JP2000241391A - Method for diagnosing stress of steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately diagnose a stress to a steel pipe even when not only a stress in an elastic range, but an external stress exceeding a yield stress acts to the steel pipe. SOLUTION: A plurality of predetermined measurement points P in a circumferential direction of a surface of a steel pipe 4 having a compressive residual stress are excited with an a.c. with use of a magnetic head 1 consisting of an excitation head 2 and a detection head 3. One of planes intersecting the predetermined measurement points of the surface and including a center line of a pipe axis T of the steel pipe 4 is set as a reference face S, and each measurement point is displayed by an angle between the reference face and the plane including the measurement point. Measurement points Pa and Pb which are spaced by an interval of 180 deg. and show effective value voltages of minimal values (minimum values) are obtained from a relationship of the angles and an effective value voltage of Barkhausen noises. A maximum value of the effective value voltage of a central point Pc is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for non-destructively diagnosing a stress generated in a buried steel pipe due to land subsidence or stratum deformation by utilizing Barkhausen noise generated from the steel pipe. .

[0002]

2. Description of the Related Art Steel pipes such as gas supply pipes and water pipes are buried in the ground, and when land subsidence occurs, bending stress is generated between steel pipe portions having different settlement amounts. If the stress acts on the steel pipe for a long period of time, there is a risk that stress corrosion cracking will occur, and if the stress is excessive, the steel pipe may be damaged. In particular, in order to prevent this from occurring in the gas supply pipe, the stress acting on the buried pipe must be monitored to confirm safety.

[0003] For this purpose, a thin bolt is opened from the ground surface to the surface of the steel pipe, a rod called a sinking rod is inserted into the steel pipe, and the deformation of the steel pipe generated in the ground is estimated from the amount of sinking of the rod to determine the bending stress. Has been conventionally implemented. However, this method cannot measure the displacement of the steel pipe in the horizontal direction, and the accuracy of estimating the amount of deformation of the steel pipe is insufficient due to the limited number of subsidence rods. was there. Therefore, a method has been proposed in which a magnetostrictive sensor (magnetic anisotropic sensor) utilizing magnetostriction is directly applied to the surface of a steel pipe, and the stress acting on the steel pipe is obtained from the output value.

[0004] This measurement principle is based on the fact that magnetostriction is positive in a steel material such as iron, so that when a stress acts on the surface of a steel pipe, the magnetic permeability increases in a tensile stress direction and decreases in a compressive stress direction. Things. For example, a stress release method for adjusting a value obtained by approximating a magnetostrictive sensor output measured around a steel pipe with a sine curve to a value not exceeding a security reference value or a minimum value (JP-A-3-176630) Japanese Patent Application Laid-Open No. 3-176626 discloses a method of linearly approximating the angle dependence of the output of a magnetostrictive sensor near two stress-neutral portions and estimating a bending stress from an average value of the inclinations of the two. Of approximating the difference between the approximation and the SINθ approximation by SIN2θ, and estimating the flat stress from the amplitude value (Japanese Patent Laid-Open No. 3-176627), removing the measured value of the welded portion when measuring the ERW pipe with a magnetostrictive sensor (Refer to Japanese Patent Application Laid-Open No. 5-281058), measuring the outer diameter at the position where the output of the magnetostrictive sensor is maximum, and the outer diameter at a position shifted by 90 ° from the position, and calculating the flatness factor. Method for correcting the maximum stress value in the direction (JP-A-6-288842) Report), a management method that incorporates the stress partially measured by a magnetostrictive sensor or the like into the simulation by measuring the amount of settlement to determine the maximum stress in the entire buried pipe so that it does not exceed the reference value (Japanese Patent Application Laid-Open No. 9-1997). 242933), and the like.

However, since all of these methods use a magnetostrictive sensor, it is necessary to obtain a stress in the elastic range below the yield stress of the steel pipe, and to find a stress in the plastic region above the yield stress. Have difficulty. further,
Correction was necessary because the linearity of the calibration curve became worse as the yield stress was approached.

[0006]

As described above, conventionally,
In order to accurately determine the stress acting on the steel pipe, it has been limited to the case where the stress is in an elastic region below the yield stress. However, it is expected that there are many cases where a stress higher than the yield stress is acting on the steel pipe actually buried, and the stress in such a plastic region should be monitored most carefully by nature. Therefore, the stress relief work must be immediately carried out.

[0007] The present invention measures the effective value voltage of Barkhausen noise at a predetermined measurement site around the surface of a steel pipe to which a controlled compressive stress is applied, so that only the stress within the elastic range is applied to the steel pipe. It is another object of the present invention to provide a method capable of performing stress diagnosis with high accuracy even when a stress exceeding the yield stress is applied.

[0008]

The gist of the present invention is as follows.

(1) A steel pipe to which a compressive residual stress is applied is used as a diagnosis target, and a magnetic head including an excitation head and a detection head is used. A stress diagnosis method for detecting a Barkhausen noise by frequency-separating a voltage signal induced in the detection head, wherein a plurality of predetermined measurement parts are set in a circumferential direction of a steel pipe surface, and a pipe axis center line of the steel pipe is set. Assuming a plane that intersects with a predetermined measurement site on the surface of the steel pipe, one of the planes is used as a reference plane, and each measurement site is indicated by an angle between the reference plane and the plane that includes each measurement site. From the relationship between these angles and the effective value voltage of Barkhausen noise, the mutual positions are maintained at an angle of approximately 180 °, and the effective value voltage of Barkhausen noise is a minimum value or a minimum value, respectively. Two measurement sites are determined, the maximum value of the effective value voltage of Barkhausen noise of the measurement site located at the substantially central angle between the two measurement sites is determined, and from the maximum value of the effective value voltage, Using a calibration curve that represents the relationship between the external stress and the effective voltage of Barkhausen noise obtained in advance using the same member,
A stress diagnostic method capable of diagnosing stress up to a yield stress of a steel pipe, wherein a maximum tensile stress value in an axial direction is obtained.

(2) A steel pipe in which the effective value voltage of Barkhausen noise over the circumference of the steel pipe surface is made uniform by applying in-plane compressive residual stress over the circumference of the steel pipe surface before installation on the site. (1)
4. The method for diagnosing stress in a steel pipe according to item 1.

(3) The method for diagnosing stress in a steel pipe according to the above (1) or (2), wherein a steel pipe in which residual stress is isotropically distributed in a measurement plane is used.

(4) When the depth of detection of Barkhausen noise is d, the compressive residual stress is applied to a depth of at least 0.5 d from the surface of the measurement site (1) or (2). 4. The method for diagnosing stress in a steel pipe according to item 1.

[0013]

DETAILED DESCRIPTION OF THE INVENTION Barkhausen noise of steel
Since the stress varies depending on the structure such as external stress and crystal grain size, precipitates and dislocations, it is essential that the structure is not changed in order to diagnose external stress. That is, even if external stress acts on the steel material, when it is within the elastic range,
Since there is no structural change, Barkhausen noise depends only on stress and changes reversibly with respect to stress. However, an external stress greater than the yield stress acts on the steel material, and when it enters the plastic region, dislocation multiplication and crystal rotation occur and the structure changes, so it is no longer possible to diagnose only the external stress. It will be possible.

The present inventors have made it possible to control the initial state of the residual stress at the measurement site so that the structural change hardly occurs even when the magnitude of the external stress becomes larger than the yield stress. Further, in such a state, a detailed investigation of the relationship between stress and Barkhausen noise resulted in the present invention.

That is, the present inventors have determined the relationship between the stress or strain and the magnitude of Barkhausen noise when plastic deformation is performed from the elastic region to the plastic region, and further to various strains in the plastic region. Was measured in detail. As a result, when a tensile stress is newly applied to a sample to which a measurement site is plastically deformed at one end and a compressive residual stress is applied in the in-plane direction of the site, the effective value voltage of the stress or strain and the Barkhausen noise is obtained. It has been found that the range of stress or strain where the linear correlation holds is significantly wider than the case where there is no compressive residual stress. Furthermore, when the compressive residual stress in the in-plane direction is controlled to be approximately the same as the yield stress of the steel pipe, the Barkhausen noise hardly changes or slightly increases even if a compressive stress is externally applied to that part. It was found to show some change.

Since the structure and residual stress of a normal electric resistance welded pipe or a seamless pipe are different for each part on the surface of the steel pipe, when the Barkhausen noise is actually measured, the measurement part differs by several cm even in the same steel pipe. Alone, the effective value voltage greatly differs. Therefore, it is necessary to manage the initial values for each part, and the management becomes complicated.

The inventor of the present invention has measured the Barkhausen noise at any position on the surface of a steel pipe by applying substantially the same amount of compressive residual stress in the in-plane direction of the surface of the ERW pipe or the seamless pipe. It has been found that an effective value voltage of the same magnitude can be obtained. By applying this compressive residual stress isotropically in the plane, the initial value is also isotropic, and it is possible to prevent the accuracy of stress diagnosis from lowering even if external stress is applied from any direction. become. In practice, it is easy to apply a compressive residual stress corresponding to the yield stress in that a uniform residual stress is applied.

Further, by applying a compressive residual stress corresponding to the yield stress, even if an external compressive stress is applied to the portion, the effective value voltage of Barkhausen noise hardly changes or slightly increases. To make a change. When bending moment acts on the steel pipe, compressive stress and tensile stress act on the steel pipe in the axial direction on both sides of the neutral point. The effective voltage of Barkhausen noise does not change at the neutral point, but increases on both the compressive and tensile stress sides.These changes are greater on the tensile stress side than on the compressive stress side. Points can be easily found, and the compressive stress side and the tensile stress side can be easily distinguished.
On the other hand, in the conventional magnetostrictive sensor, since the output value continuously changes from the neutral point as a boundary, there is ambiguity in determining the neutral point.

Next, the measurement procedure will be described with reference to FIGS.

FIG. 1 is a schematic perspective view showing a magnetic head using stress measurement. The magnetic head 1 includes a U-shaped core 11 made of a soft magnetic material such as a silicon steel plate or an amorphous material, and an exciting coil 12 formed by winding a copper wire such as an enamel wire around the U-shaped core 11. 2 and
For example, it comprises a detection head 3 which is an air-core coil.

Using such a magnetic head 1, Barkhausen noise is measured by exciting in a pipe axis direction at a plurality of predetermined circumferential positions on the surface of a steel pipe to which a controlled compressive residual stress is applied. . At that time, as shown in FIG. 2 (a cross-sectional view orthogonal to the pipe axis T of the steel pipe 4), a plane including the center line of the pipe axis T on the surface of the steel pipe 4 and intersecting with a predetermined measurement site P on the steel pipe surface is considered. One of the planes is set as a reference plane S, and each measurement site is indicated by an angle θ between the reference plane S and a plane including each measurement site. Any surface may be used as the reference surface. The relationship between these angles and the effective value voltage of Barkhausen noise is shown in a graph. From this graph, the mutual positions are maintained at an interval of 180 °, and the effective value voltage of Barkhausen noise is a minimum value or a minimum value, respectively. Two measurement sites (Pa, Pb in the illustrated example) having a value are obtained. These two measurement sites Pa and Pb are the neutral points described above.

FIG. 3 shows an example represented in this manner. However, FIG. 3 shows the profile of the effective value voltage of Barkhausen noise at each stress stage when a bending stress is externally applied to the steel pipe. here,
Since the number of normal measurement points is a finite number of several to several tens, the measurement site does not always coincide with the position of the neutral point. In such a case, the effective value voltages at the two locations having the minimum value are not necessarily the same value but different values. The determination of the neutral point can be easily performed by approximating the interval between the measurement points with a complementary curve.

Next, the maximum value of the effective value voltage of Barkhausen noise at a measurement site (Pc in the illustrated example) located at an approximately central angle between these two measurement sites is obtained. This part is where the tensile stress acting in the direction of the tube axis usually becomes maximum. If the neutral point is at the end of the drawn graph and is difficult to see, the reference plane of the graph may be changed to make it easier to see. Since the effective voltage at the two neutral points is the original initial value, even if the initial value is not known, it can be obtained. The maximum tensile stress value in the axial direction can be obtained using a calibration curve previously obtained from the maximum value of the effective voltage.

Here, a calibration curve representing the relationship between stress and Barkhausen noise is obtained by simultaneously measuring Barkhausen noise while applying stress to members of the same steel type to which a strain gauge is attached. It can be easily obtained.

Further, this maximum tensile stress value is defined as σmax
Then, the bending moment M can be obtained from M = Z × σmax (where Z is a section modulus). Further, by applying a compressive residual stress corresponding to the yield stress to the surface of the steel pipe, it is possible to make a diagnosis by Barkhausen noise up to an external tensile stress corresponding to about twice the yield stress.

In an ERW pipe, there are a weld zone and heat-affected zones on both sides of the weld zone. However, in these zones, the Barkhausen noise may greatly change. If the coating or coating is not applied, these parts can be visually checked, so they can be removed from the measurement site in advance. What is necessary is just to exclude the value corresponding to a site | part. In the graph showing the relationship between the angle of the measured part and the RMS voltage of Barkhausen noise in the weld and heat affected zone, the RMS voltage changes discontinuously, so it is easy to find those parts. it can. If a known non-contact magnetic head (JP-A-7-174730) is used, measurement can be performed even from above the coating material.

Since the attenuation increases as the Barkhausen noise generated from a deeper part of the sample decreases, the voltage generated in the detection coil decreases. This is skin depth
depth) effect, which is quantitatively expressed as follows. Assuming that the depth of the source at which Barkhausen noise attenuates to 1 / e on the sample surface, that is, the detection depth is d, d
= (Ρ / πfμ) ^1/2 , where (ρ is the electrical resistance, f is the detection frequency of Barkhausen noise, and μ is the magnetic permeability).
The depth at which the residual stress is applied must be at least 0.5d or more. If it is less than 0.5 d, in the relationship between Barkhausen noise and stress or strain, the stress range in which a linear correlation between the two holds is reduced.

Examples of a method of applying a compressive residual stress to a measurement site of Barkhausen noise include a method of causing small steel balls such as air blast and shot blast or ceramic particles to collide with a sample surface at a high speed, polishing with a sander, Although air blast and shot blast are suitable for imparting isotropic residual stress to the sample surface. Even in the case of using a sander, the residual stress can be isotropically applied by isotropic polishing.

When the measuring method of this embodiment is actually used, a calibration curve indicating the relationship between the external stress in the member to be measured and the effective voltage of Barkhausen noise is measured in advance, and the actually measured effective curve is measured. This calibration curve may be used when converting the maximum value of the value voltage into the stress.

[0030]

The present invention will be specifically described below with reference to examples.

(Example 1) The initial value of the effective value voltage of Barkhausen noise when there is a controlled residual compressive stress on the steel pipe surface and when it does not exist, that is, the effective value when no bending stress acts on the steel pipe The voltage was measured according to the measurement site. The test tube has an outer diameter of 31
It is a seamless steel pipe with a thickness of 8 mm and a thickness of 7.9 mm. The residual compressive stress was applied uniformly by performing shot blasting using a steel-based abrasive on the entire surface of the steel pipe surface.

The measurement of Barkhausen noise was performed as follows. An excitation head in which a 1000-turn enamel wire is wound on a U-shaped excitation core in which silicon steel sheets are laminated, and an acrylic bobbin having a cross-sectional area of 2 mm × 8 mm are 500
The effective value voltage of Barkhausen noise was measured by placing a magnetic head consisting of a detection head wound with an enameled wire on a sample surface of a steel tube having a compressive residual stress and a steel tube having no compressive residual stress. Excitation directions at each measurement site are two directions of the axial direction and the circumferential direction of the steel pipe. The excitation frequency is 100 Hz, and the detection frequency is 10 kHz to 100 kHz. In this test, a steel pipe without controlled compressive residual stress is a steel pipe before shot blasting.

The measurement site of the steel pipe includes a plane including the center line of the pipe axis and intersecting with a predetermined measurement site on the surface of the steel pipe. One of the planes is used as a reference plane, and the measurement site is defined by the reference plane and each measurement site. Indicated by the angle between the plane and the plane. Actually,
22. Since any surface is used as a stainless steel tube,
Measurements were made for one round over the circumference of the steel pipe at 16 locations at 5 ° intervals.

The distribution of the magnitude of the residual stress on the surface of the steel pipe after the shot blasting in the depth direction was determined by an X-ray residual stress measuring method after etching a predetermined thickness from the surface in the thickness direction. As a result, it was confirmed that the compressive residual stress of the same magnitude entered isotropically uniformly in the plane up to a depth of about 150 μm from the surface. Skin dept
The detection depth of Barkhausen noise obtained from the calculation formula d) of (h) = (ρ / πfμ) ^1/2 is about 160 μm.

FIG. 6 shows the effective value voltage (R) of the Barkhausen noise measured when the steel pipe was excited in the axial and circumferential directions.
MS). Looking at the result of the comparative example in which there is no controlled compressive residual stress, it can be seen that the effective value voltage in the axial direction greatly varies depending on the measurement position. The same applies to the circumferential direction. Furthermore, the measured value in the axial direction and the measured value in the circumferential direction are significantly different even at the same measurement site. On the other hand, in the example of the present invention in which the compressive residual stress is uniformly applied to the surface of the steel pipe by the shot blast treatment, it can be seen that the variation in the effective value voltage between the measurement sites is almost eliminated and the steel tube is made uniform. Further, the effective value voltages in the axial direction and the circumferential direction are also substantially the same.

As described above, by applying a compressive residual stress in the in-plane direction around the surface of the steel pipe, the effective value voltage of Barkhausen noise over the circumference of the surface of the steel pipe can be made uniform.

(Example 2) Yield stress is 24 kgf / mm
The relationship between Barkhausen noise and external stress was investigated using the ERW tube of No. ² . The measurement sample has an outer diameter of 318 mm and a thickness of 6.
It is a 9 mm, 6000 mm long steel pipe. However, a steel tube surface to which a compressive residual stress was applied by shot blasting or not was used. Barkhausen noise was measured while performing a bending test on each steel pipe, and the relationship between the two was examined. The bending test was performed at a two-point load using a 200-ton testing machine. The loading point interval is 700 mm, and the fulcrum interval is 4800 mm.

The measurement sites of each steel pipe are 22.5 ° intervals around the circumference, for a total of 16 locations. Note that 33
The reference plane was selected so that the 7.5 ° measurement site was the weld. A three-axis strain gauge that can measure the external stress applied to the measurement site up to the plastic region attached adjacent to the site
I asked for it. The measurement of Barkhausen noise is the same as in the first embodiment. However, the excitation direction is the tube axis direction.

The distribution of the magnitude of the residual stress on the surface of the steel pipe after the shot blasting in the depth direction was determined by an X-ray residual stress measurement method after etching a predetermined thickness from the surface in the thickness direction. As a result, a compressive residual stress (−24 kg) having the same magnitude as the yield stress from the surface to a depth of about 200 μm.
f / mm ² ) was confirmed to be isotropic in the plane. The detection depth of Barkhausen noise calculated from the skin depth calculation formula d = (ρ / πfμ) ^1/2 is about 160 μm
It is.

FIG. 7 shows the axial strain profile at each measurement site when the total load of the two-point loading was increased in the shot blasting material. Although the unit of the total load is represented by kN in the figure, 1 kgf = 9.
8N. 90 ° and 27 as total load increases
Tensile strain increases at a portion between 0 ° and compressive strain increases at portions on both sides thereof. 90 ° and 270
° is the neutral point. The results in FIG. 7 were the same for the material without shot blasting.

FIG. 3 shows a profile of an effective value voltage (RMS) of Barkhausen noise measured at a place adjacent to the measurement point in FIG. As can be seen from FIG. 3, the effective value voltage at the time of no load is a uniform value except for the welded portion. It can be seen that the effective value voltage hardly changes at the neutral points of 90 ° and 270 °, and greatly increases between the neutral points where the tensile stress is applied in the axial direction. The increase in the effective value voltage is small at the part where the compressive stress is applied on both sides of the neutral point. The portion at 337.5 ° is a welded portion, and it can be seen that the effective value voltage at this portion changes discontinuously. It can be seen that the two neutral points show a minimum value when the load is low (except for the welded portion), but have a minimum value when the load is high. The maximum tensile stress is located at a substantially central angle between the two neutral points.

FIG. 4 is a calibration curve obtained in this experiment. The tensile stress in the axial direction and the Barkhausen noise obtained from the strain gage value at the position of 180 ° where the maximum value of the effective value voltage is obtained are shown. It shows the relationship with the effective value voltage. It should be noted that the same correlation was observed in the measurement site located between 90 ° and 270 °. It can be seen that the linear correlation between the ^two holds up to a stress range of about 47 kgf / mm ² corresponding to about twice the yield stress. Normally, since Young's modulus E changes in the plastic region, it is not possible to calculate stress from strain with E being constant. However, as in the case of the present invention, the stress changes from the compressive yield stress state to the tensile yield stress state. The Young's modulus E = σ / ε = 21000
Since kg / mm ² (stress σ, strain ε) holds almost reversibly, calculation from strain to stress becomes possible.
However, since the relationship of E = σ / ε does not hold in the stress range beyond that, it is impossible to obtain the stress from the strain.

The abscissa in FIG. 4 indicates the number in parentheses.
By using FIG. 4 as a calibration curve, it is possible to determine the stress from the effective value voltage.

As a comparison, a similar experiment was performed on a steel pipe that was not subjected to the shot blast treatment. The measurement point at which the maximum tensile stress is applied, that is, 180 in FIG.
FIG. 5 shows the relationship between the stress at a position corresponding to ° and the effective value voltage of Barkhausen noise. It can be seen that there is almost no linear relationship between the two, and the stress dependence of the effective value voltage is small.

(Embodiment 3) When the depth of detection of Barkhausen noise is d and the depth of existence of compressive residual stress is D,
The range in which a linear correlation between external stress and the effective value voltage of Barkhausen noise when / d changed was examined. Actually, assuming that the detection depth d of Barkhausen noise is constant,
D was changed by changing the shot blast conditions. The method of measuring Barkhausen noise and the method of measuring residual stress are the same as in Example 1. When the shot blast conditions are changed, the magnitude -σr of the compressive residual stress changes simultaneously with the depth D of the residual stress, so the evaluation of the stress range in which the linear correlation is established is based on the fact that the measured linear correlation range is σliner. In this case, σliner / (σr + σy), σy
Is the yield stress. This is because the range σliner where the linear correlation is established is (σr + σy) at the maximum, and this σliner
A larger value of / (σr + σy) means that the range in which the linear correlation is established is wider. The measurement results are shown in Table 1 below.

[0046]

[Table 1]

As can be seen from the above, when the detection depth of Barkhausen noise is d, the residual stress is applied at least to a depth of 0.5 d from the surface of the measurement site, so that the external stress and the Barkhausen noise can be effectively reduced. It can be seen that the linear correlation of the value voltage holds over a wider stress range.

Example 4 The same steel pipe as that used in Example 2 was subjected to a shot blast treatment to impart a compressive residual stress. The magnitude of the residual stress was the same as in Example 2. Next, it was investigated whether or not the present invention can diagnose the external stress in a state where the steel pipe is bent and an arbitrary amount of external stress is applied. The number of measurement points around the steel pipe and the method of measuring Barkhausen noise are the same as in Example 2. The value obtained by converting the actually measured maximum value of the effective value voltage of Barkhausen noise into stress using the calibration curve of FIG. 4 is 32 kgf / mm ^2.
Met. This value almost coincided with the value obtained by measuring the curvature of the bent steel pipe with high accuracy and converting the value of strain obtained by calculation into stress.

Therefore, it can be understood that the present invention makes it possible to diagnose a stress equal to or higher than the yield stress.

[0050]

According to the present invention, the Barkhausen noise of a steel pipe having a compressive residual stress applied to its surface is measured at a predetermined portion around the circumference of the steel pipe, so that the stress acting on the steel pipe is reduced to the elastic region. In addition, it is possible to accurately diagnose even stress in the plastic region exceeding the yield stress. By using the present invention, the accuracy of diagnosing stress in a plastic region where buried steel pipes should be monitored with the utmost care and where stress relief work must be immediately performed in some cases is significantly improved. improves.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing a magnetic head used for stress measurement.

FIG. 2 is a schematic diagram simply showing a stress measurement method.

FIG. 3 is a characteristic diagram illustrating a change in an effective value voltage profile of Barkhausen noise.

FIG. 4 is a characteristic diagram showing a calibration curve representing a relationship between a stress and an effective value voltage of Barkhausen noise.

FIG. 5 is a characteristic diagram showing a calibration curve representing a relationship between stress and an effective value voltage of Barkhausen noise.

FIG. 6 is a characteristic diagram illustrating a profile of an effective value voltage of Barkhausen noise.

FIG. 7 is a characteristic diagram showing a change in a strain profile of a steel pipe.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Magnetic head 2 Excitation head 3 Detection head 4 Steel pipe 11 U-shaped core 12 Excitation coil

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shigehiko Yamana 2-6-3 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Steel Corporation (72) Inventor Takao Sasaki 2-6-3, Otemachi, Chiyoda-ku, Tokyo Within Nippon Steel Corporation (72) Inventor Jun Tsujimoto 2-6-3 Otemachi, Chiyoda-ku, Tokyo F-term within Nippon Steel Corporation (reference) 2G053 AA12 AA19 AB20 BA12 BA26 BB05 BC02 BC14 CA03 CA18 CB21 DA01

Claims (4)

[Claims] 1. A steel pipe to which a compressive residual stress has been applied is used as a diagnosis target, and a magnetic head including an excitation head and a detection head is used to perform AC excitation of a measurement portion of the steel pipe by the excitation head. A stress diagnosis method for detecting Barkhausen noise by frequency-separating a voltage signal induced in the detection head, wherein a plurality of predetermined measurement sites are set in a circumferential direction of a steel pipe surface, and a pipe axis center line of the steel pipe is set. Assuming a plane that intersects a predetermined measurement site on the surface of the steel pipe, including one of the planes as a reference plane, displaying each measurement site at an angle between the reference plane and a plane including each measurement site, From the relationship between those angles and the effective value voltage of Barkhausen noise, the mutual positions are approximately 18
Two intervals are measured at an angle of 0 °, and the effective value voltage of the Barkhausen noise has a minimum value or a minimum value. The maximum value of the effective value voltage of Barkhausen noise of the measurement site is determined, and from the maximum value of the effective value voltage, the relationship between the external stress previously obtained using the same member and the effective value voltage of Barkhausen noise is determined. A method for diagnosing stress in a steel pipe, wherein a maximum tensile stress value in an axial direction is obtained by using a represented calibration curve. 2. Over the circumference of the steel pipe surface before installation on site,
The method for diagnosing stress in a steel pipe according to claim 1, wherein a steel pipe having a uniform effective value voltage of Barkhausen noise around the surface of the steel pipe by applying a compressive residual stress in an in-plane direction is used. 3. The method for diagnosing stress in a steel pipe according to claim 1, wherein a steel pipe having residual stress isotropically distributed in a measurement plane is used. 4. The steel pipe according to claim 1, wherein, when the detection depth of Barkhausen noise is d, the compressive residual stress is applied to a depth of at least 0.5 d from the surface of the measurement site. Stress diagnosis method.