Method of reduced variables for stiffness degradation process of unidirectional CFRP composites subjected to alternating bending

The stiffness of carbon fiber reinforced polymer (CFRP) composites under alternating bending has been measured as a function of the number of loading cycles at various temperatures and deflection amplitudes. The stiffness of the specimens decreases gradually with an increase in the number of loading cycles. Such a stiffness degradation is closely correlated with the residual strength degradation, which suggests that the stiffness degradation process corresponds to the accumulation of microscopic damages under alternating bending. The stiffness degradation rate increases with an increase in temperature and deflection amplitude. By means of the method of reduced variables, a master curve for stiffness degradation that makes it possible to estimate the fatigue life has been composed from the stiffness degradation curves at various temperatures and loading stress levels. The activation energy and activation volume for the elementary process of the stiffness degradation are estimated to be 26 ± 3 kcal/mol and 1.1 × 10−28 m3, respectively. A molecular process for the stiffness degradation is discussed on the basis of the thermally activated process theory.