overall torque is zero.A finite element model simulates this process and reveals the transformation that occurred.Moreover, the twist makes the weaving of yarns easy as it stiffens them with a radial stress directed towards the yarn core.This aspect is also observed when analyzing the transverse behavior of the yarn cross section.Some ex-situ mechanical tests representing the transverse compaction behavior of twisted yarns are analyzed.Different levels of twist are investigated for this purpose, and an elastoplastic behavior is observed.In modeling such behavior, akin to a 2D granular media describing the yarn cross section, a cohesive Mohr-Coulomb elastoplastic is proposed to account for the mechanical behavior in the continuum. Such a law provides a good compromise between a low number of mechanical parameters to be identified and a more complete and realistic mechanical description.It thereby allows for a good agreement with the experimental results.The proposed model is intended to be a viable alternative to models present in literature, also taking into account the presence of twist and sizing.The elastoplastic behavior of the cross section is further observed when analyzing the yarn bending behavior.A loose tightening knot aims to observe the variation of the cross section shape (width and thickness) as a function of the imparted curvature.It is confirmed how even in this case, the level of twist greatly influences these parameters but also the variation of curvature along the yarn path.The modeling of weaving in the loom was decided not to be addressed in the current PhD study.Finally, after weaving, a tomography of the woven reinforcement (3D interlock) often shows a slightly deformed structure compared to an ideal model of yarn columns in regularly spaced planes and perpendicular between warp and weft.An original digital volume correlation analysis is proposed to match the real fabric with its model.