JPH0364830B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a crack detection method for detecting cracks generated in metal structural members, and is particularly suitable for detecting the shape of surface cracks with high accuracy. Regarding.

[Background of the invention]

As a conventional crack detection method using a potential method, there is a so-called four-terminal method. In this method, a probe having a pair of power supply terminals and a pair of measurement terminals arranged in a row inside the probe scans the surface of a structural member to detect cracks from changes in the potential difference distribution. In determining a crack, the potential difference in a region where no crack is expected to be present is used as a reference potential difference, and a crack is determined to exist where the potential difference is larger than that. Therefore, although the four-terminal method can determine the presence or absence of a crack and the shape of the crack to some extent, it has the disadvantage that the shape of the crack cannot be determined with high accuracy.

[Purpose of the invention]

An object of the present invention is to provide a method that is simple but capable of accurately detecting the shape of defects or surface cracks occurring in structural members using a small computer.

[Summary of the invention]

Using test pieces with surface defects of various aspect ratios, we applied a direct current in a direction perpendicular to the defects and measured the potential distribution or potential difference distribution around the defects. The potential difference at the tip of the defect changed significantly and reached its maximum value at the deepest point of the defect. Although there was a unique relationship between the defect depth and potential difference at each location of the surface defect, the relationship between the two differed depending on the aspect ratio of the defect. However, it was found that when the aspect ratio becomes smaller than 0.25, the relationship between the two tends to become independent of the aspect ratio. Furthermore, the electric field of the member with surface defects was analyzed using the finite element method, and the results were compared with the results measured on the test piece, and it was found that the two coincided well. Therefore, defect elements with several different aspect ratios and depths are created, the potential difference distribution around the defect on the surface of the member is measured, and the potential difference is extracted by extracting elements with aspect ratios that closely correspond to the potential difference distribution. Compare the distributions, and if there is a difference in the potential difference distribution, partially correct the contact position of the element, analyze the electric field, and use the defect shape when they match as the actual defect shape to accurately determine the defect shape. I understand. However, although this method has very high accuracy, it takes time to determine the shape and requires a relatively large computer that can perform calculations using the finite element method. Therefore, in order to determine the shape of cracks formed on the inner surface of stainless steel pipes of various thicknesses due to fatigue from the potential difference measurement results, the potential difference measurement terminal was By creating a master curve of potential difference ratio and crack depth, or aspect ratio and crack depth from the ratio of the gap distance to the plate thickness, the crack shape can be determined with relatively high accuracy. I found out that it can be done.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an equipotential diagram showing the potential distribution near a surface crack when a direct current is applied. This is the result of analysis using the finite element method for a case where a 20 mm thick flat plate has a semicircular crack with a surface thickness of 30 mm and a depth of 15 mm. If we pay attention to the potential distribution on the crack surface, the equipotential lines will sink into the crack surface. The number of equipotential lines that penetrate into the crack surface changes depending on the crack depth. It can also be seen that the potential distribution shows a symmetrical distribution with respect to the crack surface. That is, since the potential shows an inverse distribution across the crack, it is easy to determine the position of the crack. Of course, when measuring the potential difference across a crack, the potential difference becomes larger where there is a crack, so it can be detected.

Next, FIG. 2 shows the results of calculating the potential distribution around the crack. This was obtained for the crack shown in Figure 1, and from the crack 1, 2, 3, 4, 5,
This is the potential difference distribution based on the crack surface at a position 10 mm away. As can be seen from Figure 2, from the crack
It is possible to determine the crack shape to some extent even at a distance of 10 mm. However, accurate detection of the crack shape is difficult. In particular, it is difficult to identify the crack tip on the surface. However, when the measurement position is brought closer to the crack, a singular point appears at the tip of the crack on the surface, making it easier to determine the tip of the crack on the surface. It can also be seen that the potential is proportional to the crack depth. Therefore, the crack shape can be determined by measuring the potential distribution along the crack in the immediate vicinity of the crack and from in front of the crack tip, or by measuring the potential difference across the crack.
However, as a result of a detailed investigation of the relationship between crack depth and potential difference by varying the crack aspect ratio a/c (a: maximum crack depth, 2c: crack length at the surface), we found that the relationship between crack depth and potential difference was The relationships vary depending on the aspect ratio. Therefore, in order to accurately determine the crack shape from the potential difference measurement results, it is necessary to analyze the electric field using the finite element method, compare it with the measured value, and when the two match, calculate the actual crack shape input using the finite element method. It is necessary to have a crack shape of . However, although this method has good accuracy,
Since a fairly large computer capable of applying the finite element method is required to analyze the electric field, this method is suitable when complex crack shapes must be determined with high precision.

FIG. 3 is an external view of the defect detection device according to the present invention. In FIG. 3, the flaw detection head driving device 1 has a structure capable of detecting cracks or defects on the surface of a nearly flat structure. The flaw detection head 20 using the DC potential method has two power supply terminals 5 for supplying DC current and two measurement terminals 10 for measuring potential difference.
There are lines. The flaw detection head 20 is rotatable around an axis (Z axis) perpendicular to the surface by a stepping motor 25, and is equipped with an air cylinder 30 for pressing measurement and power supply terminals against the surface of the member. Furthermore, in order to make the flaw detection head 20 movable on a two-dimensional plane, it is provided with drive mechanisms for an X-axis 51 and a Y-axis 56, and each coordinate axis is driven by stepping motors 52 and 57. The Y-axis 56 is fixed to a side plate 60, and a compressor 6 is attached to the side plate 60.
A suction cup 62 operated by compressed air supplied from 1 is attached, and has the function of fixing the drive device 1 to the surface of the member. Therefore, it is possible to detect not only wall-like defects but also ceiling-like defects. The coordinate axis drive motors 52 and 57 are connected to a drive control device 65, and the drive control device 65 is controlled by a computer 100.

FIG. 4 shows the structure of a flaw detection head 20 for potential difference measurement. The substrate 21 of the flaw detection head 20 is made of a nonconductor such as Bakelite or acrylic. A large number of power supply terminals 5 for supplying direct current are arranged at equal intervals in two rows in parallel, with the terminals facing each other. The two rows of measurement terminals 10 are provided at equal intervals so that their centers are located at the center of the two rows of power supply terminals 5 and are located between adjacent power supply terminals. Further, a direct current 66 is provided independently to each pair of power supply terminals, and a switching device 67 is also provided. The switching device 67 is for canceling the thermoelectromotive force generated between the measurement terminal 10 and the structure by switching the polarity of the direct current applied to the structure at regular intervals. In this case, the potential difference must be measured after the DC current has stabilized, and the best time is just before switching the polarity.

Next, FIG. 5 shows a structure for attaching the power supply terminal 5 and the measurement terminal 10 to the substrate 21. FIG. 5 shows only one row of terminals when the number of terminals is six. It is necessary to press the measurement terminal 1 and the power supply terminal 5 to the structure to the extent that contact resistance does not occur between them, and even if the structure has some unevenness or curvature, they must all be in contact with each other in the same way. Must be. Furthermore, in order to accurately determine the shape of a defect, it is necessary to measure the potential distribution within 1 to 2 mm from the defect, as shown in FIG. Therefore, the tip of the measurement terminal 10 is made conical, a flange is provided behind it, and a coil spring is inserted between the flange and the substrate 21. When the flaw detection head 20 is pressed against the structure, the spring allows the terminal to be evenly attached to the structure. Since it is important that the measurement terminals be pressed against each other and that the distance between the measurement terminals be accurate, the holes drilled in the substrate 21 must be long and finished as guide surfaces.

Below, a method for measuring potential distribution and a method for determining defect shape will be described. In FIG. 1, DC current is equally applied from a plurality of DC power supplies 66 to each of the power supply terminals 5 provided in the flaw detection head 20 via a switching device 67 to form a uniform electric field in the structural member. The potential difference generated between the many pairs of measurement terminals 10 is taken into the micropotentiometer 71 via the scanner 70 and measured, and is input to the computer 100 through the interface 72. A recording device 1 connected to a computer 100 as a distribution
03. The computer 100 determines the crack position from the recorded potential difference distribution, and measures the detailed potential difference distribution around the crack. Next, a potential difference is calculated based on the potential distribution in the direction perpendicular to the crack surface at the center of the crack among the electric fields analyzed for cracks with various aspect ratios by a large computer stored in the electric field storage device 102. Create a relationship between the crack depth at the crack center and the potential difference ratio corresponding to the ratio of the distance between the measurement terminals and the plate thickness of the structural member to be measured, determine the crack depth at the crack center from them, and then determine the overall This determines the crack shape.

Figure 6 shows a flowchart of crack shape determination using the DC potential method. First, using the driving device 1 shown in FIG. 1, the flaw detection head 20 is roughly scanned over the entire area within the driving device to examine the potential distribution. At this time, since the direction in which the crack occurs is roughly determined by the structural member, the direction of the flaw detection head 20 is set by the stepping motor 25 so that a direct current flows orthogonally to the crack surface. If there is a crack, it can be easily detected because a potential difference distribution as shown in FIG. 3 will occur. from cracks
It is fully detectable even at a distance of 10 mm, but shallow cracks may be overlooked. Since it is safe to measure at a distance of 5 mm, it is sufficient to keep the distance between the measurement terminals at 10 mm or less. If you want to improve the measurement accuracy of crack shape, the best distance between the measurement terminals is 2mm, but this will increase the measurement time, so 4mm is the best distance.
It is best to set it to about mm. In actual measurement, the potential difference distribution is measured at the same measurement interval as the measurement terminal, and the approximate location of the crack is determined. As shown in FIG. 3, a potential difference distribution occurs around the crack, so it is determined that the potential difference is near where a potential difference larger than the reference potential when there is no crack is measured. In order to accurately determine the crack shape, the crack must be placed in the center between the measurement terminals, so the measurement head 20 should be placed perpendicular to the crack surface near the position where the potential difference distribution is maximum. Measure the potential distribution by scanning finely in the direction. There is a crack in the center between the measurement terminals when the potential difference is maximum. Next, the potential difference distribution is measured in detail along the crack surface at that location.

Next, based on the potential difference distribution in the direction perpendicular to the crack surface of cracks having various aspect ratios stored in the electric field storage device 102, the potential difference at a position corresponding to the ratio of the distance between the measurement terminals to the plate thickness is determined. The relationship between the ratio and the crack depth is determined by the computer 100. Here, electric fields are determined in advance for cracks with aspect ratios of, for example, 1.0, 0.5, 0.25, and 0.125 using a large computer. From this relationship between the potential difference ratio and the crack depth, the computer 100 determines the relationship between the potential difference ratio and the aspect ratio for each crack depth.
In this case, the relationship between the two should be approximated by n-th order approximation, for example, 5-th order approximation, for the best fitting. Next, using this relationship between the potential difference ratio and aspect ratio for each crack depth, a master curve of the relationship between the crack depth and potential difference for each aspect ratio is determined for each 0.01 aspect ratio, for example, from 0.125 to 1.0. Next, the crack length 2c on the surface is determined from the measured potential difference distribution. The maximum potential difference ratio V/V ₀ max corresponding to the deepest point of the crack is substituted into a master curve with an appropriate aspect ratio a/c to obtain the crack depth a*, and a*/c* is calculated. a*/
Change the master curve until c*=a/c,
The overall crack shape is determined from the potential difference distribution using the master curve when they match. At this time, crack depth a*, crack length at the surface 2c
*Both plate thicknesses must be corrected. The specific method is shown below.

Figures 7, 8, 9, and 10 have aspect ratios a/c of 1.0, 0.5, 0.25, and 0.125, respectively.
This is the potential distribution in the direction perpendicular to the crack surface at the center of the crack, obtained by analyzing the electric field using the finite element method for a 20 mm thick plate material with a surface crack. The depth a/t of the crack standardized by plate thickness is the deepest point at the center of the crack: 0, 0.125, 0.25, 0.375, 0.5, 0.625, and 0.75. If there is no crack, the potential difference increases in proportion to the distance from the crack, but if there is a crack, the electric field around the crack will be disturbed, and if the crack is deep, the potential difference will increase in proportion to the distance from the crack. The electric field is disturbed. Furthermore, the smaller the aspect ratio, the more severe the disturbance of the electric field, and the larger the potential difference measured across the crack. Usually the first
The distance l between the measurement terminals of the potential difference distribution measuring device as shown in the figure is constant. However, the thickness of the member to be measured varies. As shown in Figure 3, the relationship between the potential difference or potential difference ratio and the crack depth varies depending on the measurement position, so it is necessary to determine the relationship between the potential difference ratio and the crack depth corresponding to the measurement position in advance. . However, it requires a lot of effort and cost. Rather than that, Figure 7, Figure 8, Figure 9,
It is more rational to store a basic potential difference distribution as shown in FIG. 10 in the electric field storage device 102, and to use the computer 100 to determine the relationship between the potential difference ratio corresponding to the measurement position and the crack depth each time the potential difference is measured.
From the potential differences at positions corresponding to the measurement positions in Figures 7, 8, 9, and 10, the relationship between the potential difference ratio V/V ₀ and the crack aspect ratio a/c as shown in Figure 11 is determined for each case. Created for crack depth a/t. Next, the relationship between the two is approximated to the nth order, for example, to the fifth order as shown in the following equation.

V/V ₀ =A ₀ +A ₁ a/c+A ₂ a/c ² +A ₃ a/c ³ +A ₄ a/c ⁴ +A ₅ a/c ^5Use this to set the aspect ratio a/c in steps of 0.01. The potential difference with respect to the crack depth is determined, and finally a master curve of the potential difference ratio V/V ₀ and the crack depth a/t for each aspect ratio is created as shown in FIG. In this case as well, the master curve of the potential difference V/V ₀ and the crack depth a/t is approximated to the nth order, for example, to the fifth order as shown in the following equation.

V/V ₀ =B ₀ +B ₁ a/t+B ₂ a/t ² +B ₃ a/t ³ +B ₄ a/t ⁴ +B ₅ a/t ^5Here , the aspect ratio changes from a/c=0.125 to a/c= Since the electric field was analyzed for cracks up to 1.0, a master curve for the potential difference ratio V/V ₀ and the crack depth a/t is created for the range from a/c=0.125 to a/c=1.0. However, the 12th
In the figure, if you draw all the curves for each a/c = 0.01, it will be complicated and difficult to understand, so a/c = 0.125, 0.25, 0.5
I drew only 4 pieces, 1.0 and 1.0. Next is a method for determining the crack shape from the measured potential difference distribution. If the potential difference is measured in the vicinity of a surface crack, a potential difference distribution as shown in Figure 3 will be obtained, and the crack length 2c* on the surface of the member can be understood as a point where the potential difference changes rapidly and can be easily determined. Ru. Regarding the depth of the crack, first, the aspect ratio of the crack is determined using the maximum value of the potential difference that is thought to correspond to the deepest point of the crack. That is, by substituting the maximum value of the potential difference ratio V/V ₀ max into the master curve of the appropriate n-th order approximated potential difference ratio V/V ₀ and crack depth a/t as shown in FIG. fissure depth a*, then a*/
Determine c* and compare it with the aspect ratio a/c of the master curve. If the two do not match, find the deepest crack depth a** again using the master curve of a*/c*, then find a**/c* and compare it with the aspect ratio a/c of the master curve. . This operation is repeated until the two match, and when they match, the crack depth is determined as the deepest crack depth. Then, the shape of the entire crack is determined by substituting the potential difference ratio at each measurement position into the master curve when they match. This will be explained below using a specific example.

FIG. 13 shows the potential difference distribution obtained around the crack after a fatigue test was performed on a 12-inch diameter stainless steel pipe with a slit on the inner surface, and the slit was removed by turning. The horizontal axis is the measurement position x (mm) in the surface direction with the origin at a position thought to correspond to the center of the crack, and the vertical axis is the potential difference V (μV).
Here, V ₀ is the potential difference where there is no crack,
As you can see in Figure 13, where there are no cracks,
V ₀ is almost constant. Where there is a crack, the potential difference increases as in FIG. 3. As in FIG. 3, a singular point appears in the potential difference distribution at the tip of the surface crack, so the surface crack length 2c can be easily determined. Various methods can be considered for determining the surface crack length 2c. Connect the point before the potential difference ratio suddenly rises with the point after it rises with a straight line, and the point where it intersects the potential difference ratio V/V ₀ = 1.02, the point after the potential difference ratio suddenly rises. It is determined by the n-th approximation of the potential difference distribution at several locations and the intersection of this with the reference potential difference V ₀ , or the n-th approximation of the entire potential difference distribution at the position corresponding to the crack, and the intersection of this and the reference potential difference V ₀ . Three methods were adopted. Regardless of the method, the surface crack length can be determined with high accuracy as long as the vicinity of the crack tip is precisely measured. In Figure 13, the potential difference distribution at the position corresponding to the crack is approximated to the fifth order,
As a result of adopting a method of determining the intersection point between this and the reference potential difference V ₀ , 2c = 22.5 mm. Next, the aspect ratio a/c of the crack, in other words, the maximum crack depth is estimated. The point where the potential difference ratio is maximum corresponds to the deepest point of the crack. The potential difference ratio at the deepest point is V/
V ₀ max=38.0/24.75=1.535. Next, a master curve of the relationship between the potential difference ratio and the crack depth with respect to the aspect ratio of 0.01 corresponding to the potential difference measurement position is created. The distance between the measurement terminals is now l = 5mm.
The thickness of the pipe is t=15.7mm. The corresponding position of the master curve obtained by the finite element method is l′=
5/15.7×20=6.4mm. The above-mentioned FIGS. 11 and 12 are master curves obtained from the potential difference at a distance l' between the measurement terminals of 6.4 mm. First, calculate the maximum crack depth using any one of the relationships in this master curve. For example, a/c=
The potential difference ratio V/ at the deepest point on the master curve of 0.8
Substituting V ₀ max=1.535, the crack depth is a*=
It becomes 6.32mm. When a*=6.32 is corrected for plate thickness, a*
=4.99mm, a*/c*=4.99/11.25=0.44
It is. This is different from the master curve's a/c = 0.8, so by substituting V/V ₀ max = 1.535 into the master curve of a/c = 0.44, the crack depth is a** = 5.17 mm, plate thickness Corrected a**=4.08
mm, and a**/c*=0.36. This process was repeated until finally a/c=0.34 and a=3.89mm, converging.

Next, the inner surface of the pipe was further turned using a lathe to reduce the plate thickness to t=14.4 mm, and the potential difference distribution was again measured for the same crack. The result is the first
Shown in Figure 4. In the same manner as in Fig. 13, the surface crack length 2c = 22.5 mm, the maximum potential difference ratio V/V ₀ max =
1.282 is found. The corresponding position of the master curve obtained by the finite element method is l'=5/14.4×20=7.0mm
becomes. From FIGS. 7, 8, 9, and 10, the relationship between the potential difference ratio V/V ₀ and the aspect ratio a/c in FIG. 15 is obtained from the potential difference at l=7.0 mm,
From Figure 15, the potential difference V/V ₀ in steps of a/c = 0.01
The relationship between crack depth a/t is obtained as shown in FIG. The maximum potential difference ratio V/V ₀ max = 1.282 is the first
Substitute into the n-th approximation formula of a/c = 0.8 in Figure 6 and get a*
= 5.05mm, with plate thickness correction a* = 3.64mm, a
*/c*=0.32. Substitute into the n-th approximation formula of a/c=0.32 and get a**=3.88mm, correct it and get a**
=2.79mm, a**/c*=0.25. By substituting into the n-th approximation formula of a/c=0.24, a****=3.71 mm was corrected, a****=2.67 mm, and a****/c*=0.24, which converged. From the crack depth a = 3.89 mm in the first lathe process, the cutting allowance in the second lathe process
Subtracting 15.8-14.4=1.4mm gives a=2.49mm, which agrees well with a=2.67mm.

Next, the crack depth from the deepest point of the crack to the crack tip on the surface is calculated by using the relationship between the potential difference ratio and the crack depth when finally determining the crack at the deepest point. It was decided from. The results are shown in FIG. In the figure, the solid line is a beach mark on a fracture surface obtained by fatigue testing a stainless steel tube with a slit of the same shape as the one in which the potential difference distribution was measured, under slightly different stress conditions. The shaded area indicates the slit made by electrical discharge machining. The two broken lines are the crack shapes obtained by measuring the potential difference distribution before and after the lathe processing described above. The two dashed lines match well with each other and also match well with the fifth beach mark. The agreement near the crack tip on the surface is somewhat poor, but this is because the potential difference measurement interval was rough, and it is thought that the agreement would be even better if the measurements were made more closely.

In this way, according to this method, it is possible to detect the surface crack shape with sufficient accuracy using a small computer without the need for a large computer.

〔Effect of the invention〕

As explained above, according to the present invention, the shape of a surface crack that has occurred in a plate-shaped or pipe-shaped member can be detected with sufficient accuracy using a small computer rather than a large computer by measuring the potential difference distribution. be.

[Brief explanation of drawings]

Figure 1 shows the potential distribution around the surface crack determined by analysis, Figure 2 shows the potential difference distribution in the vicinity of the crack in the potential distribution shown in Figure 1, and Figure 3 shows the potential difference according to the present invention. Fig. 4 is a diagram showing the structure of a flaw detection head for measuring potential difference distribution;
The figure shows the terminal shape and the mounting situation on the board, Figure 6 is a flowchart for simple crack shape determination, and Figures 7 to 10 are obtained by analyzing members with cracks of various aspect ratios using the finite element method. Figure 11 shows the relationship between the potential difference ratio and aspect ratio for the distance between the measurement terminals of 6.4 mm, and Figure 12 shows the relationship between the potential difference ratio and the crack depth obtained from Figure 11. , Figures 13 and 14 are the potential difference distribution measured around the surface crack of the stainless steel pipe, Figure 15 is the relationship between the potential difference ratio and aspect ratio for a distance of 7.0 mm between the measurement terminals, and Figure 16 is from Figure 15. FIG. 17 is a diagram showing the relationship between the potential difference ratio and the crack depth obtained, and a comparison of the beach mark on the fracture surface of a broken stainless steel pipe and the crack shape obtained by measuring the potential difference distribution. DESCRIPTION OF SYMBOLS 1... Drive device, 5... Power supply terminal, 10... Measurement terminal, 20... Flaw detection head, 21... Board, 25... Stepping motor, 30... Air cylinder, 51... X
shaft, 52...stepping motor, 53...reducer,
56...Y axis, 57...Stepping motor, 58...
Reducer, 60... Side plate, 61... Compressor, 62
... Suction cup, 65... Drive control device, 66... DC power supply,
67...Switching device, 70...Scanner, 7
1...Microvoltmeter, 72...Interface, 10
0...Computer, 102...Potential distribution storage device.

Claims (1)

[Claims] 1. Direct current is applied to the surface of a member through one or more pairs of power supply terminals spaced apart from each other, and a potential distribution is measured by scanning a pair of potential difference measuring terminals between the pair of power supply terminals. In the method of detecting the shape of a defect from the potential distribution, a device for scanning a measurement terminal for measuring the potential difference distribution and a control device for driving the device are used to detect crack shapes with various aspect ratios in a memory circuit. The crack length at the surface is determined from the measured potential difference distribution by a calculation device that stores the analysis results of the potential distribution when the crack depths are different, and the maximum potential difference among the measured potential differences is used. The crack depth is determined using a master curve for a specific aspect ratio among the relationships between the potential difference and the crack depth for cracks with various aspect ratios stored in the memory circuit, and the crack depth and surface crack are determined using a master curve for a specific aspect ratio. Find the aspect ratio from the ratio of fissure lengths,
Compare this aspect ratio with the aspect ratio of the master curve described above, and if they are different, calculate by changing the aspect ratio of the master curve to be used until they match, and when they match, calculate the crack depth on the surface. The maximum crack depth of the crack is determined, and the crack depth from the maximum crack depth to the crack tip on the surface is determined from the potential difference at each measurement position using the master curve when they match. A surface crack shape detection method characterized by determining the shape. 2. In the method described in claim 1,
A surface crack shape detection method characterized in that the aspect ratios of cracks to be stored in a memory circuit are 1.0, 0.5, 0.25, and 0.125. 3. In the method described in claim 1,
The surface potential difference distribution in the direction perpendicular to the crack surface at the center of the crack when the crack depths of cracks with different aspect ratios are different is stored in a memory circuit, and the distance between the potential difference measurement terminals is calculated. The relationship between the aspect ratio and the potential difference ratio for each crack depth is determined by an arithmetic circuit from the potential difference at the measurement position, which is standardized by the thickness of the member, and from this relationship, the relationship between the potential difference ratio and the crack depth for any aspect ratio is determined. determining the maximum crack depth from the maximum potential difference using the relationship, and determining the crack shape at each measurement position from the potential difference ratio at each measurement position. Crack shape detection method. 4. In the method described in claim 3,
From the relationship between the aspect ratio and the potential difference ratio for each crack depth, a master curve of the relationship between the potential difference ratio and the crack depth is determined in steps of 0.01 aspect ratio, and the measured maximum potential difference ratio is used to calculate the arbitrary first aspect ratio. A first crack depth is determined using a master curve for the ratio, a second aspect ratio is determined from the ratio of the crack depth to the surface crack length, and the aspect ratio is the aspect ratio of the master curve. Compare and if they are different, use the second aspect ratio master curve to find the second crack depth, and then calculate the third aspect ratio again from the ratio of the second crack depth to the surface crack length. is determined and compared with the aspect ratio of the aspect ratio master curve.This is repeated and the crack depth when both match is the maximum depth of the surface crack, and the crack depth at each measurement position is A method for detecting a surface crack shape, characterized in that the crack shape is determined by substituting the potential difference ratio in the master curve having the same aspect ratio. 5 The point where the potential difference ratio is 1.02 in the potential difference ratio distribution measured along the crack by the method according to any one of claims 1, 3, or 4 is defined as the crack tip on the surface. A surface crack shape detection method characterized by determining the shape of a surface crack. 6. Obtained by nth-order approximation of the potential difference ratio distribution near the crack in the potential difference ratio distribution measured along the crack by the method according to any one of claims 1, 3, or 4. A method for detecting the shape of a surface crack, characterized in that the intersection point between the calculated curve and a reference potential difference is determined to be the tip of a crack on the surface.