JPS61111460A - Method for detecting disconnected strand of parallel cables for bridge - Google Patents

Abstract

PURPOSE:To detect a disconnected strand by fitting a converter to the end of a strand to be tested of parallel cables and injecting an ultrasonic pulse having transmittable frequency corresponding to the diameter of the strand to be tested into the strand to be tested within a specific frequency range to detect the reflected pulse from a disconnected position. CONSTITUTION:An ultrasonic pulse adjusted at 20KHzx-5MHz testing frequency is outputted from an ultrasonic tester 5 and transmitted to the converter 4. The converter 4 converts the electric signal into mechanical vibration to inject the ultrasonic pulse into the strand to be tested. If the strand to be tested is disconnected, the ultrasonic pulse is reflected at the disconnected point, transmitted again to the converter 4 through the strand and converted into an electric signal. The electric signal is sent to the tester 5, amplified and removed at its noise by a filter circuit and the noise- removed signal is displayed on a display device 6. The display device 6 displays both the transmitted pulse and the reflected pulse, so that the disconnected position can be found out from the time interval of both the pulses and the transmission speed. Thus, respective strands are successively tested and the existence of disconnection of all the strand of the cables and the disconnected positions can be precisely grasped.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting broken strands in parallel cables used in suspension bridges such as cable-stayed bridges and long span bridges.

[Conventional technology]

Parallel wire cables used in suspension bridges such as cable-stayed bridges and long span bridges have high breaking strength, fatigue strength, and a large modulus of longitudinal elasticity, so they have excellent performance as tensile structural members. In order to maintain the excellent performance of such a parallel wire cable for a long period of time, the outer layer of the cable is coated with plastic or the like, so that rainwater does not directly shine on the inner strands of the cable. However, during long-term use, the coating deteriorates, cracks occur, and rainwater infiltrates into the cable, causing stress corrosion cracking in the strands, which may grow and eventually break.

In addition, over a long period of time, excessive repeated tension is exerted on the cable due to the repeated passing of heavy vehicles and strong winds from typhoons, which can cause fatigue rupture of some of the strands in the cable. Conceivable.

Generally, a socket is attached to both ends of a parallel wire cable, and within the socket, the strands are spread into a broom shape and fixed with a casting material. For this reason, the strands close to the outer layer of the cable are strongly bent outward at the entrance of the socket, so stress due to bending is added in addition to the tensile stress, and strand breakage is concentrated in this part. There is a possibility that it may occur. Therefore, it is very important to non-destructively detect such broken wires to ensure the safety of bridges.

A method for detecting wire breakage is AE (^causticEmis), which detects the sound emitted when a wire breaks.
sion) method, a method used in mines, etc., in which a wire rope is partially saturated with direct current magnetization using a magnetizer, and leakage magnetic flux from the broken part of the wire is detected with a detection coil or magnetic sensor.An electromagnetic flaw detection method is used for steel plates, etc. One possibility is to use ultrasonic flaw detection, which is a method for non-destructively detecting internal defects.

However, these methods have the following problems.

[Problem that the invention seeks to solve]

For example, the AE method is a constant monitoring method, and it is impossible to detect if the device accidentally breaks down when a wire breaks. Furthermore, it is less reliable as it may malfunction due to vibrations or earthquakes caused by passing vehicles. Also, for electromagnetic flaw detection, the wire rope must be sent at a constant speed, or the flaw detection device itself must be moved at a constant speed! However, the
This method is effective when the diameter of the wire rope is at most a few tens of degrees* or less, but in the case of parallel wire cables, the diameter is often more than 100 degrees, and the outer layer is coated with an anticorrosive layer with a thickness of about 1ON. Since the detection coil and magnetic sensor cannot be brought close to the broken part of the strand, the detection sensitivity decreases. Therefore, if a wire near the surface of the cable is broken, it can be detected, but in the case of a break in an internal wire, almost no leakage magnetic flux comes out to the surface of the cable, so it is difficult to detect the break. . Also,
Disconnection in the socket and approximately 500m from the base of the socket
A break in the wire cannot be detected due to physical limitations.

In the case of ultrasonic flaw detection, the flaw detection range is limited to several meters even in the case of steel plates, and there are no examples of its application to flaw detection ranges of tens or hundreds of meters.

[Means for solving problems]

The inventors investigated various methods to solve the above-mentioned problems, and found that the manner in which ultrasonic pulses propagate inside a thin round bar is similar to that in which ultrasonic pulses propagate through a steel plate. In contrast, we found that there are sound waves with frequencies that are easy to propagate and sound waves with frequencies that are difficult to propagate, and that the propagation speed changes depending on the frequency. It was also found that the frequencies that are easy to propagate depend on the diameter and elastic modulus of the round bar, and that there are multiple frequencies in the frequency range from IKHz order to I M II z order.

To explain this concretely, by solving the wave equation (11, (2)) of a longitudinal wave propagating in a round bar expressed in cylindrical coordinates, we can calculate the sound wave with a frequency fMHz propagating in a steel round bar with a diameter dm. When determining the propagation velocity, the product of f and d at which the propagation velocity reaches its maximum value is as follows.

f-d(MH2-N) value at maximum propagation velocity: 1.4
．． 4.9. 8.4. 11.9. 14.7. 1
B, 2°22.4. 26.6. 30.2.・・・
Therefore, if we take a round bar with a diameter of 711 m as an example, the frequency f at which the propagation velocity becomes maximum is as follows.

f (MHz): 0.2.0.7.1.2.1
．． 7.2.1゜2.6.3.2.3.8.4.3.・・・
...Sound waves with a frequency at which the propagation velocity takes a maximum value are ultrasonic waves that are easy to propagate, and sound waves that deviate from this frequency do not propagate in the round bar.

Generally, as the diameter of a round bar becomes smaller, the frequencies that are easier to propagate shift to higher frequencies, and conversely, as the diameter increases, the frequencies that are easier to propagate to lower frequencies. Therefore, by using ultrasonic pulses with a frequency that is easy to propagate according to the diameter of the strands of a parallel cable, it is possible to detect breaks in the strands in the cable over a range of tens or even hundreds of meters. be.

The gist of the present invention is to attach a converter to the strand end of a parallel bridge cable, and to convert the frequency from 20 KHz to 5 MHz.
Within the range of The purpose is to detect broken wires. That is,
If the strand to be inspected is not broken, the injected ultrasonic pulse will propagate through the strand while being diffused and attenuated, so no reflected pulse will be detected, but if the strand to be inspected breaks on the way. If the broken wire is broken, a reflected pulse will return from the broken point, and this is received by the transducer, amplified by the ultrasonic testing device, and then displayed on the display device in order to detect the broken wire. It is something to do.

As described above, according to the present invention, it is possible to reliably detect a broken strand in a parallel wire cable used in a suspension bridge at any time while the cable is under tension without having to remove it. At the same time, it is also possible to determine the fracture position. Therefore, by detecting the breakage status of each strand using the present invention, it is possible to always ensure the safety of the suspension bridge by taking measures such as replacing the cable before the number of broken strands reaches the design allowable standard. can.

Hereinafter, the present invention will be explained in detail based on the figures.

! FIG. 1 shows an apparatus for carrying out the method of the present invention. 1 is a group of inner strands of a parallel wire cable, and 2 is an anti-corrosion layer coated with plastic or the like to prevent the strands from being exposed to wind and rain. 3A and 3B are sockets for the full length of the cable, and inside the socket, the bundle of strands is untied and spread out like a broom, and the space between each strand is filled with sewing material 7 to fit into the socket. It is fixed.

Further, 4 is a converter attached to one end of the strand to be inspected. Reference numeral 5 denotes an ultrasonic test device, which emits ultrasonic pulses adjusted to a predetermined test frequency and transmits them to the transducer 4. The transducer 4 converts this electrical signal into mechanical vibration, and injects an ultrasonic pulse into the strand to be inspected. When the wire is broken, the ultrasonic pulse is reflected at the break point, and this reflected pulse travels through the wire and reaches the transducer 4 again, where it is converted into an electrical signal and sent to the ultrasonic testing device 5. After being sent, the signal is amplified, noise is removed by a filter circuit, and then sent to the display device 6 for display. Since the display device 6 displays both the first transmitted pulse and the reflected pulse from the rupture position, the rupture position can be determined from the time interval between the two pulses and the propagation velocity of the ultrasonic wave.

In this way, by sequentially inspecting each strand,
It is possible to reliably grasp the cable breakage status, including whether or not all the wires in the cable are broken and the location of the breakage.

〔Example〕

Next, the effects of the present invention will be illustrated more specifically by examples.

In the cable with the structure shown in Figure 1, the diameter of the strands is 7
The method of the present invention was applied to a parallel cable with 500 strands and a total cable length of 500 m. The Young's modulus E of these wires is 2. I x 10'kg 7wm2
．． The Poisson's ratio μ is 0.29, and the results of finding the ultrasonic frequency that is easy to propagate and its propagation speed from the wave equation are the first
Shown in the table.

Table 1 All strands of this cable were inspected using ultrasonic pulses with a frequency of 2.6 MHz, and as a result, the display shown in FIG. 2(a) was obtained for one strand. In the figure, T is the transmitted pulse signal, B+ is the reflected pulse from the break point, and the time interval t between the T and B1 pulses was 12 m5.

Next, the same wire was inspected using ultrasonic pulses with a frequency of 0.2 MHz, and as a result, the display shown in FIG. 2(b) was obtained. Here, T is the transmitted pulse, B1 is the first reflected pulse from the break point, B2 is the second reflected pulse, and B
3. B4 indicates the third and fourth reflected pulses, respectively. Also, the time interval between T and 81 pulses, B1 and B2,
The time intervals between B2 and B3 and between B3 and B4 were all the same, 12 m5. The distance from the inspection end of this wire to the breaking point is Lm, and the propagation velocity of the ultrasonic pulse is 500.
Assuming 0m/s, the fracture position was calculated using equation (3), and the result was L
= 30m.

As a result of disassembling the parallel cable and investigating the breakage of the strands, it was confirmed that only the above-mentioned strands were broken at a position exactly 30 m from the inspection end. In addition, the above frequency 0.2
When inspecting a broken strand using an MHz ultrasonic pulse, up to the fourth reflected echo was obtained, indicating that using this frequency it is possible to detect a broken strand up to a length of 120 m from the end of the strand. It is.

〔Effect of the invention〕

As described above, according to the method of the present invention, it is possible to use bridges!
During actual operation of parallel cable, frequency 20KH
A broken strand in a parallel cable can be reliably detected by injecting an ultrasonic pulse with a predetermined frequency within the range of 5 MHz to 5 MHz from the end of the strand and detecting the reflected echo from the breaking point. In other words, not only will breakage occur within the socket, but also breakage of the outer wire of the parallel cable (even if it is an I11 wire),
It is possible to detect broken wires and their fracture positions over a range of 100 m or more from the end of the wire, and it is possible to reliably detect wire breaks at the base of sockets, where wire breaks are likely to occur. This is an extremely reliable method to ensure safety.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is an example of a record of detection of a broken wire by the apparatus shown in FIG. 1: Wire group, 2: Anticorrosion layer, 3A, 3B: Socket,
4: Transducer, 5: Ultrasonic testing device, 6: Display device,
7: Casting material.

Claims (1)

[Claims] An ultrasonic pulse with a predetermined frequency that is easy to propagate depending on the wire diameter within the frequency range of 20 KHz to 5 MHz is injected into the wire from a transducer installed at the wire end of the parallel wire cable, and the ultrasonic pulse is transmitted from the wire breakage point. A method for detecting broken strands of parallel cables for bridges, which is characterized by detecting reflected pulses.