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The article presents the results of a study on the mechanical properties of glass (T-10-14 filler) and carbon-fiber (Formosa TC-35 12K) reinforced plastics in an epoxy matrix T-107. It was found that the mechanical properties of carbon fiber were superior to fiberglass. Regardless of the composition of the composite, there are two distinct stages on the deformation curves of experimental samples based both on carbon and glass fibers, which correspond to elastic deformation and softening stages associated with the failure of composite fibers. Taking account for the significant difference between the elastic moduli and tensile strength of carbon and glass fibers, on the one hand, and the polymer matrix, on the other, it was suggested that the observed effect of softening of the composites is associated with the process of sequential breaking of the fibers as the applied load increases. Basing on the proposed idea, a model of deformation of a polymer composite material based on the sequential fracture of the filler fiber with increasing the applied load has been developed. According to this model, the observed increase in the applied load at the stage of strain hardening is associated with the presence of dispersion of the fibers of the samples by the values of strength, i. e. there is a distribution of the strength characteristics of the fiber along the length. With an increase in the length of a fiber in the sample, and, consequently, the length of the sample itself, the probability of appearance of thread sections with an even lower level of strength increases. Thus, in general, the tensile strength of the composite measured in the experiment should decrease. This dependence of the strength characteristics of the composite on the geometrical parameters of the product is determined by the fiber strength distribution function along its length, which can be determined from the experimental curves of tensile specimens of composites. Taking into account the proposed model, the obtained experimental results are used to estimate the distribution function g(f) of the probability of detection of its strength value per unit length of fiber.

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