JPS63101742A - Defect inspection - Google Patents

Abstract

PURPOSE:To inspect the position and shape of a cracking, by using measuring terminals arranged in matrix to measure bidirectional potential difference distribution in the perimeter of an inspecting section for an object to be inspected. CONSTITUTION:Terminals 20 concurrently serving for supply of DC current and for measurement of potential difference are mounted by spot welding or the like at equal intervals axially and circumferentially in the perimeter of a weld part 19 of a piping 1. The terminals 20 are grouped into one cable 21 and connected to a measuring device 100. DC current supplied from a DC power source 7 is switched in the polarity with a current polarity converter 8 controlled with a computer 3 through an interface 5 and supplied to a specific terminal 20 through a multiplexer 9. A potential difference between the numerous terminals 20 is measured by a potential difference meter 12 with the selection of a measuring terminal through multiplexers 10 and 11. The potential difference being measured is transferred to the computer 3 through an interface 6, which judges the shape of a cracking from axial and circumferential potential difference distributions of the piping.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a crack detection technique for detecting the shape of a crack that occurs in a metal structural member, and in particular, the present invention relates to a crack detection technique for detecting the shape of a crack that occurs on a metal structural member. The present invention relates to a device suitable for accurate online detection.

[Prior Art] As a conventional crack detection method, there is an ultrasonic flaw detection method. There are various ultrasonic flaw detection methods, including the edge peak echo method, aperture synthesis method, and holography method.

Each method has its own characteristics, but there are cases in which echoes from the crack tip, which are particularly important for crack detection, cannot be obtained, and in that case, the shape of the crack cannot be determined. In addition, during operation, the probe cannot scan because the insulation material is wrapped around the piping, making detection impossible. 1975-215545 Crack detection device f
i! (Mitsubishi Heavy Industries), but it is only possible to qualitatively determine the presence or absence of a crack, but it is impossible to detect the shape.

[Problem that the invention seeks to solve]

The above conventional techniques have physical problems such as the inability to obtain reflected echoes from near the crack tip in the ultrasonic method, or the inability to set up the probe during operation, and the potential problem in which reflected echoes occur near the crack tip. There was a problem that the crack shape could not be detected because the unique electric field disturbance was not accurately grasped.1 The purpose of the present invention is to detect the shape of surface cracks that occur on the inner surface of the pipe during operation using the potential method. It's about doing.

[Means for solving problems]

The above purpose is to apply a direct current to the surface of a member through a pair of power supply terminals spaced apart from each other, and to measure the potential difference by providing one or more pairs of potential difference measuring terminals between the pair of power supply terminals. In this method of detecting the shape of a defect from the potential difference, terminals that serve both as power supply terminals and potential difference measurement terminals are arranged in a matrix on the surface of a structure where cracks may occur, and the potential difference with the power supply terminals is measured. This is accomplished by switching terminals and using testing methods.

[Effect]

In order to measure the potential difference distribution on the surface of the specimen, terminals that serve both for power supply and potential difference measurement are arranged in a matrix on the outside surface of the pipe, and by switching between the power supply terminal and the potential difference measurement terminal, the axial and local directions of the outside surface of the pipe can be measured. By measuring the potential difference distribution, the surface crack shape can be determined with high accuracy.

[Embodiments of the invention]

An embodiment of the present invention will be described below. FIG. 1 shows an embodiment of the piping monitoring device of the present invention, and FIG. 2 is a system diagram of the control, measurement, and calculation system of the piping 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. The computer 3 controls the measuring device via the interface 5 and the GP-IB interface 6, takes in and processes the measured values, and at each result interval, the terminal 20, which serves both for DC current supply and potential difference measurement, is connected by spot welding or the like. It is installation.

Terminal 20 is typically an oxidation resistant material SUS 30
Use a thin wire such as 4, SUS 316, or Ni. Although the terminals 20 are not shown in FIG. 1, they are led out of the heat insulating material through a passage provided inside the heat insulating material, and are connected together to a single cable 21 to a measuring device. In this case, terminal 2
0 must be insulated from the piping 1 and the terminals during the wiring, so it is necessary to pass it through a short insulator tube, cover the terminal surface with an insulating material, or bundle the terminals 20 outside of the insulation material. It is also possible to fit it into one cable, but
Since the temperature outside the insulation material is low, it is better to connect it with a normal multi-core cable. Note that all terminals 20 are connected to the multiplexer 9 for switching the power supply terminal and the multiplexers 10 and 11 for switching the two potential difference measurement terminals. The current is connected to three multiplexers, the polarity of which is switched by a current polarity converter 8 controlled by the computer 3 via the interface 5, and then supplied to the multiplexer 9. Current is distributed and supplied to specific terminals 20 . The potential difference between the many terminals 20 is measured by switching the terminals to be measured using one or two multiplexers 10 and 11 and connecting them to the minute potentiometer 12.

The measured potential difference is transferred to the computer 3 via the GP-IB zein interface 6. The computer 3 determines the shape of the crack from the potential difference distribution in one circumferential direction in the axial direction of the pipe using a method described later. Here, multiplexer 9,
10, 11 and the micropotentiometer 12 are controlled by the computer 3 via the GP-IB Zein interface 6 or interface 5.

FIG. 3 shows a developed view of the arrangement of the terminals 20 on the outer surface of the pipe, and FIG. 4 shows an axial cross section.Generally, defects in atomic couplants and chemical plants are caused by stress corrosion cracking and corrosion fatigue. Stress corrosion cracking occurs where tensile residual stress exists near the weld, and corrosion fatigue occurs at the root of the weld metal, which is discontinuous in shape in addition to residual stress.

Therefore, the terminal 20 is placed near the welding part. Stress corrosion cracking occurs in the circumferential direction of the weld heat-affected zone, but in rare cases it may occur at an angle to the circumferential direction.6 When detecting defect shapes using the potential method of the present invention, the electric field should preferably be perpendicular to the defect. However, the potential difference distribution must be measured across the defect.
When measuring the potential difference distribution online with the terminal fixed, it is impossible to predict in which direction a crack will occur, so the terminal 20 should be arranged in such a way that the potential difference distribution can be measured in both the circumferential and axial directions. One method for achieving this is the terminal arrangement shown in FIGS. 3 and 4. In FIG. 3, only the positions of the terminals are shown. As described above, since cracks occur near the weld heat affected zone, the terminals 20 are arranged at equal intervals in the axial direction so as to cover this region.

Similarly, the terminals 20 are arranged at equal intervals in the circumferential direction over the entire circumference. In other words, are the terminals 20 arranged in a matrix? Place it in At this time, in the axial direction, the terminals at both ends are used only as power supply terminals.

In order to make measurements with a uniform electric field, it is better to install the terminal with a distance equal to or greater than the thickness of the pipe from adjacent terminals.

In FIG. 4, the terminal 20 is also placed on the weld metal 19, but since no cracks will occur in the weld metal 19,
In addition, in the case of atomic couplant piping, even if cracks are discovered by ultrasonic testing during periodic inspections, if the cracks are small, they may not be repaired and may continue to be used. Sometimes. Crack growth is predicted using a fracture mechanics method, and cracks do not grow significantly. It is sufficient to arrange the terminals 20 only around the fissure.

The flowchart of potential difference distribution measurement is shown in Figure 5. First, by controlling the multiplexer 9, a DC current is supplied to the terminals dedicated to power supply at both ends of the pipe in the axial direction, thereby forming an electric field in the axial direction of the pipe. .

Next is the measurement of potential differences between multiple terminals. First, the potential difference between terminals adjacent in the axial direction is determined by the multiplexer 10.
, 11 to measure the potential difference between terminals adjacent in the circumferential direction. Once the potential difference between the axial direction and the circumferential direction is measured, the polarity of the DC current is switched by the current polarity converter IT8, and the potential difference between the axial direction and the circumferential direction is measured again.
9 to form an electric field in the circumferential direction. For example, the 6th
The figure shows the terminal arrangement in one section in the circumferential direction, and DC current is first supplied from terminals A and C, which face each other 180 degrees. There is a significant potential drop 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 is as shown in Figure 6, for example, ■■■■■■1
3 14 Is 1617 0th order A that measures the potential difference between the 18 terminals.

To measure the potential difference near both terminals of A.

Direct current is supplied from the terminals B and D, which are 90 degrees apart from the terminal C, and the potential difference between the terminals at ■■■@11 12 19 20 is measured. In this case as well, the polarity of the supplied DC current is changed, and the amplitude is evaluated by measuring the amplitude twice: when a current of 10 is passed and when a current of - is passed. 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 data processed. The shape of the crack is determined based on the potential difference distribution measurement result, and the determined shape of the crack is displayed on the CRT screen of the computer 3, and the printer is caused to output the result and a hard copy of the formed shape.

The reason for passing current in two directions, the axial direction and the circumferential direction, is as mentioned above, but the details are as follows.If the crack is parallel to the axial direction of the pipe, then the current is passed in the axial direction. However, since the electric field is in the axial direction, the electric field is not disturbed by the crack, so the measured potential difference distribution will be exactly the same as when there is no crack, and it will be determined that there is no crack. .

However, when a current is passed in the circumferential direction through such an axial crack in the pipe, the circumferential electric field is greatly disturbed by the crack, resulting in a potential difference distribution. If a crack occurs at an angle to both the axial and circumferential directions of the pipe, the size of the crack can be determined from the potential difference distribution measured by passing current from both directions. It is possible to determine the shape including the inclination.

Now, when current is flowing in the axial direction, if no cracks occur in the piping, 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. be. When a crack occurs in the circumferential direction, the current flows around the tip of the crack, so the current density is higher on the outside of the pipe, and the potential difference between the terminals in the axial direction that sandwiches the crack is lower than that in the area where there is no crack. It takes on a value larger than the potential difference between the terminals.

At the same time, a potential distribution occurs in the direction parallel to the crack, so 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 where the 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 by the potential difference distribution. Judging from this is a matter of wisdom. In addition, it is determined that there is a nearly parallel crack between the terminals where the potential difference in the circumferential direction is greater than zero, and a comparison with the potential difference between the adjacent terminals in the axial direction shows that the potential difference between the upstream terminals in the circumferential direction is large. Ba,
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 determined that there is a crack on the upstream side. However, if the crack is inclined with respect to the axial direction, it is determined that the crack exists at the point where the center of the terminals where the potential difference ratio is greater than 1 is connected. Fig. 7 shows a method for determining the crack position. In Fig. 7, the 0 mark indicates the position of the terminal 20,
The solid line indicates the crack position. When measuring the potential difference distribution by applying an electric field in the axial direction, the potential difference between the terminals that sandwich the crack becomes the largest. It is unknown where the crack is between the terminals, so if it is in the center, it will be at the position indicated by the 0 mark.If an electric field is applied in the circumferential direction, it will be the same as the ◇ mark. The broken line shows the result of connecting both the 00 mark and the ◇ mark, which are determined to be in position. Two examples are shown in FIG. 7, and in both cases, the actual crack position and the crack position determined from the potential difference distribution approximately match.

The method for determining the forged shape from the potential difference distribution along the crack is shown below.1 The flowchart of the method for determining the surface crack shape is shown in Figure 8. Various aspect ratios, e.g., a/o = l, OeO,5,0,
25,0,1 cracks, and store the potential difference distribution in the direction perpendicular to the cracked surface on the surface opposite to the crack in the storage device of the computer 3 or the external storage device 4; As an example of the potential difference distribution to be memorized, the potential difference distribution for each crack depth with an aspect ratio a/c = 0.5 is shown in Figure 9. Figure 8 shows a case where a crack is in the center of a flat plate with a thickness t = 20-. This was obtained by analyzing the electric field using FEM for a certain case. The crack depth a/t standardized by plate thickness t is 0 at the deepest point at the center of the crack, 0.125°0.
．． 25, 0.375, 0.5, 0.625 and 0
．． It is 75. When there is no crack (a/t=O), the potential difference is 1-a, and the potential difference gradually increases as the distance from the crack increases, and after a certain distance, the potential difference increment remains almost constant. These potential difference distributions are approximated to the nth order and stored in the computer 3 or external storage device 4. When determining the forging shape, the surface crack length 2c and the maximum potential difference are determined from the first measured potential difference distribution around the crack. Find the ratio V/Voma x.

- As an example, Figure 10 shows the potential difference ratio distribution around the crack obtained by analyzing the electric field using FEM.The aspect ratio of the crack is a/o = 0, 25, and the maximum crack depth is a = 1
2.5 (a/t = 0.625), where there is no crack, the potential difference is almost constant, and the potential difference ratio distribution is shown using the potential difference at such a location as the reference potential difference. The potential difference is large where there is a crack,
The potential difference distribution in this part is approximated to the nth order. From the approximate curve, the maximum potential difference ratio V/V om corresponding to the deepest point of the crack
Determine ax. In the case of Figure 10, V/Voma
x=1,30 was obtained. In the potential difference distribution on the surface opposite to the crack, the potential difference increases gradually near the crack tip, so it is difficult to identify the crack tip.0 Potential difference distribution and crack tip for cracks with various aspect ratios As a result of examining the relationship with position, it was found that the crack tip was located around 0.15 of the peak of the maximum potential difference ratio VZVo*aX. In Figure 10, V / V oll
Note that aX = 1.30, V / V o =
If the crack length 2c at the surface is determined from the intersection of 1 + 0, 3

Next, from the potential difference distribution shown in FIG. 9, the potential difference ratio V/V for cracks with various aspect ratios a/o is determined.

In order to create the relationship between the crack depth a/t, the relationship between the potential difference ratio V/Vo and the aspect ratio a/c is created. In this case, in the electric field analysis using FEM, the plate thickness t=2
Since we are analyzing a flat plate of 0 m, the potential difference ratio V / at the measurement position rIld corresponding to the distance d between the measurement terminals is
A relationship between Vo and the aspect ratio a/c must be created. Therefore, d = d x 2Q / corrected by the plate thickness t of the member to be measured is calculated as each crack depth a / at the skewer position. Find the potential difference with respect to t and calculate the potential difference ratio V /
The relationship between Vo and the aspect ratio a/O is created as shown in FIG. The relationship between the potential difference ratio V/Vo and the aspect ratio a/O is determined by approximating each crack depth a/to the nth order and storing it in the memory bag M4 of the computer 3.
Using the relationship between the phase difference ratio V/Vo and the aspect ratio a/C, the master curve of the relationship between the potential difference ratio V/Vo and the crack depth a/t for the aspect ratio a/o = 0.5 is calculated as the 12th Create as shown.

In the second case, the potential difference ratio V/V o and the crack depth a/
The relationship of t is approximated to the nth order, for example, to the fifth order. By substituting the maximum potential difference ratio V/Vomax obtained by fourth-order approximation of the potential difference distribution into this master curve, the crack depth Bsa is determined. The surface crack length 2c (==
2cX20/l*), the aspect ratio of the crack is a fist/C
Find the skewer and compare it with the aspect ratio a/o of the master curve. If the two do not match, calculate the aspect ratio a/o again using the relationship between the potential difference ratio V/Vo and the aspect ratio a/o.

'= 6 Skewer/o Potential difference ratio for fist V/Vo
A master curve of the relationship between crack depth a/t is created, and the maximum potential difference ratio V/Vomax is substituted to find the crack depth a. Repeat this process until the two match, for example, until the difference between i/o and a*10 is 0.01 or less. When they match, the potential difference ratio to the aspect ratio V/Vo and the 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 t. In this case, the potential difference ratio at each measurement position may be substituted for the potential difference ratio, or an nth-order approximated potential difference ratio distribution may be substituted.

Figure 10 shows the results of specific calculations of the potential difference distribution around the crack.The plate thickness is t=20.0m, and the distance between the measurement terminals is d=20m, so d=dX
Find the potential difference for each crack depth for each aspect ratio at the position where 20/ is fist = 20-. However, since the potential difference is measured with the crack located at the center of the measurement terminal,
Find the potential difference at the position of 2=da/2=10-, and calculate the potential difference ratio V/Vo and aspect ratio a/C as shown in Figure 11.
Create a relationship. Using these relationships, a master curve of the relationship between the potential difference ratio V/Vo and the crack depth a/t for the aspect ratio a/a=0.5 is created as shown in FIG. This curve shows the maximum potential difference ratio V/Vo
By substituting max==1.30, am/l+=0.7
6, and a-=15.2 cabinets are obtained++ 2o*=
From +110mm, the 7spect ratio of the crack is a・/C fist==
15.2155=0.276. Therefore, next a
/ c = 0, 276 Create a master curve and find the crack depth, a""12.86mm
is obtained, and a fist/c fist=0.234. Again, a/
When we create a master curve for c=0.234 and find the crack depth, we get a=12.4, a=0.225, and a10=0.225.
By creating a master curve for and finding the crack depth, a *; 12.3 ■ is obtained, and a umbrella/C fist = 0.2
24, and the aspect ratios are almost the same. Figure 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 0 surface is somewhat poor, but apart from that, they match very well. ing.

This can be applied when there is a straight S, but it cannot be applied directly to a crack that is tilted as shown in Figure 7. In such a case, the points marked 0 and ◇ in Figure 7 should be applied. The coordinate points are linearly approximated by the method of least squares to find the angle with respect to the vertical direction, and the crack length 2o is found from the coordinates at both ends.

At this time, if the angle between the normal direction of the crack and the direction of the electric field is e, the potential difference ratio V/Vo' is smaller than the potential difference ratio V/Vo when the crack is perpendicular to the electric field. , as a first approximation, V/Vo' = V/Vo-
It becomes cos θ. Therefore, when determining the crack shape using the above method, the measured potential difference ratio V/V o'
Corrected by e, V/Vo ==V/Vo'/cosθ
It is necessary to evaluate the However, θ is 45'
If the value exceeds 45', the accuracy deteriorates, so it is better to make a determination using the measured value for the electric field where θ is smaller than 45'.

〔Effect of the invention〕

As described above, according to the piping crack monitoring device of the present invention, cracks can be detected in one direction and in the other direction around the inspection part on the surface of the object to be inspected. The position and shape of cracks can be detected by measuring the potential difference distribution in both directions of the test object using measurement terminals arranged in a matrix at equal intervals in both directions, allowing accurate inspection of the health of the test object. is possible.

[Brief explanation of the drawing]

Fig. 1 is an external view of a piping crack monitoring device according to the present invention, Fig. 2 is a system diagram of the crack monitoring device of Fig. 1, and Figs. 3 and 4 are terminal diagrams of the device of Fig. 1. 5 is a flowchart of potential difference distribution measurement according to the present invention, FIG. 6 is a diagram showing the positions of the power supply terminal and measurement terminal in circumferential potential difference distribution measurement according to the present invention, and FIG. Figure 8 is a flowchart of the method for determining the crack shape according to the present invention, and Figure 9 is a diagram showing the method for determining the crack position according to the present invention. A graph showing an example of the potential difference distribution on the surface. FIG. 10 is a graph showing the potential difference distribution around the crack on the surface opposite to the crack of the cracked member obtained by the FEM of the present invention. FIG. 11 is a graph showing an example of the potential difference distribution on the surface. A graph diagram showing an example of the relationship between potential difference ratio and aspect ratio, 12th r! A is a graph showing the relationship between potential difference ratio and 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. DESCRIPTION OF SYMBOLS 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 110, 11...Multiplexer for switching measurement terminals 112...Minimum potentiometer, 19...Welding metal, 20...Terminal, 21... Cable.

Claims (1)

[Claims] 1. Direct current is applied to the surface of the member through a pair of power supply terminals spaced apart from each other, and one or more pairs of potential difference measuring terminals are provided between the pair of power supply terminals to measure the potential difference. measure,
In this method of detecting the shape of a defect from the potential difference, terminals that serve both as power supply terminals and potential difference measurement terminals are arranged in a matrix on the surface of a structure where cracks may occur, and the potential difference with the power supply terminals is measured. A defect inspection method characterized by detecting the location and shape of a crack by switching terminals and measuring the potential difference distribution. 2. In the item described in claim 1, terminals that serve as both power supply terminals and measurement terminals are arranged in a matrix on the outer surface of the pipe so as to be parallel to the axial direction and the circumferential direction, and are arranged at both ends in the axial direction. The position and shape of circumferential cracks are detected by applying a direct current between the circumferential terminal rows and measuring the potential difference distribution between axially adjacent terminals placed in the middle. A defect inspection method characterized by: 3. In the device described in claim 1, a direct current is provided between one set of terminal rows arranged in the axial direction and one set of terminal rows facing the terminal row and separated by 180 degrees with respect to the terminal row. A defect inspection method characterized by detecting the position and shape of an axial crack by applying a voltage and measuring the potential difference distribution between circumferentially adjacent terminals. 4. In the item described in claim 3, a direct current is provided between one set of terminal rows arranged in the axial direction and one set of terminal rows facing the terminal row and separated by 180 degrees with respect to the terminal row. is applied, and the potential difference distribution between the terminals adjacent in the circumferential direction is measured, excluding the potential difference between the terminal that supplies the DC current and the terminal next to it, and the row of terminals that supply the DC current are 90 degrees from each other. Applying a direct current between two sets of distant terminal rows,
By measuring the potential difference distribution between circumferentially adjacent terminals, excluding the potential difference between the terminal that supplies direct current and its neighboring terminal, we can measure the potential difference distribution in the circumferential direction in a uniform electric field. A defect inspection method characterized by detecting the position and shape of a crack. 5. In the device described in claims 1 to 4, a direct current is applied from the terminal rows at both ends in the axial direction to measure the potential difference between the terminals adjacent in the axial direction, and the potential difference between the terminal rows in the circumferential direction is measured. A DC current is applied between the terminals to measure the potential difference between circumferentially adjacent terminals, and the shape of a crack that is inclined to the axial or circumferential direction is detected from the potential difference distribution in the axial and circumferential directions. Defect inspection method.