# Operation State Evaluation of Miter Gate Based on On-Line Monitoring and Finite Element Analysis
## 4 Signal Analysis and Discussion
### Crack Signal Analysis  
Based on the new cracks found during the gate maintenance in the past years, it was found that the cracks mainly occurred in the 23 rd to 27 th layers of the gate, which have been submerged for a long time. At the same time, according to the stress signal analysis and finite element model calculation, it was found that the stress concentration area during the gate operation was on the gate shaft column and the transverse rear flange plate, which were chosen to install the crack sensors. Therefore, the relevant crack sensors are mainly installed in these five layers, and the discovered cracks are numbered 1-7, as shown in Figure 10. The distributed FBG crack sensors were installed to collect the crack propagation displacement, and three sensors were grouped on one crack. The collected signal waveform is shown in Figure 11, and the peak-peak value of the signal waveform indicates the maximum crack width. Therefore, here, we collected the waveform peak-peak of the different location sensors. Four sets of crack sensor data collected during the navigation cycle in different sampling times were selected to analyze the fatigue crack degree of the miter gate and the crack propagation displacement variation, which is the peak-peak value of the signal waves, as shown in Table 3.  
Figure 11: Crack signal waveform collected by distributed fiber Bragg grating crack sensor. Figure 10: Schematic diagram of crack sensors layout near the door shaft area. From Table 3, there is one sensor (marked by font-weight) in every group, in which displacement variation is always the biggest in four-time sampling, so that this sensor data can present the crack state. According to Table 3, the sensors LY60, LY64, LY70, LY71, LY98, LY72, and LY89 are selected from every group to analyze the crack state near the door shaft column of the miter gate. The maximum variation of crack propagation displacement at the different positions is shown in Figure 12.  
As seen from Figure 12a, the displacement of every crack propagation fluctuates a little in one year. The data collected the second time are the biggest; possibly, the reason for this is seasonal water level fluctuations. We can find that the crack variation located near the bottom (sensor LY89) and the right of the door shaft column (sensor LY72 and LY98) of the gate is more significant, as shown in Figure 12b. The result is consistent with the location and the degree of actual cracks found in maintenance, also with the maximum stress distribution of the gate obtained by simulation. In Figure 12c, for the No. 1 crack located near the gate shaft column, the sensor (LY60) data waveform on the bottom of the crack has the most significant fluctuation compared to the upper sensors (LY61 and LY62), which indicates the direction of crack propagation from the bottom to the top of the crack. Similarly, the No. 5 crack propagation direction is from the bottom of the crack, and the crack width increases with time cumulatively, as shown in Figure 12d. Comparing the stress distribution at different locations, it can be found that the stress is more significant in the transverse rear flange plate area, and the crack width expansion is fastest.