Development of deformations in unidirectional carbon fiber – thermoplastic matrix composite materials under long‐term and cyclic loads

Abstract The composites were prepared by impregnating carbon fiber yarns with a polysulfone solution and subsequently removing them from the solvent. The development of deformations in the samples was studied by processing photographs in combination with scanning electron microscopy to reveal the local character of the deformation processes is revealed and the main patterns of damage accumulation processes under long‐term and cyclic loads are identified. It is shown that the development of deformations inside carbon threads impregnated with thermoplastic polymer is nonuniform; there are local areas with dimensions of 10–50 μm, where the deformations develop more actively than in the main volume, and the development of transverse deformations outpaces the development of longitudinal ones. The Poisson ratio at the initial moment is close to that of pure polymer (~0.40) and decreases to 0.19–0.22 with increasing stress as deformation develops. The processes of creep and stress relaxation are determined by the plasticity of the polymer matrix and its flow in microvolumes under the action of applied external stress, which leads to the equalization of the tension of individual fiber bundles inside the impregnated thread and an increase in the effective modulus of elasticity. Low‐cycle fatigue tests showed that the conditional fatigue strength limit according to the failure‐free criterion after 10,000 loading cycles is 2.5 GPa for high‐strength fibers and 2 GPa for high‐modulus fibers, which are 51% and 47% of the total strength of the carbon fibers used. Highlights Polysulfone reinforced with unidirectional carbon fibers via solution technology. Polysulfone's ability to deform under load determines composite behavior. Polymer flow leads to bundles, local reorientation, and stress equalization. Polymer yielding onset leads to strain–stress curve nonlinearity. After stress removal, the polymer deformation and fiber reorientation were kept.