The fibers in unidirectional composites were treated at two levels: fiber bundle and array of fiber bundles. The fibers were assumed to be placed in hexagonal arrays in the fiber bundle model. And the fiber bundles were assumed to be placed in planar array in unidirectional composites. Based on the statistical theory of crack evolution and the perfect evolution process of random crack cores, the evolution probability of a random crack core with consideration of the influence of length was given, and a two-level model of random crack cores for forecasting the longitudinal tensile strength of unidirectional composites was built. In this two-level model, the average stress concentration factor of the bundle and the stress concentration factor of the fiber inside bundle were calculated respectively. Behavior of various composites was predicted by the model. Comparisons show that several predicted values fit well with the experimental results, but there are gaps between the experimental and predicted values for other materials. The reason for this difference is discussed in this paper.
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