JP2005099045A - C-scan ultrasonic flaw detection method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a C-scan ultrasonic flaw detection method and apparatus that can test a full section in a plate thickness direction with a single scan by eliminating a dead zone near the surface of a tested plate and can detect even the form of a micro internal flaw. <P>SOLUTION: An ultrasonic transmitter emits point-focused ultrasonic waves onto a tested plate substantially perpendicularly. An ultrasonic receiver receives both reflected waves of the ultrasonic waves reflected by the top of an internal flaw toward the obverse surface of the tested plate, reflected by the obverse surface and propagated into a liquid from the reverse surface until received, and reflected waves of the ultrasonic waves reflected first by the reverse surface of the tested plate toward the internal flaw, reflected by the bottom of the internal defect and propagated into the liquid from the reverse surface until received. From amplified received signals, the signal by the reflected waves from the internal flaw having the longer propagation distance is extracted. According to the extracted signal, the internal flaw is detected in the tested plate. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a C-scan ultrasonic flaw detection method and apparatus, and more particularly to a C-scan ultrasonic flaw detection method and apparatus suitable for use in detecting an internal defect of about 10 to 100 μm in a cut sample of a rolled metal sheet. It is.

In recent years, thin steel sheets used as materials for automobiles, cans, etc. have been made thinner in order to reduce weight and reduce material costs, and to reduce production costs by reducing the number of parts and materials used in processing such as pressing and drawing. Strong processing with significant deformation of the material is applied.
When performing strong processing on the steel sheet, cracks occur if there are internal defects consisting of non-metallic inclusions, etc., in the part where the deformation is significant, but the occurrence of cracks due to internal defects becomes more pronounced as the thickness of the steel sheet is thin, and The size of internal defects that cause cracking is also reduced. There is also a relationship between the form of defects and the occurrence of cracks. Defect forms include spherical single bodies, single bodies that extend in one direction, and aggregates of microspherical defects. Is seen. In addition, products with severe usage conditions such as thick steel plates used in sour gas line pipes are increasing, and even small inclusions with a size of about 10 μm are considered to be harmful and cause hydrogen-induced cracking. Ease of occurrence is different.

For this reason, it is required that the above-described steel sheet has as few internal defects as possible, and that the defect form is less likely to be cracked. It is necessary to do.
As a means for detecting the internal defect of such a steel sheet and evaluating its form, a part of the product is cut out as a sample, and the internal defect in this sample is detected using a device called a C-scan ultrasonic flaw detector. Has been widely used. FIG. 11 shows a flaw detection method using a conventional C-scan ultrasonic flaw detector.

  That is, the point-focusing type ultrasonic transceiver 102 above the inspection plate 101 immersed in the solvent liquid is scanned by the scanning device 104 that is moved by the signal of the controller 114 and is spaced from the electric pulse generator 116 at a constant time interval. The electrical pulse transmitted in step 1 is converted into an ultrasonic wave, and the ultrasonic beam 103 is transmitted substantially vertically toward the inspected plate 101, and the internal defect of the inspected plate 101 and the reflected wave from the surface are received. Convert to signal.

  The received signal is amplified by the receiving amplifier 111, and the reflected wave from the defect is extracted by the gate circuit 112. The extracted signal is sent to the peak value detection circuit 113, where the amplitude of the defect reflected wave is detected and transmitted to the controller 114. The controller 114 outputs the amplitude of the defect reflected wave and the position signal of the scanning device 104 to the display 115, and the display 115 displays a two-dimensional distribution map of the internal defects, thus detecting the internal defects. .

  In such a method in which ultrasonic waves are transmitted substantially perpendicularly to the inspected plate 101 by one point-focusing type ultrasonic transmitter / receiver 102 and a reflected wave from the inspected plate 101 is received to detect a defect, an ultrasonic beam is used. When a surface is incident on the surface, a surface echo having a large amplitude and reverberation is generated for a while, so that a defect reflected wave in the vicinity of the surface overlaps the surface echo or the reverberation, and the presence cannot be identified. There was a problem that it was not possible to detect defects.

Moreover, as a prior art regarding the C-scan ultrasonic flaw detection method or apparatus, there are Patent Documents 1 and 2 using high-frequency ultrasonic waves. The former is 30 to 100 MHz, and the latter is 15 to 50 MHz, both using high frequency ultrasonic waves and reducing the beam diameter, thereby improving resolution and improving internal defect detection ability. The latter also optimizes the ultrasonic frequency, focal length, and the relationship between the inspected plate and the focal position, thereby ensuring the detection of minute defects existing near the surface and enabling quantitative evaluation of flaw detection results. It is a thing.
JP 59-17153 JP-A-5-333000

  However, as described in Patent Documents 1 and 2, it is generally known that, when high-frequency ultrasonic waves are used and the focal beam diameter is reduced, the focal depth decreases (for example, R. Saglio et al. "THE USE OF FOCUSED PROBES FOR DETECTION, IMAGING, AND SIZING OF FLAWS", in Proc. First Intrenational Symposium on Ultrasonic Materials Characterization-Gaithersburg Md. (1978)).

  The depth of focus is, for example, the length of the range in which the sound pressure on the central axis of the ultrasonic beam to be transmitted and received is within −6 dB compared to the sound pressure at the focus position, and the greater the depth of focus, the greater the depth of focus. It is possible to detect internal defects over a wide range in the direction of the plate thickness of the plate to be inspected. According to the above document, when the frequency of ultrasonic waves is f, the velocity is C, the diameter of the transducer built in the ultrasonic transceiver is D, and the focal length is F, the ultrasonic beam diameter d at the focal position is as follows. And the focal depth L are expressed by the equations (1) and (2), respectively.

d = (C / f) × (F / D) (1)
L = (C / f) × (F / D) ² ………………… (2)
From the equation (1), it can be seen that when the frequency f is increased in order to reduce the ultrasonic beam diameter d at the focal position, the focal depth L is reduced from the equation (2). For this reason, in flaw detection using high-frequency ultrasonic waves, it is difficult to detect the entire cross section in the plate thickness direction of the inspected plate with uniform sensitivity, and the ability to detect defects existing at depths other than the focal position is greatly reduced. There is a drawback in that detection omissions occur frequently. For this reason, in order to detect a defect without omission in the thickness direction, it is necessary to change the focal position and perform the flaw detection as many times as necessary.

Further, in Patent Document 2, although the dead zone directly under the surface is reduced, it cannot be said that there is none, and the problem that a micro defect still existing in the vicinity of the surface cannot be detected in the C-scan ultrasonic flaw detection by the vertical flaw detection method. Is left.
By the way, in order to solve the above-mentioned problems, the present inventors have already faced the line focus type ultrasonic transmission sensor and the one-dimensional array type ultrasonic sensor with Japanese Patent Application No. 6-7176 across the board to be inspected. And transmitting a band-like ultrasonic beam from the transmitting sensor toward the inspection plate substantially perpendicularly, and reflecting a reflected wave from an internal defect caused by the ultrasonic wave incident on the inspection plate to the one-dimensional array type ultrasonic wave. Proposing an ultrasonic flaw detection method and apparatus characterized by detecting the presence or absence of a reflected wave that has reached a predetermined amplitude after amplifying the received ultrasonic wave received by a receiving sensor and extracting only the reflected wave, As a result, it became possible to detect a linear region having a constant width all over the thickness without a dead zone immediately below the surface.

However, with this method, the presence or absence of a minute defect can be clearly seen, but since the ultrasonic waves to be transmitted and received are not two-dimensionally focused, the resolution in the width direction is low, and there is a problem that the form of the defect cannot be identified. It was.
The present invention has been made to solve the problems of the prior art, and in C-scan ultrasonic flaw detection, there is no dead band near the surface of the plate to be inspected, and flaw detection in the entire cross section in the plate thickness direction in one scan. It is an object of the present invention to provide a C-scan ultrasonic flaw detection method and apparatus capable of detecting even fine internal defect forms.

The present invention is as follows.
1. A point-focusing type ultrasonic transmitter and a point-focusing type ultrasonic receiver are placed opposite to each other with a test plate immersed in the liquid in between and scanned, and point focusing is performed from the ultrasonic transmitter. Is incident on the inspected plate approximately perpendicularly, and the ultrasonic wave is reflected on the upper side of the internal defect and directed to the surface of the inspected plate, reflected on the surface, and propagated into the liquid from the back surface and received. The reflected wave and the reflected wave that is reflected first on the back surface of the inspected plate, directed toward the internal defect, reflected on the lower side of the internal defect, and then propagated into the liquid from the back surface and received. Of the received signals received and amplified by the ultrasonic receiver, a signal due to a reflected wave from an internal defect having a longer propagation distance is extracted, and the internal defect of the inspection plate is detected based on the extracted signal. A C-scan ultrasonic flaw detection method characterized by detecting.
2. When (τ 0 + τ 1) and after, when τ 0 is the time at which the transmitted ultrasonic wave reaches the ultrasonic receiver and τ 1 is the time required for the ultrasonic wave to propagate in the thickness direction of the plate to be inspected And (τ 0 + 2 × τ 1), a signal by a reflected wave from an internal defect is extracted from a signal received before (τ 0 + 2 × τ 1).
3. A point-focusing-type ultrasonic transmitter that transmits ultrasonic waves to the surface of the board to be inspected substantially perpendicularly, and a position that faces the point-focusing-type ultrasonic transmitter with the board to be inspected interposed therebetween. The sound wave is reflected on the upper side of the internal defect and is directed to the surface of the inspected plate, reflected on the surface, propagated into the liquid from the back surface and received, and the ultrasonic wave is first reflected on the back surface of the inspected plate. A point-focusing ultrasonic receiver that receives both reflected waves that are reflected, faced to an internal defect, reflected on the lower side of the internal defect, then propagated into the liquid from the back surface, and the ultrasonic transmitter And a support arm that supports the ultrasonic receiver with a test plate interposed therebetween, a scanning device that scans the support arm, and an electric pulse that is transmitted to a piezoelectric vibrator built in the ultrasonic transmitter An electric pulse generator, an amplifying device for amplifying the received signal, and an amplified received signal Chi, C scan ultrasonic inspection apparatus characterized by comprising a gate means for propagation distance extracts the signal due to reflected waves from the internal defects of the longer.
4. When the gate means sets τ0 as the time when the transmitted ultrasonic wave reaches the ultrasonic receiver, and τ1 as the time required for the ultrasonic wave to propagate in the thickness direction of the inspection plate, 4. The C-scan exceeding the C scan according to claim 3, wherein the signal is set to extract a signal due to a reflected wave from an internal defect from a signal received after (τ0 + τ1) and before (τ0 + 2 × τ1). Sonic flaw detector.

  According to the present invention, a point-focusing type ultrasonic transmitter and a point-focusing type ultrasonic receiver are arranged to face each other with a test plate immersed in a liquid interposed therebetween, and the ultrasonic transmission is performed. Ultrasound that has been point-focused from the child is incident substantially perpendicularly into the inspected plate, and the ultrasonic wave transmitted by the ultrasonic wave and the reflected wave from the internal defect generated by the ultrasonic wave are received by the ultrasonic wave receiver. Since the signal of the reflected wave from the internal defect is extracted from the amplified signal, and the internal defect of the inspection board is detected based on the extracted signal, the vicinity of the front and back surfaces of the inspection board It is possible to detect even an internal defect with a uniform sensitivity over the entire cross section without a dead zone. This can contribute to quality control of the product and improvement of the quality itself.

FIG. 1 is a block diagram including a partial perspective view showing the configuration of an embodiment of the present invention.
In FIG. 1, 11 is a point-focusing ultrasonic transmitter (hereinafter simply referred to as an ultrasonic transmitter), 12 is a point-focusing ultrasonic receiver (hereinafter simply referred to as an ultrasonic receiver), Oppositely arranged with a sandwich. Reference numeral 14 denotes a U-shaped support arm that supports the ultrasonic transmitter 11 and the ultrasonic receiver 12. Note that water that is suitably used as an ultrasonic propagation medium is interposed between the ultrasonic transmitter 11 and the ultrasonic receiver 12 and the board 13 to be inspected.

A scanning device 15 scans the support arm 14. Reference numeral 16 denotes an electric pulse generator that transmits electric pulses from a built-in clock circuit (not shown) to a piezoelectric vibrator (not shown) built in the ultrasonic transmitter 11 at regular time intervals. Reference numeral 17 denotes a receiving amplifier that receives a signal from the ultrasonic receiver 12, 18 denotes a gate circuit, 19 denotes a peak value detection circuit, 20 denotes a controller, and 21 denotes a display.
The ultrasonic transmitter 11 converts the electric pulse transmitted from the electric pulse generator 16 at a predetermined time interval into an ultrasonic wave, and transmits the ultrasonic transmission beam 11A substantially perpendicularly to the inspected plate 13 through water. . The ultrasonic receiver 12 receives an ultrasonic reception beam 12A including a reflected wave from an internal defect caused by the ultrasonic wave incident on the inspection plate 13 through water. Then, the support arm 14 is scanned by the scanning device 15 driven by a signal from the controller 20, and the ultrasonic transmitter 11 and the ultrasonic receiver 12 which are arranged to face each other are squarely scanned on the surface of the inspected plate 13. Investigate its internal defects.

  The received signal amplified by the receiving amplifier 17 is extracted by the gate circuit 18 from the received signal. The extracted signal is transmitted to the peak value detection circuit 19, and the peak value detection circuit 19 detects the amplitude of the reflected wave and outputs it to the controller 20 as an analog amount or a digital amount. The controller 20 outputs the amplitude of the reflected wave and the position signal of the scanning device 15 to the display 21 to create a two-dimensional distribution map of internal defects.

Here, the function of the gate circuit 18 will be described in detail with reference to FIG.
First, when the thickness of the inspected plate 13 is t and the internal defect 22 is near the surface 13a of the inspected plate 13, that is, the distance d from the surface 13a is equal to or less than t / 2, ultrasonic transmission is taken as an example. When the ultrasonic transmission beam 11A transmitted from the child 11 propagates in the liquid and reaches the surface 13a of the inspected plate 13, it enters the inspected plate 13 and propagates therein.

  At this time, a part of the ultrasonic wave propagating into the inspected plate 13 goes straight and is received by the ultrasonic receiver 12 as a direct transmitted wave 30. Further, when the internal defect 22 exists in the ultrasonic wave propagation path, first, it is reflected once on the upper side of the internal defect 22, directed to the surface 13 a, reflected by the surface 13 a, and within the thickness t of the inspected plate 13. 0.5 reciprocally propagates, and the reflected wave 31 propagates in the liquid from the back surface 13b and is received, and first propagates 0.5 reciprocally within the thickness t of the inspected plate and is reflected by the back surface 13b toward the internal defect 22, After being reflected once on the lower side of the defect 22, a reflected wave 32 is generated which propagates from the back surface 13b into the liquid and is received.

The reflected waves 31 and 32 are further reflected once or more on the back surface 13b, and the reflected wave received by the ultrasonic receiver 12 is generated one or more times within the thickness t of the board 13 to be inspected. However, illustration is omitted here.
As described above, the present invention extracts the reflected wave signal from the internal defect from the received and amplified signal, and detects the internal defect based on the extracted signal. When the internal defect 22 has a distance d from the surface 13a of the inspection plate 13 of t / 2 or less, the reflected wave 32 reaches the ultrasonic receiver 12 later than the reflected wave 31.

  Furthermore, when d is t / 2 or less, the temporal transition of the signal received by the ultrasonic receiver 12 will be described with reference to FIG. In this figure, τ0 is the time when the directly transmitted wave 30 propagated 0.5 times in the thickness t of the inspection plate 13 reaches the ultrasonic receiver 12, and τ1 is the ultrasonic wave in the thickness t of the inspection plate 13. This is the time required for 0.5 round-trip propagation. Reference numeral 33 denotes a transmitted wave signal which propagates 0.5 times in the thickness t of the board to be inspected 0.5 and further travels once in the board 13 to be inspected.

  Thus, since the propagation distance of the reflected wave 32 is larger than the propagation distance of the reflected wave 31, the reflected wave 32 is received later than the reflected wave 31. From this figure, the reflected wave 32 due to the internal defect 22 is reflected from the time τ 0 when the directly transmitted wave 30 that has propagated 0.5 times in the thickness t of the inspection plate 13 reaches the ultrasonic receiver 12. Appears after the time τ1 required for 0.5 reciprocal propagation within the thickness t of 13 has elapsed from the dead zone region of the direct transmitted wave 30 and before (2 × τ1) has elapsed since time τ0. Appears earlier than the transmitted wave 33.

  Further, the closer the position of the internal defect 22 is to the surface 13a, the more τ2 (τ2 is the time required for the ultrasonic wave to make one round trip to the distance d to the internal defect 22 in the inspected plate 13, that is, the time required for propagation by the distance 2d). Although it becomes smaller, it appears earlier than the transmitted wave 33. Therefore, even if the internal defect 22 exists directly under the surface 13a, the reflected wave 32 caused by the internal defect 22 can be clearly identified and extracted. Therefore, it is preferable to extract this and detect the internal defect 22.

The time τ when the reflected wave 32 appears due to the internal defect 22 is expressed by the following formula (3).
(Τ0 + τ1) ≦ τ ≦ (τ0 + 2 × τ1) ……………… (3)
When the time from the arrival of the directly transmitted wave 30 to the end of the reverberation of the transmitted wave is τD (see FIG. 3), it is received after the time (τ0 + τD) and before (τ0 + 2 × τ1). If the processed signal is extracted, it is possible to detect both the reflected waves 31 and 32.

The above description is about the case where the internal defect exists near the front surface. Next, FIG. This will be described with reference to FIG.
In this case, unlike FIG. 2, the propagation distance of the reflected wave 31 due to the internal defect 22 is longer than the propagation distance of the reflected wave 32, so that the reflected wave 31 is received with a delay.

As described above, the reflected wave 31 appears outside the dead zone region due to the direct transmitted wave 30 and appears earlier than the transmitted wave 33. Therefore, even if the internal defect 22 exists directly under the back surface 13b, the reflected wave 31 due to the internal defect 22 can be clearly identified and extracted, so that the internal defect 22 can be detected and is preferable.
As explained so far, the direct propagation wave 30 that has propagated 0.5 reciprocatingly through the plate 13 to be inspected and the transmitted wave 33 that has propagated 1 reciprocating propagation through the substrate 13 to be inspected from the internal defect with the longer propagation distance. It is preferable to extract the reflected wave because a miscellaneous echo component that becomes noise is small.

However, in the present invention, the ultrasonic wave propagates 0.5 reciprocatingly through the plate 13 to be inspected, and further propagates through the inspected plate 13 by an integer number of reciprocations of one or more reciprocating times, and the inspected plate 13 1 more than the transmitted wave. You may make it extract the reflected wave from the internal defect with a longer propagation distance which appears between the transmitted waves which propagated back and forth.
In this case, in the present invention, a point-focusing type ultrasonic transmitter and a point-focusing type ultrasonic receiver that are two-dimensionally focused are used, and the ultrasonic transmitter and the ultrasonic receiver are relative to the plate to be inspected. Therefore, the resolution in the width direction is high, and the form of internal defects can be detected. Furthermore, in the present invention, as shown in FIG. 6, the ultrasonic transmission beam 11A from the ultrasonic transmitter 11 is focused on the surface of the ultrasonic receiver 12, and the ultrasonic wave of the ultrasonic receiver 12 is set. The focal point G of the reception beam 12A may be the surface of the ultrasonic transmitter 11.

  In this way, the intensity of the ultrasonic transmission beam 11A transmitted from the ultrasonic transmitter 11 becomes lower as it approaches the surface of the inspected plate 13, so the intensity of the reflected wave from the internal defect existing there is small. On the other hand, since the reception efficiency of the ultrasonic receiver 12 is larger near the focal point F, it becomes larger as it is closer to the surface of the inspected plate 13. Therefore, the intensity of the reflected wave from the internal defect received by the ultrasonic receiver 12 can be made almost constant even if the position where the internal defect exists is changed by canceling the effect of both, and the thickness direction This is because the sensitivity can be uniform over the entire area.

FIG. 7 shows the result of detecting an artificial defect by the method of the present invention and the conventional method.
That is, the method of the present invention was measured using an ultrasonic transmitter 11 and an ultrasonic receiver 12 having a frequency of 25 MHz and an underwater focal length of 38 mm arranged so that the focal points F and G are connected as shown in FIG. In the conventional method, as shown in FIG. 11, measurement is performed by transmitting and receiving ultrasonic waves from one direction to one point focusing type with a frequency of 25 MHz and an underwater focal length of 38 mm. It is a thing. The artificial defect was manufactured by changing the distance from the surface of the inspected plate 13 having a thickness of 5.5 mm and making a 0.2 mmφ horizontal drill hole perpendicular to the thickness direction.

  From this figure, compared with the conventional method, the method of the present invention has a constant amplitude of the reflected wave regardless of the distance from the surface of the defect, has a remarkably excellent focal depth, and is uniform over the thickness direction. It can be seen that it can be detected with sensitivity.

Next, the positional relationship among the point-focusing type ultrasonic transmitter 11, the point-focusing type ultrasonic receiver 12, and the inspected plate 13 will be described in detail based on experimental data.
In FIG. 8, the ultrasonic transmitter 11 and the ultrasonic receiver 12 are arranged so that the focal points F and G are connected as shown in FIG. 6, and the ultrasonic transmitter 11 and the ultrasonic receiver 12 at that time are arranged. The distance between them is LS, and the inspected plate 13 having a thickness t of 4.5 mm is moved between the ultrasonic transmitter 11 and the ultrasonic receiver 12 to move between the ultrasonic receiver 12 and the inspected plate 13. This is a measurement of the amplitude and S / N of the reflected wave from the internal defect of the inspected plate 13 when the distance L2 is changed.

The used ultrasonic transmitter 11 and ultrasonic receiver 12 have a frequency of 25 MHz and an underwater focal length of 38 mm.
As a result, the amplitude and S / N of the reflected wave due to the internal defect hardly change at any position between the ultrasonic transmitter 11 and the ultrasonic receiver 12 in the inspection target plate 13. That is, it can be seen that the inspected plate 13 may be at any position between the ultrasonic transmitter 11 and the ultrasonic receiver 12.

  When the focal lengths of the ultrasonic transmitter 11 and the ultrasonic receiver 12 are equal, that is, as shown in FIG. 6, the focal point F of the ultrasonic transmission beam 11A coincides with the surface of the ultrasonic receiver 12, When the ultrasonic transmitter 11 and the ultrasonic receiver 12 are arranged so that the focal point G of the ultrasonic reception beam 12A coincides with the surface of the ultrasonic transmitter 11, the ultrasonic transmitter 11 and the ultrasonic receiver 12 Assuming that the distance between them is LS0, the distance LS0 is expressed by the following equation (4).

LS0 = FL − {(CM / CL) −1} × t (4)
Where FL: focal length of the point-focusing type ultrasonic transmitter 11 or focal length of the point-focusing type ultrasonic receiver 12, t: plate thickness of the plate to be inspected, CM: ultrasonic wave in the material to be inspected Is the propagation velocity of ultrasonic waves in the liquid.
FIG. 9 shows the distance between the ultrasonic transmitter 11 and the ultrasonic receiver 12 in water when the focal lengths of the ultrasonic transmitter 11 and the ultrasonic receiver 12 are equal in this way, with the distance LS0 as the reference distance. The amplitude of the reflected wave from the internal defect of the board to be inspected is measured. The ultrasonic transmitter 11 and the ultrasonic receiver 12 used had a frequency of 25 MHz, an underwater focal length of 38 mm, and a thin steel plate having a thickness t of 4.5 mm was used as the inspected plate.

  From the result shown in FIG. 9, it can be seen that when the distance LS is smaller than the reference distance LS0, the amplitude of the reflected wave is slightly large, but when the distance LS is larger than the reference distance LS0, the amplitude of the reflected wave rapidly decreases. A reduction of 3 dB or more in the amplitude of the reflected wave from the reference distance LS0 leads to a decrease in S / N in defect detection, which is not preferable. Therefore, the distance between the ultrasonic transmitter 11 and the ultrasonic receiver 12 LS (mm) must satisfy the following formula (5).

LS ≤ LS0 + FL x 5/38 (5)
That is,
LS ≦ FL − {(CM / CL) −1} × t + FL × 5/38 (6)
Represented as:
In addition, the coefficient 5/38 of FL in the equation (6) is generalized in consideration of the case where the focal lengths of the ultrasonic transmitter 11 and the ultrasonic receiver 12 are other than 38 mm.

Even when the focal length of the ultrasonic transmitter 11 and the focal length of the ultrasonic receiver 12 are not equal, the same effects as those shown in FIGS. 7 and 8 can be obtained by arranging as follows. be able to.
For example, when the focal length of the ultrasonic transmitter 11 is larger than the focal length of the ultrasonic receiver 12, the ultrasonic transmitter 11 so that the focal point F of the ultrasonic transmission beam 11 A coincides with the surface of the ultrasonic receiver 12. And an ultrasonic receiver 12 are arranged. At this time, if the inspected plate 13 is positioned closer to the ultrasonic receiver 12 than the focal point G of the ultrasonic reception beam 12A, the same thing as that described with reference to FIG. Further, when the focal length of the ultrasonic receiver 12 is larger than the focal length of the ultrasonic transmitter 11, the ultrasonic transmitter 11 so that the focal point G of the ultrasonic reception beam 12A coincides with the surface of the ultrasonic transmitter 11. And an ultrasonic receiver 12 are arranged. At this time, if the inspected plate 13 is positioned closer to the ultrasonic transmitter 11 than the focal point F of the ultrasonic transmission beam 11A, the same thing as described with reference to FIG.

Note that the focal length of the ultrasonic transmitter 11 is larger than the focal length of the ultrasonic receiver 12 with respect to the distance LS between the focal length of the ultrasonic transmitter 11 and the ultrasonic receiver 12 when they are not equal. The case will be described in detail with reference to experimental data shown in FIG.
At this time, when the ultrasonic transmitter 11 and the ultrasonic receiver 12 are arranged so that the larger focal length is FL and the focal point F of the ultrasonic transmission beam 11A coincides with the surface of the ultrasonic receiver 12, Assuming that the distance between the sound wave transmitter 11 and the ultrasonic wave receiver 12 is LS0, the distance LS0 is expressed by the following equation (7).

LS0 = FL − {(CM / CL) −1} × t (7)
FIG. 10 shows the measurement of the amplitude of the reflected wave from the internal defect of the inspected plate by changing the distance between the ultrasonic transmitter 11 and the ultrasonic receiver 12 in water using the distance LS0 as a reference distance. is there. The ultrasonic transmitter 11 used has a frequency of 25 MHz and an underwater focal length of 38 mm, and the ultrasonic receiver 12 has a frequency of 25 MHz and an underwater focal length of 25 mm. A thin steel plate having a thickness t of 4.5 mm was used as the inspected plate.

  As in FIG. 9, the amplitude of the reflected wave is slightly large when the distance LS is smaller than the reference distance LS0, but it can be seen that the amplitude of the reflected wave rapidly decreases when the distance LS is larger than the reference distance LS0. The fact that the amplitude of the reflected wave is reduced by 3 dB or more as compared with the case of the reference distance LS0 leads to a decrease in S / N in defect detection, and it is judged that it is not preferable. Therefore, the ultrasonic transmitter 11 and the ultrasonic receiver 12 The distance LS (mm) between the two must satisfy the following equation (8).

LS ≤ LS0 + FL x 5/38 (8)
That is,
LS ≦ FL − {(CM / CL) −1} × t + FL × 5/38 (9)
Represented as: In addition, the coefficient 5/38 of FL in the equation (9) is generalized in consideration of cases where the focal lengths of the ultrasonic transmitter 11 and the ultrasonic receiver 12 are other than 38 mm.

  When the focal length of the ultrasonic receiver 12 is large, the ultrasonic transmitter 11 has a large focal length; the frequency as the ultrasonic transmitter 11; 25 MHz, the underwater focal length; 25 mm; the frequency as the ultrasonic receiver 12; 25 MHz, underwater When an experiment was conducted using a focal length of 38 mm, the result was almost the same as that shown in FIG. Therefore, when the focal length of the ultrasonic transmitter 11 and the focal length of the ultrasonic receiver 12 are not equal, the larger one is FL, and the distance LS between the ultrasonic transmitter 11 and the ultrasonic receiver 12 is the same as the above. The ultrasonic transmitter 11 and the ultrasonic receiver 12 may be arranged so as to satisfy the equation (9).

  The present invention method was applied when defect inspection was performed on a thin steel plate having a thickness of 1.2 to 5.5 mm as the inspected plate 13. As the ultrasonic transmitter 11 used at this time, an ultrasonic wave having a frequency of 25 MHz and an underwater focal length of 38 mm was used to transmit an ultrasonic wave having a frequency of 25 MHz, and as the ultrasonic receiver 12, a frequency of 25 MHz and an underwater focal length of 38 mm was used. We used a thing to detect flaws. As a result, it was possible to detect an ultra-fine defect of 10 μm φ regardless of the position in the thickness direction of the defect.

  In the above-described embodiment, the ultrasonic transmitter 11 and the ultrasonic receiver 12 which are disposed to face the board to be inspected by holding the ultrasonic transmitter 11 and the ultrasonic receiver 12 by the support arm 14. However, the present invention is not limited to this, and it goes without saying that it may be configured to scan the inspected plate side.

It is a block diagram including the partial perspective view which shows the structure of one Example of this invention. It is explanatory drawing which shows the relationship between the propagation path of the reflected wave from a defect, and a defect position. FIG. 3 is a characteristic diagram showing a relationship between a received ultrasonic signal and time at the defect position in FIG. 2. It is explanatory drawing which shows the relationship between the propagation path of the reflected wave from a defect, and a defect position. FIG. 5 is a characteristic diagram showing a relationship between a received ultrasonic signal and time at the defect position in FIG. 4. It is explanatory drawing which shows the focus of the ultrasonic beam of a transmission element and a receiver. It is a characteristic view which shows the relationship between the distance from the surface of a defect, and the amplitude of a reflected wave. It is a characteristic view which shows the amplitude and S / N of the reflected wave from an internal defect. It is a characteristic view which shows the amplitude of the reflected wave from an internal defect. It is a characteristic view which shows the amplitude of the reflected wave from an internal defect. It is a block diagram including the partial perspective view which shows the structure of a prior art example.

Explanation of symbols

11 Ultrasonic Transmitter (Point Focusing Ultrasonic Transmitter)
11A Ultrasonic transmission beam
12 Ultrasonic receiver (Point-focusing type ultrasonic receiver)
12A Ultrasonic receiving beam
13 Board to be inspected
13a surface
13b reverse side
14 Support arm
15 Scanning device
16 Electric pulse generator
17 Receiver amplifier
18 Gate circuit
19 Peak value detection circuit
20 Controller
21 Display
22 Internal defects
30 Direct transmitted wave
31, 32 Reflected wave
33 Transmitted wave

Claims (4)

  A point-focusing type ultrasonic transmitter and a point-focusing type ultrasonic receiver are placed opposite to each other with a test plate immersed in the liquid in between, and the point-focused ultrasonic transmitter is scanned from the ultrasonic transmitter. A sound wave is incident substantially perpendicularly into the inspected plate, and the ultrasonic wave is reflected on the upper side of the internal defect, is directed to the surface of the inspected plate, is reflected on the surface, is propagated into the liquid from the back surface, and is received. Both the wave and the ultrasonic wave are first reflected on the back surface of the inspected plate, directed to the internal defect, reflected on the lower side of the internal defect, and then propagated into the liquid from the back surface to receive the reflected wave. Of the received signals received and amplified by the sonic wave receiver, a signal due to a reflected wave from the internal defect having a longer propagation distance is extracted, and the internal defect of the inspected plate is detected based on the extracted signal. A C-scan ultrasonic flaw detection method.   When τ0 is the time at which the transmitted ultrasonic wave reaches the ultrasonic receiver, and τ1 is the time required for the ultrasonic wave to propagate in the thickness direction of the plate to be inspected, after (τ0 + τ1), 2. The C-scan ultrasonic flaw detection method according to claim 1, wherein a signal based on a reflected wave from an internal defect is extracted from a signal received before (.tau.0 + 2.times..tau.1). A point-focusing type ultrasonic transmitter that transmits ultrasonic waves substantially vertically to the surface of the inspection plate;
Arranged at a position facing the point-focusing ultrasonic transmitter across the board to be inspected, the ultrasonic wave is reflected on the upper side of the internal defect and directed to the surface of the board to be inspected, reflected on the surface, and from the back surface The reflected wave that is propagated and received in the liquid and the ultrasonic wave are first reflected on the back surface of the board to be inspected, directed to the internal defect, reflected on the lower side of the internal defect, and then propagated into the liquid from the back surface and received. A point-focusing ultrasonic receiver that receives both reflected waves, and
A support arm that supports the ultrasonic transmitter and the ultrasonic receiver with an inspection plate interposed therebetween;
A scanning device for scanning the support arm;
An electric pulse generator for generating an electric pulse to be transmitted to a piezoelectric vibrator incorporated in the ultrasonic transmitter;
An amplifying device for amplifying the received signal; and gate means for extracting a signal due to a reflected wave from an internal defect having a longer propagation distance among the amplified received signal;
A C-scan ultrasonic flaw detector characterized by comprising: The gate means has a time (τ0) when τ0 is a time at which the transmitted ultrasonic wave reaches the ultrasonic receiver and τ1 is a time required for the ultrasonic wave to propagate in the thickness direction of the inspection plate. 4. The C-scan ultrasonic flaw detection as set forth in claim 3, wherein the signal is set to extract a signal due to a reflected wave from an internal defect from a signal received after + τ1) and before (τ0 + 2 × τ1). apparatus.