JP2007024674A - Surface/surface layer inspection device and surface/surface layer inspection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect flaws, such as exfoliation or cracks of a surface/surface layer, with high sensitivity. <P>SOLUTION: Heat is applied to a surface to be inspected by a pulse heating means, and a laser interfering image is formed by a shearing optical system 20, by using a laser interfering method; while the formed laser interfering image is subjected to time-resolved measurement by a time-resolved type camera 13 and the laser interfering image signals time-resolved by an image signal processor 14 are respectively measured, before and after heat is applied, to evaluate the flaw by the differential processing of the measured laser interfering image signal. At this time, the time-resolved laser interfering image signal is subjected to logarithmic processing, and flaws are detected with high sensitivity, by discriminating between a steady change part and a non-steady change part. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a surface / surface layer inspection apparatus that performs nondestructive inspection of a surface and a surface layer to be inspected with high sensitivity using laser interferometry, and a surface / surface layer inspection method that is executed by the apparatus.

  As a rapid inspection technique for the surface and the surface layer, in recent years, a thermography method or a laser shearography method is used for inspection of peeling of FRP, which is an aircraft material.

  First, in the thermography method, the measurement surface is heated and the temperature distribution is observed with an infrared camera, and the defective part such as delamination has a different thermal conductivity than the healthy part. The defect is detected using the fact that it appears. More recently, as shown in Non-Patent Document 1, high-sensitivity inspection is possible by heating in pulses and capturing minute time changes in temperature distribution.

  On the other hand, the laser shearography method is similar to the thermography in that the measurement surface is heated, but is characterized in that a thermal strain generated around the defect accompanying the heating is detected as a vertical displacement of the surface. The surface displacement can be measured with high accuracy on the order of several tens of nanometers using the interference phenomenon of laser light, and as shown in Non-Patent Document 2, a wide range can be measured at once, so that rapid measurement is possible.

  For example, speckle interferometry and shearing interferometry are also known as laser interferometry. Among them, speckle interferometry is a method in which light scattered by minute irregularities on the surface of an inspection object irradiated with laser light is slightly shifted with respect to reference light through an optical system and imaged with a CCD camera. Since coherent light is used, fine speckled patterns called speckle patterns are recorded (see FIGS. 3A and 3B described later). In this way, the gradient of the surface displacement perpendicular to the direction of the shift can be known. Therefore, it is possible to know the surface displacement gradient by shifting from various directions, and to create a map relating to the gradient of the surface displacement from these, and based on these, an image as shown in FIG. .

The strain interferometry method uses an optical system to create an image of the reflected laser light from the surface of the inspection object and an image of the reflected laser light from the surface of the same device that has been distorted by operation or mechanical stress. When the image is picked up by a CCD camera with a slight shift, a fine spot-like pattern (interference pattern) can be observed as in the speckle interferometry described above. In this case, the intensity of a certain pixel depends on the intensity of light from two points separated by a minute distance of the sample and the phase difference between them. Can make the device distortion directly visible as a function of time at nanometer resolution.
SAMPE Journal, vol.39, No.5, September / October 2003, P53-58 Wolfgang Steinchen, Lianxiang Yang, "Digital Shearography" Chapter 6, SPIE PRESS, 2003.

  However, in the thermography method, the peeling can be inspected, but the detection of the surface crack (crack entering the direction perpendicular to the surface) to be inspected cannot be expected. In addition, since the laser shearography method can only measure at a certain time, there are cases where the sensitivity is insufficient and information in the depth direction cannot be obtained. Furthermore, since laser shearography is a further development of speckle interferometry and strain interference, there are similar problems with laser shearography in speckle interferometry and strain interferometry. It was.

  The present invention has been made in view of the actual situation of such a prior art, and its purpose is not only the inspection of peeling in the inspection object, but also the detection of surface cracks (cracks that enter the direction perpendicular to the surface) An object of the present invention is to detect various defects near the surface with high sensitivity and information in the depth direction.

  In order to achieve the above object, the first means is a surface / surface inspection apparatus comprising a heat application means for applying heat to a surface to be inspected, and a surface displacement measurement means for measuring surface displacement by laser interferometry. It is characterized by comprising time-resolved image measuring means for time-resolved measurement of a laser interference image generated by laser interferometry.

  In this case, a streak camera or a CCD line sensor camera is used as the time-resolved image measuring means, a pulse light source or a pulse high-frequency source is used as the heat applying means, and a laser shearography method or laser speckle is used as the surface displacement measuring means. A measuring device that uses the interferometry is used. Furthermore, it is possible to provide signal processing means for logarithmically processing the laser interference image signal measured by the time-resolved image measuring means.

  The second means is characterized in that a defect detection means for detecting a defect on the surface to be inspected based on inspection data obtained by a surface / surface inspection apparatus is provided for the first means.

  A third means is a surface / surface layer inspection method in which heat is applied to a surface to be inspected and surface displacement is measured by laser interferometry, and a laser interference image generated by the laser interferometry is time-resolved to measure It is characterized by inspecting the surface and surface layer.

  The fourth means applies heat to the surface to be inspected, generates a laser interference image by laser interferometry, time-resolves the generated laser interference image, and applies the time-resolved laser interference image signal to heat. It is characterized by a surface / surface inspection method in which each measurement is performed before and after, or at two different times after heat application, and the measured laser interference image signal is subjected to differential processing to evaluate defects.

  In this case, it is possible to evaluate a defect by using a line image at an initial time of the time-resolved laser interference image signal as a reference and differentially processing the subsequent line image. Further, the defect can be evaluated by logarithmically processing the time-resolved laser interference image signal and discriminating between the steady change portion and the non-steady change portion.

  According to the present invention, since the laser interference image generated by the laser interferometry is time-resolved and inspected on the surface to be inspected, not only the inspection of the surface to be inspected but also the surface crack (perpendicular to the plane) It is possible to detect cracks that have entered and various defects in the vicinity of the surface with high sensitivity and information in the depth direction. In addition, since the surface to be inspected is heated, a defect inspection is possible even with a material having extremely high thermal conductivity.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of a time-resolved laser shearography apparatus according to an embodiment of the present invention. In the figure, a time-resolved laser shearography apparatus includes a laser beam emitting device 3, a lens 4 that spreads the laser beam emitted from the laser beam emitting device 3 and irradiates the surface of the inspection object 51, and the surface of the inspection object 51. A condensing lens 6 that captures reflected light, a shearing optical system 20 that guides the reflected light captured from the condensing lens 6 to a time-resolved camera 13, and an image signal that is processed by the time-resolved camera 13 for time-resolved image signals The measurement system mainly includes a processing device 14 and pulse heating means 1 that heats the surface of the inspection target 51 surface.

The shearing optical system 20 includes a half mirror 7 disposed downstream of the condenser lens 6, a mirror 9 disposed on the side facing the condenser lens 6 with the half mirror 7 interposed therebetween, and the half mirror 7 disposed therebetween. 1 includes a mirror 8 disposed on the side facing the time-resolved camera 13 and an adjusting mechanism 10 for adjusting the angle of the mirror 9. The mirror 8 is disposed on the side where the half mirror 7 reflects the reflected light incident from the condenser lens 6, and the mirror 9 is disposed on the side where light is transmitted. In place of the half mirror 7, a prism having the same function can be used.
In the time-resolved laser shearography apparatus generally configured as described above, first, the surface of the inspection target 51 is locally heated by the pulse heating means 1 to generate thermal distortion on the surface of the inspection target 51. At this time, if there is a crack or separation 52 in the inspection object 51, thermal distortion at that portion increases, and a slight displacement (surface displacement) occurs in a direction perpendicular to the inspection object surface. Therefore, the laser beam 5 emitted from the laser beam emitting device 3 is spread by the lens 4 and irradiated onto the surface of the inspection object 51, and the surface displacement generated on the inspection object surface is increased by a laser shearography method using laser interference. Detect to sensitivity. The pulse heating means 1 emits pulse heating light 2 to the inspection object, and a flash lamp, a pulse laser, or the like can be used as the pulse light source. Further, the high frequency 2 may be emitted by a pulsed high frequency source.

  The principle of detecting surface displacement by the laser shearography method is shown in FIG. The laser beam 5 irradiated on the surface of the inspection target 51 causes interference due to minute unevenness on the surface of the inspection target, and a speckle pattern image generated by the interference is measured. At that time, the image to be inspected is divided into two by the half mirror 7 of the shearing optical system 20 installed in front of the time-resolved camera 13 shown in FIG. 1, and the angle of the mirror 9 with the angle adjusting mechanism 10 is changed. By adjusting, the image is double-exposed on the time-resolved camera 13 by shifting in an arbitrary direction by a certain number of pixels. FIG. 1 shows an arrow 11 indicating the optical path by the mirror 8 and an arrow 12 indicating the optical path by the mirror 9 with an angle adjusting mechanism. As a result, the first-order differential value of the surface strain to be inspected is directly measured. FIG. 2A is a diagram showing the relationship between the position and the actual surface displacement, and FIG. 2B is a diagram showing the relationship between the position and the observed differential value. Since the detected image is a displacement first-order differential value in an arbitrary direction, the detected image of the defect is a characteristic image called a butterfly pattern as shown in FIG.

  FIG. 3 shows an example of an image in a conventional laser shearography method that is not time-resolved. In the conventional laser shearography method, the time-resolved camera 13 as shown in FIG. 1 is not used, but a normal CCD camera or the like is used. As shown in FIGS. 3A and 3B, the measurement image in that case is a spatial axis on both the vertical axis and the horizontal axis, and FIG. 3A is a speckle pattern image of a reference before heat application. 3 (b) is a speckle pattern image after heat application. Conventionally, the image shown in FIG. 3C can be finally obtained by separately performing difference processing between the two images. Also in this case, the presence or absence of a defect can be determined based on the presence or absence of a butterfly pattern. The finally obtained image of FIG. 3C is equivalent to the image of FIG. 2C, but in FIG. 2C, the presence or absence of the result is obtained without performing image processing such as difference processing. Can be judged.

  As described above, this embodiment is characterized in that a time-resolved image is obtained using the time-resolved camera 13. As the time-resolved camera 13, a streak camera or a CCD line sensor camera is used. A streak camera having a high time resolution from the femtosecond level to several hundreds of microsecond level can be used. In addition, it is possible to provide a time resolution at a level of several hundred microseconds at the fastest speed. FIG. 4 is a diagram comparing the visual field when using the time-resolved camera 13 with the visual field of a conventional CCD camera. Since the field of view when using the time-resolved camera 13 has only one space axis, the space X is measured in this figure, but the space Y is limited to a certain line.

  Next, two methods can be applied for obtaining the reference in the present embodiment. First, the first method is shown in FIG. This method is the same as the conventional method, and is a method of taking a reference image before applying heat and taking a difference from the image after applying heat. In FIG. 5, the vertical axis is the time axis, that is, the result of time-resolved measurement. As a supplement, since it is a time-resolved image, it is not a dot-like speckle pattern as shown in FIG. 3, but a time change of speckle, so that a line-like pattern is measured in the time direction. There are features. Further, instead of taking a reference image before applying heat, a reference image may be taken at an arbitrary time after applying heat and a difference between the reference image and an image at a different time may be taken. Thereby, there exists an advantage by which the measurement before heat provision becomes unnecessary. In this case, if the image of FIG. 5B is a reference image, it means that a difference from an image having a different time start point on the vertical axis is taken.

  Next, FIG. 6 shows how to take the second reference. Since the present invention takes a time-resolved image, the line being measured is fixed as described above. Therefore, a line with a time-resolved image (dashed pixel row in FIG. 6) can be used as a reference. Therefore, it is possible to obtain a difference image as shown in FIG. 5C and FIG. 6B by performing a difference process on the value of this pixel row from the measurement value at each time, and the time resolution of the defect. An image can be obtained. The signal processing as described above is performed by the image signal processing device 14.

  In the above description, the laser shearography method is mainly described as the surface displacement measuring means. However, the laser speckle interferometry is also possible.

  Further, in the difference image of FIG. 5C or FIG. 6B, the location and width (size) where there is a defect can be specified, but cannot be detected at the depth of the defect. Therefore, in the present embodiment, as shown in FIG. 7C, the size of the defect in the depth direction is also known by performing signal processing with the vertical axis representing the logarithm of strain displacement. That is, FIG. 7A is a difference image obtained as shown in FIG. 5C or FIG. 6C, where a broken line in the time axis direction indicates a defective portion and a solid line indicates a healthy portion line. Yes. FIG. 7B shows the change over time with the strain displacement as the vertical axis for these two lines. In the case of a very small defect, the difference between the defective portion and the healthy portion is also small as shown in FIG. Although FIG. 7B intentionally displays a difference, it is actually smaller. In the case of such a very small defect, as shown in FIG. 7C, when signal processing is performed with the logarithm of strain displacement on the vertical axis and the time axis on the horizontal axis, the difference between the defective portion and the healthy portion is determined. Appears greatly, and the difference between the two can be clearly shown.

  This is effective in detecting the amount of minute thermal strain accompanying thermal excitation, and can be realized only by performing time resolution. Further, since the time position of this time change includes information in the depth direction, it is possible to grasp the depth of the defect by capturing the time position with the change. The signal processing as described above is performed by the image signal processing device 14.

  FIG. 8 shows a schematic configuration of a time-resolved laser shearography apparatus according to another embodiment. FIG. 8 is obtained by adding a defect determination device 15 for determining defects on the surface / surface of the inspection object from the image signal processed by the image signal processing device 14 to the device configuration of FIG.

  The position, size, depth, and the like of the defect are performed by the image signal processing device 14, but the defect determination device 15 determines the presence or absence of a defect. The presence / absence of a defect is set according to the size and depth of the defect according to the application and importance of the inspection object. At that time, a criterion for determining whether to be regarded as defective or sound is set as a threshold, and the determination is made as appropriate. Of course, if there is even a defect, the case where it is determined as a defect is also included.

1 is a diagram illustrating a schematic configuration of a time-resolved laser shearography apparatus according to an embodiment of the present invention. It is a figure which shows the principle which detects a surface displacement by the laser shearography method. It is a figure which shows the example of the image in the conventional laser shearography method which is not a time-resolving type. It is the figure which compared the visual field in the case of using a time-resolved camera with the visual field of the conventional CCD camera. It is a figure which shows the 1st method of the method of taking the reference in embodiment of this invention. It is a figure which shows the 1st method of the method of taking the reference in embodiment of this invention. It is a figure which shows the specific example of the signal processing for the high sensitivity detection in embodiment of this invention. It is a figure which shows schematic structure of the time-resolved laser shearography apparatus which concerns on other embodiment of this invention.

Explanation of symbols

1 Pulse heating means 2 Pulse heating light (high frequency)
3 Laser beam emitting device 4 Lens 5 Laser beam 6 Condensing lens 7 Half mirror (prism)
8, 9 Mirror 10 Angle adjustment mechanism 13 Time-resolved camera 14 Image signal processing device 15 Defect determination device 20 Shear optical system 51 Inspection object 52 Crack (peeling)

Claims (13)

Heat application means for applying heat to the surface to be inspected;
Surface displacement measuring means for measuring surface displacement by laser interferometry;
In the surface / surface inspection apparatus having
A surface / surface layer inspection apparatus comprising time-resolved image measuring means for time-resolved measurement of a laser interference image generated by the laser interferometry.   2. The surface / surface inspection apparatus according to claim 1, wherein the time-resolved image measuring means is a streak camera.   2. The surface / surface inspection apparatus according to claim 1, wherein the time-resolved image measuring means is a CCD line sensor camera.   The surface / surface layer inspection apparatus according to claim 1, wherein the heat application unit is a pulse light source.   4. The surface / surface layer inspection apparatus according to claim 1, wherein the heat applying means is a pulse high frequency source.   6. The surface / surface inspection apparatus according to claim 1, wherein the surface displacement measuring means is a measuring apparatus that performs measurement by laser shearography or laser speckle interferometry.   7. The surface / surface layer according to claim 1, further comprising: a signal processing unit that performs logarithmic processing on the laser interference image signal measured by the time-resolved image measurement unit. Inspection device.   8. The surface / surface inspection apparatus according to claim 1, further comprising defect detection means for detecting defects on the surface / surface of the inspection object based on the obtained inspection data. . In the surface / surface inspection method in which heat is applied to the surface to be inspected and surface displacement is measured by laser interferometry,
A surface / surface layer inspection method, wherein a laser interference image generated by the laser interferometry is time-resolved to inspect a surface / surface layer to be inspected. Applying heat to the surface to be inspected,
Laser interference image is generated by laser interferometry,
Time-resolved measurement of the generated laser interference image,
Measure the time-resolved laser interference image signal before and after heat application,
A surface / surface inspection method, wherein a defect is evaluated by performing differential processing on a measured laser interference image signal. Applying heat to the surface to be inspected,
Laser interference image is generated by laser interferometry,
Time-resolved measurement of the generated laser interference image,
Time-resolved laser interference image signals are measured at two different times after heat application,
A surface / surface inspection method, wherein a defect is evaluated by performing differential processing on a measured laser interference image signal.   The surface / surface layer inspection method according to claim 10 or 11, wherein a defect is evaluated by using a line image of an initial time of the time-resolved laser interference image signal as a reference, and performing differential processing on the subsequent line image.   12. The surface / surface layer inspection method according to claim 10 or 11, wherein the defect is evaluated by logarithmically processing the time-resolved laser interference image signal and discriminating between the steady change portion and the non-steady change portion.

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