Abstract. On the example of a unidirectional fibrous composite made of unidirectional carbon fibre composite and cold-hardening epoxy XT-118, tests of specimens with a stacking scheme [+45 0 /-45 0 ]2s, where s is the number of monolayers, were carried out for cyclic loading by tension and compression. When the samples are stretched, both the fibers and the epoxy of composite are under shear stresses and stretching in the transverse direction by normal stresses, and under compression, under shear stresses and compression by normal stresses across the fibers. It is shown that in each loading cycle there is a consolidation of the material associated with the rearrangement of its microstructure, an increase in the secant modulus of elasticity and a decrease in the magnitude of the deformation increment at each cycle. The mechanism of accumulation of residual deformations under multiple loading of test samples was qualitatively studied and their level formed in the composite after each loading cycle was quantitatively found.
Statement of the problemIn the products of space technology for special purposes, nowadays, structural elements from fibrous composite materials (CM), in particular, carbon plastic, are widely used. Such structural elements in the process of operation under multiple (multi-cycle) loading (in particular, thermal loading) should preserve the stability of geometric sizes and shapes. It was found that when ground-based testing of such products and during their operation, distortions of the measured parameters are always fixed, apparently due to the formation of residual deformations in the design. All the formation mechanisms of such residual deformations in fibrous composites in the process of exploitation of the structural elements made from them have not yet been fully revealed, which raises an urgent problem in the world science in the field of mechanics and machine building. We note that a numerical study of the geometrically and physically nonlinear problem of the three-point bending of test samples from layered fibrous composites having a rectangular cross-section was carried out in [1]. The problems of the bending of sandwich plates with a transversally soft core are discussed in [2-6].
Experiments on cyclic deformationAn extensive scientific literature [7][8][9][10][11][12], etc is devoted to the study of nonlinear deformation under cyclic loading of unidirectional fibrous composites. In particular, in work [12] such a study is carried out for a unidirectional composite on the basis of glass fibers under shearing conditions and repeated cyclic loading of specimens with a stacking scheme [+45 0 /-45 0 ]2s (s is the number of monolayers). On the basis of the obtained experimental data, the secant shear modulus G12 is calculated and a conclusion is made about its fall after loading the sample with ten cycles. The authors explain the fall of the shear modulus by the degradation of the composite under conditions of shear deformations forming in the composite.The carried out studies on the shear ...