JPH0599900A - Flaw detection by use of optomagnetic field - Google Patents

Abstract

PURPOSE:To rapidly and highly accurately detect a defect by making an optomagnetic effect element film whose axis of easy magnetization is perpendicular to the film surface close to a surface to be inspected, applying a strong magnetic field in parallel with the surface to be inspected, and optically scanning perpendicular components of the magnetic field leaking from a surface detect. CONSTITUTION:A thin film 5 of an optomagnetic effect element having an axis of easy magnetization perpendicular to the film surface is placed close to a surface of a material 1 to be inspected. When the material 1 is strongly magnetized by a magnetizer 2, a strong background magnetic field begins to scatter after the material 1 to be inspected is magnetically saturated completely. If no surface detect 7 exists, a direction of a magnetic flux 3 is horizontal with no magnetic component in a perpendicular direction. Sector light 7 linearly polarized through a polarizer 8 from a sector light source 6 is projected from a perpendicular direction and reflected on a bottom of the film 5 to be detected. If the detect 4 exists, a perpendicular magnetic field occurs in the vicinity, so that magnetization of the film 5 along the axis of easy magnetization proceeds, and rotation of a polarizing surface occurs due to a Faraday effect. This rotation is converted into a change in light amount by a light detector 9 and is made incident to a linear sensor 10 to be detected as a voltage signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting surface defects and surface internal defects of a ferromagnetic material.

[0002]

2. Description of the Related Art A method of magnetizing a ferromagnetic material and detecting a leakage magnetic flux leaking from a defect by a hall element or a detection coil is that an inspection speed is faster than a magnetic particle flaw detection and a correlation of a detection output with a depth. In addition, since it is high in temperature, and less affected by variations in surface roughness and scale and magnetic permeability of the ferromagnetic material compared to other methods such as eddy current flaw detection, it is widely used in automatic flaw detectors for steel pipes. In addition, JP-A-2-22768
As seen in Japanese Patent No. 3 publication, a device for applying the optical magnetic field measuring method to the detection of the leakage magnetic flux has been devised.

[0003]

However, since the conventional method detects the leakage magnetic flux as a change in the magnetic field, the range in which the magnetic field leaks rapidly decreases as the size of the defect decreases. In order to detect, it is necessary to arrange a large number of small magnetic field detecting elements close to the surface of the material to be inspected and to follow the other party. However, it is very difficult to manufacture a mechanism that follows a steel head or a thick plate that moves a sensor head having thousands of signal wirings at high speed, and it is not practical in terms of cost.

Further, the above-mentioned Japanese Patent Application Laid-Open No. 2-227683 relates to the point measurement for the detection of minute defects, and the equipment cost such as in the case of flaw detection of a steel strip or a thick plate having a large surface area is required. Conventionally, there has been no means for solving the mutual contradiction between the inspection speed and the inspection accuracy.

Further, in recent years, the quality requirements of customers' products have become stricter, and it has become necessary to guarantee small defects that cannot be detected at the same magnetization level as in the conventional magnetic flaw detection method. The magnetic flux density of 0.
It was said that a magnetization level of about 8 times was optimum, but for example, in detecting small inclusions in a thin plate, a magnetic field strength of about 40 times that is required. In general, a highly sensitive sensor can be used only in a weak magnetic field, but for magnetic flaw detection, it is necessary to detect a weak magnetic field change in a strong background magnetic field, which is a new issue.

In view of such a problem, the present invention provides a method for radically solving this problem, enabling a wide range of magnetic leakage inspection with a single detection unit, and enabling high-speed and highly accurate detection of surface defects. The purpose is to provide.

[0007]

According to the present invention, in a magnetic flux leakage flaw detection method, a thin film of a magneto-optical effect element having an easy axis of magnetization in a direction perpendicular to a film surface is provided near the surface of a material to be inspected. It is arranged parallel to the surface of the inspection material, and a strong magnetic field is applied in the parallel direction to the inspection surface of the inspection material by the magnetizer, and the vertical component of the leakage magnetic field leaking from the defect on the surface of the inspection material is It is characterized in that the surface defect of the material to be inspected is detected by measuring with an optical scanning means from above.

In a preferred embodiment, the optically scanned means of the optical scanning means and the magnetizer are fixed, and the thin film of the magneto-optical element is made to follow the surface of the material to be inspected, and in a direction orthogonal to the scanning line on the surface of the material to be inspected. The feature is that the material to be inspected is moved to detect flaws.

In a further preferred aspect, the thin film of the magneto-optical effect element, the optical scanning means, and the magnetizer are perpendicular to the scanning line on the surface of the inspection material of the optical scanning means with respect to the stationary inspection material. The feature is that it detects flaws while simultaneously moving and following the directions.

[0010]

The present invention will be described in detail below. The present invention has been made based on the following findings. When a ferromagnetic material to be inspected is magnetized, the magnetic flux passes through the material to be inspected while the material to be inspected is not magnetically saturated, but when it is close to saturation, the magnetic flux leaks into the air from the defective portion. Inspecting this leakage magnetic flux using a magnetic sensor is the leakage magnetic flux flaw detection method, which is the most effective flaw detection method for steel materials. However, if the surface defects are very small cracks, or if there are small inclusions inside the thin plate, the defects are so small that the range of leakage of magnetic flux is very narrow.In the past, many small detectors were arranged in parallel or a small number. The only option was to mechanically scan the sensor. Therefore, there are problems that the price becomes high and the inspection speed cannot be increased. When the magnetization is increased in order to detect minute defects, the three-dimensional strength of the magnetic field leaking into the space affects the flaw inspection, and the magnetic sensor using a ferromagnetic material changes the sensitivity and the magnetic field. Unstable measurement itself occurs due to saturation.

Therefore, as a result of various researches and investigations in order to solve the above problems all at once, among the materials of Faraday rotator elements recently developed for the isolator in optical communication, the film surface is A rare earth metal / iron / garnet thin film having an easy axis of magnetization only in the vertical axis is a magnetic domain seen from a vertical direction in a vertical direction even when a strong magnetic field of about 4000e as described above is applied parallel to the hard-to-magnetize surface. It has been discovered that the pattern does not change, and that the magnetic domain pattern viewed from the vertical direction changes even with a weak vertical magnetic field of 10e or less due to defects. The method of the present invention is based on this new finding.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the overall structure of the apparatus used in the present invention. Reference numeral 1 is a material to be inspected, and 2 is a magnetizer for magnetizing the material. 3 is a magnetic flux line, which indicates that it exits from one N pole and enters the other S pole. Reference numeral 4 is a surface defect, and a thin film 5 of a magneto-optical effect element having an easy axis of magnetization in the direction perpendicular to the film surface is arranged in the vicinity of the surface of the inspection object 1. Reference numeral 6 indicates a fan-shaped light source, and reference numeral 7 indicates fan-shaped light emitted from the light source. Reference numeral 8 denotes a polarizer that linearly polarizes the fan-shaped light 7 only into a vibration component in a direction perpendicular to a plane formed by the fan-shaped light. Reference numeral 9 is an analyzer, the polarization axis of which is inclined by 45 ° with respect to the polarizer 8. A linear image sensor 10 photoelectrically converts incident fan-shaped light and outputs it as a voltage time-series signal.

Now, in order to detect minute defects, the magnetizer 2
When a stronger magnetization is applied, a strong background magnetic field will fly after the material to be inspected is completely magnetically saturated, but if there is no defect, the direction of the magnetic flux 3 is horizontal near the middle of both poles of the magnetizer 2. Yes There is no vertical magnetic field component. As the magneto-optical effect element 5, a magneto-optical effect element 5 is used which has a non-magnetizing property except in a direction perpendicular to the surface and which does not cause magnetic domain movement or magnetic saturation in a horizontal magnetic field at this level.

Further, the fan-shaped light 7 linearly polarized from the fan-shaped light source 6 through the polarizer 8 is projected from a substantially vertical direction, is reflected by the bottom surface of the element, and is transmitted by a distance of twice the thickness. -Depending on the measurement principle of the effect, the polarization level is rotated in proportion to the magnetization level of the light traveling direction component and the transmission distance, but since the magnetic field direction and the light traveling direction are almost orthogonal, the horizontal component of the magnetic field is Is not detected. However, in the vicinity of the surface flaw, a vertical magnetic field component is generated due to the magnetic pole generated on the flaw surface. As a result, the magnetization of the magneto-optical effect element advances in the easy axis direction, which causes rotation of the plane of polarization of light, which is converted into a change in the amount of light by the analyzer 9 and is incident on the linear image sensor 10 to generate a voltage signal sequence. Will be detected as.

Therefore, it is only the magneto-optical effect element that needs to follow the material to be measured. For this reason, compared to the conventional method in which a large number of sensors are arranged and followed by dragging a detection head, the weight is much lighter and more compact, and the tracking performance is greatly improved and troubles in the sensor and wiring system are reduced. I can let you do it.

There are various methods other than those shown in the figure for generating a strong and constant horizontal magnetic field over a long distance while suppressing the vertical magnetic field. For example, JP-A-59-1
The same rod-shaped electromagnet that generates a uniform horizontal magnetic field as shown in Japanese Patent No. 60750 is disclosed in Japanese Utility Model Laid-Open No. 62-11153.
As shown in Japanese Patent Publication No. 9, if two test pieces are arranged in parallel and the material to be inspected is arranged along the axis-symmetrical line, vertical magnetic fields cancel each other out, and horizontal magnetic fields reinforce each other.
A strong and uniform horizontal magnetic field can be obtained. By doing so, it is possible to generate a strong horizontal magnetic field while suppressing the vertical magnetic field in the width direction of the strip-shaped inspection material. Although various methods are conceivable as described above, a method suitable for the method may be selected according to the characteristics of the measurement target.

Further, in the present invention, since the optical scanning is performed, the scanning speed can be made sufficiently higher than that in the case where the target member is vibrated, and the same signal processing as a normal image signal is applied in the post-processing. It is possible and disclosed in JP-A-2-227
The effect of the change in the magnetic field due to the lift-off change, which is a problem in Japanese Patent No. 683, can be removed only by a simple spatial differentiation process, and it is also possible to recognize the pattern of flaws and determine the flaw type.

2 and 3 show the configuration of a preferred embodiment corresponding to claim 2. In FIGS. 2 and 3, 21 is a fan-shaped light source, 22 is a polarizer, 23 is a fan-shaped light, 24 is a magneto-optical effect element having an easy axis of magnetization in the direction perpendicular to the film surface, 25 is an analyzer, and 26 is a linear image. -Jisensor-. Reference numeral 27 is a thin plate of the material to be inspected, and 28 is a magnetizing roll. FIG. 3 shows the details of the tracking mechanism of the magnetization roll 28 and the thin film of the magneto-optical effect element 24. Reference numeral 29 is an air-core non-magnetic metal roll, 30 is a magnetizer, and 31 is an air cushion boat. The magneto-optical effect element 24 is attached to the center of the tip of the air cushion boat 31. Three
3 is an air cylinder, 34 is an air-core piston rod, 3
5 is an operation air supply port for pressing and retracting, and 36
Is a bellows.

The fan-shaped light source 21 is composed of a semiconductor laser, a collimator lens, a cylindrical lens, etc., and has a collimator so that an elongated linear light is imaged on the surface of the linear image sensor 26. Adjusted. Semiconductor laser
As the wavelength of, the one having the optimum wavelength determined by the thin film 24 of the magneto-optical effect element is used. In order that the gap with the thin film 24 does not change due to the shape and vibration of the steel plate 27,
The steel plate 27 is wound around the magnetizing roll 28 with a predetermined tension. The sectional configuration of the magnetizing roll 28 is shown in FIG.
And the roll 29 is made of stainless steel,
An electromagnet (30) is installed in it.

The air cushion boat 31 has a bottom 4
A slit is cut inwardly around the circumference, and air is blown from the slit to form an air curtain and trap air to form an air cushion layer. As a result, the surface of the steel plate 7 rises by about 30 to 300 μm and flaw detection can be performed without contact. The thin plate 24 is fixed in a mold. The levitation air is supplied through the hollow piston rods 34 of the pressing air cylinders 33 installed on both sides of the air boat, and further through the metal bellows 36.
The bellows 36 has the functions of a spring and a universal joint.

FIG. 4 shows the configuration of a preferred concrete embodiment corresponding to the third aspect. In FIG. 4, 41 is a material to be inspected, 42 is a bed, 43 is a magneto-optical effect measuring unit, and 44 is a magnetizer. In this embodiment, the magnetizer is installed from above the measurement optical system. The magneto-optical effect measuring unit 43 and the magnetizer 44 are configured to have a constant gap with the material to be inspected by a touch roll. Reference numeral 45 denotes a movable carriage, which can be moved at a predetermined speed by an electric motor. Reference numeral 46 is a rail for a moving carriage, and 47 is a gate frame. Such a method is effective when the material to be inspected is difficult to move, correct the shape and magnetize from the back surface like an extremely thick plate.

The scanning optical system shown in FIG. 1 and FIG. 2 is a flying image type in which the image pickup side moves, but in contrast to this, it is constructed as a flying point type scanning optical system for scanning a laser spot. It is possible to obtain exactly the same effect by changing the above, and the present invention is not limited to the embodiment shown in the drawings.

[0024]

In recent years, the field of optoelectronics has made remarkable progress, and high-sensitivity Faraday rotating materials for optical output semiconductor lasers and isolators, large contact sensors for large facsimiles, etc. are cheaper than conventional ones. Since a wide fan-shaped light source, a high-sensitivity perpendicularly magnetized film, and a wide image scanner can be realized at low cost, the device adopting the method of the present invention can be used as a conventional small sensor. The number of circuits is several hundredth of that of a large number of parallelized circuits, and the economical efficiency can be remarkably improved.

Further, by changing the magnification of the optical system, it becomes possible to detect a defect smaller than before, and the detection accuracy is improved.

Further, since the number of constituent parts is reduced, breakdowns are reduced and maintainability is remarkably improved.

Further, the method of the present invention can be used in all areas where the accurate leakage magnetic flaw detection method could not be used due to economical difficulty, and the effect of labor saving and improvement of productivity through automation is great.

[Brief description of drawings]

FIG. 1 is a perspective view showing the configuration of an apparatus for carrying out the present invention.

FIG. 2 is a perspective view showing a device configuration of a specific embodiment.

3 is a front view showing the device of FIG. 2. FIG.

FIG. 4 is a front view showing a device configuration of a modified example.

[Explanation of symbols]

1: Inspected material 2: Magnetizer 3: Magnetic flux line 4: Defect 5: Magneto-optical effect element 6: Fan-shaped light source 7: Fan-shaped light 8: Polarizer 9: Analyzer 10: Linear image sensor-21:
Light source of fan-shaped light 22: Polarizer 23: Fan-shaped light 24:
Magneto-optical effect element 25: Analyzer 26: Linear image sensor 27: Thin plate of material to be inspected 28: Magnetization roll 29:
Non-magnetic metal roll 30: Magnetizer 31: Air cushion boat 33: Air cylinder 34: Piston rod 35: Operation air supply port for pressing and retracting 36:
Bellows 41: Inspected material 42: Bed 43:
Magneto-optical effect measuring unit 44: magnetizer 45: moving carriage 46:
Rail 47: Gate frame

Claims (3)

[Claims] 1. A flaw detection method using leakage magnetic flux, wherein a thin film of a magneto-optical effect element having an easy axis of magnetization in a direction perpendicular to a film surface is provided on the surface of the material to be inspected, close to the surface of the material to be inspected. By arranging in parallel, a strong magnetic field is applied in the parallel direction to the inspection surface of the material to be inspected by the magnetizer, and the vertical component of the leakage magnetic field leaking from the defect on the surface of the material to be inspected is optically scanned from above the film surface. An optical magnetic field flaw detection method characterized by detecting surface defects of a material to be inspected by measuring with a means. 2. The optical scanning means and the magnetizer are fixed, the thin film of the magneto-optical effect element is made to follow the surface of the material to be inspected, and the material to be inspected is moved in a direction orthogonal to the scanning line on the surface of the material to be inspected. The optical magnetic field flaw detection method according to claim 1, wherein 3. The thin film of the magneto-optical effect element, the optical scanning means, and the magnetizer are simultaneously applied to a stationary inspection object in a direction orthogonal to a scanning line on the inspection object surface of the optical scanning means. The optical magnetic field flaw detection method according to claim 1, wherein flaw detection is performed while following the movement.