JP2003294714A - Electromagnetic acoustic wave inspection device using cross-correlation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic acoustic wave inspection device using cross-correlation method for material surface inspection which can solve a problem such that an erroneous detection occurs because random noises prevent a small ultrasonic wave reflected by a surface defect from being detected due to low transmit-receive efficiency of ultrasonic wave. <P>SOLUTION: An electromagnetic acoustic wave probe for sending an ultrasonic wave and an electromagnetic ultrasonic wave probe for receiving an ultrasonic wave are located to face each other with a small clearance on the surface of an object to be inspected. The electromagnetic acoustic wave prove for receiving an ultrasonic wave receives a direct ultrasonic wave emitted from the electromagnetic acoustic wave probe for sending an ultrasonic wave and propagates in the clearance, and a defect ultrasonic wave reflected by a surface defect. After the received ultrasonic waves are amplified and digitized by an AD converter, a cross-correction function of the direct ultrasonic wave and the defect ultrasonic wave is calculated, and thereby the defect ultrasonic wave submerged with random noises can be detected. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inspection method for detecting defects inside or near a surface of a material by using surface waves or plate waves of ultrasonic waves.

[0002]

2. Description of the Related Art Conventionally, ultrasonic surface waves are often used for inspecting the surface layer of materials such as thick steel plates produced in steelworks. For example, Japanese Unexamined Patent Publication No. 6-331603 discloses a surface defect inspection method using a tire probe having a variable angle oscillator inside. However, in this method, in order to transmit the ultrasonic waves emitted from the variable angle transducer in the tire probe to the surface of the material, the tire probe is brought into contact with the material surface through a medium liquid such as water, and the tire probe is It is necessary to establish an acoustic coupling between the child and the material surface. The liquid medium such as water used in this case does not always stay only around the tire probe, but may flow on the material surface and spread to the propagation path of the surface wave. However, there is a problem that the medium liquid of (3) cannot be distinguished from the surface defect and is erroneously detected.

In addition, ultrasonic plate waves are often used for inspecting the surface layer portion of materials such as thin steel plates, but the tire probe used therefor has the same problem.

As a method for solving such a problem, an electromagnetic ultrasonic method (EMAT) has been invented, which does not need to use a medium liquid such as water and can emit and receive ultrasonic waves in a non-contact manner with a material,
JP-A-7-77465 and "Ultrasonic Handbook"
(Publisher: Maruzen, Publication date: August 1999), Chapter 3, Section 4. However, the electromagnetic ultrasonic method is inferior in efficiency of transmitting and receiving ultrasonic waves because of its low electric-acoustic energy conversion efficiency in principle, and to compensate for this, an amplifier with a large amplification factor is used to amplify a small ultrasonic signal. There is a need to. An amplifier with a large amplification factor also amplifies a small amount of random noise at the same time, so small ultrasonic signals reflected from the target surface defect or internal defects near the surface tend to be buried in random noise, resulting in false detection. Was the biggest cause of

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and extracts a small ultrasonic signal buried in random noise reflected from a surface defect or an internal defect near the surface. An object of the present invention is to provide an electromagnetic ultrasonic inspection method capable of performing the above.

[0006]

[Means for Solving the Problems] Means for solving the above-mentioned problems relating to inspection by surface waves include: an electromagnetic ultrasonic probe for transmitting surface waves and an electromagnetic ultrasonic probe for receiving surface waves; A direct surface wave transmitted by the electromagnetic ultrasonic probe for transmitting the surface wave and propagating the short distance and a surface wave that is reflected from a surface defect or an internal defect near the surface. Random by calculating the cross-correlation function between the direct surface wave and the defective surface wave after receiving both of them by the electromagnetic ultrasonic probe for receiving the surface wave, amplifying them, and digitizing them by the AD converter. This is to extract a defect surface wave buried in noise.

A means for solving the above-mentioned problems relating to flaw detection by plate waves is to provide an electromagnetic ultrasonic probe for transmitting a plate wave and an electromagnetic ultrasonic probe for receiving a plate wave on the surface of the material to be inspected over a short distance. Plates both a direct plate wave transmitted separately from the plate-wave transmitting electromagnetic ultrasonic probe and propagating the short distance and a defective plate wave reflected from a surface defect or an internal defect near the surface. A defect buried in random noise by calculating a cross-correlation function between the direct plate wave and the defective plate wave after being received by an electromagnetic ultrasonic probe for wave reception, amplified, and digitized by an AD converter. It is to extract plate waves.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described based on examples with reference to the drawings. FIG. 1 is a schematic diagram showing an outline of an electromagnetic ultrasonic inspection apparatus according to an embodiment of the invention.
In FIG. 1, 1 is a steel plate to be inspected, 2 is a surface defect, 3 is a pulse transmitter, 4 is an electromagnetic ultrasonic probe for transmitting surface waves, and 5 is an electromagnetic ultrasonic probe for transmitting surface waves. An electromagnetic ultrasonic probe for receiving surface waves, which is provided at a short distance (8 cm) from the probe 4, 6 is an amplifier, 7 is an AD converter, 8 is a data accumulator, 9 is a calculator, and 10 is an output device. Is. The data storage 8 is composed of an IC memory of a computer.
A hard disk of a computer may be used instead of the C memory. The waveform arrow 11 indicates how the surface wave transmitted by the electromagnetic ultrasonic probe 4 for surface wave propagation propagates on the surface of the steel material and returns even after being reflected by the surface defect 2.
2 (a) is a bottom view of the electromagnetic ultrasonic probe 4 for transmitting surface waves, and FIG. 2 (b) shows the electromagnetic ultrasonic probe 4 for transmitting surface waves and the thick steel plate 1 which is the subject. It is the figure which showed the relationship. Figure 3
(A) is a bottom view of an electromagnetic ultrasonic probe 5 for receiving surface waves,
FIG. 3B is a diagram showing the relationship between the electromagnetic ultrasonic probe 5 for receiving surface waves and the thick steel plate 1 which is the subject. 12, 1
Reference numeral 4 is a permanent magnet, and 13 and 15 are coils having reciprocating repetitive lines. Although a permanent magnet is used in FIGS. 2 and 3, an electromagnet may be used. As can be seen from FIGS. 2 and 3, the surface acoustic wave transmitting electromagnetic ultrasonic probe 4 and the surface wave receiving electromagnetic ultrasonic probe 5 have exactly the same structure. It is generally well known that the electromagnetic ultrasonic probe having such a structure can transmit and receive surface waves, as disclosed in FIGS. 9 and 10 of JP-A-7-77465. The principle will be briefly described. When a transmission current is applied from the pulse transmitter 3 to the coil 13 of the electromagnetic ultrasonic probe 4 for transmitting surface waves in FIG. 2, an eddy current flows on the surface of the thick steel plate 1 due to electromagnetic induction. A surface wave is transmitted by the Lorentz force between the eddy current and the magnetic field generated by the permanent magnet 12 and the magnetostrictive effect of the thick steel plate 1. When the surface wave passes directly below the electromagnetic ultrasonic probe for receiving the surface wave in FIG. 3, an electric signal is generated in the coil 15 by a physical process completely opposite to the transmission, and the surface wave is received.

As another example of the implementation, instead of the electromagnetic ultrasonic probe shown in FIGS. 2 and 3, "Ultrasonic Handbook" (publisher:
Maruzen, Publication date: August 1999) Chapter 3 Section 4 Disclosed in Fig. 3.9.1 Electromagnetic for Lamb wave of the type using a magnet having a magnetic field component parallel to the material surface Even if an ultrasonic probe having the same structure as that of the ultrasonic probe is used, the surface wave can be emitted and received, so that this may be used.
An electromagnetic ultrasonic probe of the type that emits and receives SH waves on the surface disclosed in FIG. 1 of the publication 00-187022 may be used.

The pulse transmitter 3 transmits a burst wave current having a frequency of 1 MHz, a wave number of 10 and a repetition frequency of 400 Hz to the electromagnetic ultrasonic probe 4 for transmitting a surface wave.
Then, a surface wave is transmitted to the surface of the thick steel plate 1 according to the principle described above. In FIG. 1, the surface wave first propagates to the right and is directly reached by the surface acoustic wave receiving electromagnetic ultrasonic probe 5 provided at a distance of 8 cm from the surface acoustic wave transmitting electromagnetic ultrasonic probe 4. Electromagnetic wave for receiving surface waves, which is received as a wave, propagates further to the right, is reflected by a surface defect 2 at a distance of 28 cm from the surface-transmitting electromagnetic ultrasonic probe 4, and propagates to the left. The probe 5 receives the defect surface wave. The amplifier 6 amplifies the direct surface wave and the defective surface wave.

In FIG. 4, when the surface defect 2 is sufficiently large (length: 3.5 cm, depth: 1 mm), the surface wave signal that has been amplified in this way, that is, the cross-correlation function is not calculated, is obtained. Shows an example of. The horizontal axis represents time, and the vertical axis represents the magnitude of the surface wave signal. In FIG. 4, reference numeral 16 is an electric signal generated by electromagnetic induction with a space separated by a burst wave current for ultrasonic wave transmission. Reference numeral 17 is a direct surface wave, 18 is a defect surface wave, and 19 is random noise. In this case, the surface defect 2 is sufficiently large (length 3.5c
m, depth 1 mm), the defect surface wave 18 is also large, and there is no problem because it is received clearly as shown in FIG.

In FIG. 5, the surface defect 2 is small (length 1.5).
(cm, depth 0.3 mm) shows an example of a surface wave signal that has been amplified, that is, a conventional method in which a cross-correlation function is not calculated. In FIG. 5, reference numeral 16 is an electric signal generated by electromagnetic induction with a space separated by a burst wave current for ultrasonic wave transmission. Reference numeral 17 is a direct surface wave, and 19 is random noise. In this case, the direct surface wave 17 is clearly detected, but the defect surface wave is buried in the random noise 19 and cannot be identified. That is, according to FIG. 5, when the surface defect 2 is small (length: 1.5 cm, depth: 0.3 mm), the received defect surface wave is also small and therefore buried in random noise, and is identified as a defect surface wave. Turns out to be impossible.

The inventor of the present invention first magnified and investigated the direct surface wave 17 and the defect surface wave 18 reflected from the surface defect, and as a result, they are different in size but very similar in shape. Found. Furthermore, by utilizing this fact, it was found that the defect surface wave can be clearly extracted by calculating the cross-correlation function between the direct surface wave and the defect surface wave buried in random noise. That is, the ultrasonic signal amplified by the amplifier 6 in FIG.
It is digitized by and is stored in the data storage device 8. The calculator 8 calculates the cross-correlation function between the direct surface wave and the defect surface wave from the accumulated data by the following formula. Here, A (i × Δt) is the i-th digital value obtained by digitizing the direct surface wave at Δt time intervals,
B ((i−j) × Δt) represents the (i−j) th digital value obtained by digitizing the defect surface wave at Δt time intervals. C (j × Δt) is the j-th digital value of the calculated cross-correlation function. The cross-correlation function calculated in this way is shown in FIG. In FIG. 6, reference numeral 20 is random noise that is reduced as a result of calculating the cross-correlation function, and 21 is a defect surface wave that clearly appears as a result of calculating the cross-correlation function. In this way, the defect surface wave which was buried in the random noise and could not be identified in FIG. 5 became able to clearly identify the defect surface wave 21 as shown in FIG. 6 by calculating the cross-correlation function. Recognize.

When the operation speed of the operation unit 8 is sufficiently high, the operation can be completed between the burst wave currents repeatedly transmitted, and in that case, the data storage unit 8 is not required. In addition, instead of calculating the cross-correlation function for each ultrasonic wave signal that is repeatedly received, the arithmetic mean of multiple ultrasonic wave signals that are repeatedly received is calculated to improve the signal-to-noise ratio. Needless to say, if the cross-correlation function is calculated, the time required will be longer, but the effect will be even greater. The target defects are not limited to surface defects, and this method can be applied to internal defects near the surface. The material to be inspected is not limited to the thick steel plate, but can be applied to the inspection of any material as long as it is a conductive material such as aluminum alloy or copper. In addition, a plate wave (La having the same structure as the electromagnetic ultrasonic probe for surface waves)
This method can also be applied to the inspection of a plate-shaped conductive material with a plate wave (Lamb wave and SH plate wave) using an electromagnetic ultrasonic probe for mb wave and SH plate wave.

[0015]

By calculating the cross-correlation function, it is possible to extract a small defect surface wave signal or defect plate wave signal buried in random noise reflected from a surface defect or an internal defect near the surface. A sound wave inspection method can be realized, and as a result, false detection of defects is eliminated, and reliability is dramatically improved. It is not necessary to use a couplant as in the conventional ultrasonic inspection, and as a result, inspection costs can be significantly reduced. The present invention thus has excellent practical effects.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an electromagnetic ultrasonic inspection apparatus for explaining an electromagnetic ultrasonic inspection method using a cross-correlation method of the present invention.

FIG. 2 is a diagram for explaining an electromagnetic ultrasonic probe for transmitting surface waves.

FIG. 3 is a diagram for explaining an electromagnetic ultrasonic probe for receiving surface waves.

FIG. 4 is a diagram showing a result of inspecting a thick steel plate having a large surface defect by a conventional method.

FIG. 5 is a diagram showing a result of inspecting a thick steel plate having a small surface defect by a conventional method.

FIG. 6 is a diagram showing a result of inspecting a thick steel plate having small surface defects by an electromagnetic ultrasonic inspection method using the cross-correlation method of the present invention.

[Explanation of symbols]

1 Thick Steel Plate 2 Surface Defect 3 Pulse Transmitter 4 Electromagnetic Ultrasonic Probe 5 for Surface Wave Transmission 5 Electromagnetic Ultrasonic Probe for Surface Wave Reception 6 Amplifier 7 AD Converter 8 Data Accumulator 9 Computing Unit 10 Output Device 11 Surface Wave Propagation Path 12 Permanent Magnet 13 Coil 14 Permanent Magnet 15 Coil 16 Electric Signal Due to Electromagnetic Induction Across Space 17 Direct Surface Wave 18 Defect Surface Wave 19 Random Noise 20 Result of Calculation of Cross-Correlation Function Noise 21 Defect surface waves that clearly appeared as a result of calculating the cross-correlation function

Claims (2)

[Claims] 1. An electromagnetic ultrasonic probe for transmitting a surface wave and an electromagnetic ultrasonic probe for receiving a surface wave are provided on the surface of a material to be inspected at a short distance, and the electromagnetic wave for transmitting the surface wave is provided. An electromagnetic ultrasonic probe for receiving both the direct surface wave transmitted by the ultrasonic probe and propagating in the short distance and the defect surface wave reflected from a surface defect or an internal defect near the surface. It is characterized by extracting a defect surface wave buried in random noise by calculating a cross-correlation function between the direct surface wave and the defect surface wave after being received by, amplified by, and digitized by an AD converter. Electromagnetic ultrasonic inspection method 2. An electromagnetic ultrasonic probe for transmitting a plate wave and an electromagnetic ultrasonic probe for receiving a plate wave are provided on a surface of a material to be inspected at a short distance, and the electromagnetic wave for transmitting the plate wave is provided. Both the direct plate wave transmitted by the ultrasonic probe and propagating the short distance and the defective plate wave reflected from the surface defect or the internal defect near the surface are detected by the electromagnetic ultrasonic probe for receiving the plate wave. An electromagnetic wave characterized by extracting a defective plate wave buried in random noise by calculating a cross-correlation function between the direct plate wave and the defective plate wave after receiving, amplifying and digitizing by an AD converter. Ultrasonic inspection method