Degradation of the Mechanical Properties of Fiber Reinforced Plastic under Cyclic Loading

The brief review and analysis devoted to the problem of the degradation processes of materials, including fiber reinforced plastics, is carried out. As a specific object, a unidirectional carbon fiber reinforced plastic with [±45] 2s lay-up under cyclic loading was selected. In the theoretical description of this process, it was assumed that the strain includes the elastic, viscoelastic, and viscoplastic components and the strain, formed as a result of the microdamages accumulation in the material. Modeling the degradation process is based on a phenomenological approach; moreover, the kinetic equation for the degradation parameter contains as the arguments both physical time and number of cycles transformed into a continuous variable. When determining the parameters from the experimental results, that are included in the constitutive relations for the strain components, the hypothesis is used, that with a small number of cycles, the strain caused by degradation is much less the strain caused by the rheological properties of the material. In addition, a number of hypotheses were introduced (a generalization of the Kachanov hypothesis, as well as the assumption that the rates of various inelastic strain cannot always be of the same order at all times of loading). It makes possible to simplify the problem of mechanical characteristics identification, as well as to reduce the variety and amount of experiments. The results of experiments and the problems solved for determining the parameters included in the physical relationships proposed were presented, and their good agreement was obtained.