Longitudinal compression and Poisson ratio of fiber yarns in meso-scale finite element modeling of composite reinforcements

Meso-scale finite element modeling is a powerful tool to analyze the deformation of textile composite reinforcements. At the meso-scale, the yarns of the reinforcement are considered to be solids made of a continuous material in contact with their neighbors. These yarns are generally under tension, but some loadings of the reinforcement lead to a longitudinal compression state. The yarns, made up of a large number of fibers, present a specific behavior when under longitudinal compression. Local buckling of the fibers causes the compressive stiffness of the continuous material representing the yarn to be much weaker than when under tension. In addition, longitudinal compression causes an important transverse expansion. It is shown in the present work that the transverse expansion could be depicted by a Poisson ratio that remained roughly constant when the yarn length and the compression strain varied. Buckling of the fibers significantly increases the transverse dimensions of the yarn which leads to a large Poisson ratio (up to 12 for a yarn analyzed in the present study). The longitudinal compression and transverse expansion were integrated in a mechanical model of the yarn. Meso-scale finite element simulations of reinforcements with binder yarns submitted to longitudinal compression showed that these improvements led to results in good agreement with micro-CT analyses.