## 2 Experimental Study on Fatigue Crack Propagation of High-Strength Steel Wire
### Specimen and Load Setting
Hydraulic oil is also subject to friction resistance when passing through local obstacles. These will cause the loss of oil in the oil circuit and the reduction of oil pressure, ultimately consuming the energy of the whole system. Because of the resistance in the oil circuit, there is an error between the load acting on steel wire and that provided by FTM. A force sensor is used to monitor the actual load acting on steel wire and adjust it to the value set by FTM. Considering that the wire is circular, a hollow shaft type annular force sensor is selected to monitor the fatigue loads, as shown in Figure 6. The working principle of this sensor is to convert the force received by the steel  
Figure 4: Fatigue testing machine—INSTRON 8802. Figure 5: Diagram of synchronous pulsation fatigue loading device. wire into voltage signal. A data acquisition device, named National Instruments BNC-2111, is used to collect the voltage signal output from the sensor as shown in Figure 7. The force sensor is connected to the BNC-2111 by transmission data line. The BNC-2111 is connected to the computer. The computer can record and read the tension in each wire during fatigue loading. The Laboratory Virtual Instrument Engineering Workbench (LabVIEW) software is used to calibrate the collected signal, so as to realize the conversion of voltage signal to force signal and perform the monitoring and recording of fatigue load. Figure 8 demonstrates the process and working principle of data collection by LabVIEW software, through which force loads acting on each steel wire can be collected synchronously. LabVIEW uses the graphical editing language G to write the program in block diagram form. Figure 9 shows the fatigue loads recorded by LabVIEW software, that is, the real-time forces applied to multiple steel wires and one of the wires respectively. When crack propagation occurs, the plastic zone near crack tip increases, and the deformation of steel wire under maximum fatigue load increases compared with that without crack propagation, resulting in the change of the peak of loading waveform. After many tests, it is found that the peak of the loading waveform will change from single peak to short platform when the crack expands. During the loading process, pay attention to the fatigue load waveform and the change of crest in the user interface of LabVIEW, as shown in Figure 10. When the crest changes from a single peak to a short platform, the change of the crest can be measured by the software measurement tool, the values of \(X_{m}\) and \(X_{n}\) are recorded, and the values of \(\Delta X=X_{n}-X_{m}\) are defined. When it changes, the fatigue load is downloaded and the loading numbers are recorded. Figure 6: Hollow shaft type annular force sensor. Figure 7: Acquisition device—National Instruments BNC-2111. After the load drops, the peak of the fatigue loading waveform is restored to a single peak. Under the action of the new fatigue load, when the crack grows again, the peak will have the same change as before, that is, from a single peak to a short platform. Similarly, the loading numbers will be recorded and the load will be lifted. This is repeated until the steel wire breaks. The change of the crest after each load drop or load rise means that the fatigue crack has been expanded. The fatigue crack propagation process was analyzed by the recorded loading numbers and the \"beach-like pattern\" left by the steel wire section. The test was conducted at room temperature, and the loading waveform was sinusoidal wave. Here, \(\sigma_{max}\) is defined as the maximum stress, \(\sigma_{min}\) is the minimum stress, \(\sigma_{a}\) is the stress range, which can be expressed as \(\sigma_{a}=\sigma_{max}-\sigma_{min}\), \(R\) is the stress ratio, which can be expressed as \(R=\sigma_{min}/\sigma_{max}\), \(f\) is the loading frequency. The parameters of the original fatigue load are set as follows: \(\sigma_{max}=643\) MPa, \(\sigma_{min}=283\) MPa, \(\sigma_{a}=360\) MPa, \(f=0.5\) Hz, \(R=0.44\).