## 1 Introduction  
Carbon Fiber Reinforced Plastic (CFRP) has the advantages of high specific strength, fatigue resistance, corrosion resistance, and heat resistance. [PERSON] et al. [1, 2, 3, 4] described how, due to their excellent properties, carbon fiber composite materials are widely used in aerospace, new energy, rail transit, and other fields. However, the CFRP shaft tube structure is often subjected to alternating loads during service. Although the stress is lower than its ultimate strength, the internal micro-cracks will slowly expand, thereby causing matrix cracking, delamination and fiber fracture, and other fatigue damages in the structure. As a result, serious accidents are likely to be caused by the structural strength, stiffness, and other mechanical properties of the attenuation, which brings hidden danger to the use of composite components. Therefore, by exploring its fatigue characteristics under a three-point bending fatigue load, and mastering its fatigue damage evolution mechanism, it is of great significance to formulate timely inspection and maintenance strategies for CFRP shaft tubes and improve the reliability and safety of the tubes during service. At present, scholars both at home and abroad have done a lot of basic research on the fatigue damage mode of composite materials, the evolution mechanism of fatigue damage, fatigue life, and damage detection. [PERSON] et al. [5] proposed that the three-point bending fatigue characteristics of CFRP laminates under different fatigue load levels and fatigue loading frequencies were studied. [PERSON] et al. [6] put forward the stiffness failure criterion and the residual stiffness degradation model. They also established the theoretical analysis model of the fatigue life of CFRP laminates. The anisotropy of composite materials enables its fatigue characteristics to be improved, and experimental testing is therefore required to explore the evolution mechanism of composite material fatigue characteristics. [PERSON] et al. [7; 8; 9; 10; 11; 12; 13] mentioned that the commonly used methods for damage detection of composite materials are ultrasonic testing, acoustic emission testing, vibration testing, and radiographic testing, etc. Specifically, ultrasonic testing and radiographic testing are both static damage detection methods, and real-time dynamic damage monitoring cannot be performed during the operation of the structure. As a new type of optical sensor, Fiber Bragg Grating (FBG) sensor is superior to traditional electrical sensors in terms of stability, anti-interference, and durability, and also lighter in weight and small in size. When the FBG sensor is loaded, the grating will deform, causing the center wavelength of the FBG sensor to drift. The central wavelength information of the FBG sensor is collected and converted into the strain information of the point to be measured. [PERSON] and [PERSON] et al. [14; 15; 16; 17] put forward that through the introduction of technology into the damage monitoring of composite materials, the shortcomings of conventional damage detection technologies such as ultrasonic testing and radiographic testing can be effectively overcome. [PERSON], [PERSON], and [PERSON] [18] pointed out that the feasibility of using FBG sensors to evaluate composite material damage has been verified. [PERSON] et al. [19] found through experiments that FBG sensors have good fatigue resistance. [PERSON] [20] et al. used FBG sensors to monitor the fatigue damage evolution process of graphite/epoxy resin composites. The results show that the fatigue damage evolution process of composite materials can be preliminarily identified through the changes in the spectral shape and center wavelength of the FBG sensor. In this paper, the CFRP shaft tube under a three-point bending load is taken as the research object.