Homogenization Analysis and Experimental Verification for Creep of Unidirectional Fiber Reinforced Composites

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1. A homogenization theory was formulated for time-dependent nonlinear composites with periodic internal structures, so that perturbed displacement rate is decomposed into time-independent elastic and time-dependent viscoplastic parts. Then a macroscopic constitutive relation and a microscopic evolution equation of stress were derived.2. The theory mentioned above was applied to analyzing transverse creep and elastic-viscoplastic deformation of unidirectional composites reinforced with continuous fibers. Assuming the plane strain condition in the fiber orientation, and employing the square and hexagonal arrays of fibers, it was shown that the square and hexagonal arrays exhibit significant and negligible anisotropy in macroscopic behavior, respectively.3. The field of perturbed displacement rate was discussed by assuming the point symmetry of internal distributions with respect to a cell center. It was thus shown that the field of perturbed displacement rate is point-symmetric with respect to cell facet centers as well as the cell center, and that perturbed displacement rate vanishes at these centers. This finding led to introducing a semiunit cell, on the boundary of which the point symmetry is imposed on perturbed displacement rate as its boundary condition.4. The present theory was employed to analyze off-axial elastic-viscoplastic deformation of a hexagonally fiber-arrayed composite. It was then found that a small off-axis angle gives rise to significant decrease of viscoplastic flow stress, but that viscoplastic flow stress changes little for off-axis angles larger than 45 degree.5. Comparison of the analytical result mentioned above with experimental results on a hybrid composite GLARE 2 showed that the dependence of elastic-viscoplastic behavior on off-axis angle revealed in the present analysis is observed experimentally.