JPH10260164A - Laser ultrasonic wave inspecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser ultrasonic wave inspecting device which, based on a new method, detects the ultrasonic wave reflected or scattered at a defective place with a high SN ratio, without damaging an object to be inspected. SOLUTION: Eight laser beams emitted from each channel of a multi-channel acoustic optical element are poured on each vertexes A, C, F, and H of a square and center points B, D, E, and G of each side, on the surface of a steel material 16. These points are the center point of square, while slightly deviated from a point O to which the laser beam for ultrasonic wave detection is poured through a mirror 21 from a laser interferometer. Irradiation of each laser beam to a specified point is assured by providing an acoustic optical element of each channel of a multi-channel acoustic optical element t an appropriate position.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser for inspecting the state of an object to be inspected by irradiating an object to be inspected with a laser beam to generate ultrasonic waves, and detecting the ultrasonic waves after propagating through the object to be inspected. Related to ultrasonic inspection equipment.

[0002]

2. Description of the Related Art As one of methods for detecting internal defects and the like in various materials, there is a so-called laser ultrasonic method. For this, for example, "Ultrasonic TECHNO
May issue "(vol.5, No.5, p38 (1993) Nippon Kogyo Shuppan). Since this method detects an ultrasonic wave using a laser beam, it can be used for a non-contact inspection for examining an internal state of a material or the like that cannot be inspected by contact. Therefore, application to quality inspection in the iron making process, inspection of welding in steel processing, quality inspection of ceramic materials, internal inspection of aircraft parts, quality inspection of other metals and composite materials, etc. are expected.

A typical laser ultrasonic inspection apparatus is, for example, a laser light source (for example, a Q-switched YAG laser) for exciting ultrasonic waves inside an inspection object, and a probe for detecting ultrasonic waves propagating in the inspection object. Laser light source (for example, He-Ne laser). When an appropriate excitation laser beam is applied to the test object, ultrasonic waves are generated in the test object due to thermal stress or evaporation reaction force. When the ultrasonic wave propagates through the inspection object, if there is a defect inside, the ultrasonic wave is reflected or scattered there. The detection of the ultrasonic wave propagating in the inspection target can be performed by observing the phase change and the Doppler shift of the probe laser beam due to the ultrasonic displacement of the surface of the inspection target.
Then, it is possible to estimate the presence or absence of the internal defect or the position where the defect exists from the change of the time and amplitude required for the ultrasonic wave to propagate from the generation position of the ultrasonic wave to the detection position.

[0004]

By the way, the intensity of the ultrasonic wave reflected or scattered by the defect is very weak, and it is difficult to detect this signal at a high SN ratio. In particular, when the inspection target is a steel material, ultrasonic waves are reflected or scattered in various directions at grain boundaries that are boundaries between crystal grains, and this is superimposed on signal waves as grain noise. It becomes difficult to distinguish the signal from the original defect in the signal. Further, as a method of increasing the intensity of the ultrasonic wave reflected or scattered by the defect, it is conceivable to increase the output of the excitation laser to increase the intensity of the generated ultrasonic wave.
However, increasing the output of the excitation laser beam, which irradiates the laser beam in one location on the surface of the inspection object, causes damage to the surface of the inspection object due to the laser beam. Has limitations.

The present invention has been made based on the above circumstances, and is based on a new technique for detecting ultrasonic waves reflected or scattered from a defect at a high SN ratio without damaging the inspection object. It is an object to provide an inspection device.

[0006]

In order to solve the above-mentioned problems, the present invention provides an excitation laser light source, a beam splitting means for converting a laser beam emitted from the excitation laser light source into a plurality of laser beams, An acousto-optic means having at least the same number of channels as the plurality of laser beams and allowing the plurality of laser beams to pass through acousto-optic elements provided independently for each channel, and a plurality of light beams emitted from the acousto-optic means A laser beam, irradiation position control means for controlling each to be irradiated to a different predetermined position on the surface to be inspected, and an acousto-optic element of each channel of the acousto-optic means, a predetermined amount from each channel of the acousto-optic means. Signal generation means for supplying an electric signal for controlling each laser beam to be emitted in order and at time intervals; Ultrasonic detection means for detecting ultrasonic waves generated in the inspection object by a laser beam applied to the surface of the object, and inspection based on ultrasonic signals corresponding to the respective laser beams detected by the ultrasonic detection means Signal processing means for performing a process of extracting a signal based on a defect in the object.

According to the present invention, as described above, when an object to be inspected is irradiated with a laser beam for excitation, the ultrasonic waves generated there propagate to the inside of the object to be inspected, and if there is a defect, they are reflected or scattered there. It returns to the surface and is detected by the ultrasonic detecting means. Grain noise is superimposed on the detected ultrasonic signal. When the irradiation position of the excitation laser beam is changed on the surface to be inspected, the intensity of the ultrasonic wave that is reflected or scattered by internal defects and returns to the surface does not change much. Has the property that it greatly changes depending on the irradiation position of the laser beam. Therefore, inspection is performed by irradiating a plurality of excitation laser beams to different predetermined positions on the surface to be inspected and performing predetermined processing on a plurality of ultrasonic signals detected based on the respective laser beams. A signal based on a defect in the object can be extracted, and the S / N ratio can be increased.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing the entire configuration of a laser ultrasonic inspection apparatus according to one embodiment of the present invention,
FIG. 2 is a schematic view of a multi-channel acousto-optic device, FIG. 3 is a diagram showing a laser beam irradiation position and a detection position of an ultrasonic wave on the surface of a steel material to be inspected, and FIG. FIG. 5 is a diagram showing the relationship between one pulse of a CO ₂ laser and each pulse irradiated on the surface of a steel material from each channel of a multi-channel acousto-optic device, and FIG. 6 is an ultrasonic detector. FIG. 7 is a block diagram of a circuit for processing a signal output from the laser interferometer of FIG. 7, FIG. 7 is a diagram for explaining that the intensity of grain noise changes depending on the difference in the irradiation position of the laser beam, and FIG. 4 is a graph in which the number of waveforms used for signal processing is plotted on the horizontal axis and the change in SN ratio is plotted on the vertical axis, using the SN ratio of the input signal as a parameter.

As shown in FIG. 1, the laser ultrasonic inspection apparatus according to the first embodiment has an ultrasonic generator 1 for generating ultrasonic waves on a steel material 16 to be inspected, and a defect which propagates inside the steel material 16 and is defective. It has an ultrasonic detector 2 that detects reflected or scattered ultrasonic waves on the surface of the inspection target. The ultrasonic generator 1
It has a laser light source 10, a beam separator 12, a multi-channel acousto-optic element 13, and an oscillator 15 for supplying a predetermined signal to the multi-channel acousto-optic element 13. The ultrasonic detector 2 includes a laser interferometer 20, a mirror 2
The ultrasonic displacement of the surface of the steel material 16 is detected by using 1 or the like.

The laser light source 10 of the ultrasonic generator 1 is a laser light source for excitation of ultrasonic waves. In this embodiment, a CO ₂ laser having a pulse width of about 200 μsec is used.
However, as long as the laser has a pulse width of about 200 μsec or more, the laser is not limited to the CO ₂ laser, and another laser can be used. A laser beam 11 emitted from a laser light source 10 enters a beam separator 12,
Here it is split into a number of parallel laser beams. In the present embodiment, the number of the laser beams is eight. As the beam separator 12, a commercially available optical element that splits a laser beam into a plurality of parts by reflection and transmission by a film in which a dielectric or metal body is coated on both surfaces of an optically polished parallel flat substrate can be used. . As shown in FIG. 2, each of the divided laser beams enters at least one of the channels of the multi-channel acousto-optic device 13 having at least the same number of channels as the number of beams. The multi-channel acousto-optic element 13 is composed of, for example, eight independent acousto-optic elements, and a predetermined signal is supplied to each acousto-optic element from an oscillator 15 which is a signal generating means, so that the The element exerts an operation described below on the laser beam incident on the channel.

As shown in FIG. 3, the eight laser beams emitted from each channel of the multi-channel acousto-optic element 13 have respective vertices A, A of a square on the surface of the steel material 16.
C, F, H and the midpoints B, D, E, G of each side are irradiated. These points are the center points of the square and are slightly separated from the point O where the laser beam for ultrasonic detection is irradiated from the laser interferometer 20. Irradiation of each laser beam to a predetermined point in this manner can be achieved by providing the acousto-optic device of each channel of the multi-channel acousto-optic device 13 at an appropriate position. Alternatively, a suitable optical means (not shown) may be arranged after the multi-channel acousto-optic element 13 and used to control the irradiation of the laser beam to a plurality of predetermined positions on the surface of the steel material.

When the laser beam is irradiated, the steel 16
Ultrasonic waves are generated at the points irradiated with the laser beam by thermal stress or evaporation reaction force. The ultrasonic waves include those propagating inside the steel material 16 and those propagating on the surface of the steel material 16. In the present embodiment, the ultrasonic wave propagating inside is used for defect detection. When an ultrasonic wave propagating into the steel material 16 hits a defect, the ultrasonic wave is reflected or scattered, and an ultrasonic wave originating from the defect portion is generated. The ultrasonic wave is detected by measuring the ultrasonic displacement of the surface of the steel material generated when the ultrasonic wave returns to the surface of the steel material 16 with the ultrasonic detection unit 2. Also, the ultrasonic wave reaching the back surface of the steel material 16 is reflected there and returns to the front surface.
Since the thickness of the ultrasonic wave and the sound speed of the ultrasonic wave propagating through the steel material 16 are known in advance, it can be easily distinguished whether the detected ultrasonic wave is based on a defect or reflected on the back surface. In addition, when a defect exists, the depth of the defect can be known from the timing of irradiating the laser beam and the timing of detecting the ultrasonic wave.

Next, the principle of the multi-channel acousto-optic device 13 will be briefly described with reference to FIGS. Acousto-optical element is obtained by bonding the piezoelectric element to the acoustic-optic medium, such as tellurite glass or lead molybdate single crystals, acousto-optic and supplies an electric signal to the piezoelectric element from the oscillator 15 _{1-15 8} for each channel When an ultrasonic wave is generated in the medium, a periodic change in the refractive index occurs in the acousto-optic medium due to the photoelastic effect, which acts as a diffraction grating. The interaction distance between light and ultrasonic L (see FIG. 4), ₀ the wavelength of light λ in vacuum, the acoustic velocity of the medium V, and ultrasonic frequency (driving frequency) f _a, the refractive index of the medium n ^{_{when, 2πλ 0 Lf a 2 / nV}} 2> Bragg diffraction occurs when 4π (1) is satisfied, other zero-order light which is not subjected to diffraction, obtaining the first-order diffracted light of high light intensity shown in FIG. The direction of the first-order diffracted light (diffraction angle) θ _B is given by θ _B = sin ⁻¹ (λ ₀ f _a / 2V) (2) in consideration of the refraction at the light entrance / exit surface of the medium. Obtained or highest diffraction efficiency when a light incident angle is theta _B in Bragg diffraction.

[0014] intensity I ₁ of the primary diffracted light depends on the power P _a of the ultrasound in the medium, the relationship ^{_{is, I 1 α sin 2 (K}} 1 (MeP a) 1/2 / λ 0) (3 ). Here, Me is a constant determined by the physical property value of the acousto-optic medium, and the larger this value is, the higher the diffraction efficiency is obtained, which is called the index of performance of the medium. Also, K ₁ is a constant dependent element. Note that the above description regarding the acousto-optical element is described in HO
This is based on the catalog of optical application products of YA-SCHOTT Co., Ltd.

As can be seen from the above equation (2), the diffraction angle θ _B of the first-order diffracted light depends on the frequency f _a of the ultrasonic wave. Therefore, by changing f _a , the angle θ _B of the first-order diffracted light can be controlled. In addition, as can be seen from equation (3), when the value in ultrasonic power P _a in the acoustooptic medium, i.e. the intensity of the primary diffracted light when in the acoustooptic medium, ultrasonic waves generated I ₁ also has a certain value corresponding to P _a to the ultrasonic power, when ultrasound power P _a is zero, i.e., the intensity of the primary diffracted light when no ultrasonic wave is generated in the acoustooptic medium I _{1 is} also zero. Ultrasonic waves in the acousto-optic medium are generated by supplying electric power to the piezoelectric element.

In this embodiment, utilizing this fact, the on / off of the laser beam applied to the surface of the steel material 16 is switched. Specifically, a laser pulse for ultrasonic excitation, which is simultaneously incident on each channel of the multi-channel acousto-optical element 13, by an electric signal supplied from the oscillator 15 to the multi-channel acousto-optical element 13,
The laser beam is emitted from each channel in a predetermined order and at a predetermined time interval within the duration of the pulse.
Is controlled so as to sequentially irradiate the surface of.

Here, the multi-channel acousto-optic device 1
The time interval of the laser beam irradiated from each channel of No. 3 to the steel material 16 will be described. In the present embodiment, a steel material 16 having a thickness of 50 mm is used as an inspection target. Since the sound speed of the ultrasonic wave inside the steel material is about 6 mm / μsec, the time t until the ultrasonic wave generated on the front surface of the steel material 16 is reflected on the back surface and returns again is t = 100 (mm) ÷ 6 ( mm / μsec) ≒ 16.7
μsec. Therefore, if the laser beam of each channel is irradiated at a longer time interval, the ultrasonic signals caused by the respective laser beams do not influence each other.

FIG. 5 is a diagram showing the relationship between one pulse of the CO ₂ laser and each pulse irradiated on the surface of the steel material 16 from each channel of the multi-channel acousto-optic device 13. As described above, the pulse width of the CO ₂ laser used in the present embodiment is about 200 μsec. Check the time interval for irradiating the laser pulse of each channel with a margin 2.
Assuming 0 μsec, when irradiating eight laser beams, the time from irradiating the first pulse to A to irradiating the last pulse to H is 140 μsec. C
Of the 200 μsec width pulse of the O ₂ laser, 140 μsec at the central portion where the intensity is uniform is selected, and each channel of the multi-channel acousto-optic device 13 is turned on / off. As a result, the ultrasonic excitation laser having a uniform intensity was applied to each point from A to H on the surface of the steel material 16 for 20 μs.
Irradiation can be performed sequentially at ec intervals. Steel material 1 from each channel
The ultrasonic waves generated by the ultrasonic excitation laser radiated on the surface of 6 are individually detected by the ultrasonic detection unit 2.

Next, the signal processing performed on the ultrasonic signal detected by the ultrasonic detector 2 will be described. FIG. 6 is a block diagram of a circuit that processes a signal output from the laser interferometer 20 of the ultrasonic detection unit 2. In the figure, the “A signal” means a signal obtained by detecting an ultrasonic wave generated by the laser beam applied to the point A on the steel material 16 in FIG. "B signal", ..., "H signal"
The same applies to. Each of these signals is processed by the signal
0, and the processing described later is performed.

In general, ultrasonic waves propagating in the interior of iron such as steel are reflected or scattered at grain boundaries during propagation, and spread in various directions with various intensities. Then, such an ultrasonic wave is superimposed on the ultrasonic wave originally reflected or scattered from the defect and which is to be originally detected as grain noise. When the defect becomes minute, the intensity of the ultrasonic wave reflected or scattered from the defect becomes small and cannot be distinguished from the grain noise, so that it is difficult to detect the defect with high accuracy. By the way, since the grain noise is generated due to the above-mentioned cause, even if the position of the source of the ultrasonic wave is slightly changed, the intensity of the noise detected at the same position on the surface greatly changes. On the other hand, for the ultrasonic waves reflected or scattered by the defect, the detected signal intensity does not change as much as the grain noise when the position of the source of the ultrasonic wave is slightly changed.

Therefore, the eight points A to H shown in FIG.
When the excitation laser beam is irradiated to these points and the eight signals detected when these points are each used as a source of ultrasonic waves are compared, if there is a defect inside the steel material, if there is a defect inside the steel material, it is reflected or reflected by this defect. The scattered ultrasonic waves should have almost the same signal intensity regardless of the point where the laser beam is irradiated, and the time from irradiation of the excitation laser beam to detection should be the same. Regarding noise, it is expected that the intensity of noise, even if detected at the same time, greatly changes depending on the location where the excitation laser beam is irradiated. FIG. 7 is a diagram for explaining this. FIGS. 7A, 7B, and 7C show schematic waveforms of ultrasonic signals detected at the same position when a slightly different position is irradiated with a laser beam. Among these signal waveforms, a large amplitude of the waveform before and after the time t ₁ is the contribution from the defect, the amplitude and phase of this portion is substantially uniform. In contrast, the waveform of the other parts is due to the grain noise, for example, look at the time t _2, the value of the three waveforms are different.

Based on this consideration, the signal processing unit 30 shown in FIG. 6 performs the following processing. First, the input A
The signals H to H are A / D converted at a sampling frequency sufficiently higher than the ultrasonic frequency to obtain eight digital signal waveforms r ₁ (r),..., R ₈ (t). Then, the digital values of the eight signal waveforms at the respective sampling times are compared, and the one having the smallest absolute value, that is, min {| r _j (t) |, j = 1, 2,. And the original value, that is, the value s (t) (|
s (t) | = min ｛| r _j (t) |, j = 1, 2,.
8｝) is the value of the signal at that time. Then, one signal waveform is reconstructed based on the values of all sampling times. The amplitude of the signal waveform obtained in this manner remains almost unchanged in a portion where a defect contributes, and becomes smaller in a portion due to grain noise. That is, this is equivalent to obtaining a signal waveform having a high SN ratio.

Next, when the above processing is performed, S
The degree to which the N ratio is improved is quantitatively evaluated using a statistical method. As a premise, the noise signal is a random signal of Gaussian distribution, and its average value is set to zero. When σ ² is dispersed, the signal intensity from a true defect is m, and the number of waveforms (same as the number of laser beams in FIG. 3) used for signal processing is N, the SN ratio is

[0024]

(Equation 1)

Is given by In this equation, “erf” is an error function. In this regard, Ultrasomics, 199
0, Vol. 28, March, p. 90-96 and J. Acoust. Soc. Am
(87) 2, February 1990 p.728-736. The change in the SN ratio in the above equation (4) is plotted with the number of waveforms used for signal processing on the horizontal axis and the improvement ratio of the SN ratio on the vertical axis, using the SN ratio (SN _in ) of the input signal before processing as a parameter. Then, the result is as shown in FIG. It should be noted that the dotted line in the figure shows the state of improvement of the SN ratio when the simple averaging process is performed for comparison. As can be seen from FIG. 8, when the processing of the present embodiment is performed, the SN ratio is greatly improved as the number of waveforms used for signal processing increases,
For example, even if the S / N ratio of the input signal is 1, the number of waveforms is 1
At about 0, only about the same improvement as the simple averaging process can be seen, but where the number of waveforms is more than 10,
The signal-to-noise ratio is much improved compared to the simple averaging process. On the other hand, when the S / N ratio of the input signal is 2, the S / N ratio improves about 10 times even when the same tooth coefficient as in the present embodiment is 8.

As described above, by irradiating a steel material with a plurality of laser beams at slightly different positions and performing the above-described processing on the obtained signal, it is possible to detect ultrasonic waves from a defect with a high SN ratio. I understand. This means that even a minute defect inside the steel material can be detected with high accuracy. In addition, since the S / N ratio is improved by irradiating a plurality of laser beams, the intensity of each laser beam does not need to be so high, and therefore, the damage to the surface of the steel material by the irradiation of the laser beam can be reduced. Can be.

The present invention is not limited to the above embodiment, and various changes can be made within the scope of the invention. In the above embodiment, eight ultrasonic excitation laser beams were used, but this number depends on the required detection accuracy and the like, and is not necessarily limited to eight. In the above-described embodiment, the irradiation positions of the eight ultrasonic excitation laser beams are set at the vertices of the square and the midpoints of the sides. However, the present invention is not limited to this. A laser beam for ultrasonic excitation may be irradiated on the circumference at equal intervals. Also in this case, the laser beam can be irradiated on the circumference by arranging each acousto-optic element of the multi-channel acousto-optic element at an appropriate position. Further, an appropriate optical means may be provided at the subsequent stage of the multi-channel acousto-optical element, so that the irradiation position of each laser beam may be controlled to be on the circumference.

[0028]

As described above, according to the present invention,
It is possible to increase the S / N ratio by performing a process of extracting a signal based on a defect in an inspection target from a plurality of ultrasonic signals detected based on a plurality of laser beams applied to different predetermined positions on a surface of the inspection target. It is possible to detect minute defects in the inspection target with high accuracy, and it is possible to increase the S / N ratio by using multiple laser beams, so that the intensity of each laser beam can be kept low. Therefore, it is possible to provide a laser ultrasonic inspection apparatus that does not damage the inspection target due to the irradiation of the laser beam.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an overall configuration of a laser ultrasonic inspection apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a multi-channel acousto-optic device.

FIG. 3 is a diagram showing an irradiation position of a laser beam and a detection position of an ultrasonic wave on a surface of a steel material to be inspected.

FIG. 4 is a diagram for explaining light diffraction in the acousto-optic element.

FIG. 5 is a diagram showing a relationship between one pulse of a CO ₂ laser and each pulse emitted from each channel of a multi-channel acousto-optic element to the surface of a steel material.

FIG. 6 is a lock diagram of a circuit for processing a signal output from a laser interferometer of an ultrasonic detection unit.

FIG. 7 is a diagram for explaining that the intensity of grain noise changes depending on the irradiation position of a laser beam.

FIG. 8 is a graph in which the number of waveforms used for signal processing is plotted on the horizontal axis and the change in SN ratio is plotted on the vertical axis, using the SN ratio of the input signal before processing as a parameter.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 ultrasonic generating unit 2 ultrasonic detecting unit 10 laser light source 12 beam separator 13 multi-channel acousto-optic element 15 oscillator 16 steel material 20 laser interferometer 21 mirror 30 signal processing unit

Claims (3)

[Claims] 1. An excitation laser light source, beam splitting means for converting a laser beam emitted from the excitation laser light source into a plurality of laser beams, and at least the same number of channels as the plurality of laser beams. Acousto-optic means for allowing the plurality of laser beams to pass through the acousto-optic element provided independently for each, a plurality of laser beams emitted from the acousto-optic means are respectively applied to different predetermined positions on the surface to be inspected An irradiation position control unit for controlling the acousto-optic device of each channel of the acousto-optic unit to emit each laser beam from each channel of the acousto-optic unit in a predetermined order and at a time interval. Signal generation means for supplying a laser beam irradiated on the surface of the inspection object. Ultrasonic detecting means for detecting ultrasonic waves generated in the inspection object; and processing for extracting a signal based on a defect in the inspection object based on an ultrasonic signal corresponding to each laser beam detected by the ultrasonic detecting means. And a signal processing means for performing the following. 2. The laser light source for excitation is a CO ₂ laser, and the signal generating means is one of the CO ₂ lasers.
The laser ultrasonic inspection apparatus according to claim 1, wherein a signal is supplied to the acousto-optic means so that the plurality of laser beams are sequentially irradiated to the predetermined position of the inspection object within a duration of a pulse. 3. The signal processing means performs A / D conversion on each of the input ultrasonic signals at a sampling frequency higher than the ultrasonic frequency, and performs digital conversion of each signal waveform at each sampling time. The values are compared, the one having the smallest absolute value is selected, the original value of the selected one is used as the signal value at the sampling time, and one signal waveform is reconstructed based on the value at each sampling time. The laser ultrasonic inspection apparatus according to claim 1, which performs processing.