JP7338132B2 - Apparatus for evaluating surface stress and/or hardness of magnetic material - Google Patents

Description

TECHNICAL FIELD The present invention relates to an apparatus for evaluating the surface stress and/or hardness of a magnetic material, and a method for evaluating the surface stress, hardness, and maximum principal stress axis of a magnetic material employed in the evaluation apparatus.

Structural members that make up infrastructure such as bridges and power plants are subject to stress changes due to residual stress generated in the manufacturing process and changes in the environment in which they are used. Stress causes deterioration such as metal fatigue. In particular, since surface stress generated on the surface of structural members causes corrosion fatigue and stress corrosion cracking, there is a demand for equipment and methods for accurately evaluating surface stress for the purpose of diagnosing deterioration of infrastructure equipment. Ta.

Since steel, which is a magnetic material, is often used for such members, devices and methods for evaluating surface stress from Barkhausen noise generated during the magnetization process of steel have been proposed (e.g., Patent Document 1-3).

In Patent Documents 1 and 2, Barkhausen noise is detected on a steel pipe surface to which compressive residual stress is applied in advance, and a calibration curve representing the relationship between the external stress obtained in advance and the effective value voltage of Barkhausen noise is used. Therefore, a stress evaluation method for diagnosing the external stress applied to the steel pipe is disclosed. In addition, in Patent Document 3, a calibration curve of the reciprocal of the maximum value at a given frequency of the Barkhausen noise generated from the surface of the magnetic material and the stress is created, the Barkhausen noise at the given frequency is measured, and the surface A stress evaluation apparatus and method for evaluating stress are disclosed.

In addition, since the hardness generated on the surface of structural members also causes corrosion fatigue and stress corrosion cracking, there has been a demand for an apparatus and evaluation method for accurately evaluating hardness.

A device and method for evaluating hardness based on the aforementioned Barkhausen noise have also been proposed. For example, Patent Document 4 discloses a method of estimating the hardness of a magnetic material from the strength of the magnetic field required to generate Barkhausen noise generated from the surface of the magnetic material. Further, Patent Document 5 discloses a method of estimating the hardness of a magnetic material from at least one parameter of Barkhausen noise generated from the surface of the magnetic material.

Barkhausen noise is known to be affected by many factors such as surface stress, material structure related to hardness, precipitates, and defects (see, for example, Non-Patent Document 1).

It is also known that when a magnetic material has magnetic anisotropy, Barkhausen noise depends on the direction of applied magnetic field. In particular, ^when the magnetic material has uniaxial magnetic anisotropy, the Barkhausen noise depends on the magnetic field application direction expressed by (see, for example, Non-Patent Document 2).

Patent No. 4029119 Patent No. 4128297 Patent No. 5981420 Japanese Patent Application Laid-Open No. 2001-133441 Special Table No. 2009-507220

Yuichi Ito, Toyota Central R&D Labs. R&D Review, vol. 27, No. 4, pp. 1-11 (1992) T. W. Krause, L.; Clapham, D. L. Atherton, Journal of Applied Physics, vol. 75, No. 12, pp. 7983-7985 (1994)

Based on Barkhausen noise, it is required to further improve the accuracy of evaluation for each of surface stress and hardness.

As a result of intensive studies, the inventors have focused on the dependence of Barkhausen noise on the magnetic field application direction. In other words, by rotating the application direction of the magnetic field within the surface of the material to be evaluated and detecting the Barkhausen noise, the physical quantity based on the Barkhausen noise is evaluated by separating the stress-dependent term and the hardness-dependent term. I came up with the idea of doing so, and completed the present invention.

That is, according to one embodiment, the present invention is an evaluation apparatus for surface stress and/or hardness of a magnetic material, which is an excitation apparatus for generating a magnetic field that magnetizes the surface of an evaluation target material, wherein the magnetic field is An excitation device whose application direction can be rotated within the surface of the evaluation target material, a detection sensor that detects Barkhausen noise generated by the evaluation target material, and a control device that controls the strength and application direction of the magnetic field. and a calculating device that records the Barkhausen noise detected by the detection sensor and calculates the surface stress and/or hardness of the evaluation target material from the strength and application direction of the magnetic field and the detected Barkhausen noise. .

In the evaluation device, the excitation device preferably includes a yoke around which an excitation coil is wound, and a magnetic field rotation mechanism that rotates the yoke about a rotation axis perpendicular to the surface of the material to be evaluated.

In the evaluation device, the excitation device includes two yokes around which excitation coils are wound, the two yokes having different magnetic field application directions, and controlling the magnetic field intensity generated by the two yokes. It is preferable that the application direction and intensity of the obtained synthetic magnetic field are configured to be controllable.

In the evaluation apparatus, it is preferable that the detection sensor includes a coil having a central axis in a direction perpendicular to the surface of the material to be evaluated.

In the evaluation apparatus, it is preferable that the detection sensor includes a Hall element whose measurement direction is a direction perpendicular to the surface of the material to be evaluated.

In the evaluation apparatus, it is preferable that the detection sensor includes a magnetoresistive element whose measurement direction is a direction perpendicular to the surface of the material to be evaluated.

In the evaluation device, the calculation device calculates the recording device based on the stress dependent constant obtained from the intensity and application direction of the magnetic field and the detected Barkhausen noise and the surface stress conversion formula stored in the calculation device and/or the hardness obtained from the intensity and direction of application of the magnetic field and the detected Barkhausen noise stored in the computing device It is preferable to execute a program for calculating and outputting the hardness from the Barkhausen noise recorded in the recording device, based on the dependence constant and the hardness conversion formula.

According to another embodiment of the present invention, there is provided a method for evaluating the surface stress of a magnetic material, comprising the steps of: applying an alternating magnetic field to a material to be evaluated in a predetermined direction; and rotating the application direction of the alternating magnetic field within the surface of the evaluation target material, so that Barkhausen noise detected in different magnetic field application directions, the applied alternating magnetic field strength, and the magnetic field application direction and calculating the surface stress of the material to be evaluated.

In the surface stress evaluation method, the Barkhausen noise obtained by integrating the Barkhausen noise intensity for the alternating magnetic field that magnetizes the evaluation target material or the alternating current for generating the alternating magnetic field with respect to the alternating magnetic field or alternating current. The step of determining the magnetic field application direction that maximizes the integrated intensity, wherein the step of calculating is the cosine of the angle of the magnetic field application direction with respect to the magnetic field application direction that maximizes the Barkhausen noise integrated intensity. It is preferable to include the steps of: calculating a change in Barkhausen noise integrated intensity with respect to the square; and evaluating the surface stress based on the change.

According to another embodiment of the present invention, there is provided a method for evaluating the hardness of a magnetic material, comprising a step of applying an alternating magnetic field to a material to be evaluated in a predetermined direction of application; Based on the Barkhausen noise detected in different magnetic field application directions, the applied alternating magnetic field strength, and the magnetic field application direction, by rotating the application direction of the alternating magnetic field within the evaluation target material surface, and calculating the hardness of the material to be evaluated.

In the hardness evaluation method, the Barkhausen noise integral obtained by integrating the Barkhausen noise intensity with respect to the alternating magnetic field that magnetizes the evaluation target material or the alternating current for generating the alternating magnetic field with respect to the alternating magnetic field or alternating current Further comprising the step of determining the application direction of the magnetic field with the maximum strength, wherein the step of calculating is the square of the cosine of the angle of the magnetic field application direction with respect to the application direction of the magnetic field with the maximum Barkhausen noise integrated strength. , plotting the integrated intensity of Barkhausen noise, calculating the value of the intercept when fitted with a linear function, and evaluating the hardness from the intercept.

According to still another embodiment of the present invention, there is provided a method for evaluating the maximum principal stress axis of a magnetic material, comprising the steps of: applying an alternating magnetic field to a material to be evaluated in a predetermined direction; and determining the magnetic field application direction that maximizes the Barkhausen noise intensity by rotating the application direction of the alternating magnetic field within the evaluation target material surface.

According to the apparatus and method of the present invention, it is possible to improve the accuracy of surface stress evaluation and hardness evaluation of a magnetic material using Barkhausen noise. In addition, it becomes possible to evaluate the maximum principal stress axis of the magnetic material, which could not be obtained conventionally based on the Barkhausen noise measurement.

FIG. 1 is a perspective view schematically showing an evaluation device and an evaluation target material according to one embodiment of the present invention. FIG. 2 is a diagram schematically showing the application direction of the magnetic field, the maximum principal stress axis, and the arbitrary reference axis shown on the plan view from the Z-axis direction in FIG. FIG. 3 is an example of an excitation device that constitutes an evaluation device according to an embodiment of the present invention, and is a perspective view schematically showing an excitation device that realizes rotation in the magnetic field application direction by a synthetic magnetic field. FIG. 4 is a graph for explaining the waveform of an alternating current for generating an alternating magnetic field that magnetizes the magnetic material of the material to be evaluated, where the horizontal axis represents time t and the vertical axis represents current i. FIG. 5 is a diagram for explaining the Barkhausen noise profile and calculation of the Barkhausen noise integrated intensity _IMBN , where the horizontal axis represents the applied current i and the vertical axis represents the Barkhausen noise intensity ( _Vrms ). FIG. 6 is a perspective view schematically showing an evaluation apparatus and an evaluation target material used in the examples of the present invention. FIG. 7 shows the dependence of the Barkhausen noise intensity (V _rms ) on the applied current intensity (i) when a tensile stress of 413 MPa is applied to the evaluation steel material in Example 1. It is a graph which shows the result of having measured by 15 degree intervals in between. 4 is a graph showing results of plotting Barkhausen noise integrated intensity (I _MBN ) against cosine squared cos ² φ of angle φ indicating the magnetic field application direction in Example 1. FIG. 4 is a graph showing the relationship between the amount of change in integrated Barkhausen noise intensity (α value) and the applied stress with respect to the amount of change in cos ² φ in Example 1. FIG. 4 is a graph showing the results showing the relationship between Barkhausen noise integrated intensity and applied stress in Comparative Example 1. FIG. 10 is a graph showing the results of plotting Barkhausen noise integrated intensity (I _MBN ) against the cosine squared cos ² φ of the magnetic field application angle φ when tensile stress is applied to the evaluation steel material in Example 2. FIG.

Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited by the embodiments described below.

[Evaluation device]
The present invention, according to one embodiment, relates to a magnetic material surface stress and/or hardness evaluation apparatus. FIG. 1 is a conceptual perspective view of a magnetic body surface stress and/or hardness evaluation apparatus according to an embodiment of the present invention. Hereinafter, in the present specification, the term "apparatus for evaluating the surface stress and/or hardness of a magnetic material" refers to a device that evaluates only the surface stress of a magnetic substance, a device that evaluates only hardness, and a device that evaluates both of them. shall be included. In addition, the evaluation device can also be said to be an evaluation device for the maximum principal stress axis of the magnetic material.

The evaluation apparatus shown in FIG. 1 mainly includes an excitation device 1 , a power supply 2 , a Barkhausen noise detection sensor 4 , a magnetic field control device 5 , a Barkhausen noise recording device 6 and a calculation device 7 .

The excitation device 1 is a device that generates a magnetic field that magnetizes the surface of the evaluation target material 10 , and is configured such that the direction of application of the magnetic field can be rotated within the surface of the evaluation target material 10 . The excitation device 1 may be provided with, for example, a yoke 8 for introducing a magnetic field, an excitation coil 9 wound around the yoke, and a magnetic field rotating mechanism 3, which are located close to the evaluation target material 10 . The yoke 8 is a yoke made of a C-shaped (U-shaped) magnetic material, and by energizing the exciting coil 9, the yoke 8 and the evaluation target material 10 form a closed magnetic circuit, and the evaluation target material Any magnetic field can be formed in the vicinity of the evaluation site 10 so as to be substantially parallel to the surface of the evaluation target material 10 . Although the winding position of the excitation coil 9 is not particularly limited, it is preferably positioned away from the detection sensor 4 to the extent that it is not affected by mutual inductance with the detection sensor 4 . The magnetic field rotation mechanism 3 is a mechanism capable of rotating the yoke 8 around which the exciting coil 9 is wound in the direction of the arrow R. As shown in FIG. The rotation axis Z of the magnetic field rotation mechanism 3 is parallel to the normal direction of the surface of the evaluation target material 10 . Thereby, the application direction of the magnetic field can be rotated within the surface of the evaluation target material 10 while the application direction of the magnetic field is kept parallel to the surface of the evaluation target material 10 . That is, the application direction of the magnetic field can be rotated within the XY surface of FIG.

Here, the application direction of the magnetic field will be described with reference to FIG. FIG. 2 is a conceptual diagram of the XY plane normal to the rotation axis Z of the magnetic field rotating mechanism 3 shown in FIG. is indicated by a virtual line. In FIG. 2, the arrow indicated by H indicates the application direction of the magnetic field. The application direction of the magnetic field can be represented by a rotation angle φ from the reference axis A that can be arbitrarily set on the XY plane. In FIG. 2, the arrow indicated by S indicates the maximum principal stress axis applied to the surface of the material 10 to be evaluated. The rotation axis Z of the magnetic field rotation mechanism 3 is parallel to the perpendicular direction of the evaluation target material 10, so that the maximum principal stress axis of the evaluation target material 10 can be represented by the rotation angle θs from the reference axis A. . 1 and 2 exemplify the case where the evaluation object material 10 has a substantially flat plate shape. The rotation angle φ from an arbitrary reference axis and the maximum principal stress axis θs in the XY plane are set by making the rotation axis Z of the magnetic field rotation mechanism 3 parallel to the normal line of the evaluation part. be able to. Therefore, in the present invention, "configured to be rotatable within the surface of the evaluation target material" means a plane including the evaluation site of the evaluation target material, and the excitation device is formed at the evaluation site of the evaluation target material. In other words, it is configured to be rotatable within a plane parallel to the magnetic field direction. In addition, the direction perpendicular to the surface of the evaluation target material is the normal direction of a plane that includes the evaluation site of the evaluation target material and is parallel to the magnetic field direction formed by the excitation device at the evaluation site of the evaluation target material. can be rephrased.

The excitation device is not limited to the embodiment shown in FIG. It is sufficient if the direction of application of the magnetic field can be changed by rotating the direction of application of the magnetic field while maintaining the .phi. Another example of an exciter is shown in FIG. The excitation device shown in FIG. 3 includes two yokes 8a and 8b around which excitation coils 9a and 9b are respectively wound, and which have magnetic field application directions different from each other. The illustrated two yokes 8a and 8b are arranged so that the magnetic field application directions are perpendicular to each other, but the arrangement angle of the two yokes 8a and 8b is not limited to a specific angle. By controlling the magnetic field intensity generated by these two yokes, it is possible to generate a synthetic magnetic field parallel to the surface of the evaluation target material 10. You can control the intensity. From the viewpoint of convenience, the two yokes 8a and 8b around which the exciting coils 9a and 9b are wound are C-type yokes capable of generating a similar magnetic field strength, and the two C-type yokes Although it is preferable that the ends are on the same surface, it is not limited to these aspects. In addition, the excitation device is composed of one C-shaped yoke and a rotation mechanism for rotating the evaluation target material, which rotates the evaluation target material on a rotation axis parallel to the direction perpendicular to the surface. may be

The power supply 2 can apply an alternating current to the excitation coil 9 and generate a magnetic field on the surface of the evaluation target material 10, and applies an alternating current with a suitable excitation frequency and waveform described later. It may be a stabilized power supply that can

The magnetic field controller 5 controls the intensity and possibly the direction of the alternating electric field by controlling the output of the power supply 2 . The magnetic field control device 5 may be a computer incorporating a power supply control algorithm, and may be integrated with a recording device 6 and a computing device 7, which will be described later.

The detection sensor 4 may be any sensor capable of detecting Barkhausen noise generated from the surface of the evaluation target material 10 . As an example, as shown in FIG. 1, a coil whose central axis is parallel to the perpendicular direction Z of the surface of the evaluation target material 10 can be used. The coil may be an air-core coil or a coil with a core. The detection sensor 4 detects Barkhausen noise defined by the time differential dB/dt of the leakage magnetic flux B generated from the surface of the object 10 to be measured. Barkhausen noise can be detected as a voltage generated in the coil by using the detection sensor 4 composed of a coil. As another example of the sensor, it is also possible to use a Hall element whose measurement direction is the perpendicular direction Z of the surface of the evaluation object material 10, or a magnetoresistive element whose measurement direction is the perpendicular direction Z of the evaluation object material 10 surface. can. When these elements are used, Barkhausen noise can be similarly detected as a voltage generated in the element.

The Barkhausen noise recording device 6 records the Barkhausen noise detected by the detection sensor 4 . The recording device 6 may be a computer incorporating a predetermined algorithm, and may be integrated with the computing device 7 .

The computing device 7 computes and outputs the surface stress and/or hardness from the intensity and direction of application of the magnetic field applied by the magnetic field control device 5 and the Barkhausen noise recorded in the recording device 6 . The computing device 7 may be a computer that incorporates a predetermined algorithm and executes a program that computes and outputs the surface stress and/or hardness from the intensity and direction of application of the magnetic field and Barkhausen noise. Further, the calculation device 7 may store the relationship between the surface stress and the later-described slope α, and the relationship between the hardness and the later-described intercept β, which are measured in advance for a predetermined evaluation target material. When the computing device 7 is integrated with the recording device 6, the computing device 7 records the Barkhausen noise, and the intensity and direction of application of the magnetic field and the surface stress from the Barkhausen noise recorded in the recording device 6 and/or function as a device for calculating and outputting hardness.

A method for evaluating surface stress and/or hardness of a material to be evaluated using such an evaluation apparatus will be described. The evaluation target material 10 may be any magnetic material that can generate Barkhausen noise by magnetization. The shape of the material to be evaluated 10 is not particularly limited, and may be an object having a curved surface such as a steel pipe as well as an object having a substantially flat plate shape as shown. In the method of the present invention, the surface stress and hardness of the evaluation target material 10 can be measured non-destructively. Although possible, the material under evaluation 10 is not limited to any particular product.

As shown in FIG. 1, the material 10 to be evaluated is installed so as to be close to two ends of a C-shaped (U-shaped) yoke 8 . The yoke 8 may be in contact with the evaluation target material 10 or may be separated to some extent, as long as the yoke 8 and the evaluation target material 10 can form a closed magnetic circuit. When the yoke 8 and the material 10 to be evaluated are not in contact with each other, it is desirable to keep the gap constant so that the strength of the applied magnetic field and the Barkhausen noise detection sensitivity do not fluctuate.

The detection sensor 4 can be installed in the vicinity of the surface of the evaluation target portion of the evaluation target material 10 . As an example, as shown in FIG. 1, the surface of the measurement object 10 on the same side as the yoke 8 can be placed near the center between one end and the other end of the yoke 8 . Alternatively, when the measurement object 10 is a relatively thin plate-shaped sample, the detection sensor 4 is located on the surface of the measurement object 10 opposite to the yoke 8, near the center between one end and the other end of the yoke. can also be placed. Further, the detection sensor 4 may be in contact with the evaluation target material 10, or may be installed at a certain distance from the evaluation target material 10, but when the detection sensor 4 and the evaluation target material 10 are not in contact, the gap is constant. It is desirable to

[Surface stress evaluation method]
According to another embodiment, the present invention relates to a method for evaluating surface stress of a magnetic material. The surface stress evaluation method can be implemented using the evaluation device described above. The surface stress evaluation method according to the present embodiment includes the steps of applying an alternating magnetic field to an evaluation target material in a predetermined application direction, detecting Barkhausen noise generated by the evaluation target material, and changing the application direction of the alternating magnetic field to calculating the surface stress of the evaluation target material based on the Barkhausen noise detected in different magnetic field application directions, the applied alternating magnetic field intensity, and the magnetic field application direction by rotating the evaluation target material within the surface; including.

The step of applying an alternating magnetic field to the material to be evaluated in a predetermined direction can be performed by fixing the direction of application φ of the alternating magnetic field and applying an alternating current to the excitation coil 9 . More specifically, the power supply 2 controlled by the magnetic field control device 5 can be used to apply, for example, an alternating current i changing like a triangular wave shown in FIG. The waveform is not limited, and may be, for example, a sine wave or a sawtooth wave. The excitation frequency is preferably lower than several kHz to several tens of kHz, which is the frequency band of Barkhausen noise, and is preferably 1 kHz or less, but is not limited to a specific frequency. The application of the alternating magnetic field may be performed from the _maximum value _imax to the minimum value imin of the alternating current i, or from the minimum value _imin to _{the maximum} value imax. should be applied over a period of time. However, the application may be repeated over 1/2 period or longer.

In the step of detecting Barkhausen noise generated by the material to be evaluated, the detection sensor 4 detects Barkhausen noise. When the detection sensor 4 is a coil, in the process of applying an alternating magnetic field, an induced current is generated in the air-core coil 16 due to the time change of the leakage magnetic flux B generated from the surface of the material to be evaluated, so Barkhausen noise is generated in the coil. It can be detected as a voltage. The detected voltage can be amplified by a differential amplifier as necessary, passed through a band-pass filter to remove low frequency components and high frequency components, and captured in the Barkhausen noise recording device 6 .

After performing the step of applying an alternating magnetic field and the step of detecting Barkhausen noise generated by the material to be evaluated in a predetermined application direction φ of the alternating magnetic field, similar measurements are performed while changing the application direction φ. In the embodiment shown in FIG. 1, the change (rotation) of φ can be performed by the magnetic field rotation mechanism 3 and the controller 5 that controls it. Alternatively, in the embodiment shown in FIG. 2, by controlling the intensity of the magnetic field generated from the two yokes 8a, 8b and the excitation coils 9a, 9b by a control device, the direction of the combined magnetic field of the magnetic fields generated from the two yokes is changed to can be controlled. The magnetic field application direction φ is preferably measured at three or more points in the range of φ=0 to 90°, but is not limited to a specific mode. The step of applying the alternating magnetic field and the step of detecting Barkhausen noise generated by the material to be evaluated can be performed in several seconds, depending on the magnitude of the applied frequency and the application time. Barkhausen noise can be detected by continuously changing φ in the range of φ=0 to 90°, or it can be detected discretely. not.

Next, a step of calculating the surface stress of the material to be evaluated is performed based on the intensity of the applied alternating magnetic field, the direction in which the alternating magnetic field is applied, and the detected Barkhausen noise. More specifically, this step includes a step of determining the magnetic field application direction that maximizes the Barkhausen noise integrated intensity, and determining the magnetic field application direction based on the magnetic field application direction that maximizes the Barkhausen noise integrated intensity. The method includes calculating a variation in Barkhausen noise integrated intensity with respect to the square of the cosine of the angle, and evaluating surface stress based on the variation.

In the present invention, the Barkhausen noise intensity is defined as the root mean square of the voltage detected by the detection sensor 4, also denoted as _VRMS . In addition, _the Barkhausen noise integrated intensity is defined by plotting the Barkhausen noise intensity V _RMS against the alternating current i (hereinafter referred to as the Barkhausen noise profile), and by the area of the resulting Barkhausen noise profile. represent. FIG. 5 schematically shows a Barkhausen noise profile and _IMBN defined by its area. In FIG. 5, the shaded area corresponds to the integrated Barkhausen noise intensity.

The integrated Barkhausen noise intensity defined by the above area can also be expressed by the following equation (1), where i is the alternating current to be applied, i _max is the maximum value of the alternating current, and i _min is the minimum value of the alternating current.
Also, the relationship between the applied alternating current i and the applied magnetic field H is represented by the following equation (2).
In the formula, a is the constant of proportionality. From the equations (1) and (2), the integrated Barkhausen noise intensity I _MBN can also be expressed by the following equation (3), where H _max is the maximum value of the alternating magnetic field H to be applied, and H _min is the minimum value. can.
Therefore, even if a Barkhausen noise profile plotting the Barkhausen noise intensity V _RMS against the alternating magnetic field H is used, the Barkhausen noise integrated intensity I _MBN can be similarly defined. Therefore, the Barkhausen noise integrated intensity I _MBN is obtained by integrating the Barkhausen noise intensity for the alternating current for generating the alternating magnetic field that magnetizes the evaluation target material or the alternating magnetic field with respect to the alternating magnetic field or the alternating current. can be defined to be a value.

The inventors have found the relationship represented by the following equation between the integrated Barkhausen noise intensity I _MBN and the magnetic field application angle φ.
In the formula, φ is as defined with reference to FIG. 2 and is an angle formed by an arbitrary reference axis A defined in a plane that includes the measurement site and is substantially parallel to the magnetic field and the magnetic field application direction H. is 0 to 90°, and θs is the angle between the reference axis A and the maximum principal stress axis S of the material to be evaluated, and is 0 to 90°. The slope α is a constant that depends only on the stress of the material to be evaluated, and β is a constant that depends on other factors such as the material structure and does not depend on the stress.

In addition, the present inventors found that when a magnetic field is applied to a material to be evaluated by changing φ, the direction of application of the magnetic field when the integrated Barkhausen noise intensity _IMBN takes the maximum value is the maximum principal stress axis of the material to be evaluated. It was found to match θs. Therefore, for example, in FIG. 2 above, if the magnetic field direction at the angle φ at which the Barkhausen noise intensity takes the maximum value is defined as the reference axis A and redefined as θs=0, the integrated Barkhausen noise intensity is
From this relational expression, the slope α corresponding to the amount of change in the integrated Barkhausen noise intensity can be obtained.

Next, the value of the surface stress of the material to be evaluated can be obtained from the slope α obtained based on the detected Barkhausen noise based on the conversion formula between the slope α and the surface stress obtained in advance. The conversion formula (conversion curve) representing the relationship between the slope α and the surface stress value is, for example, the relationship between the surface stress value measured by X-ray or other method and the slope α, depending on the composition and / or the manufacturing method. A desired material to be evaluated can be obtained and stored in the computing device 7 .

According to the method of the present embodiment, it is possible to measure the surface stress of an evaluation target material made of a magnetic material in a short measurement time and non-invasively. In the conventional method of evaluating the surface stress from the integrated Barkhausen noise intensity without considering the angular dependence of the magnetic field application direction, the term dependent only on stress (α term) and the term due to other factors (β term) could not be separated. In the method of the present embodiment, it is possible to determine α that depends only on surface stress, evaluate the surface stress from the magnitude of α, and separate the α term and β term, so that it is not affected by other factors. , the surface stress can be evaluated with high accuracy.

[Hardness evaluation]
According to another embodiment, the present invention relates to a method for evaluating hardness of a magnetic material. The hardness evaluation method can be carried out using the evaluation device described above. The hardness evaluation method according to the present embodiment comprises the steps of: applying an alternating magnetic field to a material to be evaluated in a predetermined direction; detecting Barkhausen noise generated by the material to be evaluated; and calculating the hardness of the evaluation target material based on the Barkhausen noise detected in different magnetic field application directions, the applied alternating magnetic field intensity, and the magnetic field application direction by rotating within the evaluation target material surface. .

In the hardness evaluation method, the step of applying an alternating magnetic field in a predetermined direction, the step of detecting Barkhausen noise, and rotating the direction of application within the surface of the magnetic material are performed in the same manner as in the stress evaluation method. be able to. In the hardness evaluation direction, the step of calculating the hardness includes the step of determining the magnetic field application direction that maximizes the Barkhausen noise integrated intensity, and the magnetic field application direction that maximizes the Barkhausen noise integrated intensity. Plotting the integrated Barkhausen noise intensity against the square of the cosine of the angle of the magnetic field application direction, calculating the intercept value when fitting with a linear function, and evaluating the hardness from the intercept .

That is, the application direction of the magnetic field that maximizes the integrated Barkhausen noise intensity is determined, and the intercept value β is calculated based on the equation (5) with reference to the direction in which θs is 0 in the equation (4).

Next, the hardness value of the material to be evaluated can be obtained from the intercept β obtained based on the detected Barkhausen noise based on the conversion formula between the intercept β and the hardness obtained in advance. The conversion formula representing the relationship between the intercept β and the hardness is, for example, the relationship between the hardness value and the slope α measured by an indentation test using an indenter or other method, for desired evaluation objects with different compositions and / or manufacturing methods The material can be obtained and stored in the computing device 7 .

According to the evaluation method of the present embodiment, it is possible to measure the hardness of an evaluation target material made of a magnetic material in a short measurement time and non-invasively. In the conventional method of evaluating hardness from the integrated Barkhausen noise intensity without considering the angular dependence of the magnetic field application direction, a term that depends only on hardness (β term) and a term that depends on other factors ( α term) could not be separated. In the method of the present embodiment, β, which depends only on hardness, can be determined, the hardness can be evaluated from the magnitude of β, and the α term and β term can be separated. can be evaluated with high accuracy.

[Maximum principal stress axis evaluation]
The present invention, according to another embodiment, relates to a maximum principal stress axis evaluation method. The maximum principal stress axis evaluation method according to the present embodiment includes the steps of applying an alternating magnetic field to a material to be evaluated in a predetermined application direction, detecting Barkhausen noise generated by the material to be evaluated, and applying the alternating magnetic field. and identifying an application direction that maximizes the integrated Barkhausen noise intensity by rotating the direction within the magnetic surface.

Also in the present embodiment, the step of applying an alternating magnetic field, the step of detecting Barkhausen noise, and rotating the alternating magnetic field are explained in the surface stress evaluation method and the hardness evaluation method using the evaluation apparatus described above. It can be carried out according to the conditions. Further, the application direction in which the integrated Barkhausen noise intensity is maximum, that is, the specification of the angle φ can be the same as in the previous embodiment.

In general, the surface stress evaluation does not reveal the maximum principal stress axis. Even in that case, according to the method of the present embodiment, the dependence of the integrated Barkhausen noise intensity on the direction of the applied magnetic field is measured, and the direction of the angle that maximizes the integrated Barkhausen noise intensity is the maximum principal stress. It can be specified as an axis. Preferably, the Barkhausen noise integrated intensity is fitted with a squared cosine function. This allows the zero point to be determined and the maximum principal stress axis to be evaluated even with variations in the integrated Barkhausen noise intensity.

EXAMPLES The present invention will be described in more detail below with reference to Examples of the present invention. However, the invention is not limited to the scope of the following examples.

[1. Stress evaluation]
(Example 1)
An evaluation apparatus shown in FIG. 6 was manufactured and used to evaluate the surface stress of the material to be evaluated. As the material 10 to be evaluated, a material obtained by processing martensitic stainless steel, which is a ferromagnetic material, into a plate shape and applying a tensile stress using a bending tester was used. In the evaluation apparatus shown in FIG. 6, an excitation device 1 was composed of a C-shaped yoke 11 made of NiZn ferrite and an excitation coil 12 wound around the yoke 11 . The excitation coil 12 was made by winding an enameled wire 200 times. A power supply 13 for supplying an alternating current to the exciter 1 used an AC stabilized power supply, and the current output was controlled and changed by a computer 14 . The application direction of the alternating magnetic field was changed by connecting the yoke 11 to the rotating stage 15 and rotating the rotating stage 15 while controlling it with the computer 14 . As a detection sensor for detecting Barkhausen noise, an air-core coil 16 with 100 turns of enameled wire was used. In this example, both ends of the yoke 11 were brought close to the evaluation target material 10, and the gap was fixed at 0.1 mm. The air-core coil 16 was arranged between one end and the other end of the C-shaped yoke 11 as shown.

The voltage detected by the air-core coil 16 was amplified by a differential amplifier 17, passed through a band-pass filter 18 to remove low frequency components and high frequency components, and input to a computer. As a result of examination, the cutoff frequencies of this example were set to 10 kHz and 200 kHz.

In order to change the strength of the alternating magnetic field applied to the evaluation target material 10, the current output of the stabilized power supply was changed like the triangular wave shown in FIG. 4, and the excitation frequency was set to 1 Hz. Moreover, the maximum value of the amplitude of the alternating current output was set so that the maximum value of the alternating magnetic field was larger than the saturation magnetic field by evaluating the saturation magnetic field of the evaluation steel material in advance.

FIG. 7 shows the Barkhausen noise profile of the evaluation target material to which a tensile stress of 413 MPa was applied. In this embodiment, the maximum principal stress axis is known, and the angle from the stress application direction is equal to φ−θs. The measurement was performed by changing from 0° to 90° in increments of 15°. The Barkhausen noise profile and the Barkhausen noise integrated intensity calculated therefrom tended to decrease as the angle φ increased. The relationship between Barkhausen noise integrated intensity and cos ² φ was then plotted. The results are shown in FIG. From FIG. 8, it was confirmed that the relationship between the integrated Barkhausen noise intensity and cos ² φ has the angular dependence of I _MBN =αcos ² φ+β shown in the above relational expression (5).

Subsequently, the α value was calculated while changing the tensile stress value, and a conversion curve was obtained. FIG. 9 shows the relationship between the α value and the applied stress in this example. A dotted line in the figure indicates a conversion curve obtained by fitting. Variation from the conversion curve at each measurement point was very small, and it was found that the stress could be evaluated with high accuracy from the α value. The maximum stress evaluation error defined by the variation from the conversion curve was 16 MPa.

(Comparative example 1)
As this comparative example, the stress was evaluated from the integrated Barkhausen noise intensity without depending on the relational expression (5), as in the conventional technique. Barkhausen noise integrated intensity was determined using the same evaluation apparatus and evaluation steel material as in the above example, and a stress conversion curve was created. FIG. 10 shows the relationship between applied stress and integrated Barkhausen noise intensity. A dotted line in the figure indicates a conversion curve obtained by fitting. The maximum stress evaluation error defined by the variation from the conversion curve was 84 MPa. This is a large value compared to the example. Therefore, the effect of improving stress evaluation accuracy according to the present invention was confirmed.

[2. Hardness evaluation]
(Example 2)
Using the same evaluation device as in Example 1, hardness evaluation of the measurement object was performed. As in Example 1, the object 10 to be measured was formed by processing martensitic stainless steel into a plate shape to which a tensile stress was applied.

FIG. 11 shows the Barkhausen noise integrated intensity I _MBN plotted against cos ² φ when a tensile stress is applied to the steel material of the measurement object 10 while changing from 0 MPa to 400 MPa. The direction of application of the alternating magnetic field was measured by changing the angle from the direction of application that maximizes the integrated Barkhausen noise intensity from 0° to 90° in increments of 15°. The integrated Barkhausen noise intensity I _MBN changed linearly with respect to cos ² φ, and the slope α when fitted with a linear function increased as the applied stress increased. On the other hand, the intercept β obtained by linear function fitting was almost independent of the applied stress.

Table 1 shows the results of evaluating the relationship between the intercept β calculated by the method of the present invention and the hardness while changing the quenching temperature of the steel material used in this example. Hardness was measured with a Vickers hardness tester. The value of the intercept β changed according to the difference in the hardness of the steel material, and it was shown that the Barkhausen noise intercept evaluation is possible. Hardness and Barkhausen noise intercept are thought to change due to progress of martensite transformation and increase in dislocation density due to quenching.

(Comparative example 2)
In Comparative Example 2, the relationship between Barkhausen noise integrated intensity and hardness was evaluated. Table 2 shows the Barkhausen noise integrated intensity I _MBN obtained in the measurement performed in the example. The Barkhausen noise integrated intensity I _MBN has a large range of variation. This is because the Barkhausen noise integrated intensity I _MBN changed with applied stress and magnetic field angle. Therefore, hardness cannot be evaluated from Barkhausen noise integrated intensity I _MBN . In general, the magnitude and direction of residual stress are unknown, and the value of Barkhausen noise integrated intensity varies even for steels with the same hardness. Therefore, it is difficult to evaluate hardness from Barkhausen noise integrated intensity. It is believed that there is.

1 excitation coil, 2 power supply, 3 magnetic field rotation mechanism, 4 Barkhausen noise detection sensor 5 magnetic field intensity/application direction control device, 6 Barkhausen noise recording device 7 hardness calculator, 8, 8a, 8b yoke, 9, 9a, 9b excitation coil 10 excitation device, 11 yoke, 12 excitation coil, 13 power supply 14 control computer, 15 rotation stage, 16 air core coil 17 differential amplifier, 18 band pass filter

Claims (3)

A step of applying an alternating magnetic field to the evaluation target material in a predetermined application direction;
A step of detecting Barkhausen noise generated by the material to be evaluated;
By rotating the application direction of the alternating magnetic field within the surface of the evaluation target material, the evaluation target material based on the Barkhausen noise detected in different magnetic field application directions, the applied alternating magnetic field strength, and the magnetic field application direction and calculating the surface stress,
A magnetic field that maximizes the Barkhausen noise integrated intensity obtained by integrating the Barkhausen noise intensity for the alternating magnetic field that magnetizes the evaluation target material or the alternating current for generating the alternating magnetic field with respect to the alternating magnetic field or alternating current further comprising determining the direction of application of
In the calculating step, the amount of change in Barkhausen noise integrated intensity is calculated with respect to the square of the cosine of the angle of the magnetic field application direction with reference to the magnetic field application direction in which the Barkhausen noise integrated intensity is maximized;
and a step of evaluating the surface stress based on the amount of change. A step of applying an alternating magnetic field to the evaluation target material in a predetermined application direction;
A step of detecting Barkhausen noise generated by the material to be evaluated;
By rotating the application direction of the alternating magnetic field within the surface of the evaluation target material, the evaluation target material based on the Barkhausen noise detected in different magnetic field application directions, the applied alternating magnetic field strength, and the magnetic field application direction and calculating the hardness,
A magnetic field that maximizes the Barkhausen noise integrated intensity obtained by integrating the Barkhausen noise intensity for the alternating magnetic field that magnetizes the evaluation target material or the alternating current for generating the alternating magnetic field with respect to the alternating magnetic field or alternating current further comprising determining the direction of application of
In the step of calculating, the integrated Barkhausen noise intensity is plotted against the square of the cosine of the angle of the magnetic field application direction based on the magnetic field application direction in which the Barkhausen noise integrated intensity is the maximum, and the integrated Barkhausen noise intensity is expressed as a linear function a step of calculating the value of the intercept when fitting;
A method for evaluating hardness of a magnetic material, comprising a step of evaluating hardness from the section. A step of applying an alternating magnetic field to the evaluation target material in a predetermined application direction;
A step of detecting Barkhausen noise generated by the material to be evaluated;
By rotating the application direction of the alternating magnetic field within the surface of the evaluation target material, the Barkhausen noise intensity with respect to the alternating magnetic field that magnetizes the evaluation target material or the alternating current for generating the alternating magnetic field is obtained by the alternating magnetic field or the alternating magnetic field. Determining a magnetic field application direction that maximizes the Barkhausen noise integrated intensity obtained by integrating with respect to the current;
and fitting the integrated Barkhausen noise intensity with the square of the cosine function of the angle in the magnetic field application direction with respect to the magnetic field application direction at which the Barkhausen noise integrated intensity is maximized. How the axis is evaluated.

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