Fatigue of polyacrylonitrile (PAN) carbon fiber monofilaments

As a high-strength and high-modulus fiber, carbon fiber is widely used in the reinforcement of advanced composite materials, which will suffer fatigue when subjected to cyclic loading with stress lower than tensile strength. However, the testing of the fatigue properties of monofilament carbon fibers presents a significant challenge due to the lack of effective testing methods for such micron-scale brittle fibers. Herein, we used a self-excited vibration principle fatigue device to investigate the fatigue behavior of carbon fiber monofilament (T800) by applying cyclic bidirectional bending. At a stress of 1041 MPa, the fiber exhibited a fatigue life exceeding 10⁷ cycles and a fatigue resistance of 19 % of its tensile strength. With progressive damage on both sides of the fiber surface, bidirectional cracks initiate at the fiber surfaces and coalesce to the middle region accompanied by flat crack growth regions. In addition, the vibration frequency decreases significantly with the increasing fatigue life indicating the slow crack growth, while molecular dynamic simulations further reveal the accumulation of atomic fracture indicating the progressive damage mechanism during the fatigue process.