Micromechanics of Unidirectional Viscoelastic Fibrous Composites-Homogenized vs Local Characterization

Abstract

In this paper homogenized elastic and viscoelastic material properties are evaluated and compared for symmetric and randomly distributed fibers within the matrix of a unidirectional composite laminate. In general, homogenized properties are different from their localized values. In homogenized characterization one assumption rests on a systematic distribution of fibers which at many circumstances might not be true. We select representative volume elements (RVE) in terms of repeating unit cells (RUC) off our types of fiber distributions; two systematic types (square—SQR and hexagonal—HEX packing), and two randomly distributed (one with identical fibers—RND, and one with random geometry—RNDG packing). Employing the principles of micromechanical characterization, RUCs of the composites were exposed to six distinctive types of loadings(three axial loadings and three shear loadings) under periodic boundary conditions.Inverse analysis is conducted to characterize and determine the comprehensive volume-averaged homogenized elastic and viscoelastic material properties of the composite. To investigate the impact of constituents’ material anisotropy on the behavior of the compound material, both isotropic and orthotropic fibers are employed. To examine the effect of the fiber/matrix volume ratios, the results are presented for three different fiber volume ratios. The local stress/strain distributions within each RUC are monitored in comparison with the corresponding volume-averaged stresses and strains. The local stresses and strains are compared with the volume-averaged ones by which the material is characterized and the differences are highlighted. One conclusion is that the impact of random distribution and geometry of fibers on homogenized stiffness properties might be minor, but it has considerable effect on local stress/strain distribution and hence on the strength of the composite. Among the different types of RUCs the HEX packing provides more uniform distributions of stresses and strains.