## 3 Experimental Procedure
### Fatigue Tests
#### 3.2.2 Experimental Conditions  
1. Determination of fatigue loading load In the three-point bending fatigue test, the Instron 1341 electro-hydraulic servo fatigue testing machine was used to load the CFRP shaft tube. The loading mode was force-controlled loading, and the loading frequency was 5 Hz. Three-point bending fatigue experiments were carried out on the experimental parts of Group B, C, and D, and the load coefficients corresponding to the fatigue load were 0.7, 0.75, and 0.8, respectively. According to the results of the quasi-static three-point bending experiment of CFRP shaft tube, the elastic limit of CFRP shaft tube is 8546.5 N, and the specific expressions of the three fatigue loads of CFRP shaft tube while the load coefficient is 0.7, 0.75, and 0.8 are as follows:  
\[F_{\text{fatigue}} =-3290.40+2692.15\sin(10\pi t) \tag{6}\] \[F_{\text{fatigue}} =-3525.43+2884.44\sin(10\pi t)\] (7) \[F_{\text{fatigue}} =-3760.46+3076.74\sin(10\pi t) \tag{8}\]
2. Fatigue damage evolution experiment of CFRP shaft tube based on ultrasonic scanning Before the ultrasonic scanning of the CFRP shaft tube, the basic operating parameters of the Mentor ultrasonic phased array flaw detector need to be set. It is divided into 4 steps sound velocity measurement, TCG calibration, sensitivity calibration, scanning detection, etc. The specific settings of each step are as follows:  
* Sound speed measurement The purpose of sound velocity measurement is to measure the sound velocity of the material under test to reduce measurement errors. In this paper, the backside echo method is used to measure the sound velocity of carbon fiber composites. This method only requires a carbon fiber specimen of known thickness, and two consecutive backside echoes are received by two A gates to calculate the propagation time of ultrasonic waves in the specimen. Thereby, the speed of sound of the carbon fiber composite material can be calculated. The thickness of the test block used to measure the speed of sound in this paper is 8 mm, the gain during the measurement is set to 28 dB, the probe delay is set to 3.2 us, and the endpoint of the range is set to 17 mm, the reference 1 is set to 8 mm, and the reference 2 is set to 16 mm; the carbon fiber is finally measured. The speed of sound for the composite is 2981 mps. Figure 13: Fatigue test system. (**a**) Schematic diagram of the fatigue test system. (**b**) Site diagram of the fatigue test system. * TCG calibration  
TCG calibration is also called time gain calibration. Its purpose is to correct the gain of the reflected echoes at different depths of the test piece to make them reach the same amplitude so that defects of the same size at different depths can be successfully detected. In this paper, flat-bottomed specimens with different depths of flat-bottomed holes are used for TCG calibration. The thickness of the specimens is 8 mm. Three flat-bottomed holes with a diameter of 3 mm are drilled on the bottom of the flat-bottomed specimen. The depths of the three holes are 1 mm, 2 mm, and 4 mm respectively. The type of TCG calibration is the back surface echo of different depths, the number of TCG points is set to 3, and the amplitude tolerance is set to 10%. During the calibration process, use A-scan to record the emission wave of each flat-bottomed hole in turn, and ensure that for each TCG point the echo amplitude error is within 10%. After TCG calibration is completed, the reflected waves of defects with the same amplitude but located in different thickness directions of the material will be displayed as echoes of the same amplitude through A-scan. * Sensitivity calibration  
The purpose of sensitivity calibration is to perform gain correction on the reflected echoes of defects with the same depth and the same size but different sound paths, so that they can reach the same amplitude (generally 80% of the wave height), to avoid the angle sensitivity error. The specimen for sensitivity calibration in this paper is the same as the specimen for TCG calibration.