Epoxy resin is widely used as a matrix resin of CFRP (Carbon Fiber Reinforced Plastics). It is known that the initial phase of the fatigue fracture of CFRP is the damage of matrix resin near fibers as well as interfacial debonding between fibers and matrix resin on the microscale. Thus, to understand the fatigue property of CFRP, it is necessary to consider the fatigue behavior of epoxy resin. However, permanent deformation occurs under the cyclic loading because of non-linear mechanical properties such as visco-elasticity, plasticity, and visco-plasticity. This research focused on the permanent strain, which is called “accumulated strain” in this research, under fatigue loading. Fatigue tests of epoxy resin were carried out to obtain the relationship between stress, number of cycles, and accumulated strain, in addition to S − N relationship. The test results showed that the values and increasing rates of the accumulated strain depend on the elapsed number of cycles and magnitude of stress. After that, a numerical simulation of CFRP was conducted considering the accumulated strain of the epoxy resin, and fatigue test of CFRP which matrix resin was the same as the previous tests was carried out under the same condition of the fatigue analysis. The results of analysis and test showed agreement except for the high stress condition. Focusing on the numerical results of the accumulated strain on the microscale, it was found that the maximum equivalent accumulated strain when first fatigue damage occurs is a similar value at each stress level and analysis model.
MORI et al. (Thu,) studied this question.