JP2007003235A - Non-destructive inspection method of change in wall thickness of measuring target - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-destructive inspection method which enables a simple and precise measurement of a change in the wall thickness of a measuring target. <P>SOLUTION: A plurality of potential difference measuring terminals are arranged on the surface of the measuring target in a matrix state so as to be mutually separated at a predetermined interval and the potential difference or rate of change in potential difference produced between the respective potential difference measuring terminals is measured while supplying a current to the surface of the measuring target through a pair of the electrodes provided so as to hold a plurality of the potential difference measuring terminals to calculate the potential difference distribution or rate-of-change distribution of potential difference in a measuring region. On reference to the relation between the preliminarily related potential difference distribution or the rate-of-change distribution of potential difference and the change in the wall thickness of the measuring target, the change position and change quantity in the wall thickness of the measuring target, the shape of the region changed in the wall thickness of the measuring target and the advance state of the region changed in the wall thickness of the measuring target are detected and measured. The supply current is preferably measured with respect to a plurality of directions and preferably set to a DC or AC pulse. Further, in order to eliminate the change in potential difference due to a factor other than the change in the wall thickness, a plurality of reference terminals are arranged on a reference plate comprising the same material as the measuring target and the reference plate is preferably installed under the same environment as the measuring target to perform measurement. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a nondestructive inspection method, and more particularly, to a nondestructive inspection method for detecting a change in the thickness of an object to be measured that occurs during use of a material (object to be measured).

  In oil and power plants, equipment and piping (hereinafter referred to as equipment) are often exposed to a strong corrosive or erosive environment, and the materials constituting the equipment include stress corrosion (SCC) and sulfide stress. Corrosion (SSCC) or intergranular corrosion or erosion may cause the wall thickness to decrease entirely or locally. Since the overall or local thickness reduction that occurs in these materials often causes damage to the device, it is necessary to detect it from the viewpoint of ensuring the safety of the device.

Conventionally, non-destructive inspection methods such as an ultrasonic flaw detection method and an X-ray transmission method have been proposed as methods for detecting a decrease in thickness. However, these detection methods have limitations on measurement points, and it is difficult to obtain the absolute value and change amount of the wall thickness with high accuracy, or complicated and expensive devices are required. there were.
There is a potential difference method as a non-destructive inspection method capable of obtaining information on the size and shape of defects such as cracks with relatively high accuracy. When a current is passed through a material to be measured including a defect such as a crack, the defect has an electrical resistance corresponding to its size, and a potential difference corresponding to this occurs on both sides of the defect. In the potentiometric method, a current is passed through the object to be measured, the potential difference at the position where this defect is sandwiched is measured, and information on the shape and size of the defect contained in the object to be measured is obtained using a calibration curve obtained in advance from the result. Is going to get. The potential difference method includes a direct current potential method using direct current and an alternating current potential method using alternating current.

  For example, Patent Document 1 proposes a non-destructive inspection method for a three-dimensional crack by a DC potential difference method. The technique described in Patent Document 1 measures the potential difference distribution on the surface of the substrate, compares the difference between the measured value and the virtual potential difference distribution obtained from the assumed shape crack, and compares the measured value with the calculated value. The shape of the crack is estimated by changing the crack shape so as to reduce the difference, and the shape, size, and inclination of the three-dimensional crack having an arbitrary aspect ratio can be quantitatively evaluated. In addition, according to the technique described in Patent Document 1, it is said that application to a welded portion where application of an ultrasonic flaw detection method, an X-ray transmission method, or the like is difficult becomes easy. However, the method described in Patent Document 1 has a problem that the thickness change of the structure cannot be accurately measured.

For example, Patent Document 2 proposes a detection device that detects a change in the thickness of a structure and a structural defect using a potential difference method. In the detection device described in Patent Document 2, the potential difference between the measurement terminals can be easily measured without setting the measurement terminals by welding or the like, and the change in the thickness of the structure in an environment such as corrosion, It is said that structural defects can be detected.
Japanese Patent No. 3167449 International Publication WO 00/50907 Pamphlet

However, since the detection device described in Patent Document 2 measures the potential difference between the terminals set on the measurement surface, it is effective when the entire thickness of the structure changes as in the case of general corrosion. However, there is a problem that the accuracy is insufficient to detect the thickness change of local corrosion or small corrosion, and it cannot meet the recent demand for further accuracy improvement.
The present invention has been made in view of such problems of the prior art, and simply and accurately measures the absolute value of the thickness of the object to be measured, the amount of change in thickness, the rate of change in thickness, and their distribution. An object of the present invention is to propose a nondestructive inspection method for the change in thickness of an object to be measured.

  In order to achieve the above-mentioned problems, the present inventors diligently studied the factors affecting the detection accuracy of the wall thickness change in the potentiometric method. As a result, the present inventors have arranged the potential difference measurement terminals in a linear or matrix form, and preferably applied a current in multiple directions to measure the potential difference, the rate of change, and their distribution between the measurement terminals. By doing so, it has been found that the detection accuracy of the change in thickness of the object to be measured is significantly improved.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) A plurality of potential difference measuring terminals are arranged on the surface of the object to be measured at a predetermined interval, and are provided on the surface of the object to be measured via a pair of electrodes provided with the plurality of potential difference measuring terminals interposed therebetween. A nondestructive inspection method for detecting a change in the thickness of an object to be measured by measuring a potential difference generated between the plurality of potential difference measuring terminals while supplying a current, wherein the plurality of potential difference measuring terminals are linearly arranged. Alternatively, it is arranged in a matrix, and the potential difference generated between the terminals of the plurality of potential difference measurement terminals is measured intermittently or continuously to obtain the potential difference distribution in the measurement region. A non-destructive inspection method for thickness change of an object to be measured, characterized by detecting and measuring the thickness change amount and / or the rate of change in thickness of the object to be measured with reference to the relationship with the thickness .

  (2) A plurality of potential difference measuring terminals are arranged at predetermined intervals on the surface of the object to be measured, and a plurality of reference terminals for measuring the reference potential difference are provided on the surface of the region where the thickness of the object to be measured does not change. The plurality of potential differences are arranged while supplying a current to the surface of the object to be measured through a pair of electrodes arranged at a predetermined interval and sandwiching the plurality of potential difference measurement terminals and the reference terminal. A non-destructive inspection method for measuring a potential difference generated between measurement terminals and between a plurality of reference terminals to detect a change in thickness generated in an object to be measured, wherein at least the plurality of potential difference measurement terminals are linearly connected. The potential difference generated between the terminals of the plurality of potential difference measuring terminals and between the terminals of the plurality of reference terminals is measured intermittently or continuously at the same time, and the potential difference distribution in the measurement region is measured. In advance Referring to the relationship between the linked potential difference distribution and the thickness of the object to be measured, the amount of change in thickness and / or the rate of change in thickness occurring in the object to be measured is detected and measured. Non-destructive inspection method for change in thickness of workpiece.

(3) In (1) or (2), the initial thickness between the terminals of the potential difference measuring terminal is input, and the thickness of the object to be measured at the time of measurement between the terminals is calculated. A non-destructive inspection method characterized by displaying.
(4) The nondestructive inspection method according to any one of (2) and (3), wherein the reference terminal is disposed on a reference plate made of the same material as the object to be measured.

(5) The nondestructive inspection method according to any one of (1) to (4), wherein the potential difference is a potential difference change rate from a reference time, and the potential difference distribution is a potential difference change rate distribution.
(6) In any one of (1) to (5), after measuring the potential difference between the terminals in the direction of the current, while supplying the current in a different direction with respect to the direction of the current, A non-destructive inspection method characterized by measuring a potential difference between the two.

(7) The nondestructive inspection method according to any one of (1) to (6), wherein the current is a direct current or a direct current pulse.
(8) The nondestructive inspection method according to any one of (1) to (7), wherein the current is 10 to 2000 A.
(9) The nondestructive inspection method according to any one of (1) to (8), wherein the current is a current using a storage battery as a power source.

  (10) A power source, a pair of electrodes connected to the power source and applying a current from the power source to the object to be measured, and a pair of electrodes spaced apart from each other at a predetermined interval and arranged in a linear or matrix form A plurality of potential difference measuring terminals, a potential difference measuring means arranged to be connectable to each of the plurality of potential difference measuring terminals and measuring a potential difference between the potential difference measuring terminals, and each potential difference measured by the potential difference measuring means Calculation means for calculating the potential difference between the measurement terminals as input data and calculating the thickness change amount and thickness change rate of the object to be measured, and data storage means for storing the input data and calculation results, or further display And a non-destructive inspection device for a change in thickness of the object to be measured.

  (11) A power source, a pair of electrodes connected to the power source and applying a current from the power source to the object to be measured, and a pair of electrodes spaced apart from each other at a predetermined interval and arranged in a straight line or a matrix A plurality of potential difference measuring terminals, a plurality of reference terminals for measuring a reference potential difference, and a plurality of potential difference measuring terminals and the plurality of reference terminals are arranged to be connectable to each other and between the potential difference measuring terminals. A potential difference measuring means for measuring a potential difference between the reference terminals and a potential difference between the reference terminals, a potential difference between the potential difference measuring terminals measured by the potential difference measuring means and a potential difference between the reference terminals as input data, An object to be measured, comprising: a calculation means for calculating a wall thickness change amount and a wall thickness change rate; a data storage means for storing the input data and calculation results; or a display means. Non-destructive inspection equipment for meat thickness change.

(12) In (10) or (11), the calculation means further calculates an initial thickness between the potential difference measuring terminals as input data, and further measures an object at the time of measurement between the potential difference measuring terminals. A non-destructive inspection apparatus, characterized in that it is a calculation means for calculating the wall thickness.
(13) The nondestructive inspection apparatus according to any one of (10) to (12), wherein the reference terminal is disposed on a reference plate made of the same material as the object to be measured.

(14) In any one of (10) to (13), the power source is a plurality of power sources, and in addition to the pair of electrodes, another pair or a plurality of other currents that can apply a current in a direction different from the current direction A nondestructive inspection apparatus comprising a pair of electrodes.
(15) The nondestructive inspection apparatus according to (14), wherein the plurality of power supplies are replaced with a single power supply with switching means.

(16) The nondestructive inspection apparatus according to any one of (10) to (15), wherein the power source is a power source that generates a pulse current.
(17) The nondestructive inspection device according to any one of (10) to (16), wherein the power source is a storage battery.

  According to the present invention, the thickness change, thickness change rate and distribution of the object to be measured, or the absolute value of the thickness at the time of measurement can be easily and accurately measured without stopping the apparatus / equipment. In addition to being able to measure well, continuously and long-term monitoring of changes in the thickness of the object under high temperature, high pressure, high vacuum, deep sea water, corrosive environment, radiation environment, etc. Therefore, it can be applied to confirm the safety of equipment, equipment, etc., and has a remarkable industrial effect. In addition, according to the present invention, by arranging the measurement terminals in a matrix shape, the change in thickness, which can only be detected in the form of dots in the measurement of the thickness of the object to be measured by the conventional ultrasonic flaw detection method or the like, is planarized. There is also an effect that detection becomes possible. In addition, according to the present invention, it is possible to detect and measure a change in thickness due to local corrosion.

As shown in FIG. 1, the nondestructive inspection apparatus used in the present invention includes a power source 1, a pair of electrodes 11 and 11 for applying a current from the power source 1 to the workpiece W, and a plurality of potential difference measuring terminals. 2, potential difference measuring means 3, computing means 4, data storage means 5, or further display means (not shown).
In the nondestructive inspection apparatus used in the present invention, it is preferable to have a plurality of reference terminals (not shown) on the surface of the region where the thickness of the object to be measured does not change. The plurality of reference terminals are preferably disposed so as to be sandwiched between the pair of electrodes 11 and 11 together with the plurality of potential difference measuring terminals 2. The reference terminal is provided to erase a resistance change (change in potential difference) caused by a cause other than a change in thickness, such as a change in temperature of an object to be measured. The plurality of reference terminals may be disposed on a reference plate that is placed in the same environment as the device under test and is of the same material as the device under test and has no change in thickness. In that case, it is necessary to insulate the reference plate so that the same current as that of the object to be measured flows. By locating and measuring the reference terminal on the reference plate, it is possible to estimate the position, distribution, size, etc. of the thickness change, etc., even for the object to be measured that has already undergone a thickness change, etc. It becomes possible.

  A plurality of potential difference measuring terminals 2 are preferably arranged in a matrix shape as illustrated in FIG. 1 in a desired potential difference measurement region on the surface of the object W to be measured by a joining means such as pressure welding, welding, pressure bonding, or adhesion. . As the bonding by pressure-contact of the potential difference measuring terminal, a method in which a pin having a sharp tip is pushed to the object to be measured using a clamp is preferable. Note that the bonding method is not particularly limited as long as the contact resistance does not change. A pair of electrodes 11 and 11 are disposed in the vicinity of both ends of the potential difference measurement region by a joining means such as welding. A pair of electrodes 11, 11 are wired with a current supply wire, and current can be supplied from the power source 1.

  The reference terminal is placed in an area where there is no change in the thickness of the object to be measured, or in the same environment as the object to be measured, in the same manner as the terminal for measuring the potential difference. It is preferable to dispose a plurality of elements in a matrix on a reference plate having no surface. When the reference terminal is provided, the potential difference measurement area and the reference terminal area are often separated from each other. Therefore, the potential difference measurement area and the reference terminal area are electrically connected. It is preferable to arrange the electrodes 11 and 11 so as to be connected in series.

  In the present invention, in addition to the pair of electrodes 11, 11, another pair of electrodes 12, 12 or a plurality of pairs of electrodes capable of applying a current in a direction different from the direction of the current supplied by the pair of electrodes 11, 11. Can be provided on the surface of the workpiece W. The direction of the difference is not particularly limited, but it is preferably a direction different from the current direction by 45 ° or 90 °. By making the current supply direction a plurality of directions, the detection accuracy of the wall thickness change is remarkably improved, and the wall thickness change in a narrow region such as local corrosion can be detected and measured with high accuracy. In the case where a plurality of pairs of electrodes are provided, a plurality of power sources may be provided, or switching means may be provided as a single power source to switch the electrodes that supply current and supply current.

  A plurality of potential difference measuring terminals 2e, 2f,..., 2k arranged in a matrix, or a plurality of reference terminals arranged on a reference plate without a change in thickness or a thickness change. The measuring end of the potential difference measuring means 3 is connected via a potential difference measuring lead wire. The material of the lead wire for potential difference measurement is preferably properly used in the usage environment. For example, in a high temperature or high temperature and high pressure environment up to about 700 ° C., it is preferable to use a single wire of an Al—Ni alloy containing about 3 to 5% Al as the lead wire for potential difference measurement. In the case of a stranded wire, oxidation is remarkably deteriorated. Further, when durability is required, it is preferable to apply silica coating. On the other hand, it is preferable to use copper or a copper-based alloy at a low temperature of, for example, about −30 ° C. Further, in a corrosive environment, it is preferable to use a wire material obtained by applying Ni plating to a copper wire as a lead wire for potential difference measurement. In this case, either a single wire or a stranded wire can be used. In addition, when durability is requested | required, it is preferable to provide a transparent resin coating. By applying the transparent resin coating, the durability is further improved and the corrosion state of the wire can be observed. The transparent resin is more preferably polytetrafluoroethylene (trade name: Teflon), which is a kind of fluororesin.

  The potential difference measuring means 3 is connected between a pair of terminals to be measured, and measures a potential difference between the terminals. After the measurement of the potential difference between the terminals is completed, the connected terminals are then switched to measure the potential difference between a pair of different terminals. The measurement end of the potential difference measuring means 3 is preferably switched manually or automatically according to a preprogrammed order by a switching means (not shown) such as a changeover switch.

In measuring the potential difference, a plurality of reference terminals are provided as described above in order to eliminate the resistance change caused by causes other than the wall thickness change such as the temperature change of the object to be measured. Is preferably measured simultaneously with the measurement between the terminals for potential difference measurement.
The nondestructive inspection apparatus used in the present invention requires the calculation means 4. The calculation means 4 performs various calculations using the potential difference between the potential difference measurement terminals measured by the potential difference measurement means 3, or the potential difference between the reference terminals, or the initial thickness between the potential difference measurement terminals as input data. To calculate the amount of change in wall thickness, the rate of change in wall thickness and their distribution, and the absolute value of the wall thickness at the time of measurement, and output it to the display means (not shown). It is preferable that various forms can be created. The calculation means is not particularly limited as long as it can perform the above-described calculation.

Furthermore, the nondestructive inspection apparatus used in the present invention has data storage means 5 capable of storing input data and calculation results. The data storage means 5 may be any storage means (memory) that can store the above-described data, and there is no particular limitation on the type of the data storage means 5.
In the nondestructive inspection apparatus used in the present invention, the power source 1 is preferably a plurality of power sources or a single power source with switching means. The power source 1 may be either an AC power source or a DC power source, but is preferably a power source that generates a pulse current in consideration of heat generation. Especially, it is preferable to set it as a storage battery, especially a lead storage battery from a viewpoint that a stable direct current | flow pulse can be supplied. When the monitoring area is wide or when the distance between the monitoring area and the power source is long, current supply by the storage battery requires a large current loss, so that the storage battery is exhausted and the wiring needs to be thick. In such a case, it is preferable to use an AC power source as the power source, set an AD converter near the monitoring region, and convert it into a direct current. As a result, the wiring can be thinned, and as a result, monitoring can be performed with high accuracy even in an environment where the distance is wide and long.

Next, a nondestructive inspection method for thickness change of an object to be measured according to the present invention using the above-described nondestructive inspection apparatus will be described.
In the present invention, a plurality of potential difference measuring terminals 2 are preferably arranged in a matrix as shown in FIG. Here, the matrix shape includes a lattice shape or a dust lattice shape. Depending on the measurement conditions, they may be arranged in a straight line. The electric field is disturbed by the presence of the thickness change, not only between the measurement terminals where the thickness change exists, but also between the other measurement terminals, and the change in potential difference greatly differs depending on the measurement position. Of course, in the region where the thickness change exists, the change is greatly observed also in the region located downstream of the existing thickness change. For this reason, by arranging the measurement terminals in a matrix and measuring the potential difference with respect to many combinations between the measurement terminals, it is possible to detect a change in the thickness of the object to be measured with higher accuracy.

At least a pair of electrodes 11 and 11 for supplying current are provided in the vicinity of both ends of the potential difference measurement region. A current is supplied from the power source 1 to the surface of the object to be measured through the pair of electrodes 11 and 11.
The supplied current can be measured by either alternating current or direct current, but in the present invention, it is preferably a direct current. The supplied current is particularly preferably a direct current pulse in consideration of a temperature rise due to resistance heat generation. The value of the supplied current is not particularly limited as long as the potential difference between each potential difference measurement terminal can be measured, but is preferably 10 to 2000 A. In particular, when the object to be measured is thick, by setting a high current of 100 A or more, a uniform electric field can be formed in the measurement region, and the measurement accuracy of the thickness change can be improved. In addition, a wider measurement area can be secured by using a higher current.

  While supplying a current between the electrodes placed on the surface of the object to be measured, the potential difference measuring means 3 preferably connects each potential difference measuring terminal arranged in this matrix, for example, between 2i and 2j, 2j in FIG. Measure potential difference between -2k, 2e-2f, 2f-2g, etc. In the present invention, from the viewpoint of improving the detection accuracy of the wall thickness change, it is preferable to measure the potential difference in every combination between the set potential difference measuring terminals. In measuring the potential difference, a plurality of reference terminals should be provided to eliminate potential difference changes due to causes other than wall thickness changes, such as temperature changes of the object to be measured, and the potential difference between the reference terminals should be measured simultaneously. Is preferred. The plurality of reference terminals may be arranged on a straight line with a predetermined interval on a reference plate where there is no change in thickness or on a reference plate where there is no change in thickness. It is preferable from the viewpoint of improving detection accuracy. It is preferable that the electrodes are arranged so that the current supply to the region where the plurality of reference terminals are arranged is electrically in series with the potential difference measuring region.

  In the present invention, the potential difference may be measured with only one direction of the supplied current in the potential difference measurement. However, after the potential difference is measured by supplying the current in one direction, the direction of the further supplied current is measured. Current in a direction different from the one direction, for example, a direction having a predetermined angle such as 45 °, 90 °, or an intermediate angle with respect to the one direction, It is preferable to measure the potential difference.

By changing the direction of the supplied current, the disturbance of the electric field due to the change in thickness existing in the object to be measured is different, and the potential difference is measured for currents in multiple directions compared to when the current direction is only one direction. As a result, the thickness change detection accuracy can be improved, and a narrow range thickness change such as local corrosion can be detected and measured with higher accuracy.
In the present invention, the potential difference generated between the set potential difference measuring terminals or the potential difference generated between the reference terminals is simultaneously measured intermittently or continuously at an arbitrary time after the reference time, and the potential difference between the terminals is measured. Calculate the rate of change. As the rate of change of the potential difference, for example, the following formula rate of change = {(Ai / Bi) × (B ₀ / A ₀ ) −1} × 1000
(Here, Ai: Potential difference between measurement terminals at the time of i (measurement), Bi: Potential difference between reference terminals at the time of i (measurement), A ₀ : Between measurement terminals at the time of 0 (standard: start of measurement) Potential difference, B ₀ : Potential difference between reference terminals at 0 time (standard: start of measurement)
Is preferably used.

Based on the obtained potential difference change or potential difference change rate, the presence / absence of occurrence of thickness change, the position and shape of the thickness change area, and the thickness change amount are grasped, and the progress of the thickness change area To figure out.
In the present invention, the relationship between the magnitude of the potential difference between the measurement terminals or the rate of change of the potential difference and the known thickness change amount, the thickness change area size, and the shape is obtained in advance by simulation. And stored in the data storage means. Based on the measured potential difference change or the potential difference change rate, the thickness change amount included in the object to be measured, the size and shape of the thickness change region, and the like are determined with reference to this relationship. These calculations are all performed by the calculation means 4. In the present invention, by inputting the initial thickness between each potential difference measurement terminal, the absolute value of the thickness between each potential difference measurement terminal at the time of measurement can be obtained by the calculation means, and the result is displayed on the display means. Can be displayed.

  As a simulation method, for example, four or more measurement terminals are set in a matrix shape on the surface, and a thickness change region in which the thickness change amount, the size change shape, and the shape of the thickness change region are known in the region. For the introduced measurement piece, measure the potential difference between each measurement terminal while flowing the current by shifting the current direction by, for example, 1 °, and the relationship between the wall thickness change amount, the wall thickness change area size and shape, and the voltage difference. There is a way to ask for. Further, for example, the relationship between the thickness change amount, the size of the thickness change region, the shape, and the potential difference may be created in advance as a master curve by using a finite element method. Note that the present invention is not limited to this method.

  In addition, from the obtained measurement results, the thickness difference state of the object to be measured can be clarified by displaying the potential difference distribution between each measurement terminal, the change rate distribution of the potential difference, or the absolute value of the thickness. can do.

Example 1
A carbon steel plate (wall thickness: 10 mm) was selected as the object to be measured. The thickness of the object to be measured was artificially reduced from the back surface, and each time, the potential difference between the potential difference measuring terminals was measured by the following method. In addition, the thickness reduction was performed stepwise over the entire surface of the steel sheet by a mechanical method. The amount of thinning was measured separately with a depth micrometer.

On the surface of the object to be measured, a plurality of potential difference measuring terminals (2a to 2p: 16 pieces in total) were arranged by stud welding in a matrix arrangement as shown in FIG. The distance between each end point was 20 mm. In addition, in order to supply a current to a region formed by the plurality of potential difference measurement terminals, a pair of electrodes 11 and 11 are provided around the end of the potential difference measurement region.
A direct-current pulse (pulse height: 110 A, pulse time: 1.7 s) was applied between the pair of electrodes 11 and 11. A DC potentiometer was used as a current difference measuring means, and the potential difference of each pair (pair No. 1 to No. 12) was measured intermittently as a pair between the terminals for measurement as shown in FIG. Needless to say, a measurement lead wire is attached to each measurement terminal in advance and is set to be switchable by a changeover switch. The same type of carbon steel plate (thickness: 10 mm) as the object to be measured is prepared as a reference plate, and the distance between terminals is set to 20 mm, which is the same as the distance between potential difference measurement terminals. Terminals were arranged. Place this reference terminal on the plus (+) side of the applied current and place it in the same environment as the object to be measured, in order to eliminate the potential difference change due to factors other than thickness change, between the potential difference measurement terminals The potential difference and the potential difference between the reference terminals were measured simultaneously.

Based on the measured potential difference between each pair, based on the measurement start time, the following equation: change rate = {(Ai / Bi) × (B ₀ / A ₀ ) −1} × 1000
(Here, Ai: measurement potential difference between the pair (between measurement terminals), Bi: potential difference between reference terminals at the time of measurement, A ₀ : potential difference at the start of measurement of the pair (between measurement terminals), B ₀ : at the start of measurement Potential difference between reference terminals)
The rate of change of the potential difference defined by is calculated. From the change rate of the obtained potential difference, the relationship between the change rate of the potential difference and the change in thickness obtained in advance by simulation was referred to and converted into the change in thickness between the terminals.

The obtained results are shown in FIG. 3 in relation to the thickness change amount obtained by the method of the present invention and the actual thickness reduction amount. From FIG. 3, the thickness change amount obtained by the method of the present invention and the actual thinning amount are in good agreement with each pair. According to this invention, it turns out that the thickness change of a to-be-measured object can be detected accurately.
It should be noted that a current is passed between the electrodes 12 and 12 previously installed at positions shifted by 90 ° from the pair of electrodes 11 and 11, and each potential difference measurement terminal is paired as shown in FIG. The potential difference of No. 13 to No. 24) was measured in the same manner, and the same result as in FIG. 3 was obtained.
(Example 2)
A carbon steel plate (wall thickness: 10 mm) was selected as the object to be measured. In the same manner as in Example 1, a plurality of potential difference measurement terminals (2a to 2p: 16 in total) were arranged by stud welding in a matrix arrangement as shown in FIG. The distance between each end point was 20 mm. Further, in order to supply a current to a region formed by the plurality of potential difference measuring terminals, a pair of electrodes 11 and 11 are arranged around the end of the potential difference measuring terminal region. First, as shown in FIG. 5, the thickness of the object to be measured is measured from the back side by a mechanical method with the length direction between the potential difference measuring terminals 2f and 2g parallel to the direction between the potential difference measuring terminals 2f to 2g. Local thinning. The initial thinning is 5mm long x 2mm wide, with a depth of 0.1mm. The depth is gradually increased, and the potential difference between each potential difference measurement terminal is measured each time as shown below. did.

  A DC pulse (pulse height: 110 A, pulse time: 1.7 s) was applied between the pair of electrodes 11, 11 as in Example 1. The current difference measuring means was the same as in Example 1, and the potential difference between each pair was measured with the pair of potential difference measuring terminals as a pair as shown in FIG. The same type of carbon steel plate (thickness: 10 mm) as the object to be measured is prepared as a reference plate, and the distance between the terminals is set to 20 mm, which is the same as the distance between the terminals for potential difference measurement. Terminals were arranged. Place this reference terminal on the plus (+) side of the applied current and place it in the same environment as the object to be measured, in order to eliminate the potential difference change due to factors other than thickness change, between the potential difference measurement terminals The potential difference and the potential difference between the reference terminals were also measured at the same time.

With respect to the measured potential difference between each pair, the rate of change in potential difference was calculated in the same manner as in Example 1 with reference to the start of measurement. From the change rate of the obtained potential difference, the relationship between the change rate of the potential difference and the change in thickness obtained in advance by simulation was referred to and converted into the change in thickness between the terminals.
In addition, a current is passed between the electrodes 12 and 12 previously placed at positions shifted by 90 ° from the pair of electrodes 11 and 11, and the potential difference measurement terminals are similarly paired as shown in FIG. The potential difference was measured in the same way.

The obtained results are shown in FIG. 6 in relation to the change in thickness obtained by the method of the present invention and the amount of local thinning introduced. FIG. 6 (a) shows a case where a current is passed between the electrodes 11, 11, that is, a direction parallel to the length direction (pair No. 5) of local thinning, and FIG. This is a case where the gas flows between the electrodes 12, 12, that is, in a direction orthogonal to the length direction of local thinning.
Between the terminals for potential difference measurement (pair No. 5) where there is local thinning, a change in potential difference is seen and can be detected as a change in thickness, but the local thinning length shown in FIG. 6 (a) When a current is passed in parallel to the direction (between the electrodes 11 and 11), the potential difference changes little, and the measured thickness change amount does not match the actual local thinning amount. . On the other hand, when a current is passed in the direction perpendicular to the local thinning length direction shown in FIG. 6B (between the electrodes 12 and 12), the amount of change in the potential difference measured by the pair No. 5 is large. It can be seen that the change in thickness can be accurately measured by changing the current direction. Moreover, the thickness change amount measured with the pair No. 5 is in good agreement with the actual local thickness reduction amount, and according to the present invention, the thickness change of the object to be measured can be detected with high accuracy. You can see that Furthermore, from the position where the local thinning exists, no change in the potential difference is observed in the upstream pair No. 2 where the current flows, whereas in the downstream pair No. 8, the potential difference is changed, which is localized. The location of thinning can be made clearer.

It is explanatory drawing which shows typically schematic structure of the nondestructive inspection apparatus used by this invention. It is explanatory drawing which shows the combination of the arrangement | positioning of an electrode used in the Example of this invention, the terminal for a potential difference measurement, and a pair. It is a graph which shows the relationship between the amount of wall thickness changes obtained in the Example of this invention, and actual amount of thickness reduction. It is explanatory drawing which shows the combination of another electrode and arrangement | positioning of a terminal for a potential difference measurement, and a pair used in the Example of this invention. It is explanatory drawing which shows typically the position (local thinning part) which performed the local thinning. It is a graph which shows the relationship between the thickness variation | change_quantity between each pair obtained in the Example of this invention, and an actual local thinning amount.

Explanation of symbols

1 Power supply
11, 12 Electrodes 2, 2a, 2b, ... 2p Potential difference measurement terminal 3 Potential difference measurement means 4 Calculation means 5 Data storage means

Claims (7)

  A plurality of potential difference measuring terminals are arranged at predetermined intervals on the surface of the object to be measured, and current is supplied to the surface of the object to be measured via a pair of electrodes provided with the plurality of potential difference measuring terminals interposed therebetween. A nondestructive inspection method for detecting a change in the thickness of an object to be measured by measuring a potential difference generated between the plurality of potential difference measuring terminals, wherein the plurality of potential difference measuring terminals are linear or matrix-shaped. The potential difference generated between the terminals of the plurality of potential difference measuring terminals is measured intermittently or continuously, the potential difference distribution in the measurement region is obtained, and the potential difference distribution previously associated with the thickness of the object to be measured The non-destructive inspection method for the change in the thickness of the object to be measured is characterized by detecting and measuring the thickness change amount and / or the thickness change rate generated in the object to be measured.   A plurality of potential difference measuring terminals are spaced apart from each other on the surface of the object to be measured with a predetermined interval, and a plurality of reference terminals for measuring the reference potential difference are provided on the surface of the region where the thickness of the object to be measured does not change at a predetermined interval. The plurality of potential difference measurement terminals while supplying current to the surface of the object to be measured through a pair of electrodes provided with the plurality of potential difference measurement terminals and the reference terminal interposed therebetween. A non-destructive inspection method for detecting a change in thickness generated in a measurement object by measuring a potential difference generated between the plurality of reference terminals and between the plurality of reference terminals, wherein at least the plurality of potential difference measuring terminals are linear or matrix The potential difference generated between the terminals of the plurality of potential difference measuring terminals and between the terminals of the plurality of reference terminals is measured intermittently or continuously at the same time, and the potential difference distribution in the measurement region is obtained in advance. Association The object to be measured is characterized by detecting and measuring the change in thickness and / or rate of change in thickness of the object to be measured with reference to the relationship between the measured potential difference distribution and the thickness of the object to be measured. Non-destructive inspection method for wall thickness change.   The initial thickness between the terminals of the potential difference measuring terminal is input, and the thickness of the object to be measured at the time of measurement between the terminals is calculated or further displayed. Non-destructive inspection method described in 1.   4. The nondestructive inspection method according to claim 2, wherein the reference terminal is disposed on a reference plate made of the same material as the object to be measured.   The non-destructive inspection method according to claim 1, wherein the potential difference is a potential difference change rate from a reference time, and the potential difference distribution is a potential difference change rate distribution.   6. The potential difference between the terminals is measured while measuring the potential difference between the terminals in the direction of the current, while supplying the current in a direction different from the direction of the current. The nondestructive inspection method as described in any one of. The non-destructive inspection method according to claim 1, wherein the current is a direct current or a direct current pulse.

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