Longitudinal Modulus of Semi-auxetic Unidirectional Fiber Composites

In micromechanical modeling of composites, it is well known that the Voigt formula or the rule-of-mixture method gives the most general upper bound estimate of the effective Young's modulus. In this article it is shown that the effective longitudinal Young's modulus of a continuous unidirectional fiber composite significantly exceeds the rule-of-mixture when both constituents possess equal or almost equal Young's moduli and opposing Poisson's ratio signs.