Abstract: Carbon Fiber Reinforced Polymers (CFRP) has been widely used in aerospace and other fields. The ultra-high cycle fatigue of CFRP components has become more and more obvious. In this paper, the damage evolution process of CFRP is experimentally investigated via ultrasonic fatigue testing system for cyclic three-point bending. The results indicated that the S-N curve of CFRP composites presented a step-wise shape under the ultra-high testing for cyclic three-point bending, and especially, the fatigue strength decreases significantly after the cycles are more than 10⁸. By investigating the damage evolution process of CFRP in the same field of view, this paper found that the damage morphology of CFRP composites under ultra-high cycle loading is mainly characterized by the matrix damage at the intersection of fiber bundles, near-fiber bundle parallel section matrix cavity, and matrix penetration. With the increase of test cycles, the damage process is also presented in turn according to the above three characteristics.

Key words: very high cycle fatigue, carbon fiber reinforced polymer, three-point bending, fatigue morphology, damage evolution process