JPH02227655A - Method and device for magneto-optic flaw detection - Google Patents

Abstract

PURPOSE:To remove detection noises due to a defect and a magnetic domain and to obtain high detecting ability by passing the transmitted light of a magneto-optic effect element through a rectangular transmission window whose long sides are set in a specific direction. CONSTITUTION:A magnetic field which crosses a rolling direction at right angles is applied to a body M whose flaw is to be detected, a detection head 6 is put close to its surface, and a similar state is entered even in a scanning direction. The luminous flux from a light source 1 is passed through a polarizer 2, a condenser lens 3, and a half-mirror 4 and projected on the head 6, and its reflected light is passed through a condenser lens 5, the half-mirror 4, a slit 7, a lens 8, and an analyzer 9 and made incident on a photodetector 10. The long sides of the slit 7 are set at right angles to the scanning direction. Then a flow detecting device main body 11 detects a peak-to-peak distance and an amplitude with an electric signal corresponding to variation in the quantity of light from the detector 10 and outputs a flaw detection signal when they exceed their reference values. Consequently, noises due to the defect and magnetic domain of the magneto-optic element are removed and sufficient resolution is secured in a necessary direction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention mainly relates to a magneto-optical flaw detection method and apparatus for detecting surface flaws in a ferromagnetic object to be flawed.

[Conventional technology]

Conventionally, surface flaw detection methods for ferromagnetic materials include the magnetic particle flaw detection method, in which the object to be flawed is magnetized and magnetic particles are attracted to the leakage magnetic field from the defect and visually detected, or the leakage magnetic field is detected using a Hall element, coil, etc. The leakage magnetic flux flaw detection method, which uses electrical detection, is widely used.

However, while the former method has high resolution, it has insufficient quantitative properties for defect depth, and while the latter method has excellent quantitative properties, it is difficult to detect defects smaller than the size of a Hall element, etc. There was a problem that the amount was low.

As a countermeasure to this problem, a magneto-optical flaw detection method that detects a magnetic field using a magneto-optic effect element has been attracting attention in recent years.

This magneto-optical flaw detection method detects a phenomenon in which when a leakage magnetic field from a defect is applied to a magneto-optic effect element, the plane of polarization of linearly polarized light transmitted parallel to the magnetic field rotates in proportion to the magnitude of the magnetic field. This is a method that utilizes the Faraday effect.

Figure 5 shows the conventional magneto-optical flaw detection method (0, L, FiLzpa
tric;11th World conf, 0NND
1985 Vol. 1, p. 186), in which reference numeral 16 indicates a detection head and M indicates a ferromagnetic object to be inspected.

The detection head 16 has magneto-optic effect elements 16b and 16c formed on both the front and back surfaces of a transparent substrate 16a, a reflective film 16d on the lower surface facing the object M, and a bias magnetization layer around the substrate 16a. It is constructed by winding a coil 16e.

The detection head 16 is brought close to the surface of the object M to be inspected to which a magnetic field is applied, and the linearly polarized light is incident on the magneto-optic effect element 16c from the upper surface side of the detection head 16, and the magnetic field is Optical effect element 16c, substrate 16a.

The light transmitted through the magneto-optic effect element 16b and reflected by the reflective film 16d is returned to the magneto-optic effect element 16b and the substrate 16a.

After passing through the magneto-optic effect element 16c, the light is observed through an analyzer 19.

When a flaw exists in the object M to be inspected and a leakage magnetic flux is formed, the magnetic field caused by the flaw causes the magneto-optic effect element 16b.

16c, and the linearly polarized light transmitted through the magneto-optic effect elements 16b and 16c to which such a magnetic field is applied has its polarization plane rotated in accordance with the strength of the applied magnetic field, and is sent to the analyzer 19.
The amount of light that has passed through the detector changes in accordance with the applied magnetic field, and by capturing this change in the amount of light, the presence or absence of a flaw in the object M to be inspected is detected.

[Problem to be solved by the invention]

By the way, the magneto-optic effect elements 16b and 16c used as such a detection head are formed on the surface of the substrate 16a by a liquid phase epitaxial growth method, etc., but in such a process, crystal defects or ferromagnetic magneto-optic effect elements are formed. In this case, the occurrence of magnetic domains is unavoidable, but the presence of such crystal defects and magnetic domains causes noise in the detection signal and impedes defect detection ability.

Figures 6(a) and 6(b) are waveform diagrams for detecting flaws in the magneto-optical flaw detection method. Noise signals due to crystal defects and noise signals due to magnetic domains of the magneto-optic effect element as shown in the circled area in FIG. 6(b) appear as noise signals superimposed on defect leakage magnetic flux signals, and the resolution This poses a major obstacle to improving the quality of life.

One of the methods for removing such noise signals is to theoretically eliminate defects in the magneto-optic effect elements 16b and 16c.

Attempts have been made to make the luminous flux cross section of linearly polarized light that passes through the magnetic domain sufficiently larger than that of the magnetic domain, and to detect it in an averaged state. It is necessary to detect with a large field of view, and increasing the field of view will impair the high resolution that is an excellent feature of magneto-optical flaw detection.

Figure 6 (a) is a flaw detection waveform diagram after averaging the noise signals shown in Figures 6 (a) and (b). It can be seen that, although this has been resolved, the resolution of detecting the leakage magnetic flux distribution has decreased as a result of being averaged.

The present inventors aimed to eliminate noise signals caused by defects and magnetic domains in the magneto-optic effect element, and to obtain sufficiently high resolution.
As a result of conducting experiments and research, we discovered the following facts.

That is, the surface defects of the object to be flawed have a directionality due to its manufacturing history, and for example, in the case of a rolled product, the flaw itself has a shape stretched in the rolling direction. Therefore, by increasing the spatial resolution in the direction orthogonal to the direction of the stretching force, it is possible to maintain a sufficiently high detection ability as a whole without increasing the spatial resolution in the stretching direction so much.

The present invention has been made based on this knowledge, and its purpose is to eliminate detection noise caused by defects and magnetic domains in magneto-optic effect elements, and to provide magneto-optical flaw detection that can obtain high detection performance. A method and apparatus thereof are provided.

[Means to solve the problem]

The magneto-optic flaw detection method according to the present invention detects j overfilling of the magneto-optic effect element through a rectangular transmission window whose long sides are oriented in a direction related to the extending direction of the defective portion.

Furthermore, the magneto-optical flaw detection device according to the present invention includes a magneto-optic effect element that faces the surface of the object to be tested to which a magnetic field is applied, and a magneto-optic effect element that is disposed in the optical path of transmitted light of linearly polarized light that is incident on the magneto-optic effect element. a rectangular transmission window disposed in the optical path of the transmitted light in front of the photodetector, the long side of which is oriented in a direction related to the extending direction of the defect; Be equipped.

[Effect]

According to the present invention, a sufficiently high detection ability can be obtained in a specific direction associated with the extending direction of the defective portion of the object to be inspected even if the field of view is enlarged.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on drawings showing embodiments thereof.

FIG. 1 is a schematic diagram showing the implementation state of the method of the present invention, in which 1 is a light source, 2 is a polarizer, 4 is a half mirror, 6 is a detection head, 9 is an analyzer, 10 is a photodetector, and M is a Indicates the object to be inspected. The object M to be inspected is, for example, a rolled product, which has a history of being rolled in the longitudinal direction, and the flaw M' is also in the longitudinal direction (2a
: rjl, d : depth). A magnetic field H in a direction perpendicular to the rolling direction of the object M is applied to the object M, and the detection head 6 is placed close to the surface of the object M, and in this state the object M is rolled. The scanning direction is such that the direction perpendicular to the direction in which the flaw is applied, that is, the direction perpendicular to the direction in which the magnetic field is applied and the direction in which the flaw extends is the scanning direction.

Note that when a pipe such as an ERW pipe is to be tested for flaws, the pipe itself often has a rolling history in the axial direction, and the flaws also extend in the axial direction, so it is necessary to apply a magnetic field in the circumferential direction. The voltage is applied and scanning is also performed in the circumferential direction.

First, the light flux F from the light source 1 is passed through a polarizer 2, linearly polarized, and condensed by a condensing lens 3, and is incident on a half mirror 4 tilted at a required angle with respect to its tip axis. The light incident on the half mirror 4 is reflected there, condensed by a condenser lens 5, and projected onto a detection head 6.

FIG. 2 is an enlarged cross-sectional view of the detection head 6, in which a magneto-optic effect is applied to one side of the substrate 6a having a light-transmitting property, that is, the side facing the object M to be tested, by using, for example, liquid phase growth method. The element 6b is formed into a film, and then AI is further applied to the surface thereof.

A reflective film 6d is laminated by depositing Au or the like, and the plane of polarization of the linearly polarized light transmitted through this is rotated according to the strength of the magnetic field due to the leakage magnetic flux applied to the magneto-optic effect element 6b. It happens.

The rotation angle θ of the plane of polarization is given by the following equation (11).

θ, = VHf ... (1) However, ■: Verdet constant (proportionality constant) H: Magnetic field strength 1: Transmission distance As a magneto-optic effect element, ferromagnetic YIG (Y3Fe
iO+z) is also non-magnetic BSO(B i +□S
ing. ) and other Faraday elements are used.

The light flux incident on the detection head 6 passes through the substrate 6a and the magneto-optic effect element 6b, and is reflected by the reflective film 6d.
The light passes through the magneto-optic effect element 6b and the substrate 6a again and is focused by the lens 5. On the optical axis of this lens 5 is a half mirror 4. Slit 7, lens 8. An analyzer 9 is provided, and the light passing through the analyzer 9 is made to be incident on a photodetector 10.

The length 1a of the long side and the length lb of the short side of the slit 7 are in the relationship la>>1b, and the long side is set in the direction perpendicular to the scanning direction of the object M to be inspected, that is, in the case of a rolled product. It is arranged in the direction in which the flaw is extending.

The photodetector lO is composed of a photoelectric conversion element and the like, and is configured to convert the light whose intensity has changed after passing through the analyzer 9 into an electrical signal corresponding to the intensity of light, and output it to the flaw detection apparatus main body 11. In the flaw detection device main body 11, the peak-to-peak distance Lp
p and the peak amplitude of 2Vp are detected, and when these exceed a predetermined reference value, a flaw detection signal is output.

3 and 4 are graphs showing the influence of the long side 1a and short side 1b of the slit 7 on the detectability.

In Figure 3, the horizontal axis shows the slit long side j2a (μm), and the vertical axis shows Lpp (μm).
The line has a depth d of the artificial defect of 3.0 mm and a width dimension 2a of 27
For the case where the plow is 0μ, and for the b line, the depth d of the artificial defect is 0.
．． 5 m, and the width dimension 2a is 210μ plow.

As is clear from this graph, when the long side 1a is kept constant and the short side 1b is changed, there is no substantial difference in the value of the peak-to-peak distance tpp, but when the long side 1a is changed, the peak It can be seen that the distance LPp increases as the long side fa increases, in other words, smoothing increases and the detection sensitivity decreases.

FIG. 4 shows the long side 6a (μm) of the slit on the horizontal axis and the relative value Vp (dB) on the vertical axis. In the graph, the ○ mark plots the depth of the artificial defect d = 3.0. The width of the crane 1 width 2a = 270 μm, and the mouth mark plots the artificial defect depth d = 0. 5
The results are shown for the case where the width dimension 2a = 210 pm. As is clear from this graph, even if the short side 1b is changed, the decrease in the relative value is small, but when the long side 1a is changed, the relative value Vp decreases, in other words, the detection ability decreases. I understand.

Therefore, as is clear from FIG. 3, the change in the peak-to-peak distance Lpp on the long side of the slit 7 is relatively small (and, as is clear from FIG. The selection may be made taking into consideration the characteristics of.

In the above-described embodiment, a configuration using the slit 7 has been described, but a cylindrical lens may be used instead, and any lens may be used as long as it has the function of a rectangular transmission window.

Further, in the above embodiment, the case where a rolled plate is used as the object M to be tested is explained, but when the object to be tested is a vibrator such as an electric resistance welded pipe, the pipe material itself has a rolling history in the longitudinal direction. Since it is normal for the flaws to extend in the longitudinal direction, it is thought that the flaws also extend in the longitudinal direction, so applying a magnetic field in the circumferential direction,
Scanning is performed in the circumferential direction using a rectangular transmission window having a long side in the axial direction.

〔Effect of the invention〕

As described above, in the method and apparatus of the present invention, it is possible to arbitrarily select the spatial distribution of resolution, eliminate noise caused by defects in magneto-optic elements and magnetic domains, and provide sufficient resolution in the required direction. The present invention has excellent effects such as being able to ensure resolution.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the method and apparatus of the present invention, and FIG.
The figure is an enlarged sectional view of the detection head used in the method and apparatus of the present invention, and Figure 3.4 is a graph showing the influence of the relationship between the long side and the short side of the slit used in the method and apparatus of the present invention on detection performance. FIG. 5 is a schematic diagram showing the implementation state of the conventional method, and FIGS. 6 (a) and (b). (C) is a detected waveform diagram when using the conventional method. l...Light source 2...Polarizer 4...Half mirror 5...Condensing lens 6...Detection head 6a
...Substrate 6b...Magneto-optic effect element 7...Slit 8...Condensing lens 9...Analyzer lO
...Photodetector 11...Flaw detection device main body patent Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Noboru Kono (la (μm) Diagram αa (μm) Funeral map Funeral map

Claims (1)

[Claims] 1. A magnetic field that applies a magnetic field to a ferromagnetic object to be tested and detects the leakage magnetic field generated at the defective part based on the rotation of the polarization plane of linearly polarized light transmitted through a magneto-optic effect element. A magneto-optical flaw detection method, characterized in that the transmitted light of the magneto-optic effect element is detected through a rectangular transmission window whose long side is oriented in a direction related to the extending direction of the defective part. 2. A magneto-optic effect element facing the surface of the object to be inspected to which a magnetic field is applied; an analyzer and a photodetector disposed in the optical path of the transmitted light of the linearly polarized light incident on the magneto-optic effect element;
Magneto-optical flaw detection characterized by comprising a rectangular transmission window disposed in the optical path of the transmitted light in front of the photodetector, the long side direction of which is oriented in a direction related to the extending direction of the defect. Device.