JPS5841341A - Detection of crack - Google Patents

Abstract

PURPOSE:To detect the location of a crack and its length with high precision and easiness by providing a wiring of many lead wires at the periphery of a member where crack is liable to occur and measuring the potential differences in the lead wires and judging by comparing those differences. CONSTITUTION:In order to measure a potential V0 that becomes a reference two measurement terminals 4 are provided with a distance l between them at a part where a crack is not likely to occur, and many measurement terminals 5 are provided with a distance l among them at a part where the crack is liable to occur. A DC current is made to flow in an entire construction member 1 from a DC current source 3 through current supply terminals 2. When a crack 13 is formed in the member 1, a potential difference V that is measured by a minute current voltmeter 14 becomes larger than the potential difference V0. The potential difference V is affected by the measurement terminal distance l and the current supply terinal distance l0, but if l0 becomes large, the effect disapeears. The accuracy of measurement of the length of a crack a/W becomes hiher as the ratio of the potential difference V/V0 becomes larger. Because many wires are required as V/V0 becomes large, the l is made 1-2 times the width W of a plate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for detecting a crack generated in a structural member, and particularly to a method suitable for accurately measuring the position and length of a crack.

The measurement principle of crack length detection using the DC potential method is that the electric field is disturbed by the presence of discontinuities in the conductor, and the measured potential difference depends on the thickness of the sample to be measured and the discontinuity. Depends on the shape of the continuous part. Therefore, the potential difference V with respect to the crack length a or the crack length a/W made dimensionless by the plate width W, or the potential difference ■ when there is no crack. If a master curve of the potential difference ratio ■/■o is created based on , the crack length can be measured by measuring the potential difference.

This relationship between V/Vo and a/W has nothing to do with the material or thickness of the member, except for special materials such as 5US304.
It depends on the shape of the member, the shape of the crack, the current input position and the potential difference measurement position.

In the conventional crack length detection method using the DC potential method, it is assumed that the crack is located in the center between the measurement terminals, and it is also assumed that the crack exists on the surface where the terminals are provided. Therefore, if the crack is not located in the center between the terminals, there are drawbacks such as the inability to accurately measure the crack length or the inability to determine whether or not the crack is on the surface where the terminals are provided.

An object of the present invention is to provide a crack detection method that can accurately detect the position and length of a crack that has occurred in a structural member.

As a result of measuring the potential difference by changing the crack position between the terminals for crack measurement, the inventors found that as the crack approaches the terminal from the center between the terminals, the potential difference increases, and conversely, the potential difference between the adjacent terminals increases. It was found that the potential difference between terminals decreases, and that if the crack is on the opposite side to the surface where the terminal is provided, the potential difference between the terminals facing the crack increases, and the potential difference between adjacent terminals also increases. The location of the crack is determined by comparing the potential difference between the terminals, determining whether the crack is on the same surface as the terminal, and then measuring the length of the crack with precision. It is characterized by the fact that it can be obtained.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an apparatus for detecting cracks in structural members. A direct current is passed from a direct current power supply 3 through a power supply terminal 2 to the entire structural member 1 including a portion where a crack is likely to occur and a portion where there is no risk of cracking. Reference potential difference V. In order to measure the
will be established. A large number of measurement terminals 5 are provided at equal intervals t in areas where cracks are likely to occur. Here, an example of measuring potential differences at three locations was shown. ■Potential difference between each measurement terminal. .

V,,V2. V3 is scanner 6 and computer 11
is sequentially switched in accordance with the instructions of
0 and is digitized and input to the computer 11. The computer 11 calculates the measured potential difference V. ,V,
, V2. Perform V3 mutual calculation to find the crack occurrence position and crack length a/W and display them on the CFLT12 screen.

Figure 2 shows the measurement system of the DC potential method.

When a crack 13 is formed in the structural member 1, the potential difference ■ measured by the microvoltmeter 14 is the potential difference V when there is no crack. Become bigger. The measured potential difference ■ is the distance t between the measurement terminals.
and the distance t between the power supply terminals. be influenced by. When to becomes large to a certain extent, it no longer affects the potential difference V. The measurement accuracy of the crack length a/W is better as the potential difference ratio V/V o is larger, but the smaller t is, the larger V/V is. If t is made small, the area around the member will be full of wiring, so it must be set to an appropriate length. t is preferably 1 to 2 times the plate width W.

FIG. 3 shows what happens to the potential difference (2) measured when a crack changes its position when the distance Rt between measurement terminals in copper is set to 45 mm. The vertical axis is the potential difference V1
The horizontal axis is the distance from the center between the terminals to the crack. When the crack is located near the center between the terminals, the potential difference V is constant, but it suddenly increases as it approaches the terminals (L = 22.5 mm). For this reason, when the measured potential difference V is used as is and the crack length is evaluated based on the master curve when the crack is located at the center, the crack length will be overestimated. If a crack appears between the measurement terminals, that is, if a crack enters between adjacent terminals when a large number of terminals are provided, the potential difference (■) decreases to 5 characteristics, and the potential difference (■) when there is no crack.

The value becomes even lower. Here, the potential difference V when the crack length a=oran. is 435μV. At this time, as shown in Figure 3, the potential difference between adjacent terminals is the same as when there is no crack if the crack is near the center between the adjacent terminals. be equivalent to. As L increases and the crack approaches, the potential difference V becomes V. It becomes smaller. When L further increases and exceeds t/2, that is, when a crack enters between the terminals, it becomes sharper, and as it approaches the center, it decreases a little and becomes constant. From this, the potential differences between the two sets of terminals near the crack are related to each other. When the crack moves away from the center between one terminal and approaches the terminal, the potential difference between that terminal increases, and conversely, the potential difference between one terminal decreases. The trends of increase and decrease in potential difference are very similar. Then, add the potential difference (■1) between the terminals with the crack and the potential difference (■2) between the adjacent terminals that are closer to the crack to get the potential difference (■) between the distant terminals without the crack. When pulled, it becomes almost constant regardless of the position of the crack, as shown in Figure 4. Therefore, if (V + 10V2 VO), the crack length can be detected with high accuracy.

In some cases, it is not known which side of the component the crack will originate from. Figure 5 shows measurement terminals (
Potential difference ratio V/v when measured with a crack at the center between the terminals.

This is the relationship between fissure length and fissure length. The potential difference ratio on the side with the crack is larger. Therefore, simply determining the potential difference ratio cannot accurately evaluate the crack length if it is unclear whether the crack is on the terminal side or not. In order to accurately determine the crack position and accurately determine the crack length, measurement terminals must be provided on both sides of the member where the crack occurs, as shown in FIG. Based on the measured potential difference, it is determined that there is a crack in the one with the larger potential difference between the pair of opposing terminals, and as described above, the crack is determined to be present in the terminal V, , V2. The crack length is evaluated from Vo.

Even if the crack is on the opposite side of the measurement terminal, the measured potential difference differs depending on the location of the crack. FIG. 7 shows the relationship between the potential difference V and the crack position when the distance t between the measurement terminals is 45 mm. The potential difference has a maximum value when the crack is in the center between the terminals and decreases as it moves away from the center. On the contrary, it increases at the adjacent terminal. Therefore, in the same way as in the previous case, the potential difference V between terminals with a crack, the potential difference V2 between adjacent terminals, and the potential difference V between distant terminals without a crack. from(
v, 10V2 VO) is almost constant regardless of the crack position, as shown in FIG.

There is an interesting point here. In FIG. 3, when a crack freezes on a surface with measurement terminals, the potential difference V between the terminals with the crack is V. The potential difference V2 between adjacent terminals is ■. decrease more. On the other hand, in FIG. 7, when the crack is on the surface opposite to the surface on which the measurement terminal is provided, the potential difference V between the terminals facing the crack is ■. The potential difference v2 between adjacent terminals also increases to V. It is about becoming bigger. Therefore, as shown in Fig. 6, even if measurement terminals are not provided on both sides of the member, the position between the terminals where a crack has occurred and the length of the crack can be detected by comparing the potential difference between the measurement terminals provided on only one side. I can do it. Figure 9 shows the potential difference ratio (Vl+V2 VO)/VO and crack length a/W.
Indicates the relationship between The figure shows a case where the distance between the measurement terminals is t=45 mm. The relationship between (VI+V2 VO)/Vo and a/W depends on the distance t between the measurement terminals and differs depending on whether the crack is on the measurement terminal side or not, so create a master curve for each case. It is necessary to memorize it in the computer. However, if the distance t between the measurement terminals becomes wider than the plate width W, regardless of whether the crack is on the terminal side or not, (Vs + V2 VO) / Vo
The relationship between and a/W seems to be almost the same.

In FIG. 3, the position where the potential difference v1 starts to increase from the constant value when the crack is near the center of the terminals is about 10 points from the center between the terminals, in other words, about 125 degrees from the terminals. Therefore, the potential difference v2 between adjacent terminals is V. If the crack is smaller, the crack will be about 12mm from the terminal. Fzo+
It can be determined that the position is within the range 2, and a more detailed position can be determined from the degree of increase in ■ and the degree of decrease in v2. This also applies to the case where the crack in FIG. 7 is not on the surface where the terminal is provided. The numerical values described here are for the plate width W=20 m, so if the plate widths are different, the plate widths must be corrected.

According to the present invention, two lead wires for supplying direct current and a number of lead wires for measuring potential differences are wired in advance around a structural member where cracks may occur, and these potential differences are wired in advance. By simply measuring and comparing these values, it is possible to easily and accurately detect the location where a crack has occurred and the length of the crack. Therefore, for example, in cases where the field coil end of a turbine generator is covered with a retaining ring and the crack length cannot be measured visually from the outside, or when the AE
It is effective in easily detecting cracks when there is not enough space to install an ultrasonic device.

[Brief explanation of the drawing]

Figure 1 is a system diagram of the crack detection device for structural members;
The figure shows a measurement system using the DC button 7-terminal method and the four-terminal method. Figures 3 and 4 are relationship diagrams between potential difference and crack position. Figure 5 is a relationship diagram between potential difference ratio and crack length. Figure 6 is a structural member diagram. 7 and 8 are diagrams showing the relationship between potential difference and crack position, and FIG. 9 is a diagram showing the relationship between potential difference ratio and crack length. ■... Structural member, 2... Power supply terminal, 3... DC power supply, 4° 5... Measurement terminal, 6... Scanner, 7...
...Low pass filter, 8...Preamplifier, 9...
- Amplifier, 10... AD converter, 11... Computer, 12... CTtT. Figure 3 OIO2030 clause 9 y'l itic L (mytc)0
20 Hōro 4ttL gap a) umpire 60 Fig. 7 yl I'F-I L (mm, Nsa 4fL tL (mm) ′@9I21 Sabukuro χtsuW

Claims (1)

[Claims] 1. Applying a direct current to a structural member where there is a risk of cracking, and using a plurality of terminals for measuring the potential difference around the cracking area and in areas where there is no risk of cracking. In the method of measuring the crack length by comparing the value of the potential difference between the terminals provided, the potential difference between the terminals provided in the part where there is no risk of the occurrence of the crack is used as the reference potential difference, There is no crack between the terminals where the potential difference between terminals provided at the periphery is not different from the reference potential difference, and there is a crack between the terminals where the potential difference is larger than the reference potential difference, and there is a crack from the center. It is determined that a crack exists near the measurement terminal where the value of the reference potential difference has decreased, and the reference potential difference and
A crack detection method characterized by detecting a crack length and a crack occurrence position from a potential difference between terminals with a crack and a potential difference between the terminals that is lower than the reference potential difference. 2. In claim 1, terminals for measuring potential difference are provided at equal intervals facing each other on both sides of the member where cracks may occur, and if the potential difference between the facing terminals is equal, the terminals It is determined that there is no crack between the terminals, and that a crack exists between the terminals whose potential difference is larger than the value of the reference potential difference, and that the crack exists on the side where the potential difference between the opposing terminals is larger, A crack detection method characterized by detecting the crack length and crack occurrence position from the potential difference between each terminal. 3. In claim 1, if a crack exists between the terminals where the value of the reference potential difference is the largest, and the potential difference between the adjacent measurement terminals is smaller than the reference potential difference, then the crack exists. exists on the surface where the terminal is provided, and if it is thick, it is determined that the crack exists on the surface opposite to the surface where the terminal is provided, and the reference potential difference and the potential difference between the terminals with the crack, A crack detection method characterized in that a crack length is detected from a potential difference between terminals that has changed from the reference potential difference.