Stiffness Reduction Process of CFRP under the Cyclic Bending Deformation

Stiffness of carbon fiber reinforced polymer composites (CFRP) under the cyclic loading of bending deformation has been measured at various temperatures and deformation amplitudes. The stiffness of specimens decreases gradually with the number of loading cycles. The stiffness reduction correlates with the decrease in bending strength and the increase in viscoelastic loss, which suggests the stiffness reduction process corresponds to the damage accumulation process under the cyclic loading. The stiffness reduction process whose rates increase with increasing temperature and deformation amplitude, has been proved to be the thermally activated process depending on load stress (deformation amplitude). Analyzing the stiffness reduction curves at various temperatures and loads by means of reduced variable methods, a master curve that can be used to estimate a fatigue life has been composed, and an activation energy and an activation volume are estimated to be 26±3kcal/mole and 1.1×10 m 3 respectively. A molecular process for the stiffness reduction is discussed on the basis of the rate process theory.