JPH08285823A - Ultrasonic inspection apparatus - Google Patents

Abstract

PURPOSE: To provide an ultrasonic inspection apparatus by which ultrasonic waves can be generated inside a sample from a sufficiently separated distance from the sample as an inspection object and by which the ultrasonic waves can be detected simultaneously in a noncontact manner and with high sensitivity. CONSTITUTION: A laser beam 16 composed of a pulse train is emitted to a sample 17, it generates ultrasonic waves at the inside, and a part of them is used as a reference signal 21 by a photodetection part 20. An ultrasonic pulse train which is generated inside the sample is detected when the Doppler shift to which a laser beam 31 for a probe is subjected is captured by the Fabry-Perot interferometer 34. The pulse signal of the reference signal 21 and the ultrasonic pulse train inside the sample have high correlativity. As a result, when a detection signal 36 is correlation-processed by using the reference signal 21 in a correlation processing part 40, the high S/N ratio of a correlation signal 41 can be realized, and a flaw at the inside of the sample can be inspected with high accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic inspection apparatus for inspecting defects inside a sample by generating ultrasonic waves in a sample using laser light and detecting the ultrasonic waves. .

[0002]

2. Description of the Related Art In the iron manufacturing process, inspecting internal defects in the slab immediately after the molten ore in the tundish is solidified improves the yield in the subsequent slabbing process and guarantees quality for the product process. However, it is important to reduce labor in the shearing process. However, since the slab is extremely hot, it is difficult to inspect it directly. For this reason,
One of the methods for non-contact inspection of defects inside the slab is
"Theory of electromagnetic ultrasonic transducer and its application to high temperature steel measurement"
(Nobuhiro Kawashima, PhD dissertation, The University of Tokyo).

This method utilizes electromagnetic ultrasonic waves,
A high-voltage pulse current is passed through an ultrasonic wave generation coil provided at a position about 5 mm from the surface of the slab to generate an induction magnetic field inside the slab. When such an induced magnetic field is induced, ultrasonic waves are generated inside the slab. This ultrasonic wave propagates inside the slab, is reflected on the back side of the slab, and returns to the surface of the slab again. This ultrasonic echo is detected by the ultrasonic detecting coil. If there is no defect in the slab, the time interval until this ultrasonic echo returns is constant, but if there are defects such as cavities and inclusions inside and the ultrasonic waves are reflected there, the ultrasonic echo will The time interval before returning is shortened. That is, by examining this time, it is possible to determine the presence or absence of defects inside the slab.

[0004]

However, in the state of the slab, the temperature is still as high as about 1000 ° C. Therefore, if the coil for generating ultrasonic waves and the coil for detecting ultrasonic waves are brought close to about 5 mm, the deterioration is accelerated. Various problems occur. On the other hand, if the ultrasonic wave generating coil is further separated, it becomes difficult to generate ultrasonic waves, and in order to generate ultrasonic waves of sufficient strength, it is necessary to supply considerably large power to generate electromagnetic waves. I won't. Further, when the ultrasonic detecting coil is separated from the sample, it becomes difficult to detect ultrasonic waves, and there is a problem that the ultrasonic detecting device is required to have very high sensitivity.

The present invention has been made based on the above circumstances, and ultrasonic waves can be generated in a sample from a distance sufficiently away from the sample to be inspected, and at the same time, the ultrasonic waves can be non-contacted. Moreover, it is an object of the present invention to provide an ultrasonic inspection apparatus capable of detecting with high sensitivity.

[0006]

According to a first aspect of the invention for solving the above-mentioned problems, there is provided a laser pulse train generating means for generating a laser pulse train and irradiating the sample to be inspected with the laser pulse train generating means. Ultrasonic detecting means for detecting ultrasonic vibration and outputting as an electric signal, reference signal generating means for extracting a part of the laser pulse train and converting it into an electric signal, and outputting as a reference signal, and using the reference signal Correlation processing means for performing correlation processing on the output signal of the ultrasonic wave detecting means and for extracting a signal relating to the ultrasonic wave generated by the laser pulse train.

According to a second aspect of the invention, in the first aspect of the invention, the laser pulse train generating means includes first to nth optical elements (n is an arbitrary natural number) for branching a part of incident light. The first optical element has a single laser pulse as incident light, and the i-th optical element (i = 2 to n is a natural number) has incident light as light branched from the (i-1) -th optical element. The laser pulse train is obtained by returning the transmitted light from each of the optical elements to a single path.

According to a third aspect of the present invention, in the first aspect of the invention, the laser pulse train generating means branches a single laser pulse into a plurality of laser pulses,
The laser pulse train is obtained by passing each laser pulse through an optical fiber having a different length and then returning to a single path again.

A fourth aspect of the present invention is characterized in that, in the first, second or third aspect of the invention, the laser pulse trains have different pulse widths.

[0010]

According to the present invention, when the laser pulse train irradiates the surface of the sample with the above-mentioned structure, the impact generates an ultrasonic pulse train corresponding to the pulse interval and the pulse width of the laser pulse train in the sample. . On the other hand, the output signal of the photodetector is a signal obtained by extracting a part of the laser pulse train and converting it into an electric signal, and therefore has a high correlation with the ultrasonic pulse train generated in the sample by the laser pulse train. Therefore, even if the ultrasonic vibration in the sample detected by the ultrasonic detecting means includes many noise components, the correlation processing means correlates the output signal of the ultrasonic detecting means and the output signal of the photodetecting means. By performing the processing, the ultrasonic pulse train generated by the laser pulse train can be detected with high sensitivity.

The laser pulse train as described above can be obtained by using an optical element for branching a part of incident light. For example, a single laser pulse is incident on the beam splitter, partially transmitting and partially reflecting. This reflected light propagates through a predetermined path and enters another beam splitter, and also partially transmits and partially reflects. By providing n such beam splitters and returning the transmitted light from each beam splitter to a single path, a laser pulse train consisting of n pulses can be obtained. Alternatively,
It is also possible to obtain the laser pulse train by branching a single laser pulse into a plurality of laser pulses, passing them through optical fibers of different lengths, and then returning them to a single path again. Further, by making the pulse width of each pulse of the laser pulse train different, the S / N ratio when the ultrasonic wave in the sample is detected can be improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. Here, FIG. 1 is a schematic block diagram of an embodiment of the present invention, FIG. 2 is a detailed configuration diagram of the pulse train generation unit 12 shown in FIG. 1, and FIG. 3 shows a laser pulse train and an ultrasonic pulse train generated in a sample. FIG. 4 is a schematic diagram showing the relationship, FIG. 4 is a schematic diagram for explaining the contents of the correlation processing, FIG. 5 is a diagram showing another method for generating a laser pulse train, and FIG. 6 is a laser pulse train having different pulse widths. It is the schematic which showed the relationship with an ultrasonic pulse train.

In FIG. 1, an ultrasonic wave generating laser 10 is provided.
Is a pulsed YAG laser that emits laser light having a wavelength of 1.06 μm, energy of 1 J, and pulse width of 10 ns at intervals of 30 Hz. The laser light 11 emitted from the laser 10 enters the pulse train generation unit 12.

As shown in FIG. 2, the pulse train generator 12 comprises three beam splitters 13a to 13c, two mirrors 14a and 14b, and a lens 15. The transmittances of the beam splitters 13a, 13b and 13c are set to 30%, 43% and 75%, respectively. That is, 30% of the first laser beam 11 is transmitted through the beam splitter 13a, and the remaining 70% is beam splitter 13.
reflected by a. The reflected laser light is totally reflected by the mirror 14a and guided to the beam splitter 13b. The beam splitter 13b transmits 43% of the incident laser light and reflects the remaining 57%, that is, about 40% of the first laser light 11. The reflected laser light is totally reflected by the mirror 14b, and the beam splitter 13
guided by c. The beam splitter 13c transmits 75% of the incident laser light.

As a result, each beam splitter 13
The laser light transmitted through a, 13b, and 13c is about 30% of the initial laser light 11, and the energy of each laser light is about 300 mJ. The distance between each beam splitter and the mirror is set to 15 m.

The laser light transmitted through each of the beam splitters 13a to 13c is condensed by the lens 15 and superposed to form one laser beam 16. At this time, the laser light passing through each beam splitter is delayed in time as it passes through the beam splitter in the subsequent stage due to the optical path difference as described above. 3 (a) (b)
Shows this situation. When the first laser beam 11 is composed of a single pulse as shown in FIG. 3A, the laser beam 16 obtained by passing this through the pulse train generation unit 12 of FIG. 2 is shown in FIG. 3B. Thus, the laser pulse train has a pulse interval of 100 ns.

When a substance is irradiated with a predetermined laser beam, the surface of the substance is instantaneously vaporized, and the thermal stress or vaporization reaction force at that time causes a broad band within the substance whose frequency depends on the pulse width of the laser beam. Ultrasound (for example, about 10MHz)
Occurs. For this, for example, ultrasonic TECH
NO May issue (vol.5, No.5, p38 (1993) Nippon Kogyo Publishing)
Or F. Alan McDonald, Appl. Phys. Lett., Vol.56,
(3) (15 Jan. 1990). by the way,
Since the laser beam 16 shown in FIG. 1 consists of three pulse trains, the ultrasonic wave generated inside the sample 17 also becomes an ultrasonic wave pulse train corresponding to this laser pulse train. Figure 3 (c)
Shows the state of this ultrasonic wave corresponding to the laser pulse train. That is, inside the sample 17,
As shown in FIG. 3C, an ultrasonic pulse train composed of three ultrasonic pulses at 100 ns intervals is generated.

A part of the laser light 16 is converted into an electric signal 21 by the photodetector 20. This signal 21
Is used as a reference signal when the correlation processing is performed in the correlation processing unit 40 described later. The light detection unit 20 is
In addition to photoelectric conversion elements such as photodiodes, necessary circuit elements are included.

On the other hand, as shown in FIG. 1, a laser 30 for generating a laser beam for a probe for detecting an ultrasonic wave generated inside the sample 17 is provided separately from the ultrasonic wave generating laser 10. ing. As the laser 30, for example, Ar having an output of 1 W and a wavelength of 514.5 nm is used.
^{+ A} laser or the like can be used. A laser beam 31 for a probe that detects an ultrasonic wave is emitted from the ultrasonic wave detecting laser 30 and is applied to the back side of the sample 17. The laser light 32 scattered and reflected by the sample 17 is reflected by the mirror 33.
And enters the Fabry-Perot interferometer 34 via.

Since the light transmittance of the Fabry-Perot interferometer 34 has a steep wavelength dependence, if the resonator length is set in advance so that the rate of change of the light transmittance becomes the maximum, the incident light will be incident. If the wavelength (optical frequency) of the laser light is changed even slightly, the change is captured as a change in transmitted light intensity. The laser light that has passed through the Fabry-Perot interferometer 34 enters the photodetector 35, where it is converted into an electrical signal. The light detection unit 35 includes a photoelectric conversion element such as a photodiode that changes the intensity of light into an electric signal, a filter that removes noise, an amplifier, and the like. The output signal 36 of the photodetector unit 35 is temporarily stored in the wave memory 37.

When the ultrasonic waves generated on the surface of the sample 17 propagate inside the sample and reach the back side, the position of the surface on the back side in the normal direction is periodically changed by the ultrasonic vibration. When the probe laser light 31 is irradiated to the portion displaced in this way, the reflected light 32 undergoes the Doppler shift, and the optical frequency transits. This transition of the optical frequency is changed in the transmitted light intensity in the Fabry-Perot interferometer 34, and then converted into an electric signal by the photodetector. However, the intensity of the ultrasonic wave reflected from the defect or the like existing inside the sample 17 is very weak, and therefore,
The output signal 36 obtained based on this ultrasonic wave is weak. In addition, since the output signal 36 contains electrical noise having various frequencies, a weak ultrasonic signal is
It is buried in these noises, and the S / N ratio is small.

Therefore, in the present embodiment, the correlation processing section 40 performs the correlation processing on the output signal (detection signal) 36 of the photodetection section 35 using the output signal 21 of the photodetection section 20 as a reference signal. This correlation processing utilizes the fact that the pulse of the laser light 16 is a special pulse train consisting of three pulse trains, and the ultrasonic wave emitted by this pulse train is also a corresponding pulse train. This is a method of extracting the signal having the highest correlation with the reference signal 21 from among 36.

Specifically, the reference signal 21 and the detection signal 3
After properly adjusting the level of 6, the reference signal 21 is scanned in the time axis direction, and at each point at a sufficiently narrow time interval, multiplication is performed with the detection signal 36, and the calculation result is correlated. The signal 41 is output. By performing such processing, if the detection signal includes a signal based on the ultrasonic pulse, a large output is obtained in the correlation signal 41 when scanning, and if the signal based on the ultrasonic pulse is not included, the correlation signal 41 is obtained. The output value of 41 becomes small. As a result, a high S / N ratio can be realized. The waveform of the correlation signal 41 is graphically displayed by the display unit 42.

FIG. 4 conceptually shows the relationship among the detection signal 36, the reference signal 21 and the correlation signal 41. The detection signal 36 includes many noise components, and the original signal 36a based on the ultrasonic pulse is in a state of being buried in this noise. When the above correlation processing is performed on this detection signal 36, the value of the correlation signal 41 increases only when the original signal component 36a appears in the detection signal 36, and is substantially zero in other cases. . Therefore, it becomes possible to detect the ultrasonic waves in the sample 17 based on the correlation signal 41. By obtaining the intervals of the detected ultrasonic signals from the display contents on the display unit 42, it is possible to inspect whether or not there are defects such as cavities and inclusions inside the sample.

FIG. 5 is a schematic diagram of an embodiment in which a plurality of optical fibers having different lengths are used as the pulse train generator 12 shown in FIG. In FIG. 2, the beam splitter 13a
~ 13c and the mirror 14a, 14b laser light 1
Although one pulse was delayed to obtain a predetermined pulse train, the pulse train can be obtained by using the optical fibers 50a to 50c having different lengths.

By the way, in the above embodiment, the laser light 1
Although the pulse widths of the pulses of 6 are made equal, it is also possible to make a pulse train composed of pulses having different pulse widths. F.
AlanMcDonald, Appl. Phys. Lett., Vol.56, (3) (15 Ja
n. 1990) and the like, it is known that when the pulse width of the laser beam for ultrasonic wave generation is widened, the frequency of the ultrasonic wave generated in the sample decreases. When a sample is irradiated with laser pulse trains having different pulse widths in this way, the pulse train of ultrasonic waves generated inside becomes a so-called chirp wave in which the frequency of each pulse is different. FIG. 6 is a diagram schematically showing a state of such a laser pulse train (a) and a waveform (b) of the generated ultrasonic wave. Such waveforms are very special and it is very unlikely that similar waveform noise is present. Therefore, if such a laser pulse train and ultrasonic waveform are used, the S
There is an advantage that the / N ratio can be further increased.

Actually, in order to generate a laser pulse train having different pulse widths, as described in the patent application by the applicant of the present invention dated April 17, 1995 (invention title "ultrasonic inspection apparatus"), It is considered that a plurality of lasers having different excitation energies are used and a laser emitted from a low excitation energy is amplified to a predetermined intensity by a laser amplifier. This is because the pulse width of the laser light can be widened by lowering the excitation energy of the laser light, and the pulse width can be maintained even if it is amplified by the laser amplifier. Therefore,
Using lasers with different excitation energies, amplifying the output laser light as needed, and then converging these to generate one beam, laser light consisting of pulse trains with different pulse widths is obtained, By irradiating the sample with this, ultrasonic detection with a higher S / N ratio becomes possible.

The present invention is not limited to the above embodiments, but various modifications can be made within the scope of the invention. For example, in the above embodiment, the laser pulse train for ultrasonic wave generation is composed of three pulses, but the present invention is not limited to this, and laser light of any pulse number can be used. Further, in the above-mentioned embodiment, the pulse intensities of the ultrasonic generation laser pulses are set to be the same, but the present invention is not limited to this, and it is possible to use laser light in which each pulse has an arbitrary height. it can. Further, in the above-mentioned embodiment, the probe laser light 31 for ultrasonic wave detection is irradiated from the back side of the sample 17, but the front side of the sample is irradiated with the probe laser light 31 and an ultrasonic echo reflected on the back surface of the sample is generated. A known configuration of detecting is also possible.

In the above embodiment, the Fabry-Perot interferometer was used as the means for detecting the Doppler shift of the laser light for the probe. However, other interferometers such as Michelson interferometer, heterodyne interferometer and homodyne are used. An interferometer or the like can be used. In addition to the interferometer, the applicant has a predetermined absorption characteristic with respect to the wavelength of the probe light, as proposed by the applicant in the patent application (“laser ultrasonic inspection apparatus”) dated April 13, 1995. A configuration in which a cell in which iodine gas or the like is enclosed in a transparent container is used and ultrasonic waves are detected based on the change in the absorption characteristics is also possible. Particularly, in the iron making process where the temperature changes drastically, it is difficult to adjust and maintain the Fabry-Perot interferometer and other interferometers, and for such applications, it is advantageous to use a cell filled with a predetermined gas. In that case, as described in the above patent application, the probe light does not necessarily have to be laser light.

Further, in the above-mentioned embodiment, the laser for the probe is used as the ultrasonic wave detecting means from the viewpoint of detecting the ultrasonic wave in the sample in a non-contact manner. Piezo (PZT) that is commonly used as
It is also possible to use an element.

[0031]

The present invention has been made based on the above-mentioned circumstances, and an ultrasonic wave is generated in a sample by using a laser beam, and an ultrasonic wave generated in the sample by using a laser beam different from this is used. Since it is possible to detect sound waves, it is possible to generate ultrasonic waves in the sample from a distance sufficiently away from the sample to be inspected, and at the same time, it is possible to detect this ultrasonic wave in a non-contact manner, Even if the sample to be inspected is a high temperature one such as a slab in the iron making process, it can be easily inspected, and using a laser beam consisting of a pulse train
Since an ultrasonic pulse train corresponding to this laser pulse train is generated and an ultrasonic wave is detected from this by using correlation processing, it is possible to provide an ultrasonic inspection apparatus capable of detecting ultrasonic waves with extremely high sensitivity. .

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an embodiment of the present invention.

FIG. 2 is a detailed configuration diagram of a pulse train generation unit 12 shown in FIG.

FIG. 3 is a schematic diagram showing a relationship between a laser pulse train and an ultrasonic pulse train generated in a sample.

FIG. 4 is a schematic diagram for explaining the content of correlation processing.

FIG. 5 illustrates another method for generating a laser pulse train.

FIG. 6 is a schematic diagram showing a relationship between a laser pulse train having different pulse widths and an ultrasonic pulse train generated.

[Explanation of symbols]

 10 Ultrasonic wave generation laser 12 Pulse generation section 13a, 13b, 13c Beam splitter 14a, 14b Mirror 15 Lens 17 Sample 20, 35 Photodetection section 30 Ultrasonic wave detection laser 33 Mirror 34 Fabry-Perot interferometer 37 Wave memory 40 Correlation Processing unit 42 Display unit 50a, 50b, 50c Optical fiber

Claims (4)

[Claims] 1. A laser pulse train generating means for generating a laser pulse train to irradiate a sample to be inspected, an ultrasonic detecting means for detecting ultrasonic vibration generated in the sample and outputting it as an electric signal, the laser. A part of the pulse train is taken out and converted into an electric signal, and a reference signal generating means for outputting as a reference signal, a correlation process is performed on the output signal of the ultrasonic detecting means using the reference signal, and the laser pulse train is used. An ultrasonic inspection apparatus comprising: a correlation processing unit that extracts a signal related to the generated ultrasonic wave. 2. The laser pulse train generating means has first to nth optical elements (n is an arbitrary natural number) that branches a part of incident light, and the first optical element is a single laser pulse. Is the incident light, the i-th optical element (i = 2 to n is a natural number) is the light branched from the (i-1) th optical element, and the transmitted light from each of the optical elements is a single path. The ultrasonic inspection apparatus according to claim 1, wherein the laser pulse train is obtained by returning the laser pulse train. 3. The laser pulse train generating means branches a single laser pulse into a plurality of laser pulses, passes each laser pulse through an optical fiber having a different length, and then returns the laser pulse to a single path again. The ultrasonic inspection apparatus according to claim 1, wherein the laser pulse train is obtained by the above. 4. The ultrasonic inspection apparatus according to claim 1, wherein the laser pulse train has different pulse widths for each pulse.