JP2015179028A - Defect depth estimation method and defect depth estimation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an estimation technique for highly accurately estimating a defect depth of relatively small defect formed from an internal surface to an external surface of an inspection object on the basis of relatively limited data that is detectable.SOLUTION: A defect depth is estimated based on an estimation formula including an amplitude which is a width between a maximum value and a minimum value of a differential signal acquired from detection signals of a pair of magnetic sensors 22a, 22b, a tube axis direction width of the differential signal, and a tube circumferential direction width of the differential signal as independent variables, and including a defect depth as a dependent variable.

Description

  The present invention generates a magnetic field in the vicinity of an inspection object, measures a leakage magnetic flux from the inspection object, and estimates a defect depth of a defect formed on the inspection object from the measured value The present invention relates to an estimation method and a defect depth estimation apparatus using the method.

  Conventionally, a so-called leakage magnetic flux inspection method is known as a detection method for detecting defects formed on the outer surface of piping or the like due to corrosion or the like. In this leakage magnetic flux inspection method, predetermined magnetizing means (for example, a magnetizer that magnetizes different parts of the surface of the inspection object as separate poles, or a pair of magnetized coils arranged at different parts of the inspection object) Is magnetized by the magnetizer equipped with the above. If there is a defect caused by corrosion or the like in the inspection target portion magnetized in this way, the distribution of magnetic flux formed in the inspection target is disturbed by the presence of the defect, and a part of the defect is outside the inspection target. It leaks. By detecting the leakage magnetic flux leaking in this way by a magnetic sensor such as a Hall element, a defect can be detected (see Non-Patent Document 1). Usually, the number of Hall elements employed in such a leakage magnetic flux inspection method is single.

Further, a technique for estimating the defect depth of a defect existing in an inspection object based on the above-described magnetic flux leakage method is known (see Patent Document 1).
In addition, in the technique disclosed in Patent Document 1, a magnetic flux detection unit including a magnetizer and a magnetic sensor that create a test defect in a pipe serving as an inspection target and magnetizes the inspection target portion of the inspection target is provided. As a parameter from the detection signal obtained by moving along the test defect, a plurality of parameters such as a leakage magnetic flux peak value (leakage magnetic flux peak value, axial distribution width, circumferential distribution width, data acquisition speed, defect (Circumferential direction angle) is acquired, and in addition to these original parameters, their function calculation values (specified in Patent Document 1 as leakage flux peak value / circumferential distribution width) are obtained, and the original parameter and function calculation are obtained. The relationship between the value and the defect depth of the test defect is subjected to multiple regression analysis to obtain the defect depth estimation formula to derive the defect depth. In Patent Document 1, the [0006] paragraph exemplifies a defect aperture of 20 mm.

Published on October 15, 1992 "New Nondestructive Inspection Handbook" 2.11 Leakage magnetic flux test

Japanese Patent No. 4234914

The inventors detect the defect depth of the defect formed on the outer surface of the pipe as the inspection object by moving a magnetic flux detection unit including a magnetizing means and a magnetic sensor into the pipe and obtaining a detection signal. It has been studied to derive an estimation formula to estimate the defect depth from the signal.
That is, using a magnetizing means having a predetermined roughly horseshoe-shaped core, the inspection target part is magnetized between both ends of the core, and the defect depth is determined based on the detection signal of a single magnetic sensor disposed between both ends. An attempt was made to establish an estimation method for estimating the height. However, in the conventional leakage magnetic flux inspection method, the presence of a large defect formed on the outer surface of the tube (having a large defect opening diameter and a deep defect depth on the surface of the inspection object) is detected. However, it is impossible to reliably detect the presence of a defect whose opening diameter is about 10 mm, for example, and whose depth remains at about 50% of the tube thickness. It was. Therefore, the depth of the defect cannot be accurately estimated from the situation where the defect itself cannot be detected well. On the other hand, in the technique disclosed in Patent Document 1, the leakage flux peak value, the axial distribution width, the circumferential distribution width, the data acquisition speed, the circumferential angle of the defect are acquired, and in addition to these original parameters, the function calculation value is calculated. It is necessary to find out, for the reason that there are too many kinds of original parameters, it is impossible to perform a reliable multiple regression analysis, and for the small defects aimed by the inventors, in practice, It has not been put into practical use.

  The present invention has been made in view of the above-described problems, and an object thereof is, for example, a defect depth of a relatively small defect formed on the outer surface of the inspection object from the inner surface side of the inspection object. Is to provide an estimation technique that can be accurately estimated based on relatively limited data that can be detected.

In order to achieve the above object, the defect depth estimation method of the present invention is:
A defect depth estimation method that generates a magnetic field in the vicinity of an inspection object, measures a magnetic flux leakage from the inspection object, and estimates a defect depth of a defect formed on the inspection object from the measured value. The feature configuration is
Changes in magnetic flux when a pair of magnetic sensors provided along an inspection direction along a cylindrical axis of the inspection object having a hollow cylindrical shape are separated from the defect in the inspection direction. In a state where the magnetic sensor on the rear side in the inspection direction is close to the defect, the change in magnetic flux is detected at an interval that can be detected.
In a state where a magnetic field is generated in the vicinity of the inspection object, the pair of magnetic sensors are moved in the inspection direction with respect to a test defect formed on the inspection object, and detection signals of the pair of magnetic sensors are detected. From the difference signal of the magnetic flux component in the normal direction of the surface of the inspection object obtained from the above, extract at least the amplitude, the width in the tube axis direction, and the width in the tube circumferential direction, which are the widths of the maximum and minimum values of the difference signal Then, using the extracted amplitude, the tube axis direction width and the tube circumferential direction width, the amplitude which is the width between the maximum value and the minimum value of the difference signal obtained from the detection signals of the pair of magnetic sensors, the difference signal The depth of the defect is estimated based on an estimation equation in which the width in the tube axis direction and the width in the tube circumferential direction of the difference signal are independent variables and the defect depth is a dependent variable.

When detecting the magnetic flux component in the normal direction of the surface of the inspection object, the detection signal is usually detected when the magnetic sensor is approaching the defect and detected when the magnetic sensor is away from the defect. The detection signal based on the change in magnetic flux is a graph in which the direction of magnetic flux with respect to each magnetic sensor (the direction of magnetic flux in the normal direction of the inspection object) is reversed, and the horizontal axis is the inspection direction. Sometimes the result is a graph with one convex upward and the other convex downward. For this reason, these difference signals have large peaks in the vicinity of the defect. That is, such detection signals can be amplified by enabling the same defect to be sensed simultaneously by a pair of magnetic sensors arranged in the moving direction.
In other words, the present invention pays attention to this point, and first detects a change in magnetic flux when a pair of magnetic sensors are in the inspection direction and the magnetic sensor on the front side in the inspection direction is separated from the defect. By arranging the magnetic flux change at a detectable interval when the rear magnetic sensor is approaching the defect in the inspection direction, the difference signal between the detection signals of the pair of magnetic sensors detects the defect. A strong detection signal is obtained in the state of being.
In other words, since the detection signal of a specific defect can be extracted largely by appropriately selecting the distance between the pair of magnetic sensors, the size of a relatively small defect intended by the inventors can be accommodated. An appropriate detection signal (difference signal) can be obtained.
Further, according to the present invention, for each of the plurality of difference signals detected by the pair of magnetic sensors set at appropriate intervals as described above, the width of the maximum value and the minimum value is determined. In order to obtain a defect depth by using an estimation formula that extracts a certain amplitude, a width in the tube axis direction, and a width in the tube circumferential direction, and uses all of these and a limited number of information as independent variables, With reasonable criteria, even the defect depth of relatively small defects targeted by the inventors can be estimated appropriately.

In order to achieve the above object, an apparatus for estimating a defect depth according to the present invention comprises:
Magnetizing means for magnetizing the inspection object and leakage magnetic flux detection means for detecting leakage magnetic flux leaking from the inspection object magnetized by the magnetization means are moved along the inspection direction of the surface of the inspection object. A defect depth estimation device for estimating a defect depth of a defect formed in the inspection object from an output of the leakage magnetic flux detection means, and the characteristic configuration thereof is:
The leakage magnetic flux detection means includes at least a pair of magnetic sensors arranged along the inspection direction along the cylindrical axis of the inspection object having a hollow cylindrical shape, and the distance between the pair of magnetic sensors in the inspection direction. In a state in which a change in magnetic flux is detected when the magnetic sensor on the front side in the inspection direction is separated from the defect, the magnetic sensor on the rear side in the inspection direction approaches the defect. Is arranged as a detectable interval of the change in magnetic flux when
The pair of magnetic sensors is moved along the inspection direction with respect to the inspection object on which a plurality of test defects are formed, and is obtained from detection signals of the pair of magnetic sensors in the normal direction of the surface of the inspection object. Parameter extraction means for extracting at least the amplitude, the tube axis direction width, and the tube circumferential direction width, which are the difference between the maximum value and the minimum value, from the difference signal of the magnetic flux component;
The amplitude that is the width between the maximum value and the minimum value of the difference signal obtained from the detection signals of the pair of magnetic sensors, the tube axis direction width of the difference signal, and the tube circumferential width of the difference signal are independent variables, and the defect depth is A storage unit for storing an estimation formula as a dependent variable;
Among the difference signals, the amplitude of the difference signal obtained when the pair of magnetic sensors pass through the defect formed in the inspection object, the tube axis direction width, and the tube circumferential direction width are the estimation equations. And a defect depth estimation unit that estimates the defect depth by inputting the defect depth estimation formula derived by the deriving unit.

  The defect depth estimation apparatus having the above-described characteristic configuration is an apparatus that can estimate the defect depth by performing the defect depth estimation method described above, and according to the defect depth estimation apparatus, the reason described above For the same reason as described above, even the defect depth of a relatively small defect, which is the object of detection by the inventors, can be estimated well.

A further characteristic configuration of the defect depth estimation apparatus of the present invention is:
The pair of magnetic sensors are arranged at intervals less than the width of the defect to be detected in the inspection direction.

Furthermore, the inventors have arranged the pair of magnetic sensors at an interval less than the width of the defect to be detected in the inspection direction, so that one of the pair of magnetic sensors is separated from the defect. It was found that a change in magnetic flux can be detected when a change in magnetic flux is detected and a defect is approached on the other side.
That is, as in the present invention, by arranging a pair of magnetic sensors at an interval less than the width of the defect to be detected in the inspection direction, a detection signal of a defect larger than the width of the detection object is output as a large peak, Defects can be detected more appropriately.

A further characteristic configuration of the defect depth estimation apparatus of the present invention is:
The pair of magnetic sensors is provided in a plurality along the inner circumferential direction of a cylindrical pipe as the inspection object.

  According to the above characteristic configuration, a plurality of pairs of magnetic sensors are provided along the inner circumferential direction of a cylindrical pipe as an inspection object. If it arrange | positions to (1), the inspection of the perimeter of the inner peripheral surface of piping to be detected can be completed only by once scanning the inspection device in the inspection direction (the pipe axis direction of the pipe). In addition, it is possible to reduce detection failures of defects in the circumferential direction.

A further characteristic configuration of the defect depth estimation apparatus of the present invention is:
The magnetizing means is suspended in a state of forming a gap with the surface of the inspection object, and the magnetic sensor is suspended through a vibration absorbing portion, and is grounded to the surface of the inspection object. And a roller that travels along the surface in a state.

According to the above characteristic configuration, first, the support is suspended in a state where a gap is formed between the magnetizing means and the surface of the inspection object, so that the magnetizing means is magnetically attached to the inspection object by its magnetic force. Can be prevented, and the inspection apparatus can be smoothly moved in the inspection direction. With this configuration, the inspection object can be appropriately magnetized by the magnetizing means, and a magnetic flux can be generated along the surface of the inspection object.
In addition, since the pair of magnetic sensors are suspended on the support via the vibration absorbing portion, it is possible to suppress vibration accompanying movement in the inspection direction to the pair of magnetic sensors.

Schematic which shows the use condition of the defect depth estimation apparatus of this invention Sectional drawing of the inspection unit which comprises the defect depth estimation apparatus of this invention, and the functional block diagram of the control apparatus Plan view showing the arrangement of sensors provided in the inspection unit The graph which shows the detection signal which measured the change of the magnetic flux by a defect, a difference signal, and the moving average signal which carried out moving average Graph diagram corresponding to defect of φ = 10mm The figure which shows the amplitude Vpp, the pipe-axis direction width Vw, and the pipe circumferential direction width Vr in the difference signal of a pair of magnetic sensor In the case where the defect depth estimation formula is derived by performing multiple regression analysis including the pipe axis direction width Vw and the pipe circumferential direction width Vr in addition to the amplitude of the difference signal of the pair of magnetic sensors, and the defect depth is estimated by the estimation formula Graph showing the estimation error of A graph showing the estimation error when a defect depth estimation formula is derived by regression analysis of only the amplitude of the magnetic sensor and the defect depth is estimated by the estimation formula

As shown in FIG. 1, the defect depth estimation apparatus 100 of the present invention appropriately detects the defect depth of a relatively small defect 15 formed on the outer surface of a pipe 10 (an example of an inspection object). It is related with the estimation apparatus 100 which can do.
As shown in FIG. 1, the defect depth estimation apparatus 100 moves the inside of the pipe 10 along the pipe axis direction of the pipe 10 (an example of the inspection direction: the direction along the arrow X in FIG. 1). Control comprising an inspection device 50 capable of scanning the inside and a detection data analysis computer for estimating the defect depth of the defect 15 formed on the outer surface of the pipe 10 based on a detection signal detected by the inspection device 50 It is comprised from the apparatus R etc.
The defect depth estimation apparatus 100 according to the present invention is characterized by the detection method of the defect 15 by the inspection apparatus 50. First, the inspection apparatus 50 will be described.

  As shown in FIG. 1, the inspection device 50 can inspect for defects 15 such as corrosion and thinning that may be formed on the outer surface of a pipe 10 (an example of an inspection object) embedded in the ground. The inside of the pipe 10 can be scanned while being connected to the traction devices 11a and 11b and the inspection data inspected by the inspection device 50 is Ethernet (registered trademark) or the like. Are collected by a detection data analysis computer (control device R) arranged on the ground.

The inspection apparatus 50 is configured by arranging a plurality of inspection units 50a shown in FIG. 2 in the circumferential direction.
As shown in FIG. 2, the single inspection unit 50 a detects a permanent magnet 21 for magnetizing the pipe 10 (an example of a magnetizing unit) and leakage magnetic flux leaking from the pipe 10 magnetized by the permanent magnet 21. And a magnetic sensor 22 (an example of leakage magnetic flux detection means).
In the inspection unit 50 a, the permanent magnet 21 and its magnetic field are propagated to the pipe 10 so as to move along the inspection direction of the surface of the pipe 10 (the pipe axis direction which is the direction of the arrow X in FIGS. 1 and 2). The iron core 27 is suspended in a state of forming a gap with the surface of the pipe 10, and the magnetic sensor 22 is in a direction perpendicular to the inspection direction (close to and away from the pipe surface which is the direction along the arrow Y in FIG. 2). A support 24 suspended via a spring 23 (an example of a vibration absorbing portion) that can be expanded and contracted in a direction), and a pair of rollers 25 that run along the surface while being installed on the surface of the pipe 10. ing. Thereby, the inspection unit 50a can smoothly move along the inspection direction and detect the defect 15 formed in the pipe 10 by the output of the magnetic sensor 22.

The magnetizing means according to the present invention includes an iron core 27 disposed in a substantially horseshoe shape and a pair of permanent magnets 21a and 21b magnetically coupled by the iron core 27 with the magnetic sensor 22 sandwiched in the inspection direction. It is configured. As a result, the surface of the pipe 10 facing the pair of permanent magnets 21a and 21b is magnetized, and the leakage magnetic flux leaking from the magnetized pipe 10 can be measured by the magnetic sensor 22.
The width L1 (shown in FIG. 2) of the gap formed between the pipe 10 side ends of the pair of iron cores 27 (different magnetic poles are formed) and the surface of the pipe 10 is as follows. Due to the magnetic force, the width is set so as not to be magnetically attached to the surface of the pipe 10, and the width is such that the pipe 10 can be appropriately magnetized without hindering movement of the inspection unit 50 a in the inspection direction. . Specifically, the magnetism is preferably about 4000 gauss and about 1.5 mm.

  Next, the magnetic sensor 22 will be described. The inventors of the present invention generally detect the magnetic sensor 22 in accordance with a detection signal accompanying a change in magnetic flux detected when the magnetic sensor 22 is separated from the defect 15 and a change in magnetic flux detected when the magnetic sensor 22 is close to the defect 15. Focusing on the fact that the accompanying detection signal is output with the amplitude being reversed in the vertical direction, the following configuration has been adopted in order to detect a relatively small defect. As such a magnetic sensor 22, a so-called Hall element is employed.

That is, in the present invention, a pair of magnetic sensors 22 are arranged along the inspection direction (the direction indicated by arrow X in FIG. 2), and the pair of magnetic sensors 22a and 22b are inspected in the inspection direction. The magnetic sensor 22a provided on the front side in the direction (indicated by the arrow X in FIG. 2) detects a change in magnetic flux due to separation from the defect 15, and the rear side in the inspection direction (the arrow in FIG. 2). The magnetic sensor 22b provided on the opposite side of the arrow X is disposed at an interval that can detect a change in magnetic flux caused by approaching the defect 15.
In other words, the pair of magnetic sensors 22a and 22b are arranged at an interval L2 that is less than the width of the defect 15 to be detected in the inspection direction. In the present invention, since the relatively small defect 15 (having an opening diameter of about 10 mm in the inspection direction) is to be inspected, the arrangement interval L2 between the pair of magnetic sensors 22a and 22b is several. mm.

Accordingly, when the magnetic sensors 22a and 22b pass near the defect 15, when the magnetic sensor 22a on the front side in the inspection direction is separated from the defect 15, the magnetic sensor 22b on the rear side in the inspection direction has the defect 15 It will be in the state approaching. At this time, the magnetic flux component in the normal direction on the surface of the pipe 10 detected by the front magnetic sensor 22a in the inspection direction and the magnetic flux component in the normal direction on the surface of the pipe 10 detected by the rear magnetic sensor 22b in the inspection direction. However, the reverse is true.
That is, when the detection signal of the magnetic flux of the magnetic sensor 22a on the front side in the inspection direction and the detection signal of the magnetic flux of the magnetic sensor 22b on the rear side in the inspection direction are plotted with the horizontal axis as the inspection direction, Since it is output as a graph having a convex shape on the other side and a convex side on the other side, a high peak is output by taking the difference signal between them.
As a result, it is possible to appropriately detect the defect 15 having an opening diameter larger than the distance L2 in the inspection direction of the pair of magnetic sensors 22a and 22b in the inspection direction.

  The detection signals detected by the pair of magnetic sensors 22a and 22b are processed by a generally used electric circuit. Specifically, a difference is taken by a difference circuit, a low frequency component is removed by a low-pass filter circuit, amplified by a lock-in amplifier, a moving average is taken by a control circuit, and then a computer for detecting data analysis (control device) R). Here, since the difference circuit, the low-pass filter circuit, the lock-in amplifier, and the control circuit have known configurations, detailed description and illustration thereof are omitted here.

In the inspection unit 50a, as shown in FIGS. 2 and 3, a pair of magnetic sensors 22a and 22b are provided along the inner circumferential direction of the pipe 10, that is, in the direction perpendicular to the inspection direction, along the surface of the pipe 10 (see FIG. Three sets are provided at a distance L3 (118 mm in this embodiment) in the front and back direction of the paper surface 2 and in the direction along the arrow Z in FIG.
The inspection apparatus 50 according to the present invention includes eight inspection units 50a along the inner periphery of the pipe 10 (not shown). Inspection is possible.

FIG. 4 shows a detection result of the defect 15 when the inspection apparatus 50 described so far is used.
The graph of FIG. 4 is a graph based on the measurement results of magnetic flux by the pair of magnetic sensors 22a and 22b for convenience. The inspection object was a pipe 10 provided with a plurality (four in FIG. 4) of defects 15 having a diameter of 10 mm in the inspection direction along the inspection direction (tube axis direction) of the pipe 10. The plurality of defects 15 are formed deeper toward the front side (indicated by the arrow X in FIG. 4) in the inspection direction.
The distance L2 between the pair of magnetic sensors 22a and 22b is set to several mm, and is set to be less than the width (10 mm) of the defect 15 in the inspection direction.

In the graphs shown in FIGS. 4A to 4D, the vertical axis is a voltage signal indicating the strength of magnetic flux in the normal direction of the surface of the pipe 10, and the horizontal axis indicates the position in the inspection direction.
4A is a detection signal of a change in magnetic flux by the magnetic sensor 22a on the front side in the inspection direction of the pair of magnetic sensors 22a and 22b, and FIG. 4B is a magnetic signal on the rear side in the inspection direction. FIG. 4 (c) is a difference signal between FIG. 4 (a) and FIG. 4 (b), and FIG. 4 (d) is a difference signal of FIG. 4 (c). It is a moving average signal obtained by moving and averaging the difference signal.
In FIG. 4, the position indicated by the arrow α is a position where the deep defect 15 exists in the thickness direction of the pipe 10, and the change in the graph is remarkable. Hereinafter, based on the inspection signal at the position indicated by the arrow α. ,explain.
At this position, the detection signal (the signal shown in FIG. 4A) of the change in magnetic flux by the magnetic sensor 22a on the front side in the inspection direction appears in a downwardly convex state, and the magnetic sensor on the rear side in the inspection direction. The detection signal (the signal shown in FIG. 4B) of the change in the magnetic flux by 22b appears in a convex state upward. Then, when the difference signal of these detection signals is taken, as shown in FIG. 4C, a relatively large peak difference signal appears at the position where the defect 15 exists. The moving average signal obtained by taking the moving average of the difference signal is a signal from which fine noise has been removed, as shown in FIG.

Next, a moving average signal (shown in FIG. 5A) when the pair of magnetic sensors 22a and 22b pass through the center of the defect 15 in a direction orthogonal to the inspection direction, and the pair of magnetic sensors 22a and 22b. Shows a moving average signal (shown in FIG. 5B) when passing through a position 6 mm away from the center of the defect 15 in a direction perpendicular to the inspection direction.
As the defects 15, those having an opening diameter of 10 mm in the inspection direction and depths of 50%, 70%, and 90% of the thickness of the pipe 10 are arranged in the order of description in the inspection direction.

As shown in FIG. 5A, in the moving average signal when the pair of magnetic sensors 22a and 22b pass through the center of the defect 15 in the direction orthogonal to the inspection direction, the depth is 50 of the thickness of the pipe 10. It can be seen that a good peak is formed in any defect 15 of%, 70%, and 90%, and can be detected appropriately.
On the other hand, as shown in FIG. 5B, even in the moving average signal when the pair of magnetic sensors 22a and 22b pass through a position that is 6 mm away from the center of the defect 15 in the direction orthogonal to the inspection direction, It can be seen that a good peak is formed at any defect 15 of 50%, 70% or 90% of the thickness of the pipe 10 and can be appropriately detected.
Here, the result of the defect 15 having an opening diameter of about 10 mm was shown. However, as a disadvantage of adopting a configuration in which the magnetic sensors 22a and 22b are brought close to each other and the difference between them is adopted, detection of a large opening diameter is detected. However, according to the study by the inventors, defects having an opening diameter of about 30 mm and an opening diameter of about 50 mm were successfully detected as before.

Although the inspection apparatus 50 has been described above, the present invention is based on the difference signal detected by the inspection apparatus 50 described above, and the defect depth estimation formula for estimating the defect depth of a relatively small defect. Are derived, a defect depth estimation method, and a defect depth estimation apparatus 100.
Hereinafter, description will be added based on FIGS.
As shown in FIG. 2, the defect depth estimation apparatus 100 of the present invention includes a control device R that receives a differential signal transmitted by the inspection apparatus 50 via the communication line 13.
The control device R includes parameter extraction means R1 that extracts parameters from the difference signals of a plurality of test defects, a storage unit R2 that stores a defect depth estimation formula used to derive the defect depth, and a stored defect depth Substituting parameters such as the amplitude Vpp extracted from the difference signal of the defect to be detected into the depth estimation equation, the defect depth estimation means R3 for estimating the defect depth, and the monitor for outputting the estimated defect depth Output section R4. Here, the parameter extraction unit R1, the storage unit R2, and the defect depth estimation unit R3 can be realized by software installed in the control device R as a program.

(Derivation of defect depth estimation formula)
First, the method for deriving the defect depth estimation formula used for deriving the defect depth according to the present invention will be described below.
In deriving the estimation formula, first, a plurality of test defects formed in the pipe 10 (an example of the inspection object) are formed, and the plurality of test defects are individually identified, and the defect depth is separately determined. Keep measuring. Then, separately, the pipe 10 (an example of the inspection object) on which the plurality of test defects are formed is inspected by the inspection apparatus 50, and a plurality of difference signals are acquired. In this way, a plurality of differential signals acquired by the inspection apparatus 50 are received via the communication line 13, and as parameters of the defect depth estimation formula, as shown in FIGS. 6 (a) and 6 (b). From the difference signal, the amplitude Vpp, tube axis direction width Vw, and tube circumferential direction width Vr of the difference signal are extracted for the identified defect. Here, FIG. 6A shows the difference signal of the pair of magnetic sensors 21a and 21b displayed as an image in which the luminance increases as the leakage magnetic flux amount increases. The horizontal direction in the drawing corresponds to the inspection direction (tube axis direction), and the vertical direction in the drawing corresponds to the pipe circumferential direction. On the other hand, FIG. 6B is a graph of the amount of magnetic flux leakage along the tube axis direction at one tube circumferential position in FIG. 6A. The horizontal direction of the drawing corresponds to the inspection direction (tube axis direction), and the vertical direction of the drawing corresponds to the leakage magnetic flux amount. 6A and 6B show the positions in the tube axis direction in the left-right direction on the paper.

As shown in FIG. 6B, the amplitude Vpp means the difference between the minimum value and the maximum value of the difference signal, and the peak value extending upward from the peak value extending downward based on the reference value Vo. The value is between. A pair of peak values extending downward with reference to the reference value Vo is present with the peak value extending upward therebetween, but the larger peak value is used for the amplitude Vpp.
As shown in FIG. 6B, the tube axis direction width Vw is defined as an interval between two peak values extending downward with reference to the reference value Vo.
As shown in FIG. 6A, the pipe circumferential width Vr is a width in the pipe circumferential direction that is equal to or larger than a predetermined set value.

Here, in estimating the defect depth, in the present embodiment, logarithms are respectively employed for the amplitude Vpp of the differential signal, the tube axis direction width Vw, and the tube circumferential direction width Vr, which are independent variables. The reason for taking the logarithm for the parameter is to make the non-linear relationship a linear relationship.
The dependent variable in the estimation formula is the defect depth, but the respective defect depths, the amplitude Vpp of the difference signal, the tube, which have been found to determine the number of processes between the above three independent variables and the dependent variable. Multiple regression analysis is performed between the axial width Vw and the pipe circumferential width Vr.
In this multiple regression analysis, coefficients a, b, c, and d were determined with test defects having a defect depth of approximately 20% to 100% of the tube thickness as an analysis target.

Defect depth (%) = 100 × (a × LN (Vpp) + b × LN (Vw) + cLN (Vr)
+ D) (Formula 1)
Here, the defect depth (%) means a ratio to the tube thickness.
The values of the variables obtained by the multiple regression analysis are as follows.
a: 0.242
b: 0.071
c: -0.203
d: 1.044

[Defect depth estimation in the control unit]
The defect depth estimation means R3 as the control device R described above estimates the defect depth of the defect to be detected based on the defect depth estimation formula stored in advance in the storage means R2.
When the description is added, the defect depth estimation means R3, when receiving the difference signal of the defect to be detected from the inspection apparatus 50, sets the amplitude Vpp, the tube axis direction width Vw, and the tube circumferential direction width Vr as parameters from the difference signal. The extracted parameters are input to the defect depth estimation formula (previously described (Formula 1)) derived in advance and stored in the storage unit R2, and the defect depth is estimated.
In the control device R, the output unit R4 outputs the estimated defect depth to a monitor (not shown) or the like.

Next, the reliability of the estimated value of the defect depth estimated by the defect depth estimation formula used in the present invention will be described.
FIG. 6 shows the actual defect depth (%) on the vertical axis and the estimated defect depth (%) on the horizontal axis for defects α, defect β, defect γ, and defect δ having different shapes. , Plotted. That is, in FIG. 6, the closer to the straight line shown on the diagonal line in the graph, the smaller the error in the estimated value of the defect depth, and the greater the deviation from the straight line, the larger the error in the estimated value of the defect depth. means. In FIG. 6, the region between the one-dot chain line and the two-dot chain line is a region where the deviation amount (estimated error) of the estimated defect depth (%) from the actual defect depth (%) is within 20%. In this estimation, the amplitude Vpp, the tube axis direction width Vw, and the tube circumferential direction width Vr of the difference signal described above are all independent variables.
The shape and size of the defect to be detected are as follows.
Defect α: Defect with a tube axial width of 30 mm and a tube circumferential width of 10 mm Defect β: Defect with a tube axial width of 10 mm and a tube circumferential width of 30 mm Defect γ: Tube axial width of 70 mm, Tube circumferential width Defect δ: Defect δ: Defect having a mortar shape with a tube axial width of 10 mm and a tube circumferential width of 30 mm All defect depths except for some defect depth estimates from the graph of FIG. The estimated value is within an error range of 20%, and it can be seen that the defect depth can be accurately estimated with sufficient accuracy for practical use.

Next, FIG. 7 shows an error from the actual defect depth when using the defect depth estimation formula when only the amplitude Vpp of the difference signal according to the present invention is used as an independent variable.
As shown in the graph of FIG. 7, the estimated value of the defect depth of a considerable proportion is out of the error range of 20%, and the defect depth cannot be estimated with high estimation accuracy as in the present invention. It turns out that it is not practical.

[Another embodiment]
(1) In the above embodiment, the distance L2 between the pair of magnetic sensors 22a and 22b is several mm, but the distance L2 can be appropriately changed according to the width of the defect to be inspected in the inspection direction.

  The defect depth estimation method and the defect depth estimation apparatus according to the present invention compare the defect depths of relatively small defects formed on the outer surface of the inspection object from the inner surface side of the inspection object. Using information that can be obtained easily and with good reliability, it can be effectively estimated with sufficient accuracy to withstand practical use.

10: Piping 15: Defect 22: Magnetic sensor 23: Spring 24: Support 25: Roller R1: Parameter extraction means R2: Storage section R3: Defect depth estimation means R4: Output section Vpp: Amplitude Vr: Pipe circumferential width Vw : Pipe axis width

Claims (5)

A defect depth estimation method that generates a magnetic field in the vicinity of an inspection object, measures a magnetic flux leakage from the inspection object, and estimates a defect depth of a defect formed on the inspection object from the measured value. And
Changes in magnetic flux when a pair of magnetic sensors provided along an inspection direction along a cylindrical axis of the inspection object having a hollow cylindrical shape are separated from the defect in the inspection direction. In a state where the magnetic sensor on the rear side in the inspection direction is close to the defect, the change in magnetic flux is detected at an interval that can be detected.
In a state where a magnetic field is generated in the vicinity of the inspection object, the pair of magnetic sensors are moved in the inspection direction with respect to a defect formed on the inspection object, and detection signals of the pair of magnetic sensors are used. From the difference signal of the magnetic flux component in the normal direction of the surface of the inspection object to be obtained, extract at least the amplitude that is the width between the maximum value and the minimum value of the difference signal, the tube axis direction width, and the tube circumferential direction width. , Using the extracted amplitude, the tube axis direction width and the tube circumferential direction width, the amplitude which is the width between the maximum value and the minimum value of the difference signal obtained from the detection signals of the pair of magnetic sensors, A defect depth estimation method for estimating the depth of the defect based on an estimation formula in which the tube axis direction width and the tube circumferential direction width of the difference signal are independent variables and the defect depth is a dependent variable. Magnetizing means for magnetizing the inspection object and leakage magnetic flux detection means for detecting leakage magnetic flux leaking from the inspection object magnetized by the magnetization means are moved along the inspection direction of the surface of the inspection object. A defect depth estimation device for estimating a defect depth of a defect formed in the inspection object from an output of the leakage magnetic flux detection means,
The leakage magnetic flux detection means includes at least a pair of magnetic sensors arranged along the inspection direction along the cylindrical axis of the inspection object having a hollow cylindrical shape, and the distance between the pair of magnetic sensors in the inspection direction. In a state in which a change in magnetic flux is detected when the magnetic sensor on the front side in the inspection direction is separated from the defect, the magnetic sensor on the rear side in the inspection direction approaches the defect. Is arranged as a detectable interval of the change in magnetic flux when
The pair of magnetic sensors are moved along the inspection direction with respect to the inspection object on which the defect is formed, and the magnetic flux component in the normal direction of the surface of the inspection object is obtained from the detection signals of the pair of magnetic sensors. Parameter extraction means for extracting at least the amplitude, the tube axis direction width, and the tube circumferential direction width, which are the difference between the maximum value and the minimum value, from the difference signal;
The amplitude that is the width between the maximum value and the minimum value of the difference signal obtained from the detection signals of the pair of magnetic sensors, the tube axis direction width of the difference signal, and the tube circumferential width of the difference signal are independent variables, and the defect depth is A storage unit for storing an estimation formula as a dependent variable;
Among the difference signals, the amplitude of the difference signal obtained when the pair of magnetic sensors pass through the defect formed in the inspection object, the tube axis direction width, and the tube circumferential direction width are input to the estimation equation. A defect depth estimation device comprising defect depth estimation means for estimating the defect depth.   The defect depth estimation apparatus according to claim 2, wherein the pair of magnetic sensors are arranged at an interval less than the width of the defect to be detected in the inspection direction.   4. The defect depth estimation apparatus according to claim 2, wherein a plurality of the pair of magnetic sensors are provided along an inner circumferential direction of a cylindrical pipe as the inspection object.   The magnetizing means is suspended in a state of forming a gap with the surface of the inspection object, and the magnetic sensor is suspended through a vibration absorbing portion, and is grounded to the surface of the inspection object. The defect depth estimation apparatus according to any one of claims 2 to 4, further comprising a roller that travels along the surface in a state.

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