JPH0119095B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transducer for electromagnetic ultrasonic flaw detection and transmission, which is useful for flaw detection of steel materials, particularly hot materials in continuous casting processes.

As a transducer for electromagnetic ultrasonic flaw detection, as shown in Fig. 1a, a plurality of permanent magnet plates 11 _{, 11 1} , 11 ₂ , 11 ₃ .
High permeability magnetic materials 12, 12 ₁ , 12 ₂ , 12 ₃ ...
The magnetic field generating mechanism has a magnetic field generating mechanism arranged in the thickness direction so that the polarity is alternately reversed by sandwiching the magnetic field generating mechanism, and a wavy shape meandering along the arrangement of the permanent magnet plates 11 is provided near the end of the magnetic field generating mechanism. There is an electromagnetic ultrasonic flaw detection transducer having a periodic magnet structure in which a coil 13 is arranged.

In such a transducer, now,
When a current is passed through the wave-shaped coil 13, the inspected material 14
Eddy currents I with the polarities shown are generated in various parts directly below the permanent magnet plate 11 on the surface. This eddy current I and the permanent magnet plate 1
Due to the interaction with the horizontal magnetic field H directly below 1,
A Lorentz force F of the illustrated polarity acts on the surface of the material 14 to be inspected. Therefore, by passing an alternating current through the wave-shaped coil 13, longitudinal ultrasonic waves whose vibration direction and propagation direction are both perpendicular to each other are excited in the inspected material 14.

In this way, ultrasonic waves are excited in the inspected material 14, and the reflected waves of the ultrasonic waves from the defective parts are detected by, for example, the corrugated coil 13 by following the conversion process reverse to the above-mentioned ultrasonic excitation. This is how flaw detection is performed. According to such a transducer, a uniform and strong magnetic field can be obtained due to the configuration of its magnetic field generation mechanism, and the interaction between this and the eddy current induced by the corrugated coil produces a uniform and wide field. A flaw detection surface can be obtained.

However, the electromagnetic ultrasonic flaw detection transducer shown in FIG. 1a still has the following basic problems. All conventional transducers, including the transducer shown in FIG. 1a described above, have used planar coils.
That is, a wavy coil called a meander line shown in FIG. 1b, and a pair or single spiral coil shown in c and d of the same figure are used. next,
The reason for using this planar coil will be explained.
That is, such a planar coil is placed close to and parallel to the surface of the material to be inspected, and a high-frequency current i is applied to it.
When , an eddy current ie flowing in the opposite direction to the current i is induced on the surface of the opposite inspected material. Since the electromagnetic ultrasonic transducer generates ultrasonic waves by using the Lorentz force acting on this eddy current, the coil is formed into a flat shape in order to efficiently induce the eddy current. This is because it becomes necessary to flow the high frequency current i on a plane along the surface of the material to be inspected.

However, if such a structure is adopted, the efficiency and sensitivity will drop sharply if the gap (lift-off) between the planar coil and the material to be inspected is increased.

Furthermore, when performing flaw detection on a hot material in a continuous casting process or the like using such an electromagnetic ultrasonic transducer, several considerations are required, unlike when testing in a cold process. A particularly important point is that the gap distance between the transducer and the material to be inspected must be sufficiently large, for example, about 5 mm. The reason for this is that in the case of hot materials, the temperature of the material is extremely high (800 to 1100℃), the surface is extremely uneven, and the transducer is used as a cooling structure so that the end surface of the transducer and the material to be inspected are A cooling medium must flow through the gap between the two. This inevitably results in an increase in the spacing (lift-off), resulting in a decrease in efficiency and sensitivity.

Furthermore, considering the case of receiving ultrasonic echoes, it is necessary to apply a bias magnetic field perpendicular or parallel to the surface of the material to be inspected. As shown in Figure 1e, permanent magnets or high frequency A planar coil is placed under the electromagnet core, which is separate from the magnetic core, and the distance between the core and the material to be inspected increases by the thickness of the planar coil, which reduces the bias magnetic field and improves sensitivity. There is a problem of deterioration.

Therefore, from this perspective, compared to other transducers, the electromagnetic ultrasonic flaw detection transducer using the above-mentioned periodic structure magnet has a shorter air gap distance with the inspected material. Although this method has the advantage of making it possible to perform flaw detection with a relatively large value, it is still not sufficient.

Therefore, it has been desired to develop a transducer that can detect defects in an object to be inspected with high sensitivity while maintaining a sufficient gap distance between the transducer and the object to be inspected.

This invention was made in view of the above points,
It is an object of the present invention to provide a transducer for electromagnetic ultrasonic flaw detection and transmission, which is capable of performing highly sensitive flaw detection while maintaining a sufficiently large gap distance between the object and the material to be inspected.

An embodiment of the present invention will be described below with reference to the drawings. Figure 2 shows the basic configuration of a transducer for electromagnetic ultrasonic flaw detection and transmission according to the present invention.
U-shaped high frequency magnetic core 21 and this high frequency magnetic core 21
The excitation coil 22 is wound around the excitation coil 22. In addition, in FIG. 2, 23 is a material to be inspected.

Next, the flaw detection principle of this electromagnetic ultrasonic flaw detection wave transmitting transducer configured as described above will be explained. Here, the depth dimension of the high frequency magnetic core 21 constituting this transducer is assumed to be sufficiently long,
It is assumed that the inspected material 23 is a perfect conductor. now,
When a current i=Im cosωt is passed through the excitation coil 22,
A high frequency magnetic flux φ proportional to this current is generated as shown in FIG. As a result, an eddy current having a surface current density je flows on the surface of the inspected material 23 so as to satisfy the boundary condition. Here, if the high-frequency magnetic flux density on the surface of the inspected material 23 is Bac, then Bac=A(X,G)・Im cosωt...(1) je(X,G)=Hac(X,G)=1/μ _D・Bac(X,
G)=1/μ _D・A(X,G)・Im cosωt...(2) In the above formula, A(X,
G) is a coefficient that depends on the gap length G between the high-frequency magnetic core 21 and the material to be inspected 23.

Due to the interaction between the high-frequency magnetic flux density Bac and the surface current density je, per unit area on the surface of the inspected material 23, fd=je(X,G)・Bac(X,G)=1/2μ ₀ A(X
, G) ² Im+1/2μ ₀ A(X, G) ² Im cos2ωt...(3) A driving force is generated. Therefore, longitudinal ultrasonic waves proportional to the square of the current are excited. The ultrasonic waves excited in this manner travel within the inspected material 23, and if there is a defect, they are reflected there and return. Flaw detection is performed by detecting this returning ultrasonic wave with a detection coil (not shown). Furthermore, since the frequency of the ultrasonic wave is twice the frequency of the current, by passing the ultrasonic wave through a filter, it is possible to easily remove the mixed induction. Note that a current having a single pulse waveform with a narrow pulse width is passed through the excitation coil 22 wound around the U-shaped high-frequency magnetic core 21 using, for example, discharge of a capacitor. Therefore, this pulse waveform has a wide frequency component, but the center frequency is, for example, 400 to 500 kHz.

By using a high-frequency magnetic core in this way, high-frequency magnetic flux with a large magnetic flux density can be transmitted to the test material 23 even if the wire is wound at a position sufficiently far from the surface of the test material 23.
can be guided to the surface of Therefore, the material to be inspected 2
It is possible to increase the gap length between 3 and 3. Note that the position at which the excitation coil 22 is wound around the high-frequency magnetic core 21 can be freely selected.

Next, a specific example of a transducer actually manufactured based on the above-described principle will be described. Figure 3a,
b shows one example, in which the excitation coil 22 is wound around the legs of the high-frequency magnetic core 21. Copper plates 24, 24 are provided to reduce leakage of magnetic flux in the middle of the high-frequency magnetic core 21, and in this example, they are provided at the excitation coil 22. Figure 4 shows the drive and measurement circuit, with 30
is a drive section, which includes a high voltage direct current generator 31 and a resistor 3.
2 to 34, a capacitor 35, and a discharge switch 36, and uses the charging and discharging of the capacitor 35 to drive the transducer shown in FIG. 3 with a large current. 23 is the material to be inspected as described above. Further, 50 is a measuring section, and a receiving transducer 5
1, a resistor 52, an oscilloscope 53, etc.

The circuit shown in FIG. 4 is an experimental circuit for measuring the transmission characteristics of the transducer shown in FIG. . However, the material to be inspected 2
When a hot material is used as No. 3 and the hot material is subjected to flaw detection, it is naturally necessary to use a non-contact type transducer as the receiving transducer 51.

As described above, an experiment was conducted using an aluminum block as the inspected material 23 and a PZT transducer as the receiving transducer 51, and the characteristics obtained from the experiment are shown in Figs. 5 and 6. . In addition, in the characteristic diagram based on this experiment, the characteristics for when a ferrite core and a cut core are used as the high frequency magnetic core 21, and for comparison, the characteristics for a transducer using a conventional periodic structure magnet are also shown. There is.
In Figures 5 and 6, the characteristics when using a ferrite core are shown by a solid line, the characteristics when using a cut core are shown by a dashed line, and the characteristics of a transducer using a conventional periodic structure magnet are shown by a broken line. There is.

Figure 5 shows the charging voltage of capacitor 35 when the gap length to the surface of the material to be inspected is constant (0.8 mm).
It shows the received voltage V _D [V] with respect to V _C [KV]. In a conventional transducer using a periodic structure magnet, V _D is simply proportional to V _C , but in the transducer according to the present invention, Now, when a ferrite core or a cut core is used as a high-frequency magnetic core, V _D is proportional to the square of V _C for each.
Note that FIG. 5 shows experimental values showing the difference in characteristics between the transducer with the conventional structure and the transducer with the structure of the present application. A larger voltage can be extracted compared to transducers of conventional structure.

Figure 6 shows the received voltage V _D [V] with respect to the air gap length [mm] when the charging voltage V _C of the capacitor 35 is constant (10 KV). In both cases, it can be seen that even if the air gap length becomes large, the received voltage does not drop much compared to the conventional transducer. In a conventional transducer, if the gap length is set to about 5 mm, which is necessary for practical purposes, the received voltage V _D will become much smaller than when the gap length is small, but in the transducer of this invention, the gap length is small. The degree of decrease is smaller than in the case where a received voltage of about 0.2 [V] is obtained in this case. This means that while the distance between the magnetic pole centers of conventional transducers using periodic structure magnets is approximately 9 mm, the distance between the magnetic pole centers of the present invention is approximately 30 mm.
This is because it is large at 40mm.

Incidentally, in this experiment, the transducer according to the present invention was provided with a copper plate 24 for preventing magnetic flux leakage, but as shown in FIG. (shown by the solid line in the figure) has better characteristics.
Note that FIG. 7 shows the change in the output voltage V _D with respect to the air gap length when a ferrite core is used as the high frequency magnetic core 21 and V _C is set to 10 [KV].

As explained above, according to the present invention, a transducer for electromagnetic ultrasonic flaw detection and transmission is configured by a high-frequency magnetic core formed in a U-shape and an excitation coil wound around an arbitrary position of the high-frequency magnetic core. By doing so, high-frequency magnetic flux with a large magnetic flux density can be guided to the surface of the object to be inspected, and defects in the object to be inspected can be detected with high sensitivity while maintaining a sufficient gap length between the object and the object to be inspected. It becomes easy,
It has various advantages such as being cost-effective.

[Brief explanation of drawings]

Figure 1a is a side view showing an example of a transducer using a conventional periodic structure magnet, Figure 1b
1 to 1e are diagrams for explaining the problems of conventional transducers, FIG. 2 is a diagram showing the basic configuration of the transducer according to the present invention, and FIG. 3a
3 is a front view showing the configuration of a transducer according to an embodiment of the present invention, and FIG.
4 is a drive and measurement circuit diagram of the same embodiment, FIG. 5 is a diagram showing the characteristics of the received voltage with respect to the capacitor charging voltage in the same embodiment, and FIG. 6 is a diagram showing the air gap in the same embodiment. FIG. 7 is a diagram showing the characteristics of the received voltage with respect to the length, and FIG. 7 is a diagram showing the difference in characteristics depending on the presence or absence of a copper plate in the same embodiment. 21... High frequency magnetic core, 22... Excitation coil, 23...
Material to be inspected.

Claims (1)

[Claims] 1. In a device that detects defects in an object to be inspected by applying electromagnetic ultrasonic waves to the object to be inspected, there is a high-frequency magnetic core formed in a U-shape, and a U-shaped A transducer for electromagnetic ultrasonic flaw detection and transmission, characterized by comprising an excitation coil that guides a high frequency magnetic flux to the surface of an object to be inspected between both legs of a high frequency magnetic core and induces an eddy current by the high frequency magnetic flux.