JP2823116B2 - Dynamic Observation of Object Deformation Using Digital Speckle Interferometry - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for dynamically observing deformation of an object such as a mechanical structural material or a structure using speckle interferometry.

[0002]

2. Description of the Related Art For example, in a process of leading to a fracture through a plastic deformation of a mechanical structural material or a process of a fatigue fracture,
The object deforms with a microscopic structural change of the surface.
Therefore, if such deformation of the object can be observed every moment, that is, dynamically, it is useful for observing fatigue, predicting a destruction position, and the like.

Conventionally, an electric strain gauge has been used to observe the process of deforming and breaking a loaded object.
The so-called point measurement method is widely used. On the other hand, various optical measurement methods have been proposed because of the advantage that the deformation of an object can be observed in the entire field of view.

[0004] As typical optical measurement methods, (1) photoelasticity method, (2) Moire method, (3) Moire interference method, (4) holographic interferometry, (5) speckle interferometry and the like are known. Have been. Of these optical measurement methods, the present invention applies
In the speckle interferometry of (5), similarly to the holography-interferometry of (4), a two-dimensional distribution of deformation can be obtained for a rough surface without requiring special pretreatment of the sample. (4) has the advantage that a photographic plate with a required high resolution is not required.

That is, in the speckle interferometry, fringes due to the presence or absence of a correlation between speckle intensities generated when an object wave diffusely reflected on a rough surface and a reference wave can be obtained. An electronic imaging device such as a CCD camera can be used as long as it has a resolving power enough to resolve the pattern. Therefore, image data can be directly input to a computer, and all post-processing can be performed electronically or digitally. Such a method is called electronic speckle interferometry (ESPI) or digital speckle interferometry (DSPI).

Next, the principle of the speckle interferometry will be described with reference to the optical system shown in FIG. The parallel laser light is split into two by a semi-transparent mirror, and one is diffusely reflected by a rough surface which is a test surface.
The other reflection surface (reference surface) may be a rough surface or a mirror surface. The intensity distribution on the image plane is

(Equation 1) Given by Here, a ₀ and _ar are the amplitudes of the light reflected on the test surface and the reference surface, and φ is the phase difference between the two light waves. Now, assuming that the object is deformed out of plane and the phase difference changes by θ, the intensity distribution of the speckle pattern becomes

(Equation 2) Becomes Here, focusing on a ₀ , a _r , and φ being independent random variables, the correlation between the two intensities is given by the following equation.

(Equation 3) Therefore, a perfect correlation is obtained when θ = 2mπ, and θ = (2m +
1) When π, the correlation becomes zero. The intensity modulation due to the presence or absence of the correlation is caused by the non-linearity of the recording medium. This is C
2 by system consisting of CD camera and computer
By taking the difference between the two speckle patterns, a clear fringe can be obtained most easily. The phase difference is the point (x,
If the out-of-plane displacement at (y) is w (x, y),

(Equation 4) Which is the same shape as the fringe of out-of-plane deformation in holographic interferometry. Further, as shown in FIG. 2, if an object is irradiated with two light waves symmetric with respect to the normal line and an optical system for observing from the normal direction is used, speckle fringes due to in-plane deformation can be obtained. If one deformation component in the plane is u (x, y), the phase difference is

(Equation 5) Becomes Here, α is an angle between the object illumination light and the surface normal.

As described above, the speckle interferometry can form a flexible optical system according to an object to be measured, and the processing of the speckle pattern can be generally performed by a system combining a photoelectric device and a computer. That is, at a certain time, a speckle pattern in a certain state is photoelectrically recorded and stored in a memory. Next, a load is applied to the object, and the speckle pattern after deformation is photoelectrically recorded, and a difference in intensity between pixels of both speckle images is calculated. The distribution of the difference is the distribution of the correlation before and after the deformation given by the equation (1), and this is displayed on the television monitor.

When it is desired to observe vibration or deformation, first, a speckle pattern of a stationary object is recorded, and a difference image from a speckle pattern of a moving object at a certain point in time is displayed on a monitor. . In this way, real-time observation of vibration and deformation becomes possible.

First, the inventors take data of a change in a speckle pattern due to deformation at a fixed time or at an appropriately controlled time interval, and take the difference between images at adjacent time intervals to obtain the time interval. Has proposed a digital speckle interferometry capable of measuring the developed deformation, and will be described below.

FIG. 3 shows the configuration of an experimental system according to this method. The laser light from the laser light source 1 is split by the beam splitter 2, the transmitted light passes through a shutter 3a, is converted into parallel light by a lens system 4a, and irradiates the sample 5 obliquely from above in the xz plane. This sample 5 is a test piece of an aluminum plate, which is attached to a tensile tester.
The reflecting mirror 6a is installed on the yz plane perpendicular to the sample surface, and a part of the incident laser light irradiates the sample 5 directly, and the other part is reflected by the reflecting mirror 6a and then obliquely from below in the xz plane. Irradiate the sample. This is an optical system equivalent to that shown in FIG. 2. If a speckle pattern generated by this system is captured by the CCD camera 7 and the difference between two images separated in time is calculated,
According to equation (3), a speckle interference fringe representing an x-direction deformation component is obtained.

On the other hand, the light reflected by the beam splitter 2 passes through a shutter 3b and a lens system 4b, and then, as an optical system equivalent to FIG. 2, irradiates the sample 5 obliquely from the left in the yz plane. After being reflected by the reflecting mirror 6b placed in the xz plane perpendicular to the sample 5, the sample 5 is irradiated diagonally from the right.
Then, the speckle pattern generated by the optical system is imaged by the CCD camera 7. Then, the interference fringe representing the y-direction deformation component is obtained by calculating the difference between the two images separated in time in the same manner as described above.

The two shutters 3a and 3b are controlled by a driver 10 via a D / A conversion board 9 mounted on a computer 8 so as to be opened and closed alternately. Can be obtained alternately.

The speckle pattern picked up by the CCD camera 7 is stored in a color image board 12 and a video recorder 13 through an image freezer 11. The color image board 12 has three sets of memories corresponding to R, G, and B frames. The color image board 12 digitally converts an x-direction component into an R frame and a y-direction component into a G frame, and stores them in the memory.

On the other hand, a load cell 14 is provided in the tensile tester.
Is attached so that the load force on the sample 5 can be measured, and a strain gauge 15 is attached to the sample 5 so that the distortion of the portion can be measured. The signals from the load cell 14 and the strain gauge 15 are input to the computer 8 via the data logger 16 and the GPIB board 17. Reference numeral 18 denotes a power supply of the driver 10, 19
Denotes a monitor, and 20 denotes an MO disk drive as an external storage device.

The following is an example of the experimental results. Sample 5 was an aluminum plate as described above, with an effective length of 64 mm and a width of 7.
It is a 9mm, 1.7mm thick test piece, which is attached to a tensile tester. FIG. 4 shows the load force detected by the load cell 14 with respect to the passage of time when the test piece was deformed at a pulling speed of 0.75 μm / s. In this experiment, in the whole process from the start of the experiment to the fracture of the test piece, interference fringes of more than 1000 frames in each of the x-direction component and the y-direction component were obtained. In this case, while the time interval for opening and closing the shutters 3a and 3b, in other words, the imaging time interval for the x and y components is δt = 1/30 seconds, the time interval for recording the speckle pattern of each component is Δt = 9 seconds. That is, in this case, the imaging time interval between the two deformation components can be almost ignored with respect to the deformation speed.

FIG. 5 schematically shows the interference fringes at 10 points among them. The alphabets in FIG. 4 indicate the time points at which the respective stripes in FIG. 5 were observed. 5 are interference fringes representing the x-direction deformation component and the y-direction deformation component, respectively. The amounts of deformation per fringe obtained from equation (3) are 0.51 μm and 0.43 μm. (a) and (b) are interference fringes in the elastic region, and the uniform interference fringes corresponding to the substantially uniform elongation are slightly inclined due to the in-plane curvature of the entire sample. In contrast, in the interference fringes (c) to (i), a sharp change in the interference fringes characteristic of plastic deformation is observed.
(c) In the x-component fringes, substantially equidistant linear fringes having different directions and intervals appear at the upper, middle and lower portions of the test piece. The area where these equally spaced linear stripes appear is called a domain. The interference fringes are continuously curved at the domain boundaries. 3 for y-component fringes
One domain can be verified. In (d), two domains are moved to the upper middle and lower parts, and in (e), the boundary between the two domains is moved to the vicinity of the center. In (f), the upper domain is further divided into two, and at the same time, stripes are blurred at the boundary. Furthermore, as (g), (h), and (i) progress, the boundary between the block and the stripe moves around the test piece in a complicated manner, and in (j), two upper and lower domains appear and finally break at the boundary. ing. The changes in these fringes directly indicate changes in the in-plane deformation distribution, and were first clarified by the digital speckle interferometry proposed by the inventors. This experiment reveals that plastic deformation does not proceed continuously in the specimen, but rather due to discontinuous processes such as local stress concentration and relaxation at domain boundaries and rotation of the entire domain. A progressive plastic deformation model is conceivable. This can be qualitatively explained by the plastic deformation wave theory by Panin et al.

In this experiment, at a load speed of 0.75 μm, Δt = 9 seconds was set as a time interval sufficient for the appearance of interference fringes suitable for observation. It was difficult to find a continuous relationship between the interference fringes. That is, when focusing on one stripe, it is generally difficult to determine which stripe the stripe corresponds to at the next time.

[0018]

That is, data of a change in a speckle pattern due to deformation is collected at certain time intervals.
In the above-described method of obtaining a fringe image by taking a difference between images at adjacent time points, in order to obtain an interference fringe suitable for observation, if the time interval between adjacent time points is lengthened, the change of the fringe is continuously tracked. Conversely, if the time interval between adjacent points is shortened to enable continuous tracking of fringe changes, interference fringes suitable for observation cannot be obtained.
There was a conflicting task.

In the case of analyzing a strain or a stress state from a change in interference fringes, it is important to determine the direction of the change. In the above-described method, it is necessary to determine the direction from the intensity distribution of the interference fringes. Could not. An object of the present invention is to solve the above problems.

[0020]

In order to solve the above-mentioned problems, according to the present invention, an ever-changing speckle pattern due to a deforming object is collected,
In digital speckle interferometry, which obtains an interference image by two speckle patterns separated by time, the time interval at which speckle patterns are collected is such that the change in speckle phase between adjacent speckle patterns is smaller than π. And the time interval between two speckle patterns for obtaining an interference image is set longer than the collection time interval of the speckle patterns, and the two speckle patterns are separated by a plurality of speckle patterns. Obtain an interference image in sequence between the patterns and observe continuously at the above-mentioned collection time interval, and set a pair of cells of an appropriate size in the speckle interference image, and change the signal intensity of each cell. Is monitored by the value obtained by subtracting the average value, and when the sign of the signal strength of one cell changes, the sign of the signal strength of the other cell is used. We propose a dynamic method of observing variations of the object to determine the movement direction of the Watarushima.

According to the present invention, in the above configuration, the speckle patterns are sequentially collected at a collection time interval for each frame, and are set in advance after the time when two speckle patterns for obtaining an interference image are collected. It is proposed that interference images between speckle patterns separated by the number of frames are sequentially obtained at acquisition time intervals.

According to the present invention, in the above configuration, the speckle patterns are sequentially collected at a collection time interval for each of a plurality of frames, and are set in advance after the time when two speckle patterns for obtaining an interference image are collected. It is proposed that interference images between speckle patterns separated by the number of frames are sequentially obtained at acquisition time intervals.

In the present invention, in the above configuration, the speckle pattern is collected at a collection time interval for each of a plurality of frames including two speckle patterns for obtaining an interference image, and is set in advance together with the collection of the speckle pattern. It is proposed to sequentially obtain interference images between speckle patterns separated by the number of frames at an acquisition time interval.

In the above invention, the deformation to be observed of the object may be a deformation corresponding to any of the in-plane displacement and the out-of-plane displacement, and in the case of the deformation corresponding to the in-plane displacement,
The deformation of two orthogonal components can be observed simultaneously.

According to the above invention, the collection of speckle patterns that change every moment is performed at sufficiently short time intervals that the change can be continuously tracked. However, two speckle patterns for obtaining interference fringes are collected. Since patterns are used at appropriate time intervals, a clear fringe image suitable for observation,
Observation can be made at the short time intervals described above. Also, by setting a pair of cells of an appropriate size in the speckle interference image and monitoring the change in the signal strength of each, it is possible to determine the change direction of the interference fringe. Can be performed.

[0026]

Next, an embodiment of the present invention will be described. FIG. 6 shows an example of the configuration of a test system to which the present invention is applied. In this example, a test piece of an aluminum flat plate attached to a tensile tester is used as a sample 5 as in FIG. The system configuration for separately observing the deformation of the two components in the plane is the same as the configuration in FIG. 3 except for the part of capturing the speckle pattern image data from the CCD camera 7 to the computer 8. The same components as those in No. 3 are denoted by the same reference numerals, and redundant description is omitted. As described above, in the present invention,
In addition to being used for observing the deformation of two in-plane components, it can be used for observing deformation of one in-plane component or out-of-plane deformation.
In that case, the system of the system configuration in FIG. 3 can be reduced from two systems to one system, or an optical system as shown in FIG. 1 can be configured and applied.

In the system shown in FIG.
1 is a CCD camera with a built-in video device.
The analog image data recorded on the video tape by the D camera 21 is transferred to the memory of the computer 8 via the A / D conversion board 22. Therefore, for this part, a digital high-speed camera or the like having a large-capacity memory can be used.

FIGS. 7 and 8 conceptually show an example of the flow of an image signal of a speckle pattern according to the present invention. First, in the example of FIG. Channel A / D converter, and therefore:
A / D of one channel for the image signal of each component
A converter is used. On the other hand, in the example of FIG.
An A / D converter of 2p channels corresponding to the components, that is, p channels (p> 2) is used for each component.
In each of the figures, the bold line at the center indicates the passage of time, and the scale is the time of image capture, and in the configuration example of FIG. 6 is the frame interval in the video rate.

Referring first to FIG. 7, time i, i +
X-component speckle images Ix (i), Ix at 1, ..., i + p
(i + 1),..., Ix (i + p) are the memory addresses cx0, cx1,.
Is stored in Similarly, the y component speckle image Iy
(i), Iy (i + 1), ..., Iy (i + p) are the memory addresses of the computer cy0, cy1, via one channel cy of the A / D converter.
..., stored in cyp. In this way, for example, the speckle images of all the necessary frame numbers of both components of the entire process from the initial stage of the deformation of the sample to the destruction are stored in the memory. After all data collection of speckle images is completed in this way, the difference between two images separated by p frames, that is, the following two expressions ΔIx (i) = Ix (i) −Ix (i + p) ΔIy (i) = Iy (i) -Iy (i + p) is calculated and sequentially displayed on the monitor. In this way,
The movement of interference fringes between speckle images separated from the p frame can be observed and measured as a moving image with a time resolution Δt = i. As described above, the time resolution Δt = i is sufficiently short so that the change in the speckle phase between the temporally adjacent speckle patterns is sufficiently smaller than π, while the difference between the two speckle patterns that takes the difference is obtained. The interval p of
Appropriate intervals are provided so that a clear fringe image suitable for observation can be obtained.

Referring now to FIG. 8, the x-component and y-component image signals are A / A divided into two for each p channel.
At the same time as the AD conversion in each channel of the D converter, the difference between the image of the first channel and the image of the p-th channel, ΔIx (i),
ΔIy (i) is calculated and displayed. At the next time point, the channels of the A / D converter are carried up one by one, and the difference image at the next time point is calculated and displayed. In this case, the displayed interference fringe image is displayed at each time increment of one frame, so that the display rate of the interference fringe image becomes the same as the original speckle image capturing rate. In this way, a change in speckle interference fringes can be observed in real time with a time resolution Δt = i. Here, as p is larger, real-time observation with high resolution in time becomes possible.

As described above, in the case of analyzing a distortion or a stress state from a change in interference fringes, to determine the deformation direction of the object or the moving direction of the interference fringes accompanying the deformation,
As described above, by observing the change of the interference fringes having a sufficiently high time resolution in real time, or by reproducing and observing the recorded image as a moving image, it is possible to make a visual judgment. In the present invention, by using the following method, it is possible to automate the determination of the moving direction of the interference fringes. FIG. 9 is a diagram schematically illustrating a part of an interference fringe image representing a distribution of deformation of an object. Each speckle intensity in the image is seemingly random,
On average, it has a sinusoidal change. Therefore, the image is divided into cells Ca and Cb which are wide enough to include a sufficient number of speckles, but which can sufficiently follow changes in interference fringes.
The coordinates of the center of each cell Ca, Cb represent the position of that cell. The temporal change in the intensity of the two cells near each other can be described by a sine wave as shown in FIG. The two intensity signals are written as A and B, respectively. When the intensity change at two points is expressed as shown in FIG. 10, that is, when the moving direction of the stripe when B is advanced with respect to A is positive (+), it is expressed as shown in FIG. Is negative (-). Subtracting the average value from these time-varying image signals gives
The signals alternate between positive and negative in each half cycle. Here, paying attention to the sign of the signal A when the signal B changes from negative to positive,
The value of the signal A at this time will be written as Iind. When the moving direction is positive (FIG. 10), Iind> 0. When the moving direction is negative (FIG. 11), Iind <0. In this way, a pair of cells of an appropriate size is set in the speckle interference image, and the change in signal strength of each cell is monitored by a value obtained by subtracting the average value, and the signal of one cell is monitored. The determination of the moving direction of the interference fringes can be automated based on the sign of the signal strength of the cell on the other side when the sign of the strength changes.

[0032]

As described above, the present invention has the following effects. a. The collection of speckle patterns that change every moment is performed at sufficiently short time intervals that the change can be continuously tracked, but two speckle patterns for obtaining interference fringes are obtained at appropriate time intervals. Since it is used, a clear fringe image suitable for observation can be observed at the short time interval. b. It is possible to automate the direction in which the interference fringes change. c. Therefore, it is possible to continuously observe the situation where the stripes change due to the deformation of the object, for example, by continuously observing and measuring the situation where the plastic deformation progresses, it will be destroyed in the future For example, the position can be predicted.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an optical system showing the principle of speckle interferometry.

FIG. 2 is an explanatory diagram showing an optical system for obtaining speckle fringes due to in-plane deformation.

FIG. 3 is an explanatory diagram showing an example of a system configuration to which digital speckle interferometry is applied.

FIG. 4 is a graph showing load force detected by a load cell with respect to time when a test piece is deformed at a pulling speed of 0.75 μm / s, based on data obtained in a test using the system of FIG. 3;

FIG. 5 is a schematic diagram illustrating an example of interference fringes obtained by the system configuration of FIG. 3;

FIG. 6 is an explanatory diagram showing an example of a system configuration to which the present invention is applied.

FIG. 7 is an explanatory diagram conceptually showing an example of a flow of an image signal of a speckle pattern in the present invention.

FIG. 8 is an explanatory diagram conceptually showing another example of a flow of an image signal of a speckle pattern in the present invention.

FIG. 9 is an explanatory diagram schematically showing a part of an interference fringe image representing a distribution of deformation of an object.

FIG. 10 is an explanatory diagram showing an example of a change in intensity of each cell.

FIG. 11 is an explanatory diagram showing another example of the intensity change of each cell.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser light source 2 beam splitter 3 a, 3 b shutter 4 a, 4 b lens system 5 sample 6 a, 6 b reflector 7 CCD camera 8 computer 10 driver 11 image freezer 12 color image board 13 video recorder 14 load cell 15 strain gauge 16 Data Logger 17 GPIB Board 18 Driver Power Supply 19 Monitor 20 MO Drive 21 CCD Camera with Built-in Video Device 22 A / D Conversion Board

Claims (7)

(57) [Claims] 1. A digital speckle interferometry for collecting an instantaneously changing speckle pattern due to an object being deformed and instantaneously obtaining an interference image by two speckle patterns spaced apart from each other. The time interval for collecting the patterns is set short so that the change in speckle phase between adjacent speckle patterns is smaller than π, and the time interval between two speckle patterns for obtaining an interference image is set to be longer than the acquisition time intervals of the speckle pattern, to obtain a sequential interference images between the two speckle patterns across the plurality of speckle patterns, as well as continuously observed by the collection time interval, speckle A pair of cells of an appropriate size are added to the interference image.
The change in the signal strength of each cell to the average value.
Is monitored by subtracting the value of
The signal strength of the other cell at the time the signal changes
A dynamic observation method of object deformation using digital speckle interferometry, wherein a moving direction of an interference fringe is determined by a code . 2. A speckle pattern which is sequentially collected for each frame at a collection time interval, and is separated by a predetermined number of frames after the time when two speckle patterns for obtaining an interference image are collected. 2. The dynamic observation method of deformation of an object using digital speckle interferometry according to claim 1, wherein interferograms are sequentially obtained at acquisition time intervals. 3. The speckle pattern is sequentially collected at a collection time interval for each of a plurality of frames, and after the two speckle patterns for obtaining an interference image are collected, the speckle pattern is separated by a predetermined number of frames. 2. The dynamic observation method of deformation of an object using digital speckle interferometry according to claim 1, wherein interferograms are sequentially obtained at acquisition time intervals. 4. A speckle pattern is used to obtain an interference image.
Collect at the collection time interval for each of a plurality of frames including one speckle pattern, and collect the speckle patterns,
2. The dynamic observation method of object deformation using digital speckle interferometry according to claim 1, wherein interference images between speckle patterns separated by a predetermined number of frames are sequentially obtained at an acquisition time interval. Of 5. A object, the deformation of observing, using digital speckle interferometry according to any one of up to claims 1-4, characterized in that the deformation corresponding to the plane displacement body Dynamic observation method of the deformation of an object. 6. The dynamic observation method of object deformation using digital speckle interferometry according to claim 5 , wherein the in-plane displacement is an orthogonal two-component in-plane displacement. Of 7. object, is deformed to observe, using digital speckle interferometry according to any one of up to claims 1-4, characterized in that the deformation corresponding to the out-of-plane displacement body Dynamic observation method of the deformation of an object.