We study crack patterns and effective stress-strain response in unidirectional fiber-matrix composites subjected to out-of-plane and in-plane loading. The fibers are aligned in the longitudinal direction and arranged randomly, with no overlap, in the transverse plane. The fibers and the matrix are isotropic and elastic-brittle, which allows a parametrization of a wide range of composites in terms of a stiffness ratio and a strain-to-failure ratio. The analysis is carried out numerically using very fine two-dimensional spring networks permitting simulation of the crack initiation and propagation by sequentially removing bonds which exceed a local fracture criterion. Particular attention is given to the effects of scale and geometric randomness in these composites. We consider several “windows of observation” (scales) and study crack patterns, types of constitutive responses, and statistics of the corresponding scale dependent effective elastic stiffness and strength of such composites.

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