Evaluation of Fatigue Strength of Polymer Composite Materials With a Two-Phase Reinforcement Scheme Under Cyclic Stretching Conditions

The most important task in the development of structures made of polymer composite materials (PCM) is to ensure their high fatigue strength. The fatigue strength of PCM is largely determined by the properties of the matrix material used. It is the matrix material that is sensitive to the loading time, and the strength of the fibers has a statistical spread and depends on the degree of damage to the matrix. Therefore, when creating PCMs that can operate for a long time under conditions of cyclically changing loads, it is necessary to develop a matrix structure that would ensure stress relaxation and prevent the accumulation of defects during deformation. In natural materials, such as living wood, this function is performed by liquid sap. Thus, the creation of a PCM structure similar to the structure of living wood will ensure high fatigue strength of the composite structure. The paper proposes a fundamentally new approach to the reinforcement of the composite. This approach consists in the use of an additional reinforcing material, which is a monomer or elastomer that does not enter into chemical interaction with the binder material and therefore retains its original state of aggregation in the composite structure, forming an independent liquid phase. The material of the liquid phase is located in the volume of the composite according to the given schemes of reinforcement, determined by the direction and nature of the loads acting on the structure. Like the sap of living wood, the task of the material of the liquid phase is not to accumulate defects during deformation and to provide stress relaxation. The effect of liquid phase materials, which are dimethacrylate triethylene glycol, silicone sealant, and synthetic wax, on the fatigue strength of carbon fiber reinforced plastics under cyclic tension conditions has been studied. The tests were carried out by the method of block cyclic loading with a loading speed of 10 mm/min and a force equal to 70% of the absolute breaking force. The number of cycles in the blocks was 100, 500 and 1000. After the implementation of each loading block, the test specimens were tested to failure with registration of residual strength. Based on the results of the tests, it was found that the use of dimethacrylate triethylene glycol and silicone sealant as a material for the liquid phase makes it possible to increase the residual strength of carbon fiber reinforced plastics in comparison with the residual strength of samples without liquid phase material. At the same time, for this type of carbon plastics, the strength after cyclic loading is higher than the strength measured before cyclic tension, which can be taken as a qualitative characteristic of stress relaxation.