# A Basic Experimental Study on Analysis of Leak Signal and Monitoring Method for Water Supply Pipe
## 5 Discussion  
Finite element analysis was performed to confirm the application of the developed smart bolt. Regardless of the presence or absence of holes in the bolt, the maximum equivalent stress used was lower than the yield tensile strength, and the equivalent stress was approximately 30% in the case of the model with holes, compared to the intact model. In the tensile strength test, the average was 170.56 kN, which exceeded the minimum requirement of 163 kN in the 10.9 strength class of M16 bolts. It was possible to determine whether the bolt was loosened using the initial (reference) value and the measured value at the specified torque that determined the bolting action.  
Generally, when the ring frequency is related to the coupled vibration in 65 A, PVC (Poly Vinyl Chloride) has 7559 Hz, steel pipe has 23,111 Hz, and the cast-iron pipe has 23,801 Hz [12; 14]. Figure 8 shows the audible range of the leak detection equipment and the frequency range of the leak sound.  
The leakage sound frequency range was between about 500 Hz and 2000 Hz, and the frequency range in cast iron pipes was about 400 Hz to 5000 Hz. An acoustic emission sensor is used for a relatively short range, and it is difficult to process noise similar to leakage sound [12; 16]. For long-distance detection, it is common to measure the vibration of a pipe that causes a coupled vibration using an acceleration [16].  
Figure 8: Audible range of leak detection equipment and frequency range of leak noise.  
When fluid and piping are coupled, the leakage vibration appears at low frequency, unlike sound wave transmission directly connected to leak detection [24]. Although it is a cast iron pipe that has a relatively high-frequency band, it was set to a lower frequency band because of the pipe with free ends and the rubber packing of the joint.  
The results of the leak detection test with the acceleration sensor are as follows. FFT analyses of cases 1 and 2 detected no leakage signal regardless of whether there was flow in the pipe. When there was leakage from the branch pipe valve (cases 3-6), it was confirmed that the leakage signal of the first mode in the 60-Hz band was detected. When there was a large amount of leakage, even the fourth-mode signal was detected; in leakage from the main pipe (cases 7-10), it was confirmed that the leak signal was continuously detected at 60-Hz intervals after the first detection, and the same result was obtained regardless of the amount of leakage in the main pipe. The frequency response of the main pipe leakage was about 100 times greater than that of the branch pipe.  
When the main pipe leak occurred, the leak signal was larger than that of the branch pipe valve, and even higher-order mode responses were detected at regular intervals. The cause seemed to be a leakage in the valve joint and rubber packing: As the joint of the valve and the rubber packing directly act as damping, it seemed that the leakage signal measurement was clearly measured at low frequencies where the effect of damping was small, rather than at high frequencies where the effect of damping was large. In addition, when the main pipe leaked, we believe that the leak signal in the relatively high-frequency band was measured because the whole pipe was subjected to coupled vibration.  
Through these fundamental experiments, we confirmed the ability of the smart bolt to detect leak signals of the water supply valve. However, more verification through various future field experiments is needed such as with various pipe types and ground boundary conditions. Furthermore, additional development of the transmit/receive module and the power supply for the smart bolt is required for effective monitoring.