JPH076936B2 - Defect inspection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a crack detection technique for detecting the shape of a crack generated in a metal structural member, and in particular, the shape of a surface crack on the inner surface of a pipe from the outer surface of the pipe. The present invention relates to a device suitable for accurate detection online.

[Prior art]

As a conventional crack detection method, there is an ultrasonic deep scratch method. There are various ultrasonic deep wound methods, such as an edge peak echo method, an aperture synthesis method, and a holography method. Although each has its own characteristics, there are some cases in which an especially important echo from the crack tip cannot be obtained in crack detection, and in that case, the shape of the crack cannot be determined. In addition, since the heat insulating material is wound around the pipe during operation, the probe cannot be scanned, and therefore cannot be detected. The crack shape detection by the potential method related to the present invention is described in
58-215545 There is a crack detector (Mitsubishi Heavy Industries), but it is not possible to detect the shape because it can qualitatively judge the presence of cracks.

[Problems to be solved by the invention]

The above-mentioned conventional technique has a physical problem that a reflection echo from the vicinity of the crack tip cannot be obtained in the case of the ultrasonic method, or the probe cannot be installed during operation, and in the case of the potential method, the probe is not surrounded by the crack. It was not possible to detect the crack shape with high accuracy because the influence of the turbulence of the peculiar electric field that occurs and the potential drop around the power supply terminal was not accurately grasped.

An object of the present invention is to provide a defect inspection method that uses the potential method to accurately detect the position and shape of a defect generated on the inner surface of a pipe.

[Means for solving problems]

The above-mentioned object is to supply a direct current to a surface of a pipe by a pair of power supply terminal pairs separated from each other, measure a potential difference between the power supply terminal pairs by one or a plurality of potential difference measurement terminal pairs, In the method for detecting a defect from the above, terminals having a power supply terminal and a potential difference measurement terminal are arranged in a matrix on the surface of the pipe, and a set of terminal rows arranged in the axial direction of the pipe and 180 degrees apart from this. A direct current between the pair of terminal rows facing the terminal row, and measuring the potential difference between adjacent terminals arranged in the circumferential direction between the terminals supplying the direct current, and the distribution of the potential difference. To detect the position and shape of the defect in the pipe.

[Action]

According to the present invention, among the terminals arranged in a matrix on the surface of the pipe, one set of terminals arranged in the axial direction of the pipe and one set of terminals facing the terminal row 180 degrees apart from this By supplying a direct current between the column and the terminal, which is arranged between the terminals for supplying the direct current and measuring the potential difference between adjacent terminals in the circumferential direction, a non-uniform electric field due to the potential drop around the power supply terminal is generated. Since the influence can be reduced, the position and shape of the defect can be accurately detected under the uniform electric field.

Example of Invention

An embodiment of the present invention will be described below. FIG. 1 shows an embodiment of a pipe crack monitoring device of the present invention, and FIG. 2 is a system diagram of a control / measurement / arithmetic system of the pipe crack monitoring device. 3 is a computer, 4 is an external storage device such as a hard disk for storing data and programs,
2 is a CRT. Computer 3 is Interface 5
Controls the measurement device via the or GP-IB interface 6 or imports and processes measured values and outputs the results. In FIG. 1, terminals 20 for supplying a DC current and for measuring a potential difference are attached by spot welding or the like at equal intervals both in the axial direction and in the circumferential direction of the pipe 1, particularly around the welded portion 19. Terminal 20 is usually made of SUS304, which is an oxidation resistant material, or
Use a fine wire such as SUS316 or Ni. Terminal 20 is the first
Although not shown in the figure, it is guided to the outside of the heat insulating material through a passage provided inside the heat insulating material, and is wound on one cable 21 and connected to the measuring device. In this case, terminal 20 is pipe 1
Since the terminals and the terminals must be insulated from each other, it is necessary to pass them through a short insulator tube or to cover the surface of the terminals with an insulator. The terminals 20 may be bundled on the outside of the heat insulating material and stored in one cable, but since the temperature is low on the outside of the heat insulating material, it is better to connect with a normal multicore cable. The terminals 20 are all power supply terminal switching multiplexers 9 and two potential difference measurement terminal switching multiplexers.
It is connected to three multiplexers 10 and 11.

The DC currents supplied from the plurality of stabilized DC power supplies 7 are switched in polarity by the current polarity converter 8 controlled by the computer 3 through the interface 5 and supplied to the multiplexer 9, and further currents are supplied. The supply destination is distributed and the current is supplied to the specific terminal 20. The potential difference between a large number of terminals 20 is measured by switching the terminals to be measured by one or two multiplexers 10 and 11 and connecting to the minute potentiometer 12. The measured potential difference is transferred to the computer 3 via the GP-IB interface 6.
Transferred to. The computer 3 determines the shape of the crack from the potential difference distribution in the axial direction and the circumferential direction of the pipe by the method described later. Here, the multiplexers 9, 10, 11 and the minute potentiometer 12 are controlled by the computer 3 via the GP-IB interface 6 or the interface 5.

FIG. 3 is a development view of the arrangement of the terminals 20 on the outer surface of the pipe,
FIG. 4 shows an axial cross section. Generally, defects in atomic plants and chemical plants are due to stress corrosion cracking and corrosion fatigue. Stress corrosion cracking occurs where there is tensile residual stress near the weld, and corrosion fatigue occurs in the root of the weld metal where the shape is discontinuous in addition to residual stress. Therefore, the arrangement of the terminal 20 is near the welded portion. Although stress corrosion cracking occurs in the circumferential direction of the heat-affected zone of welding, it may occasionally occur inclining to the circumferential direction. In the defect shape detection by the potential method of the present invention, it is necessary to form an electric field perpendicular to the defect and measure the potential difference distribution across the defect. However, when fixing the terminals online and measuring the potential difference distribution, it is not possible to predict in which direction the crack will occur, so the terminal 20 can be arranged to measure the potential difference distribution in both the circumferential and axial directions. Must be One of the methods is the terminal arrangement shown in FIGS. Only the positions of the terminals are shown in FIG. As described above, cracks are generated in the vicinity of the heat-affected zone of welding. Therefore, the terminals 20 are arranged at equal intervals in the axial direction so as to cover this area. Similarly, the terminals 20 are arranged at equal intervals over the entire circumference in the circumferential direction. That is, the terminals 20 are arranged in a matrix. At this time, in the axial arrangement, the terminals at both ends are used only as power supply terminals, and therefore, in order to make the electric field uniform, it is better to install the terminals at a distance equal to or more than the plate thickness of the pipe from the adjacent terminals. In Fig. 4, the terminal is also on the weld metal 19.
Although 20 is arranged, it does not have to be arranged here because a crack does not occur in the weld metal 19. Further, in the case of piping of a nuclear power plant, even if a crack is found by ultrasonic flaw detection during a periodic inspection, if the crack is small, it may be continuously used without being repaired. The crack growth is predicted by the fracture mechanics method, and the crack does not grow significantly.However, the crack is predicted to be more safety in the case of monitoring the crack by this method. It suffices to dispose the terminal 20 only around the crack.

The flow chart for measuring the potential difference distribution is shown in FIG. First, by controlling the multiplexer 9, a direct current is supplied to the terminals dedicated to power supply at both ends of the pipe in the axial direction to form an electric field in the axial direction of the pipe. Next is the measurement of the potential difference between a large number of terminals. First, the potential difference between the terminals adjacent in the axial direction is measured by switching the terminals by the multiplexers 10 and 11. Next, the potential difference between the terminals adjacent to each other in the circumferential direction is measured. When the potential difference between the axial direction and the circumferential direction is measured once, the polarity of the direct current is switched by the current polarity conversion device 8,
The potential difference between the axial direction and the circumferential direction is measured again. Next, the power supply multiplexer 9 is switched to form an electric field in the circumferential direction. For example, FIG. 6 shows a terminal arrangement in one cross section in the circumferential direction. First, a direct current is supplied from the terminals A and C facing 180 degrees. The potential drop is significant around the power supply terminal, and the electric field is not uniform, so there is no point in measuring the potential difference. Therefore, the potential difference between the terminals 13 14 15 16 17 18 shown in FIG. 6 is measured. Next, in order to measure the potential difference near both terminals A and C, a DC current is supplied from terminals B and D 90 degrees apart from the terminals A and C, and the potential difference between the terminals 11 12 19 20 is measured. taking measurement. Also in this case, the polarity of the direct current to be supplied is changed, and when the positive current is passed and the negative current is passed, the amplitude of two measurements is evaluated. By switching the power supply terminals in this way, it is possible to measure the potential difference distribution in the entire circumferential direction in a uniform electric field. The measured potential difference is transferred to the computer 3 through the GP-IB interface 6 and processed as data. The crack shape is measured based on the potential difference distribution measurement result, the judged crack shape is displayed on the CRT screen of the computer 3, and the printer is made to output the result and a hard copy of the crack shape.

The reason why the current is passed in the two directions of the axial direction and the circumferential direction is as described above, but the details will be described below. Now, if the crack enters parallel to the axial direction of the pipe, the electric field is not disturbed by the crack because the electric field is in the axial direction even if a current is passed in the axial direction, so it is measured. The potential difference distribution is exactly the same as when there is no crack, and it is determined that there is no crack. However, when an electric current is applied in the circumferential direction to an axial crack in such a pipe, the circumferential electric field is greatly disturbed by the crack and a potential difference distribution is generated. The size of can be determined. If the cracks are inclined with respect to both the axial direction and the circumferential direction of the pipe, it is possible to determine the shape including the inclination from the potential difference distribution measured by passing current from both directions. is there.

Now, when a current is flowing in the axial direction, if there is no crack in the pipe, the measured potential difference is that the potential difference between the terminals in the axial direction is constant, and the potential difference between the terminals in the circumferential direction is zero. is there. When a crack is generated in the circumferential direction, the current flows around the tip of the crack, so the current density becomes higher on the outside of the pipe, and the potential difference between the terminals in the axial direction that sandwiches the crack is where there is no crack. The value becomes larger than the potential difference between the terminals.
At the same time, a potential distribution is generated in the direction parallel to the crack, so that the potential difference between the terminals in the circumferential direction becomes larger than zero. Therefore, it is determined that there is a crack between the terminals whose potential difference ratio is greater than 1.0, using the potential difference where there is no crack as the reference potential difference, and the location and shape of the crack are determined from the potential difference distribution. It is possible to judge. Also, it was determined that there were cracks in parallel between the terminals where the potential difference in the circumferential direction was greater than zero,
By comparison with the potential difference between the adjacent terminals in the axial direction, if the potential difference between the terminals in the circumferential direction on the upstream side is large, it is determined that there is a crack on the downstream side, and if the potential difference between the terminals on the downstream side is large, It is judged that there is a crack on the upstream side. However, if the crack is tilted with respect to the axial direction, it is determined that the crack is present at the position connecting the centers of the terminals where the potential difference ratio is greater than 1. FIG. 7 shows a method of determining the crack position. In FIG. 7, the mark ○ indicates the position of the terminal 20, and the solid line indicates the crack position. When the potential difference distribution is measured by applying an electric field in the axial direction, the potential difference between the terminals that sandwich the crack becomes the largest. Since it is unclear in which position between the terminals the crack exists, if it were in the center, it would be the position indicated by the □ mark. Next, when an electric field is applied in the circumferential direction, it is determined that it is at the position marked with ⋄. The dashed line shows the result of connecting both the □ and ◇ marks. Although two examples are shown in FIG. 7, in both cases, the actual crack position and the crack position determined from the potential difference distribution are substantially the same.

The method for determining the crack shape from the potential difference distribution along the crack is shown below. The flow chart of the surface crack shape determination method is shown in FIG. A general-purpose large-scale computer was used in advance to analyze the electric field for cracks with various aspect ratios, for example, a / c = 1.0,0.5,0.25,0.1, and the potential difference distribution in the direction perpendicular to the crack surface on the surface opposite to the crack. Is stored in the storage device of the computer 3 or the external storage device 4. As an example of the potential difference distribution to be stored, FIG. 9 shows the potential difference distribution for each crack depth with an aspect ratio a / c = 0.5. Fig. 8 is obtained by analyzing the electric field by FEM in the case where there is a crack at the center of a flat plate with a plate thickness t = 20 mm. The crack depth a / t normalized by the plate thickness t is 0,0.125,0.25,0.375,0.5, at the deepest point in the center of the crack.
0.625 and 0.75. When there is no crack (a / t = 0), the potential difference is proportional to the distance z from the crack. On the other hand, when there is a crack, the potential difference gradually increases as the distance from the crack increases, and the potential difference increment is almost constant when the distance increases to some extent. These potential difference distributions are approximated to the nth order and stored in the computer 3 or the external storage device 4. In determining the crack shape, the surface crack length 2c * and the maximum potential difference ratio V / V ₀ max are determined from the potential difference distribution around the crack measured first. As an example, Fig. 10 shows the potential difference ratio distribution around the crack obtained by analyzing the electric field by FEM. The aspect ratio of the crack is a / c = 0.25, and the maximum crack depth is a = 12.5mm (a / t = 0.62)
5). The potential difference is almost constant where there is no crack, and the potential difference ratio distribution is shown with the potential difference at such a location as the reference potential difference. The potential difference increases at the cracked portion, and the potential difference distribution in this portion is approximated to the nth order. From the approximate curve, the maximum potential difference ratio V / V ₀ max corresponding to the deepest point of the crack is determined. In the case of FIG. 10, V / V ₀ max = 1.30
was gotten. In the potential difference distribution on the surface opposite to the crack, it is difficult to identify the tip of the crack because the potential difference gradually increases near the tip of the crack. As a result of investigating the relationship between the potential difference distribution and the crack tip position for cracks with various aspect ratios, it was found that the crack tip was located at about 0.15 of the peak of the maximum potential difference ratio V / V ₀ max. In FIG. 10, V / V ₀ max = 1.30, so V / V ₀ = 1 + 0.3 × 0.15 = 1.
When the crack length 2c * on the surface is obtained from the intersection of 045 and the fourth-order approximation curve, 2c = 110 m is obtained.

Next, from the potential difference distribution shown in FIG.
In order to create the relationship between the potential difference ratio V / V ₀ for a / c crack and the crack depth a / t, the relationship between the potential difference ratio V / V ₀ and the aspect ratio a / c is created. In this case, the plate thickness t in the electric field analysis by FEM
Since a flat plate of 20 mm is analyzed, the relationship between the potential difference ratio V / V ₀ and the aspect ratio a / c at the measurement position d * corresponding to the distance d between the measurement terminals must be created. Therefore, d * = d × 20 / t corrected by the plate thickness t * of the member to be measured.
Figure 11 shows the relationship between the potential difference ratio V / V ₀ and the aspect ratio a / c by finding the potential difference for each crack depth a / t at the * position. The relationship between the potential difference ratio V / V ₀ and the aspect ratio a / c is approximated to the nth order for each crack depth a / t and stored in the storage device 4 of the computer 3. Next, the potential difference ratio V / V ₀ and the aspect ratio a /
Using the relationship of c, the master curve of the relationship between the aspect ratio a / c = 0.5 and the potential difference ratio V / V _{0 and the} crack depth a / t is created as shown in FIG. Also in this case, the relationship between the crack depth a / t and the potential difference ratio V / V ₀ is approximated by the nth order, for example, the fifth order. The crack depth a * is obtained by substituting the maximum potential difference ratio V / V ₀ max obtained by fourth-order approximation of the potential difference distribution into this master curve.
Next, the surface crack length corrected for plate thickness 2c * (= 2c × 20 / t *)
The aspect ratio a * / c * of the crack is calculated by using and is compared with the aspect ratio a / c of the master curve. If no numbers match, again the potential difference ratio V / V ₀ and the aspect ratio a = aspect ratio a / c using the relationship / c a * / c potential difference ratio V / V ₀ when relative *裂深of a / A master curve of the relationship of t is created, and the maximum potential difference ratio V / V ₀ max is substituted to obtain the crack depth a *. This operation is repeated until they match, for example, until the difference between a / c and a * / c * becomes 0.01 or less. Ratio of potential difference to aspect ratio when matched V / V _{0 and} crack depth a /
The shape of the entire crack is determined by substituting the potential difference ratio at each measurement position into the master curve of the relationship of t. In this case, the potential difference ratio at each measurement position may be substituted for, or the potential difference ratio distribution approximated to the nth order may be substituted for the potential difference ratio.

Fig. 10 shows the results of concrete calculation of the potential difference distribution around the crack shown in Fig. 10. Since the plate thickness is t * = 20.0 mm and the distance between the measuring terminals is d = 20 mm, d * = d × 20 / t * =
The electric potential difference for each crack depth of each aspect ratio at the position of 20 mm is obtained. However, since the potential difference is measured with the crack at the center of the measuring terminal, z = d * / 2 = 10
Calculate the potential difference at the position of mm and calculate the potential difference ratio V / V ₀ as shown in Fig. 11.
And the aspect ratio a / c. Using these relationships, as shown in Fig. 12, a master curve of the relationship between the aspect ratio a / c = 0.5 and the potential difference ratio V / V _{0 and the} crack depth a / t is created. The maximum potential difference ratio V / V ₀ max = 1.30 on this curve
Substituting for, a * / t = 0.76 and a * = 15.2 mm is obtained. Aspect ratio of crack is a * / c from 2c * = 110mm
* = 15.2 / 55 = 0.276. So, if you next create a master curve for a / c = 0.276 and find the crack depth,
A * = 12.86 mm is obtained, and a * / c * = 0.234. Again, when a master curve for a / c = 0.234 is created and the crack depth is obtained, a * = 12.4 mm is obtained, and a * / c * = 0.
When the crack depth is calculated by creating a master curve for a / c = 0.225, a * = 12.3 mm is obtained, and a * / c * = 0.224, and the aspect ratios are almost the same. Fig. 13 shows the correspondence between the surface crack shape obtained in this way and the crack shape used in the analysis. The accuracy near the crack tip on the surface is somewhat poor, but except for that, the agreement is very good.

However, the above method can be applied when the crack is perpendicular to the electric field, and cannot be applied as it is to the crack that is inclined as shown in FIG. In such a case, the coordinate points of the points □ and ◇ in Fig. 7 are linearly approximated by the method of least squares to obtain the angle with respect to the vertical direction, and the crack length 2c * is obtained from the coordinates at both ends. At this time, if the angle between the normal direction of the crack and the electric field direction is Θ, the potential difference ratio V / V ₀ ′ is smaller than the potential difference ratio V / V ₀ when the crack is perpendicular to the electric field. Therefore, the first approximation is V /
V ₀ ′ = V / V ₀ · cos Θ. Therefore, when the crack shape is obtained by the above method, it is necessary to correct the measured potential difference ratio V / V ₀ ′ with Θ and evaluate it with V / V ₀ = V / V ₀ ′ / cos Θ. . However, if Θ exceeds 45 °, the accuracy will deteriorate, so it is better to use the measured value for the electric field where Θ is smaller than 45 °.

〔The invention's effect〕

As described above, according to the defect inspection method of the present invention, it is possible to reduce the influence of the non-uniform electric field due to the potential drop around the power supply terminal, so that the position and shape of the defect can be accurately detected under the uniform electric field. Therefore, the reliability of the inspection is improved and the soundness of the pipe is enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an external view of a crack monitoring apparatus for pipes according to the present invention, FIG. 2 is a system diagram of the crack monitoring apparatus of FIG. 1, and FIGS. FIG. 1 is a diagram showing the arrangement of terminals of the apparatus, FIG. 5 is a flow chart of the potential difference distribution measurement according to the present invention, and FIG. 6 is the position of the power supply terminal and the measurement terminal in the circumferential potential difference distribution measurement according to the present invention. Fig. 7 is a diagram showing a crack position determining method according to the present invention, Fig. 8 is a flow chart of the crack shape determining method according to the present invention, and Fig. 9 is a flow chart obtained by the FEM of the present invention. Graph showing an example of the potential difference distribution on the surface of the crack member opposite to the crack, Fig. 10 of the present invention
Potential difference distribution graph around the crack on the surface opposite to the crack of the crack member obtained by FEM, FIG. 11 is a graph showing an example of the relationship between the potential difference ratio and the aspect ratio according to the present invention,
FIG. 12 is a graph showing the relationship between the potential difference ratio and the crack depth according to the present invention, and FIG. 13 is a diagram showing a comparison between the crack shape used in the analysis according to the present invention and the determined crack shape. 1 ... Piping, 2 ... CRT, 3 ... Computer, 4 ...
External storage device, 5 ... interface, 6 ... GP-IB
Interface, 7 ... DC power supply, 8 ... Current polarity converter, 9 ... Multiplexer for switching power supply terminals,
10,11 …… Mux for switching measurement terminals, 12…
… Micro potentiometer, 19 …… Weld metal, 20 …… Terminal, 21 ……
cable.

Claims (4)

[Claims] 1. A direct current is supplied to a surface of a pipe by a pair of power supply terminal pairs spaced apart from each other, and a potential difference is measured between the power supply terminal pairs by one or a plurality of potential difference measurement terminal pairs, In the method of detecting a defect from a potential difference, a terminal which also serves as a power supply terminal and a potential difference measurement terminal is arranged in a matrix on the surface of the pipe, and a set of terminal rows arranged in the axial direction of the pipe is provided.
A direct current is supplied to a set of terminal rows facing the terminal row at a distance of 0 degree, and a potential difference between terminals adjacent to each other arranged in the circumferential direction arranged between the terminals for supplying the direct current is measured, A defect inspection method comprising detecting the position and shape of the defect in the pipe from the distribution of the potential difference. 2. The method according to claim 1, wherein the position of the terminal for supplying the DC current is moved in the circumferential direction, and the potential difference is measured for all terminals arranged in a matrix. Characteristic defect inspection method. 3. The method according to claim 1 or 2, wherein, in addition to the measurement of the potential difference, a direct current is applied between circumferentially arranged terminal rows arranged at both ends in the axial direction. A defect inspection method comprising: supplying and measuring the potential difference between terminals arranged between the terminal rows and adjacent in the axial direction, and detecting the position and shape of the defect also by using the distribution of the potential difference. 4. The method according to any one of claims 1 to 3, wherein the distribution of the potential difference measured by reversing the direction of the supplied DC current is also used to detect defects in the pipe. A defect inspection method characterized by detecting the position and shape of the defect.