JPS6228654A - Detection of crack - Google Patents

Abstract

PURPOSE:To decide whether a crack had occurred on the same side or opposite side of the probe and detect accurately the position and length of the crack, by disposing a number of probes at equal spaces, comparing potential differences between probes, and operating the results. CONSTITUTION:A DC voltage is applied to a bar member 5 from a DC stabilizer power supply. Potential differences between a number of probes disposed at equal spaces on one side of a member 5 are measured by a micro-potentiometer 6. A computer 8 compares the measured potential differences, operates the position and length a/W of each of generated cracks, and outputs them to a CRT display 9. The presence or absence of a crack is discriminated based on the ratio of the potential difference V1 to a reference potential difference V, and whether it is a front or a rear crack is decided from the sum of potential differences V2, V3 between each two probes on either side of the probe concerning the crack. The lenght and the position of the crack are operated based on the decision, reference potential difference V and the potential differences V1-V3.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a crack detection technique for detecting cracks generated in metal structural members, and in particular to a technique for accurately detecting the position and length of a through crack online. This invention relates to a method suitable for

[Background of the invention]

As a conventional crack detection method using the 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. A crack is determined by using a potential difference in a region where no crack is expected to be present as a reference potential difference, and determining that a crack exists where the potential difference is larger than that. Therefore, the four-terminal method has the disadvantage that the presence or absence of a crack and the length of the crack cannot be determined unless the terminal is scanned along the surface of the member.

[Purpose of the invention]

An object of the present invention is to provide a method for detecting the position and length of a defect or crack occurring in a structural member online without scanning a power supply terminal and a measuring terminal along the surface of the member.

[Summary of the invention]

Cracks generally occur at discontinuities in shape, and the length of the crack can be measured by installing a measurement terminal across the discontinuities, but it is difficult to determine where cracks occur in parts without discontinuities. In some cases, it may be unclear whether or not the cracks will be detected, and even if there are discontinuities, cracks may need to be detected remotely. By comparing and calculating the potential difference, it is possible to determine whether the crack is on the same side as the terminal or on the opposite side, and to detect the detailed position and length of the crack with high accuracy.

[Embodiments of the invention]

An embodiment of the present invention will be described below. Figure 1 shows a detection system for single-sided through cracks using the multi-terminal potential method. Power supply terminals 3 for supplying direct current to the member are attached to both ends of the plate-shaped member 5. The power supply terminal 3 may be attached by spot welding, or if spot welding is not possible, it is sufficient to simply press it together. Direct current is supplied from the DC stabilized power supply 1, but the measurement terminal 4 and the member to be measured 5 If the materials of the terminals 4 and 5 are different, a thermoelectromotive force is generated between the terminal 4 and the member 5, resulting in a decrease in potential difference measurement accuracy and, in turn, a decrease in crack detection accuracy. 1 for removing thermoelectromotive force
One method is to reverse the polarity of the supplied DC current and subtract the potential difference when a negative current flows from the potential difference when a positive current flows. This means that the thermoelectromotive force is added to the potential difference when current is passed.In other words, the thermoelectromotive force exists as an average potential difference, so by reversing the polarity of the DC current, the thermoelectromotive force can be canceled out. It is. A switching device 2 is used to reverse the polarity of the direct current.A large number of measuring terminals 4 are attached at equal intervals on one side of the member. The shorter the distance between the measurement terminals 4, the better the sensitivity, but the lower the accuracy. Therefore, the balance between sensitivity and accuracy is a problem, but studies using the finite element method have revealed that the optimum spacing between the measurement terminals is about the plate width W of the member 5. The number of measurement terminals 4 depends on the member 5, but at least five are required. 1st
The figure shows an example in which seven sensors are provided so that potential differences at six locations can be measured. The potential difference V between each measurement terminal is taken into the micropotentiometer 6 via a lead wire, and the switching of the measurement terminals is automatically and sequentially performed by a scanner built into the micropotentiometer 6. The potential difference measured by the minute potentiometer 6 is A/D converted and sent to the computer 8 via the interface 7. The computer 8 compares and calculates the measured potential differences to determine the crack occurrence position and crack length a/W, and displays the results on the CRT screen.

Alternatively, output to the X-Y plotter 9.

The crack detection method using the multiterminal potential method will be described below. Figure 2 (a), (b)
.

(c) is an equipotential diagram obtained by analyzing a plate-shaped member having a through crack on one side using the finite element method.

Due to symmetry, only half of the member is analyzed.

In the figure, the crack length is shown in a/W standardized by the plate width W of the member, and the crack tip is indicated by an arrow. Away from the crack, the equipotential lines are perpendicular to the surface of the part, indicating that the current is flowing uniformly. It can be seen that the equipotential lines around the crack are twisted and the electric field is disturbed. The area where the electric field is disturbed by the crack depends on the crack length, but is limited to the vicinity of the crack.

As shown in Figures 3(a) and (b), the potential difference ratio V/ V, (V: potential difference when there is a crack, vo: potential difference when there is no crack), and crack length a/
The relationship between W (a: crack length, W: plate width) is shown in FIGS. 4 and 5. The crack measured here is a one-sided penetrating crack, as shown in Figure 3(a).

As shown in (b), a case where the crack is on the same side as the measurement terminal is defined as a surface crack, and a case where the crack is on the opposite side of the measurement terminal is defined as a back surface crack. Figure 4 is for surface cracks. The distance Ω between the potential difference measurement terminals was varied from 10 meters to 30 meters. Since the disturbance of the electric field is limited to the vicinity of the crack, it can be seen that the shorter the distance Ω between the terminals, the larger the potential difference ratio V/V, and the better the detection sensitivity. In actual measurements, the resolution of the micropotentiometer and the specific resistance of the object to be measured affect the measurement accuracy, and ρ
In some cases, it is better to have a larger value to some extent. Furthermore, if 0 is made smaller, the number of measurement terminals must be increased, so when applying it to an actual machine, the optimum distance between the terminals must be determined.

FIG. 5 shows the relationship between the potential difference ratio V/V and the crack length a/W in the case of a backside crack.

The detection sensitivity of backside cracks is considerably lower than that of surface cracks.

If the measurement terminal can be scanned over the surface of the material and it is known whether the crack is on the surface or the back surface, then the relationship between the potential difference ratio V/V and the crack length a/W shown in Figure 4 or Figure 5 can be used. Cleft length can be detected.

However, in actual machine parts, if you measure the crack length by scanning the measurement terminal, it can be measured visually, so there is no point in scanning the measurement terminal.The measurement terminal must be fixed to detect the crack length. The fissure is shown in Figure 3 (a), (
Since the crack is not always located at the center between the measurement terminals as in Figure 6 (a), it is necessary to consider what happens to the potential difference when the crack is not at the center between the measurement terminals.
) to (d), consider scanning the measurement terminal over a crack. Regarding the positional relationship between the crack and the measurement terminal, the distance from the center between the measurement terminals to the crack was defined as L, as shown in FIGS. 6(a) to 6(d). Therefore, when L=Om, the crack is in the center between the measuring terminals, when L=12/2, the crack is directly below the measuring terminals, and when L>Q/2, the crack is outside the measuring terminals. It will be in . FIG. 7 shows the case of a surface crack, that is, the case where the measurement terminal is scanned over a cracked surface. The distance between the measurement terminals Q is 30 mm, and the crack length is a/W = 0, 1 to 0.7. The vertical axis of the figure is the potential difference V, and the horizontal axis is the crack position [L]. Here, the potential difference is a convenient value because unit values are used as the specific resistance and current value of the member in the Ti field analysis.

The reason for this is that when evaluating the crack length, the ratio of the potential difference to the reference potential difference, that is, the potential difference ratio is used. Conversely, since resistivity depends on the material of the member and the temperature of the member, when evaluating by potential difference, the relationship with the crack length must be determined in advance by taking the resistivity into consideration. This is because if the ratio is used, there is no need to consider material or temperature. Potential difference v0 when there is no crack in Figure 7
is 150. As L increases and the crack moves away from the center between the measurement terminals, the potential difference increases, and when the crack reaches almost directly below the terminals, the potential difference reaches its maximum value.

When L becomes larger than Ω/2 and the crack goes outside between the terminals, the potential difference decreases rapidly, and the potential difference when there is no crack, v,
The value is even lower than . As the distance from the terminal increases, the potential difference increases and approaches the potential difference v0 when there is no crack.

Figure 8 shows the crack location in the case of a crack on the back side! The relationship between L and potential difference V is shown. The potential difference V has a maximum value when the crack is located at the center between the terminals, and decreases monotonically as it moves away from the center.

When L=30m, the potential difference V becomes almost equal to the reference potential difference v0. The reason for the potential difference distribution as shown in FIGS. 7 and 8 will be explained in sections 9m(a) to (Q). (a) shows the potential difference distribution in the case of a surface crack, (b) shows an equipotential diagram, and (c) shows the potential difference distribution in the case of a back surface crack. First, the case of surface cracks will be explained. When the terminal is scanned from the left side in the drawing, the equipotential lines are perpendicular to the surface at a distance from the crack and are lined up at equal intervals, so they are equal to the reference potential difference v0. When approaching the crack indicated by the thick broken line, the potential difference decreases because no current flows near the crack mouth, and the distance between the equipotential lines widens. When the right terminal crosses the crack and the crack enters between the terminals, the number of equipotential lines submerged into the crack surface increases at once, so the potential difference increases rapidly. When moving further to the right, the distance between the equipotential lines becomes wider near the mouth of the fissure, so the potential difference decreases. When the crack reaches the center between the terminals, the potential difference shows a minimum value because the potential distribution is symmetrical across the crack. If the terminal moves further to the right, the symmetry of the electric field means that the change will be the opposite of what it was before.

On the other hand, in the case of a backside crack, the equipotential lines are dense in front of the crack, so when the terminals are scanned from the left side, the potential difference increases monotonically, and the crack comes to the center between the terminals. and takes the maximum value. If the terminal moves further to the right, the potential difference monotonically decreases due to the symmetry of the electric field and asymptotically approaches the reference potential difference. Since the potential difference changes in a complicated manner depending on the crack position, the crack length cannot be easily determined even when scanning the terminal.

The area where the electric field is disturbed by the crack is limited to the area around the crack, as shown in FIG. Therefore, if the distance Q between the measurement terminals is made sufficiently large, the measured potential difference V will be constant even if the position of the crack is slightly shifted.

Now, consider the case where another measurement terminal is provided between these terminals and the potential difference is measured using two sets of measurement terminals.

The potential difference when a crack exists at a certain position is vl.

Set it to v2. Here, let vl be the potential difference between the terminals with a crack, and let v2 be the potential difference between the other terminals without a crack. At this time, V + V, = V. Next, when the measurement terminal is slightly shifted, the potential difference changes as shown in FIG. 7 or 8. vl is v1', v.

Even if it changes to V and l, the distance α between the measurement terminals is sufficiently large, so V + v2 = v1' + v2' = v.

Specifically, this is shown in Figs. 7, 8, and 10 (a) to
This will be explained in (c). Two sets of potential difference measuring terminals are arranged around the crack, and the potential difference between the terminals with the crack is defined as vl, and the potential difference between the adjacent terminals is defined as v2.

In the case of a surface crack, vl depends on the crack position, as mentioned above, when the crack is at the center between the terminals, that is, from the value of L = Om to L
increases as becomes larger, L = 15rm = Q
/2, that is, the maximum value is taken directly below the terminal.

When L becomes larger than Q/2 and goes outside between the terminals, the potential difference decreases rapidly, and as the crack gets further apart, it approaches the potential difference v0 when there is no crack. On the other hand, the potential difference between adjacent terminals v2
The relationship between the crack position and the crack position L' is shown in FIGS. 7 and 8 as L'
=30-L. That is, when the crack is located at the center between adjacent terminals, v2 is the value at L'=30-L=30 m, and is equal to the potential difference v0 when there is no crack.

As the crack approaches, v2 decreases, reaching a minimum value when the crack is almost directly under the adjacent terminal, and reaching a maximum value when the crack approaches further and enters between the terminals. It gradually decreases as it approaches the middle of the range. Looking at the change in potential difference depending on the position of the crack, as the crack moves away from the center between the terminals, the potential difference between adjacent terminals tends to decrease as the crack increases. From another perspective, this is because, as shown in FIG. 10(c), the potential distribution in the shaded area that moves depending on the position of the crack is the same because it is away from the crack.

V x +V 2 appears to be constant regardless of the crack location. Figure 11 shows the reference potential difference V from the sum of V and v2.
. The relationship between V1+V, -V, and the crack position is shown for the case of surface cracks. V, ten V□−■. is almost constant regardless of the crack location. This means that determining the crack length using the potential difference v1 between terminals with a crack overestimates it, whereas it is estimated using the potential difference v1 between the terminals with a crack and the potential difference v2 between the adjacent terminals that are closer to the crack. This shows that the crack length can be determined with high accuracy, indicating that this method is effective.

In the case of a crack on the back side, as shown in Figure 8, the potential difference v1 between the cracked terminals shows the maximum value when the crack is located in the center between the terminals, and increases monotonically as L increases. Decrease.

It can be seen that the potential difference v2 between adjacent terminals increases monotonically. In this case, the decreasing trend of vl and the increasing trend of v2 are very similar, and as shown in FIG. 12, the reference potential difference V is determined from the sum of V and v2. subtracted v1+V2-V, which is approximately constant regardless of the granulation [L].

As can be seen in Figures 11 and 12, V + V * V - plotted against the crack position is not constant at all; for the surface crack in Figure 11, it is upward to the left, and for the back side in Figure 12. The crack slopes downward to the left, and the measurement accuracy is slightly lower when the crack is in the center between the terminals. This means that when the distance between the measurement terminals is Q = 30 m, the crack is a / W = 0, 7
This indicates that if the depth is deep, the area where the electric field is disturbed is not completely covered. That is, this is due to the fact that the potential distribution in the lower shaded area in FIG. 10 is slightly different. The distance between the measurement terminals is a; 15■ and Q = 45
V work + V2 - V when I. When granulating [As a result of investigating the relationship with L, when Q = 15m, a / W = 0,
Even in 2, V, +V, -V are upward to the left, but Ω=45
At m, a/W = 0, and even at 7 it becomes an almost horizontal straight line. Therefore, simply considering the accuracy, 30 to 45 m is appropriate as the distance 0 between the measurement terminals. However, when Q is made large, crack detection sensitivity decreases, and crack detection accuracy also decreases. Figure 13 shows the relationship between the potential difference ratio V□+V, -V, /V and the crack length a/W in the case of a surface crack. It decreases slightly. On the other hand, Fig. 14 shows the potential difference ratio V1+ in the case of a backside crack.
The relationship between V2-VO/v0 and crack length a/W is shown, but the sensitivity increases, contrary to surface cracking. In the figure, ■ marks indicate variations. If the distance Ω between the measurement terminals is short, there will be large variations and the accuracy will be poor, and if Q is long, the accuracy will be good but the sensitivity will decrease. Judging from both accuracy and sensitivity, it seems that the distance between the measurement terminals should be M=20 nm, that is, about the same as the plate width W. The potential difference ratios V, +V, -V, /v for this surface crack and back surface crack.

The master curve for the relationship between the crack length a/W is an nth-order approximation of the relationship between the two, and is stored in the storage circuit of the computer 8.

Next is the method for determining the crack location. There are two methods for determining the crack location. The method shown in Fig. 15 is for surface cracks, but the potential difference ratio between terminals with cracks is
V, when the relationship between potential [L and crack length a/W =
It is created in steps of 0 and 1, and the relationship between the two is approximated to the nth order, and the result is stored in the memory circuit of the computer 8. From the measured potential difference, vl, ■2, and V are determined according to the master curve shown in Fig. 13. Determine the crack length a/W from . However, the crack length a/W is not divisible by 0.1, so for example, a/W =
If 2 and 7 are obtained, a/W = 0,
2 and a/W = 0, 3 When the master curve before and after the crack length a/W is obtained, the potential difference ratio V/V between the terminals with a crack.

Find the crack positions L1 and L2 by substituting , and the average value L=
The crack position is (L1+L,)/2. Another method is to calculate the potential difference ratio V L between terminals with cracks.
/ VO and the relationship between crack position and crack length a /
W = 0, 1 increments, the relationship between the two is approximated to the nth order, and stored in the memory circuit of the computer 8. From the measured potential difference, Vl, v2, and V.

Determine the crack length a/W from The relationship between the potential difference ratio v1/v0 corresponding to the obtained a/W and the crack position is created from the relationship between the two created at a/W = 0 and 1 increments. That is, as shown in FIG. 15, for example, the crack length a/W obtained by approximating the relationship between the potential difference ratio V/V and the crack length a/W every L=2.5 mm is nth order.
Find the potential difference ratio V, /V, and calculate the crack length a again.
/W to potential difference ratio Vt/v.

The crack position between the terminals is determined by approximating the relationship between the time and the crack position to an n-th order approximation and substituting the measured potential difference V, /V into the relationship. No. 15 also applies in the case of cracks on the back side.
The crack location can be determined by creating a master curve as shown in the figure.

As a method for determining the location of a crack, it is also possible to use the potential difference between terminals adjacent to each other with a crack. That is, the ratio V'a of the difference between the potential difference v3 between the adjacent terminals farther from the crack and the potential difference v2 between the terminals closer to the crack to the reference potential difference vo.
The relationship between V Z / Va and the crack position between the terminals is shown in Figure 16, so v a v −/
The relationship between v n and the potential PIL between the terminals is approximated to the nth order and stored in the storage circuit of the computer 8, and the potential between the terminals is determined by the same method as using the potential difference ratio V 1/V a. [1, is determined.

As can be seen by comparing FIG. 15 and FIG. 16, the change in the potential difference ratio with respect to the crack position is larger when v3 and v2 in FIG. 16 are used, and the accuracy is better.

Contrary to the case of surface cracks, for rear cracks, the difference between the potential difference v2 between the terminals closer to the crack and the potential difference v3 between the adjacent terminals farther from the crack is used.

FIG. 17 shows the relationship between the potential difference ratio V z −V 3 /V o and the crack position between the terminals. The relationship between the two is different from that for surface cracks, and appears to be approximately proportional.

Next, a method for determining surface cracks and back surface cracks will be described. 7th
As shown in the figure and FIG. 8, the potential difference distribution changes completely differently between the surface crack and the back surface crack. When the measurement terminal is scanned, the potential difference at a surface crack once decreases from the reference potential difference, then rapidly increases, then gradually decreases again to reach a minimum value, and the change is reversed. On the other hand, in the case of a backside crack, the potential difference monotonically increases, reaches a maximum value, and then monotonically decreases again.

Therefore, the potential difference v1 between the terminals with a crack is always larger than the reference potential difference v0, whereas the potential difference v2 and ■3 between the terminals on both sides of the terminal with a crack are different depending on the surface crack and the back surface crack. , for surface cracks, both v2 and v3 are smaller than the reference potential difference v0.

On the other hand, in the case of a crack on the back side, both v2 and v3 are larger than the reference potential difference v0. Therefore, when calculating the sum of v2 and V, the sum of v2 and V is smaller than twice the reference potential difference for a crack on the back side, and the reference potential difference for a crack on the back side. Since it is larger than twice the
It is possible to determine whether it is a surface crack or a back surface crack by the sum of v2 and v.

Discrimination between v2 and v3 can be made based on the fact that v2 is smaller than V in the case of a surface crack, and that v2 is thicker than v3 in the case of a back surface crack.

Further, the reference potential difference v0 is determined from the average of all the potential differences between the terminals, excluding the potential difference V□ between the terminals with the crack and the potentials v2 and v3 between the terminals on both sides of the crack.

The actual determination is as follows. First, find the maximum potential difference among all the measured potential differences between all the terminals, and set it as the potential difference v1 between the cracked terminals. The reference potential difference v0 is determined by the average value of the potential difference v1 between the two terminals and the potential difference v2 between the terminals on both sides thereof, excluding the potential difference v2 and . The potential difference ratio V 1/V o is determined, and if it is, for example, 1.02 or more, it is determined that there is a crack, and the terminal with the crack is determined to be a surface crack or a back surface crack according to the method described above. The sum of the potential differences between adjacent terminals V
, +V, is more than twice or less than the reference potential difference.

Next, the potential difference ratio between the cracked terminals V 1 / v o
By substituting into the master curve of the surface crack or back surface crack potential difference V/V and the crack length a/W, the crack length a/W is obtained.
Find W. If there is a surface crack, the potential difference ratio V, -V is obtained from the potential difference V, , V, between the terminals with the crack and the terminals on both sides.
, /V, and 5 Va-V for various crack lengths, /
9th place between V and terminal! Create a master curve corresponding to the crack length a/W using the master curve of the relationship between From the potential difference v2 between a certain terminal and the terminals on both sides, ■3, the potential difference ratio V2 is obtained.
-V.

/vo is determined, and the crack position between the terminals is determined using the same method as described above using a master curve to determine the relationship between V2-V, /V, and the gap 9th position 11L between the terminals.

Measurement of these potential differences, determination of the maximum potential difference v1, V, ,
Discrimination of surface cracks and back surface cracks by V, determination of crack length, V3-V for various crack lengths.

Using the master curve of the relationship between /V, or v, -■, /v0 and the crack position between terminals, the crack length a/W is calculated.
A series of operations of creating a master curve corresponding to the above and determining the crack position between the terminals using the master curve are all performed by the computer 9 shown in FIG. Therefore, the master curve of the relationship between potential difference ratio and crack length, and v z v −/vo for various crack lengths.
Or V, -V, /V.

A master curve for the relationship between the 9th position flL between The computer 8 also controls the switching device 2 for reversing the polarity of the signal.

Next, using the measurement system shown in FIG. 1, a slit simulating a crack was made in a plate-shaped test piece of plate [20I, thickness 8I, length 400+m], and the above method was verified. Three types of materials were used: stainless steel 5US304, carbon steel 5S41, and electrical steel. The measurement results are shown in FIGS. 18 to 21.

FIG. 18 shows the correspondence between the measured crack length and the actual crack length in the case of a surface crack. It can be seen that the two agree very well. Measurement accuracy is ±0.0 in a/W
5, and even shallow cracks of a/W = 0.05 can be detected with sufficient accuracy. FIG. 19 is a comparison between the measured value and the actual value of the crack position between the measurement terminals.

The measurement accuracy is on the order of ±I leap, which is good considering the distance between the terminals of 20 mm. Figures 20 and 21 show the case of a backside crack.6 The accuracy of the crack length is the same as that of the front side crack, but the crack position accuracy is slightly lower. In this way, according to this method, it is possible to distinguish between surface cracks and back surface cracks simply by arranging a large number of measurement terminals, and the crack length and crack position can be detected with high accuracy.

〔Effect of the invention〕

According to the present invention, surface cracks and back surface cracks can be determined by arranging a large number of measurement terminals and calculating the potential difference between them without scanning the measurement terminals. Since the fissure position can be detected with high accuracy, there is an effect that the fissure can be detected online.

[Brief explanation of the drawing]

Figures 1 to 21 are explanatory diagrams of the crack detection method of the present invention, Figure 1 is a block diagram of a through crack detection device using the multi-terminal potential method, and Figures 2 (a) to (c) are FIG. 3(a) is an equipotential diagram obtained by finite element analysis of a member having a one-sided through crack. (b) is a diagram showing the definition of surface cracks and back surface cracks, Figure 4 is a diagram showing the relationship between potential difference ratio and crack length in the case of a surface crack when the crack is placed in the center between terminals, and Figure 5 Figure 6 (a) to (d) shows the relationship between the potential difference ratio and the crack length in the case of a crack on the back surface when the crack is placed in the center between the terminals. A diagram showing the definition of L, Figure 7 is a diagram showing the change in potential difference when scanning the measurement terminal for a surface crack, and Figure 8 is a diagram showing the change in potential difference when scanning the measurement terminal for a rear surface crack. Figures 9 (a) to (Q) are diagrams explaining changes in potential difference when scanning the measurement terminals, Figures 10 (a) to (c) are diagrams illustrating the basic principle of the multi-terminal potential method, 11th
12 and 12 are diagrams showing the relationship between the potential difference obtained by subtracting the reference potential difference from the sum of the potential differences between the terminals with the crack and the terminals on both sides of the terminal with the crack, respectively, and the crack position in the case of surface cracks and back surface cracks, respectively. and FIG. 14 are diagrams showing the relationship between the potential difference obtained by subtracting the reference potential difference from the sum of the potential differences between the terminals with the crack and the terminals on both sides of the terminal with the crack and the crack length in the case of surface cracks and back surface cracks, respectively.
Figure 15 is a diagram showing the relationship between the potential difference ratio between cracked terminals and the crack position, and Figure 16 is a diagram showing the potential difference between terminals with a crack in a surface crack between terminals that are far from the adjacent crack. Figure 17 shows the relationship between the ratio of the potential difference obtained by subtracting the potential difference between the terminals closer to the crack and the reference potential difference and the crack position. A diagram showing the relationship between the ratio of the potential difference obtained by subtracting the potential difference between the terminals far from the crack from the potential difference between the terminals nearer to the reference potential difference and the crack position. Figure 18 shows the relationship between the crack position and the measurement of the crack length in a surface crack. Figure 19 is a diagram showing a comparison between the measured value and the actual crack length, Figure 20 is a diagram showing the comparison between the measured value and the actual crack position, and Figure 20 is the comparison between the measured value and the actual crack length for a crack on the back side. FIG. 21 is a diagram showing a comparison between the measured value of the crack position and the actual position. 1... DC stabilized power supply, 2... Switching device,
3... Power supply terminal, 4... Measurement terminal, 5... Member to be measured, 6... Minute potentiometer, 7... Interface, 8... Computer, 9... X-Y plotter , 1o...Crack. tZ figure 4/1su, 2 It/f section 1.4 f 3 Zuden, /i ton ratio (V〆ro) Densa 4 ratio tJp difference 1 Potential difference 7 Denshu disease fall-b only 1 Nii 9n ” Frog and 11-ni Reinashi 2-VI/Nshiden 4 sentence difference 75 Figure = Ripping device L (/77t/WL) Figure 1 Parting device L (Kanjo stone !7 Art Crafts I Stand IL (riη/?7t) Straw/g Figure Abusive /9 Figure

Claims (1)

[Claims] 1. Apply direct current to the surface of the member through a pair of power supply terminals spaced apart from each other, and measure the potential difference by providing a pair of potential difference measuring terminals between the pair of power supply terminals, and measure the potential difference from the potential difference. In the method of detecting the shape of a defect, multiple measurement terminals are provided at equal intervals and the potential difference between the measurement terminals is compared and calculated, and the potential difference between the terminals with the maximum potential difference is excluded from the potential difference between the terminals on both sides. The average value of all potential differences is determined, and the average value is used as a reference potential difference. If the ratio of the maximum potential difference to the reference potential difference is larger than the limit value, a crack exists between the terminals, and the crack is a surface crack or a back surface crack. The presence of a crack is determined by the sum of the potential differences between the terminals on both sides of the terminal where the crack exists.If the crack is a surface crack, the location of the crack between the terminals is determined by the sum of the potential differences between the terminals where the potential difference is the smallest. In the case of a crack on the back side, it is determined that the crack exists between the terminals with a larger potential difference, and the reference potential difference, the potential difference between the terminals with the crack, and the terminals on both sides of the crack are determined. A crack detection method characterized by detecting the crack length and crack initiation position from the potential difference. 2. A crack detection method according to claim 1, characterized in that the distance between the measurement terminals is made equal to the plate width of the member. 3. A crack detection method according to claim 1, characterized in that the power supply terminal is separated from the measurement terminals at both ends by at least twice the plate width of the member. 4. In the method described in claim 1, the potential difference with respect to the reference potential difference of the potential difference obtained by adding the potential difference between adjacent terminals near the crack to the potential difference between the terminals with the crack, and further subtracting the reference potential difference from the potential difference. A crack detection method characterized by detecting the crack depth based on the ratio. 5. In the method set forth in claim 1, if the sum of the potential differences between the terminals on both sides of the cracked terminal is less than twice the reference potential difference, the crack is located on the same surface as the surface on which the terminal is provided. exists,
A crack detection method characterized by determining that a crack exists on a surface opposite to a surface on which a terminal is provided if the difference is twice or more. 6. The potential difference between the terminals with a crack and the position of the crack between the terminals obtained based on the potential difference distribution obtained by analyzing the electric field in advance by the finite element method in the method described in claim 1. A crack detection method characterized by determining the crack position between terminals based on the relationship. 7. In the method described in claim 6, the relationship between the potential difference ratio between cracked terminals and the crack position is determined by creating a ratio of the crack length to the plate width of the member in steps of 0.1. The relationship is approximated to the nth order, the crack position is determined using the relationship between the ratio of the obtained crack length to the plate width of the member, and the average of these is determined to be the crack position between the terminals. Crack detection method. 8. In the method described in claim 6, the relationship between the potential difference ratio between cracked terminals and the crack position is determined by creating a ratio of the crack length to the plate width of the member in 0.1 increments, and comparing both. The relationship is approximated to the n-th order, and the relationship between the potential difference ratio corresponding to the ratio of the obtained crack length to the plate width of the member and the crack position is created from the relationship between the two created in steps of 0.1,
A crack detection method characterized by determining the position of a crack between terminals by substituting the potential difference between the terminals in the crack into the relationship. 9. Among the potential differences on both sides between terminals with a crack, which are obtained based on the potential difference distribution obtained by analyzing the electric field in advance by the finite element method in the method described in claim 1, the potential difference is close to the crack. A crack detection method characterized by determining a crack position between terminals based on the relationship between the potential difference between the terminals, the potential difference between terminals far from the crack, and the crack position between the terminals. 10. In the method described in claim 9, among the potential differences on both sides between terminals with a crack, the difference between the potential difference between the terminals close to the crack and the potential difference between the terminals far from the crack, and the crack position between the terminals The relationship between the crack length and the plate width of the member is created in increments of 0.1, and the relationship between the two is approximated to the nth order. A crack detection method characterized by determining the crack position using a relationship and determining the average of the crack positions as the crack position between terminals. 11. In the method described in claim 9, among the potential differences on both sides between terminals with a crack, the difference between the potential difference between the terminals close to the crack and the potential difference between the terminals far from the crack, and the crack position between the terminals The relationship between the crack length and the plate width of the member is created in increments of 0.1, the relationship between the two is approximated to the nth order, and the relationship corresponds to the ratio of the obtained crack length to the plate width of the member. The relationship between the potential difference ratio and the crack position is created from the relationship created in steps of 0.1, and the crack position between the terminals is determined by substituting the potential difference on both sides between the terminals with the crack into that relationship. A crack detection method characterized by: