JP7387105B2 - electromagnetic ultrasound probe - Google Patents

Description

The present invention relates to a compact electromagnetic ultrasound probe that achieves high vertical magnetic flux density.

Ultrasonic testing is widely used in the field of non-destructive testing for measuring thickness and detecting surface and internal damage to materials.
Generally, piezoelectric elements are used to generate and detect ultrasonic waves. When using a piezoelectric element, it is usually necessary to interpose a couplant between the piezoelectric element and the test specimen in order to efficiently propagate ultrasonic waves between the two. This causes some inconvenience when applying the piezoelectric element, and also makes it difficult to reproduce measurements.

On the other hand, in non-destructive testing of metal materials, an electromagnetic ultrasound probe (hereinafter referred to as EMAT) 50 is widely used. EMAT is made of a permanent magnet and an alternating current coil (Figure 10). In the electromagnetic ultrasonic probe 50 shown in FIG. 10, a core magnet 53 is formed by combining two permanent magnets 53R and 53L so that adjacent magnetic domains 53a to 53d have different polarities, and a core magnet 53 is formed under the core magnet 53. A coil 52 was installed in the. As a result, a static magnetic field B is generated on the coil 52 side of the core magnet 53, while an eddy current U and a dynamic magnetic field are generated in the test object 100 due to the alternating current (driving current) of the coil 52.
An alternating current electromagnetic force is excited by the interaction between the static magnetic field B, the eddy current U, and the dynamic magnetic field. This electromagnetic force drives the EMAT 50's magnetic energy to mechanical energy conversion mechanism as described below. A brief explanation will be given below.

In the case of non-magnetic metals, the only electromagnetic force is the Lorentz force. In the case of ferromagnetic materials, electromagnetic force consists of Lorentz force, magnetizing force, and magnetostrictive force. However, the main driving force of the conversion mechanism from magnetic energy to mechanical energy in carbon steel, which is generally used as a constituent material, is the Lorentz force.
The Lorentz force mechanism is generally expressed as body force per unit volume.

Ultrasonic vibration of the specimen in a magnetic field also generates an induced current density J ⁱ in the specimen.

This induced current density J ⁱ generates a magnetic field, thereby exciting a detectable electromotive force in the coil 52 of the EMAT 50 . Due to the above principle, EMAT generates and detects ultrasonic signals in metal specimens without the need for couplants, so it can be used in non-destructive testing of metallic materials, for example in thickness measurements or when specimens are exposed to high temperatures and/or high speeds. The range of applications is wider than that of conventional piezoelectric elements. In this case, EMAT generates a bulk wave, and the bulk wave in EMAT is generally a transverse wave. This specification deals only with bulk wave EMAT.

A drawback when applying conventional EMAT50 is the weakness of the generated ultrasound waves. This is due to poor conversion efficiency between electromagnetic energy and mechanical energy. From equation (1), the ultrasonic intensity will increase if the drive current is increased and a stronger bias magnetic field is applied.
Improving the drive current typically requires a high power pulsed power supply and impedance matching of the EMAT coil 52. If the power of the pulsed power supply is sufficiently high, sufficiently powerful ultrasound waves can be generated even without the bias static magnetic field B. However, high power pulse power supplies are expensive. Furthermore, it also increases the dead time of the received signal.

Impedance matching improves the efficiency of ultrasound generation and detection. However, since impedance matching must be performed for each different type of coil, there is little flexibility. Therefore, it seems that providing a strong bias magnetic field from a permanent magnet is cheaper and more efficient in design. Furthermore, from equation (2), a strong bias magnetic field can also improve the received signal.

Neodymium magnets are generally considered to be the strongest commercially available permanent magnets. Therefore, for the same magnet configuration, neodymium magnets produce stronger bias magnetic fields and ultrasound waves. In order to increase the ultrasonic intensity, conventional EMATs, especially their magnet arrangement, need to have a large volume. As such an EMAT, there is a device as described above (Non-Patent Document 1).

Nippon Matec Co., Ltd. “Latest Electromagnetic Ultrasonic (EMAT) System Principle” [Search date 2019/05/28], Internet <www.matech.co.jp/products/emat_principle.html>

Considering the application of EMAT in the field of non-destructive testing, miniaturization of EMAT is very important. Small EMAT not only improves spatial resolution but also expands maneuverability in narrow and complex spaces. However, miniaturization of EMAT is extremely difficult. In general, the bias static magnetic field provided by a permanent magnet is under certain conditions directly proportional to its thickness, so in particular, reducing the thickness (or height) of the EMAT will result in a corresponding reduction in the magnetic field provided by the magnet. . However, to obtain a stronger magnetic field, larger magnets would be stacked on top of each other. In other words, since the thickness of the magnet arrangement cannot be reduced, its application range is limited to larger spaces.

In light of the above, an object of the present invention is to increase the magnetic field force on the coil side of a small magnet without increasing the thickness (height) of the magnet arrangement, and to miniaturize an electromagnetic ultrasound probe (EMAT).

The invention according to claim 1 (FIG. 1)
a coil 2 disposed along the inspection surface 11 of the test object 10 and generating an eddy current U in the test object 10;
a core magnet 3 that is provided directly above the inspection surface 11 via the coil 2 and applies a static magnetic field B to a region where the eddy current U is generated;
a ring magnet 4 that is arranged to surround the core magnet 3 and enhances the bias static magnetic field B of the surface 3m of the core magnet 3 on the inspection surface 11 side;
An electromagnetic ultrasonic wave that transmits an ultrasonic wave into the test object 10 by an electromagnetic force generated by the interaction of the eddy current U and the static magnetic field B, and receives an ultrasonic wave reflected by a defect 15 of the test object 10. In probe 1,
The coil 2 is a circular coil formed of a spiral conductive wire and whose size matches the size of the core magnet 3 .
The core magnet 3 has a circular block shape , and in four magnetic domains 3a, 3b, 3c, and 3d on the upper, lower, left, and right sides, adjacent magnetic domains are magnetized with different polarities,
The ring magnet 4 has a ring shape into which the core magnet 3 is fitted, and has four magnetic domains 4a, 4b, 4c, and 4d on the inner, outer, left, and right sides that are different from each other due to bipolar magnetization on the inner peripheral surface in the radial direction. It is magnetized to the pole,
The core magnet 3 is arranged so that the polarity of the magnetic domains 3a and 3b on the coil 2 side and the polarity of the magnetic domains 4a and 4b inside the ring magnet 4 are the same , and the domain wall K of the core magnet 3 and the ring magnet It is characterized in that it is arranged so that the domain walls K of No. 4 coincide with each other .

Here, the core magnet 3 may be constructed of one circular block-shaped member, as will be described later, or may be formed by bonding and integrating two semicircular block-shaped members at their plane portions.
Similarly, the ring magnet 4 may be formed of a single ring-shaped member, or may be formed by joining a magnetized half ring.
In addition, when the inspection surface 11 is flat, a planar coil is usually used as the coil 2 so as to follow the flat inspection surface 11, but when the inspection surface 11 is a cylindrical surface such as a pipe or a curved pipe. In the case of a curved or spherical surface such as, it is formed into a curved or spherical surface that follows the curved or spherical surface.
At this time, the surfaces 3m and 4m of the core magnet 3 and the ring magnet 4 on the coil 2 side are similarly formed into curved or spherical surfaces. These points are common to the core magnet 3 and coil 2 of the present invention.

Although a circular coil is used as the coil 2 of the present invention, it is also possible to use coils of other shapes.

According to this, the bias magnetic field B of the surface 3m of the core magnet 3 on the inspection surface 11 side was able to be enhanced by the ring magnet 4 and the opposing magnet 4 arranged around the core magnet 3. It can be made smaller than the conventional electromagnetic ultrasound probe (EMAT), which is composed of a single magnet magnetized in the thickness direction (NS) and does not have an intensifying means.

(a) A perspective view of an electromagnetic ultrasound probe (Example 1) of the present invention, and (b) a perspective cross-sectional view of the center thereof. (a) A plan view of a modification of Example 1, and (b) a central sectional view thereof. (a) A perspective view of Example 2, and (b) a central sectional view thereof. (a) A perspective view of a modification of Example 2, and (b) a central sectional view thereof. (a) Perspective view of Example 3, (b) XX cross-sectional view, (c) Y-Y cross-sectional view, (d) Z-Z cross-sectional view, and (e) bottom view thereof. be. (a) A perspective view of Example 4, (b) a central sectional view of the front view, and (c) a bottom view of the same. (a) A perspective view of a modified example of Example 4, (b) a central sectional view of the front view, and (c) a bottom view of the same. (a) A perspective view of Example 5, (b) a center sectional view of the front view, (c) a bottom view, and (d) a right sectional view. (a) A perspective view of a modified example of Example 5, (b) a central cross-sectional view when viewed from the front, (c) a bottom view, and (d) a right-hand side cross-sectional view. It is a front view of a conventional example. Distribution diagrams of vertical magnetic flux density in an aluminum test specimen of a conventional example (core magnet alone), in which (a) the separation distance (lift-off) of the core magnet alone from the surface of the test specimen was 0.5 mm, and (b) 1. 0 mm, (c) is 1.5 mm, and (d) is 2.0 mm. Distribution diagrams of vertical magnetic flux density in the aluminum test specimen of the present invention (Fig. 1), in which (a) the distance of the core magnet alone from the surface of the test specimen is 0.5 mm, (b) 1.0 mm, (c ) is for 1.5 mm, and (d) is for 2.0 mm. It is a graph showing the relationship between vertical magnetic flux density and separation distance in aluminum test specimens of a conventional example (single core magnet) and the present invention (composite magnet). Distribution diagrams of vertical magnetic flux density in a carbon steel test specimen of a conventional example (core magnet alone), in which (a) the distance of the core magnet alone from the surface of the test specimen is 0.5 mm, (b) 1.0 mm, (c) is the case of 1.5 mm, and (d) is the case of 2.0 mm. Distribution diagrams of vertical magnetic flux density in the carbon steel test specimen of the present invention (Fig. 1), in which (a) the distance of the core magnet alone from the surface of the test specimen is 0.5 mm, (b) 1.0 mm, ( c) is for 1.5 mm, and (d) is for 2.0 mm. It is a graph showing the relationship between vertical magnetic flux density and separation distance in carbon steel test specimens of a conventional example (core magnet alone) and the present invention (composite magnet).

The present invention will be explained below with reference to the drawings. The EMAT 1 of the present invention includes a coil 2, a core magnet 3, and a ring magnet 4 arranged to surround the core magnet 3, or a pair (or two pairs) of opposing magnets arranged to sandwich the core magnet. It is composed of 4R and 4L (4F and 4B).
The EMAT 1 of the present invention has a circular block shape as shown in FIG. 1, a rectangular block shape as shown in FIG. 2 (Example 1), and both sides of the core magnet 3 as shown in FIGS. (Example 2) where opposing magnets 4R, 4F, and 4B are provided on four sides of the core magnet 3 as shown in FIG. 5 (Example 3). 6 and 7, in which opposing magnets 4R and 4F are provided on both sides of the core magnet 3 (Example 4: In this case, the magnetization state of the core magnet 3 is different from Example 2), FIG. Or, as shown in FIG. 9, opposing magnets 4R, 4F, 4F, and 4B are provided on four sides of the core magnet 3 (Example 5: In this case, the magnetization state of the core magnet 3 is different from Example 3. )and so on.

The coil 2 is common to all embodiments, and is made of a conductive wire made of copper or the like which is spirally wound in a circular shape (elliptical "racetrack shape", double racetrack shape, D-shaped coil, double D-shaped coil, or rectangular shape) from the center. It is a conductor wire wound several tens of times around the circumference, a conductor wire formed in a meander shape, and a conductor wire formed in a similar spiral or meander shape on a printed circuit board.
The coil 2 is selected to have a preferable shape in relation to the core magnet, as will be described later. The coil 2 is formed so that the surface formed by the spiral or meandering conductive wire follows the inspection surface 11 of the test specimen 10. For example, if the inspection surface 11 is a flat plane, a planar coil is selected; if the inspection surface 11 is cylindrical like a pipe, a curved coil with an arcuate cross section is selected; In this case, a spherical coil is selected.
In FIG. 1, the coil 2 is a single-layer spiral coil, but it is not limited to this, and may be multiplexed vertically.

Here, the shape of the coil 2 will be briefly explained.
As described above, the coil 2 is made by winding a conducting wire in a spiral shape from several turns to several tens of turns from the center so that the radius gradually increases with each turn.
The elliptical "racetrack-shaped" coil is a coil 2 composed of a straight section 2a and arcuate sections 2b provided at both ends of the straight section 2a.
A D-shaped coil (not shown) is a D-shaped coil made up of a straight section and an arcuate section connecting both ends of the straight section.
The double racetrack type and double D type coils use two coils, and their straight portions 2a are arranged in parallel.
In the case of a double racetrack type or double D type coil, the coil 2 is wound in such a way that the currents flowing in adjacent conductors flow in the same direction.
The meander shape refers to a shape in which a conductive wire or printed wiring is bent into a crank shape.

The core magnet 3 is a permanent magnet or an electromagnet, and is a means for generating a static magnetic field B on the inspection surface 11 side of the test object 10. In this embodiment, permanent magnets are used.
The shape of the core magnet 3 may be a circular block shape (cylindrical or disk shape) as shown in FIGS. 1 and 3, or a rectangular block shape as shown in the other figures. Similarly to the coil 2, when the inspection surface 11 is flat, the surface 3m of the core magnet 3 on the coil 2 side is formed flat along the flat inspection surface 11. In the case of a cylindrical surface, or a curved or spherical surface such as a curved pipe, it is formed into a curved or spherical surface that follows this.

The number of core magnets 3 may be one or two.
In the case of one piece, one circular block-shaped member is magnetized with four poles on both sides, and the four adjacent magnetic domains 3a, 3b, 3c, and 3d, which are divided into four vertically and horizontally, are made to have different polarities. Magnetize.
In the case of two pieces, use two semicircular block-shaped members 3R and 3L, and use N-S magnetization in the thickness direction to set the polarity of the front side and back side ends to N/S, respectively. The four adjacent magnetic domains 3a, 3b/3c, and 3d are made to have different polarities by bonding and integrating them at the plane portions.
The core magnet 3 is arranged so that the static magnetic field B on the surface 3m on the coil 2 side is biased by the ring magnet 4 or the opposing magnet 4 and acts on the eddy current U generated by the coil 2, as will be described later. Therefore, the shape of the core magnet 3 and the shape of the coil 2 are closely related. This point will be explained in detail in each embodiment.

Magnets that are placed around the core magnet 3 and that bias the core magnet 3 to enhance the static magnetic field B on the surface 3m of the coil 2 side of the core magnet 3 include ring magnets and arc-shaped or rectangular block magnets. .

The ring magnet 4, which is an example of a magnet that enhances the static magnetic field B, may be a cylindrical member as shown in FIG. 1 or a rectangular cylindrical member as shown in FIG. Similar to the core magnet 3, the ring magnet 4 includes one that uses one member and one that uses two members, an inner circumference side member and an outer circumference side member.
When manufacturing the ring magnet 4 with one member, the square tubular ring magnet 4 shown in FIG. By magnetization (axial magnetization), mutually adjacent magnetic domains are magnetized with different polarities in the magnetic domains 4a, 4b, 4c, and 4d, which are divided into four parts inside, outside, and on the left and right.
When using half rings, magnetize them in the radial direction so that if the inner circumference of one half ring is N and the outer circumference is S, then the inner circumference of the other half ring is S and the outer circumference is N. to be magnetized. Then, the left and right half ring members are joined to form one ring magnet 4.
The inner diameter of the ring magnet 4 is approximately equal to the outer diameter of the core magnet 3, and the ring magnet 4 is fitted into the core magnet 3 and fixed by adhesive.

The circular arc block-shaped opposing magnets 4R and 4L in FIG. 4c) is magnetized to the north pole, and the magnetic domain 4b (4d) on the outer circumference side (inner circumference side) is magnetized to the south pole.
The rectangular block-shaped opposing magnets 4R and 4L in FIG. 4 are also magnetized in the thickness direction, so that the magnetic domain 4a (4c) at one end becomes the N pole, and the magnetic domain 4b (4d) at the other end becomes the S pole. (N-S) magnetized.
The pair of rectangular block-shaped front and rear opposing magnets 4F and 4B in FIG. -S) Magnetization is performed.

Next, the relationship between the core magnet 3, ring magnet 4 (or opposing magnets 4R, 4L/4F, 4B), and the coil 2 will be explained. As mentioned above, there are various types of coils 2, and any of them can be used. Usually, a circular coil is used, but an appropriate coil in relation to the core magnet 3 is used depending on the situation.
In addition, when using a race track type, double race track type, D type coil, or double D type coil, the straight line portion 2a should be parallel to the lower side of the core magnet 3 facing the left and right opposing magnets 4R and 4L. will be installed.

(Example 1)
In the EMAT 1 in FIGS. 1 and 2, the magnetic domain wall K of the core magnet 3 and the magnetic domain wall K of the ring magnet 4 are made to match, and the polarity of the magnetic domains 3a and 3b of the core magnet 3 on the coil 2 side and the ring magnet The magnetic domains 4a and 4d inside the magnetic domain 4 are fitted and fixed with adhesive so that the polarities thereof are the same.
A core magnet 3 fitted into a ring magnet 4 is placed on top of the coil 2. As described above, coils of various shapes can be applied to the coil 2, but in FIG. 1, a circular coil is used because the core magnet 3 is in the shape of a circular block. The size of the coil 2 matches the size of the core magnet 3. If necessary, a conductive thin plate (aluminum thin plate), not shown, may be provided between the core magnet 3 and the coil 2. This point is common to all the examples.

In the case of FIG. 2, the core magnet 3 has a rectangular block shape, but normally a circular coil is used, but a racetrack-shaped coil may also be used. In this case, the straight line portion 2a corresponds to the length of the coil-side surface 3m of the core magnet 3.

(Example 2)
In the EMAT 1 of FIG. 3, a pair of circular arc block-shaped opposing magnets 4R and 4L are used. These are arranged on the left and right sides of the core magnet 3 via domain walls K extending in a direction rising with respect to the coil 2. The opposing magnetic domains 4a and 4d of the opposing magnets 4R and 4L that face each other via the core magnet 3 are arranged so as to have different polarities. The core magnet 3 is arranged so that the polarity of the magnetic domains 3a and 3b on the coil 2 side and the polarity of the magnetic domains 4a and 4d of the opposing magnets 4R and 4L adjacent to the magnetic domains 3a and 3b are the same.
The coil 2 is a circular coil that matches the surface 3m of the core magnet 3, as in FIG.

In FIG. 4, the core magnet 3 and the pair of opposing magnets 4R and 4L have a rectangular block shape, so the opposing magnets 4R and 4L are adhered to both sides of the core magnet 3 while maintaining the relationship shown in FIG. Therefore, FIG. 3(b) and FIG. 4(b) have the same shape.
As the coil 2, a circular coil is used as in FIG. 2, but a rectangular coil, a racetrack coil, or a meandering coil may also be used.

(Example 3)
In the EMAT1 shown in FIG. 5, a pair of opposing magnets 4F and 4B are further added to the front and rear of the EMAT1 shown in FIG. 4L, and are arranged to sandwich the core magnet 3 from the front and back.
The pair of front and rear opposing magnets 4F and 4B are rectangular block magnets that are magnetized with four poles on both sides (in the magnetic domains 4e to 4h/4i to 4l, which are divided into four in the front, back, left, and right, adjacent magnetic domains are magnetized with different polarities. ), the polarity of the magnetic domains 3a and 3b on the coil 2 side of the core magnet 3, and the polarity of the magnetic domains 4e, 4h/4i, and 4l of the pair of front and rear opposing magnets 4F and 4B adjacent to these magnetic domains 3a and 3b. They are arranged so that they are at opposite poles. In addition, in the above case, the opposing magnets 4F and 4B are one member magnetized with four poles on both sides, but two members of each are magnetized in the thickness direction (N-S) and combined. You can.
As the coil 2, a circular coil is used as in FIG. 3, but a rectangular coil, a racetrack-shaped coil, or a meander-shaped coil can also be used.

(Example 4)
In the EMAT 1 in FIG. 6, the core magnet 3 and the opposing magnets 4R and 4L are (NS) magnetized rectangular block-shaped magnets, and the S pole of the core magnet 3 is placed on the coil 2 side. The opposing magnets 4R and 4L are arranged such that the magnetic domains 4b and 4d on the core magnet 3 side have the same polarity.
Again, circular coils are used, but it is also possible to use double racetrack coils or double D-shaped coils, meander shapes.
The double racetrack coil is one in which the racetrack coils are line-symmetrically arranged as described above, and the directions of current flowing through adjacent straight portions 2a are the same.
A double D-shaped coil (not shown) is a double D-shaped coil arranged symmetrically with respect to a straight line portion thereof. Note that the D-shaped coil is a coil in which a straight portion and an arcuate portion are combined into a D shape.
In each coil 2, the straight portion 2a is arranged so as to coincide with the coil-side surface 3m of the core magnet 3. Note that the left and right coils 2 are indicated by 2R and 2L.

(Modification of Example 4)
In the EMAT1 shown in FIG. 8 , opposing magnets 4F and 4B are further added to the EMAT1 shown in FIG. The pair of opposing magnets 4F and 4B before and after this are (NS) magnetized, and the core magnet 3 side and the opposite outside are magnetized with different polarities.
The added opposing magnets 4F and 4B are arranged with the core magnet 3 at the center, orthogonal to the pair of opposing magnets 4R and 4L on the left and right, and are arranged to sandwich the core magnet 3 from the front and back. .
The polarity of the magnetic domain 4f/4h adjacent to the core magnet 3 of the added opposing magnets 4F/4B is as follows:
It is arranged to have the same polarity as the magnetic domain 3b of the core magnet 3 on the coil 2 side.
In the above embodiment, in the EMAT 1 using rectangular block magnets, the core magnet 3 can be shaped like a circular block, and the opposing magnet can be shaped like an arc block as shown in FIG.
In this case as well, the coil 2 is a double racetrack type coil, a double D type coil, or a meander type coil.

Next, the principle of generating ultrasonic waves by EMAT1 will be explained. The EMAT 1 is installed directly above the inspection surface 11 of the test specimen 10 with a slight gap. With this installation, a static magnetic field (bias magnetic field) B is applied from the core magnet 3 of the EMAT 1 to the test specimen 10. Next, when a high frequency current is passed through the coil 2, an eddy current U is generated on the test surface 11 of the test object 10 and its vicinity. The high-frequency current flowing through the coil 2 causes the eddy current U to generate periodically fluctuating high-frequency vibrations, which generates electromagnetic force (Lorentz force if the test object 10 is a non-magnetic material, Lorentz force if the test object 10 is a magnetic material, and magnetostrictive force). (the sum of the force and the magnetizing force) vibrates the test object 10 and generates ultrasonic waves.
Note that the ring magnet 4 of the EMAT 1 and the arc-shaped or rectangular block-shaped opposing magnets 4R and 4L (4F and 4B) change the path of the lines of magnetic force coming out of the core magnet 3, so that the divergence of the lines of magnetic force on the coil 2 side is suppressed. As a result, an enhanced bias magnetic field B is generated on the coil 2 side.

Transmission and reception of EMAT1 is as follows. The EMAT 1 is placed on the inspection surface 11 of the test specimen 10 as described above. The core magnet 3 forms the static magnetic field B in the depth direction of the test object 10 as described above. When a high frequency current is passed through the coil 2 in this state, ultrasonic waves are generated as described above.

If a defect 15 (acoustic discontinuity such as a crack in the test object 10 or corrosion on the back side of the test object 10) exists in the test object 10, the ultrasonic wave is reflected by the defect 15 and propagates in the direction of the EMAT 1 again. do. When the ultrasonic wave reaches the vicinity of EMAT1, the magnetic field in the region where the static magnetic field B exists changes over time due to the displacement of the ultrasonic wave. An eddy current U is generated in a direction that attempts to prevent this magnetic field change, and by detecting this eddy current U again by the coil 2, it is possible to receive a reflected wave from the defect 15 present in the test specimen 10.
As described above, the position of the defect 15 can be known by measuring the time it takes for the ultrasonic wave transmitted from the EMAT 1 to be reflected by the defect 15 and received by the EMAT 1 again.

In Example 1 (FIGS. 1 and 2), in order to downsize the EMAT structure, a small composite permanent magnet composed of a core magnet and a ring magnet and a circular coil with a diameter of 10 mm were used.
The core magnet has the same diameter as the coil and is 5 mm thick. The core magnet of the composite small permanent magnet was aligned with the coil and placed directly above it with a slight gap. The magnetic pole distribution in this case is shown in FIG. 1(b).
The core magnet and ring magnet are combined based on the Halbach arrangement principle, and the ring magnet, whose inner peripheral surface in the radial direction is bipolarly magnetized (axially magnetized), surrounds the circular block magnet that is the core magnet and generates a magnetic field. Concentrate. That is, the lines of magnetic force coming out of the N pole on the coil side of the core magnet are blocked by the surrounding ring magnet 4 and do not diverge in the horizontal direction, but are concentrated and reach the S pole on the adjacent coil side. This increases the downward vertical magnetic field of the core magnet. That is, with this configuration, it was possible to improve the magnetic field force on the coil side without increasing the thickness of the small magnet while maintaining the diameter of the coil at the same size as in the conventional example.

In the case of FIG. 1, since the core magnet 3, ring magnet 4, and coil 2 are circular, no wasted space is generated between the core magnet 3 and the coil 2, and the electromagnetic ultrasound probe 1 is made smaller. I was able to do that. In addition, in the case of FIG. 2, the core magnet 3 and ring magnet 4 are shaped like rectangular blocks, and the coil 2 is shaped like a racetrack, and the entire straight portion 2a is used, so the efficiency of transmission and reception can be greatly increased. .

In the case of the second embodiment (FIGS. 3 and 4), the arrangement of the magnets and coils is basically the same as in the first embodiment, so that the same effects as in the first embodiment are achieved.

In the case of Embodiment 3 (FIG. 5), the magnet arrangement is such that a pair of opposing magnets 4F and 4B are added to that of Embodiment 2 in FIG. The divergence of the magnetic lines of force of the core magnet 3 is suppressed, and the magnetic flux density on the coil 2 side can be further increased.

In the case of the fourth embodiment (FIG. 6), the arrangement of the magnets is different from the above embodiments, and the entire surface 3m of the core magnet 3 on the coil 2 side is one pole (in this case, the S pole). Lines of magnetic force are directed from the outer magnetic domains 4a and 4c of the opposing magnets 4R and 4L to the surface 3m of the core magnet 3 on the coil 2 side.
Thereby, the magnetic flux density on the coil 2 side can be further increased for the above-mentioned reason.

In the case of a modification of the fourth embodiment (FIG. 7), the magnet arrangement is such that a pair of opposing magnets 4F and 4B are added to the embodiment of FIG. 6, and the same effect as in FIG. 6 is achieved.

The static magnetic field on the coil side of the core magnet of EMAT1 shown in FIG. 1 was simulated using the finite element method.
Regarding the static magnetic field, Ampere's law is simplified as follows.

Introducing a scalar potential to explain the magnetic field.

For linear materials, the constitutive relations of the magnetic field are expressed by equation (5).

According to Gauss' law for magnetism:

Finite element (FE) simulation provides a solution to equation (4) for the magnetic scalar potential. Permanent magnets have residual magnetization derived from the magnetic domain structure and anisotropy, which is expressed as follows.

The magnetic flux density in the permanent magnet domain is expressed by equation (8).

The magnetization of a magnetic domain of a non-ferromagnetic material (including air) such as a paramagnetic material or a diamagnetic material is expressed by equation (9).

通常、非強磁性体の比透磁率μ_ｒｎｆは１に近い。よって、シミュレーションではμ_ｒｎｆを１とする。強磁性体磁区について、磁化曲線は非線形である。磁束密度と磁場力との関係をＢ－Ｈ曲線で表す。 Therefore, the magnetic flux density of the non-ferromagnetic material is expressed by equation (10).

Usually, the relative magnetic permeability μ _rnf of a non-ferromagnetic material is close to 1. Therefore, μ _rnf is set to 1 in the simulation. For ferromagnetic domains, the magnetization curve is nonlinear. The relationship between magnetic flux density and magnetic field force is represented by a BH curve.

For the purpose of numerical analysis, software (COMSOL Multiphysics 5.4) was used to calculate the magnetic flux density on the coil side surface of the permanent magnet used in the conventional EMAT and the composite magnet of the core magnet and ring magnet used in the EMAT of the present invention. A three-dimensional finite element (FE) simulation was performed for comparison.
Below, the magnetic flux densities of the composite magnet of the present invention and the permanent magnet used in the conventional EMAT on test specimens of aluminum (non-ferromagnetic material) and low carbon steel (ferromagnetic material) were analyzed using the same simulation.
Here, the composite magnet of the present invention had the magnet configuration shown in FIG. 1, and the core magnet of the present invention was used as the permanent magnet used in the conventional EMAT. This is called a single core magnet.
The thickness of the test specimen was 10 mm, and its three-dimensional size was 100×40×10 mm ³ . These two configurations were placed above the center of the test specimen with a certain space provided.

The separation distance (lift-off) between the composite magnet and the single core magnet from the test specimen was set to 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm.
In addition, an air region of 200×100×10 mm ³ surrounds the test specimen, the composite magnet, and the core magnet alone (analysis region).
The test specimen, the composite magnet, the single core magnet, and the surrounding air were all divided by tetrahedral meshes. The maximum size of the tetrahedral mesh in the region of the test specimen, composite magnet, and single core magnet was 0.5 mm, and the maximum width of the air region was 2 mm. The residual magnetic flux densities of the core magnet and ring magnet of the composite magnet and the core magnet alone were set to 1.20T (core magnet) and 1.25T (ring magnet), respectively. The relative magnetic permeability of air and aluminum was set to 1, and the relative magnetic permeability of the core magnet and ring magnet was set to 1.05. In the simulation of low carbon steel, the BH (magnetization) curve of structural rolled steel material SS400 (carbon content: approximately 0.16%) was used.

Simulation results and discussion Due to the skin effect, eddy currents mainly flow near the surface of the test specimen. Therefore, the static magnetic field near the surface of the specimen interacts with the eddy currents to generate electromagnetic forces. In addition, since this type of EMAT generates transverse waves mainly generated by the vertical static magnetic field, only the distribution of the static magnetic field in the vertical direction was considered here.

Figures 11(a)-(d) show the vertical magnetic flux density distribution produced by a single core magnet placed directly above the surface of the aluminum specimen. As the lift-off of the single core magnet increases, the color of the half-moon-shaped parts corresponding to two adjacent magnetic poles showing the vertical magnetic flux density distribution becomes lighter, indicating that the vertical magnetic flux density is decreasing. Furthermore, the vertical magnetic flux density rapidly decreases near the edge of the single core magnet and the boundary between the two magnetic poles. The former is because the lines of magnetic force diverge at the edges, increasing the horizontal magnetic flux density and decreasing the vertical magnetic flux density accordingly. From the change in color of the half-moon-shaped part mentioned above, it can be seen that as the distance from the test specimen to the core magnet increases, not only does the vertical magnetic flux density decrease, but also the vertical magnetic field density distribution becomes non-uniform. . This is one of the reasons why the permanent magnet arrangement of conventional EMATs is large.

FIG. 12 shows the vertical magnetic flux density distribution resulting from a composite magnet of the invention (FIG. 1) placed just above the surface of an aluminum specimen. Compared with FIG. 11, under the ring magnet with horizontal magnetic pole distribution, the vertical magnetic flux density is distributed in a crescent shape. This occurs due to the interaction of two permanent magnets (core magnet and ring magnet). That is, the lines of magnetic force coming out of the N pole on the coil side of the core magnet are suppressed from divergence in the horizontal direction by the ring magnet, and reach the S pole on the adjacent coil side without divergence. Furthermore, this suppressed amount increases the vertical magnetic flux density. Note that since the coil is installed only under the core magnet of the composite magnet, the magnetic field distribution under the ring magnet does not affect the intensity or distribution of the ultrasonic waves. It can be seen that this configuration increases the vertical magnetic flux density under the core magnet, as long as the separation distance is the same, compared to the case of the core magnet alone.

FIG. 13 shows the maximum perpendicular magnetic flux density on the surface of the aluminum test piece with respect to the distance from the test piece for a composite magnet of a core magnet and a ring magnet, and for a single core magnet. The vertical axis shows the maximum vertical magnetic flux density, and the horizontal axis shows the separation distance.
According to this, the maximum perpendicular magnetic flux density of the single core magnet decreased rapidly as the separation distance increased. This is another reason why the permanent magnet arrangement of conventional EMATs is large.
According to a composite magnet, the maximum perpendicular magnetic flux density can be greatly improved compared to a single core magnet at the same separation distance. If the separation distance (lift-off) of the magnets was the same, the composite magnet of the present invention was able to increase the maximum magnetic flux density by about 20%. However, it can be seen that the tendency that the maximum perpendicular magnetic flux density decreases rapidly with increasing separation distance cannot be changed.

FIGS. 14 and 15 show the vertical magnetic flux density distribution just above the surface of the low carbon steel specimen in the case of a single core magnet and in the case of a composite according to the present invention. FIG. 16 shows the change in maximum perpendicular magnetic flux density with respect to the separation distance between the composite magnet of the present invention and a single core magnet. The variation of vertical magnetic flux density with separation distance in low carbon steel specimens is similar to that for aluminum specimens. However, regarding the distribution in ferromagnets, it can be seen that the vertical magnetic flux density of low carbon steel is clearly stronger.

In a conventional EMAT having a large magnet arrangement, even if the distance between the magnets is large, the effect on the ultrasonic generation and detection effect of the EMAT is small, but this is not the case for a small magnet arrangement such as the present invention. That is, unlike the conventional EMAT with a large magnet arrangement, in the miniaturized EMAT of the present invention, the separation distance of the magnets has a very important effect on the distribution of magnetic field force in the test specimen, and this Directly affects the transmission and reception of signals.

In the magnet arrangement of miniaturized EMATs, whether the test object is a non-magnetic material or a magnetic material, with increasing separation distance, the maximum vertical magnetic flux density decreases rapidly and the distribution of the vertical magnetic field becomes non-uniform. This means that the miniaturized EMAT is not suitable for use under conditions where the separation distance is large. The composite magnet of the present invention can greatly increase its maximum perpendicular magnetic flux density and improve the uniformity of the magnetic field at the same separation distance. In other words, this shows that in the field of application of miniaturized EMATs, the composite magnet of the present invention can increase ultrasonic signals and improve lift-off characteristics.
In addition, it is thought that the above-mentioned effect can be obtained also in Example 2 and subsequent examples.

From the above, by adopting the magnet arrangement of the present invention, it was possible to achieve miniaturization of the EMAT.

1: Electromagnetic ultrasound probe (EMAT), 2: Coil, 2a: Straight section, 2b: Arc-shaped section, 3: Core magnet, 3R/3L: Semicircular block magnet, 3a/3b/3c/3d : Magnetic domain, 3m: Surface on the inspection surface (coil) side, 4: Ring magnet, 4R/4L (4F/4B): Opposing magnet, 4m: Surface on the inspection surface (coil) side, 4a, 4b, 4c, 4d ( 4e to 4h/4i to 4l): magnetic domain, 10: test object, 11: inspection surface, 15: defect, B: (bias) magnetic field, K: domain wall, U: eddy current

Claims (1)

a coil arranged along the inspection surface of the test object and generating an eddy current in the test object;
a core magnet that is provided directly above the inspection surface via the coil and applies a static magnetic field to a region where the eddy current is generated;
a ring magnet arranged to surround the core magnet and enhance a bias static magnetic field of a surface of the core magnet on the inspection surface side;
An electromagnetic ultrasonic probe that transmits ultrasonic waves into the test object by electromagnetic force generated by the interaction of the eddy current and the static magnetic field, and receives ultrasonic waves reflected by defects in the test object,
The coil is a circular coil formed of a spiral conductive wire and matching the size of the core magnet 3 ,
The core magnet has a circular block shape , and in the four magnetic domains on the top, bottom, left and right, adjacent magnetic domains are magnetized with different polarities,
The ring magnet has a ring shape in which the core magnet is fitted, and has four magnetic domains on the inner, outer, left and right sides of the ring magnet, and adjacent magnetic domains are magnetized with different polarities by bipolar magnetization on the inner peripheral surface in the radial direction.
The core magnet is arranged so that the polarity of the magnetic domain on the coil side and the polarity of the magnetic domain inside the ring magnet are the same , and the domain wall of the core magnet and the domain wall of the ring magnet are arranged so that they match. An electromagnetic ultrasonic probe characterized by: