JP7359660B2 - Deformation analysis method for rubber materials - Google Patents

Description

The present invention relates to a method for analyzing deformation of rubber materials.

Digital image correlation (DIC) is known as a technique for visualizing in-plane distortion of rubber materials for deformation analysis of rubber materials. Visualizing not only in-plane distortion during product use but also distortion during mechanical testing is useful for research and development of rubber products.

As a method for analyzing the deformation of rubber materials, for example, Non-Patent Document 1 describes a method for visualizing the distortion of a notch in a test piece made of a rubber material by using a camera that uses visible light and a digital image correlation method. Are listed. Moreover, Patent Document 1 describes a method of analyzing tearing behavior of a rubber material using a method that combines an X-ray imaging method and a digital image correlation method.

JP 2019-100814 Publication

Chang Liu, and 4 others, "Influence of Strain Amplification Near Crack Tip on the Fracture Resistance of Carbon Black-filled SBR", Rubber Chemistry and Technology, Vol.88, No.2, pp276-288 (2015)

In deformation analysis of rubber materials, the digital image correlation method can be used not only for specimens with simple contours such as strip specimens, but also for specimens with various contours such as notches. It is desirable that calculation errors are less likely to occur and calculations can be performed stably along the contour.

In view of the above points, the embodiments of the present invention aim to provide a rubber material deformation analysis method that can stably perform calculations using the digital image correlation method along the contour of a test piece.

A deformation analysis method for a rubber material according to an embodiment of the present invention includes: imparting a random pattern to a test piece made of a rubber material; obtaining a plurality of images by photographing the test piece while deforming the test piece; Obtaining the outline of the test piece from one of the images, and setting a plurality of analysis points and subsets in the image from which the outline has been obtained to set an analysis area, and at that time, each subset is set so that the outline does not pass through. and calculating the displacement of each analysis point with respect to the analysis area by a digital image correlation method.

According to the embodiment of the present invention, calculations using the digital image correlation method can be stably performed along the contour of the test piece.

1 is a flowchart showing a deformation analysis method according to an embodiment. FIG. 2 is a schematic diagram for explaining a deformation test according to an embodiment. (A) X-ray transmission image in Example, (B) its binarized image. Explanatory diagram for explaining how to set analysis points and subsets Explanatory diagram to explain how to set analysis points near the outline of the test piece

Hereinafter, matters related to the implementation of the present invention will be explained in detail.

A deformation analysis method according to one embodiment is a deformation analysis method in a mechanical test of a rubber material, and includes the following steps. FIG. 1 is a flow chart thereof.
(a) Step of imparting a random pattern to the test piece (step S1),
(b) a step of photographing and acquiring an image while deforming the test piece (step S2);
(c) obtaining the outline of the test piece (step S3);
(d) setting an analysis area by setting analysis points and subsets (step S4);
(e) a step of calculating the displacement of each analysis point by the digital image correlation method (step S5); and (f) a step of calculating distortion from the displacement of each analysis point (step S6).

Mechanical tests for rubber materials that are subject to deformation analysis include various mechanical tests in which displacement and strain within a test piece made of a rubber material are observed by applying force to deform the test piece. For example, a test in which a force is applied to a test piece in one direction such as a tensile test or a compression test may be used, a test in which a tensile or compressive strain of a predetermined magnitude is repeatedly applied to a test piece, a tear test or a crack growth bending test. A test that involves the destruction of a test piece may also be used.

A rubber material is a substance having rubber-like elasticity, and is a concept that includes not only vulcanized rubber but also elastomers such as thermoplastic elastomers. In one embodiment, a vulcanized rubber is preferably used as the rubber material, that is, a vulcanized rubber obtained by vulcanizing a rubber composition containing a rubber polymer and various compounding agents including a vulcanizing agent such as sulfur. can be mentioned. Examples of the rubber polymer include various diene rubbers such as natural rubber, butadiene rubber, styrene-butadiene rubber, or combinations thereof. In addition, as compounding agents, various compounding agents normally used in the rubber industry may be used, such as fillers such as carbon black and silica, softeners, anti-aging agents, zinc white, stearic acid, wax, and vulcanization accelerators. I can do it.

The shape of the test piece can be specified depending on the type of mechanical test targeted for deformation analysis, and is not particularly limited. For example, when acquiring an image by transmitting radiation in step (b), the shape of the test piece may be any shape as long as it can transmit radiation, and various shapes such as a sheet, a cylinder, a block, etc. can be mentioned, and preferably It is in sheet form.

In one embodiment, the test piece may be a test piece with a simple contour shape such as a strip test piece, or a test piece with irregularly shaped portions may be used. The irregularly shaped portion is not particularly limited, and includes, for example, a notch provided on the edge of the test piece, a V-shaped or U-shaped notch, and a slit or hole (i.e., an opening) provided inside the test piece. ), etc. For example, in the test piece 10 used in the mechanical test shown in FIG. 2, a notch 12 is provided in the longitudinal center of one side of the strip-shaped test piece 10 in the width direction. The cut 12 is formed by making a cut in a direction perpendicular to the longitudinal direction of the test piece 10.

In the deformation analysis method according to the embodiment, first, in step (a), a random pattern is provided to a test piece made of a rubber material (step S1).

A random pattern is a random pattern that is applied to a test specimen to perform digital image correlation methods. Therefore, the random pattern is applied to at least the measurement target site, but it may be applied to the entire test piece, for example, as long as it is applied to the measurement target site. The measurement target region is a region in which an image is taken in step (b) to perform deformation analysis on the test piece, and when a test piece having the above-mentioned irregularly shaped region is used, it is the surrounding region including the irregularly shaped region.

Random patterns can be applied to the surface or inside of the test piece. For example, when the imaging in step (b) involves obtaining a transmission image using radiation, the random pattern may be provided on the surface or inside of the test piece. Further, when the image is acquired using visible light in the photographing in step (b), the random pattern may be provided on the surface of the test piece.

When applying a random pattern to the surface of a test piece, for example, a coating liquid containing marker particles may be applied by spraying onto the surface of the test piece, or alternatively, a rubber layer containing marker particles may be applied to the surface of the test piece. It may be provided on the surface. Further, when acquiring an image using visible light, for example, a random pattern such as a mottled pattern may be applied to the surface of the test piece using a coloring agent.

When applying a random pattern to the inside of the test piece, the random pattern may be applied to the test piece by blending marker particles into the rubber material and dispersing the marker particles inside the test piece.

Marker particles are fine particles for forming random patterns used in digital image correlation methods. For example, it is a fine particle that can be detected by radiation such as X-rays, that is, a fine particle that is a detection target in a radiation imaging method. As the marker particles, particles containing a metal element (that is, metal element-containing particles) that are easily contrasted by radiation are used. One example is a stable one. Specifically, examples thereof include metal particles such as tungsten particles, silver particles, and lead particles, and alumina particles.

The particle size of the marker particles is not particularly limited, and may be, for example, a particle size that is equal to or larger than the spatial resolution (number of pixels executed) by radiation imaging method. Note that the spatial resolution varies depending on the line width and divergence of the radiation used.

In the deformation analysis method according to the embodiment, next, in step (b), the test piece is photographed while being deformed to obtain a plurality of images showing the deformation process to be analyzed (step S2).

Specifically, the measurement target site is photographed while applying force to the test piece to give it deformation, that is, distortion. The force to be applied to the test piece can be specified depending on the type of mechanical test targeted for deformation analysis, and a unidirectional tensile or compressive force may be applied to the test piece, or a force of a predetermined magnitude may be applied to the test piece. Expansion strain or compression strain may be applied repeatedly at a predetermined frequency. In the example shown in FIG. 2, a test piece 10 having notches 12 is repeatedly subjected to elongation strain at a predetermined frequency in its longitudinal direction.

By continuously photographing the test piece at predetermined time intervals while deforming the test piece in this manner, a plurality of two-dimensional still images representing the deformation behavior (temporal change in strain) of the test piece are obtained. For example, when the subject of analysis is repeated strain in a test piece, images are taken multiple times at predetermined timing during one cycle of repeated strain, thereby producing multiple images representing temporal changes in strain during one cycle. can get.

The image acquisition method is not particularly limited. For example, it may be photographed by a camera using normal visible light, or it may be a radiation imaging method in which the test piece is irradiated with radiation such as X-rays. In the radiation imaging method, a test piece is irradiated with radiation so that the radiation passes through the test piece, and a transmission image is obtained. The image acquisition method itself using the radiation imaging method can be performed by a known method and is not particularly limited. Here, radiation means radiation in a broad sense, and includes particle radiation such as neutron beams, and electromagnetic waves such as X-rays, gamma rays, and ultraviolet rays. X-rays are preferable, and therefore it is preferable to obtain an X-ray transmission image as an image.

FIG. 2 schematically shows an X-ray imaging test apparatus according to one embodiment. The test piece 10 is attached to a tensile testing machine with both longitudinal ends held by grips (not shown), and the grips on both sides are moved in a direction away from each other and also moved in a direction toward each other. By repeating this, repeated elongation strain is imparted.

The test apparatus is provided with an X-ray tube 22 as an irradiation means for irradiating the test piece 10 with X-rays, and a detector 24 for detecting the X-rays that have passed through the test piece 10. An X-ray transmission image is acquired based on the X-rays. The X-ray tube 22 and the detector 24 are arranged in a straight line with the measurement target area being the measurement target area including the notch 12 in the test piece 10 .

The X-rays used are preferably high-intensity X-rays of, for example, 10 ¹⁰ (photons/s/mrad ² /mm ² /0.1%bw) or more. Examples of synchrotrons that emit such X-rays include SPring-8 at the High Brightness Photon Science Research Center and the Aichi Synchrotron Light Center at Aichi Knowledge Center.

Furthermore, the energy of the X-rays is not particularly limited, and may be, for example, 1 to 100 keV or 10 to 50 keV. The X-ray irradiation time (ie, exposure time) is also not particularly limited, and may be, for example, 0.0001 to 10000 ms, or 1 to 1000 ms. The frame rate is also not particularly limited, and may be, for example, 1 to 10,000 fps or 1 to 2,000 fps.

In the deformation analysis method according to the embodiment, next, in step (c), the outline of the test piece is acquired from one of the plurality of images (step S3).

In this example, a reference image for deformation analysis, for example, an initial image before deformation, is used as a reference image, and the outline of the test piece at the measurement target site is determined in the reference image. As a method for acquiring the outline of the test piece, it is preferable to use a method of binarizing the image. In detail, a binarized image is created from the above reference image by binarizing using the difference in luminance value between a polymer such as rubber, a background (air), and marker particles, and from the binarized image. The outline of the test piece can be extracted. The binarization itself can be performed using commercially available software.

FIG. 3(A) shows an X-ray transmission image as a reference image. By binarizing this X-ray transmission image, a binarized image shown in FIG. 3(B) is obtained.

The outline of the test piece may be obtained at least at the site to be analyzed. For example, when using a test piece having an abnormally shaped part in the above-mentioned outline, it is sufficient to acquire the outline at the irregularly shaped part, and there is no need to extract the outline from the entire image.

In the deformation analysis method according to the embodiment, next, in step (d), a plurality of analysis points and subsets are set in the image obtained from the contour (step S4).

Specifically, an analysis region is set by setting analysis points and subsets within the test piece (that is, within a region corresponding to the test piece) in the reference image. Here, the analysis point is an arbitrary point within the specimen used to calculate displacement in the digital image correlation method, and the deformation of the specimen can be analyzed by calculating the displacement at the analysis point. .

A subset is a minute calculation area for performing pattern matching based on correlations such as luminance value distribution in order to specify the positions of analysis points before and after deformation, and is made up of a plurality of pixels. A subset is set for each analysis point, so each subset includes one analysis point, and usually the analysis point is set at the center of the subset. In this example, the subset is a rectangular region centered on the analysis point. Note that the concept of rectangle also includes square.

The analysis region is a region in which analysis points and subsets are set in the measurement target region, and within this region, calculations are performed using the digital image correlation method, and the displacement of each analysis point is calculated. The analysis region is set along the contour, and is set in the test piece region around the contour. For example, in the case of a test piece having the above-mentioned irregularly shaped part, the test specimen range is set along the outline of the irregularly shaped part and in the vicinity of the outline. As an example, in a test piece having a notch 12 shown in FIG. 2, the analysis region 28 is set to include the tip of the notch, as shown in FIG.

In this embodiment, when setting the analysis region, each subset is set so that the outline of the test piece does not pass through it. In other words, the subset is set such that the outline of the test piece falls within the area so that the air area that is the background of the test piece is not included in the area.

FIG. 4 is a diagram schematically showing this, in which the left side of the parabolic contour 30 is the test piece region 32, and the right side of the contour 30 is the background region 34. The black circles are analysis points 36, and within the test piece region 32, there are a plurality of equally spaced grids 38 extending in the Y direction (stretching direction) and a plurality of grids 38 extending in the X direction (direction perpendicular to the stretching direction). An analysis point 36 is set at each grid point that is an intersection with grid elements 40 arranged at equal intervals. A subset 42 consisting of a rectangular (square in the example of FIG. 4) region is set around each analysis point 36. Note that although only some subsets 42 are illustrated in FIG. 4, the illustration is simply omitted, and a similar subset 42 is set at each analysis point 36.

In the vicinity of the contour 30, not only the grid points existing in the background region 34 but also the grid points existing in the specimen region 32, when a subset is set around the grid point, are For grid points that pass through the contour 30, no analysis points 36 are set, and therefore no subsets 42 are set. A dotted line frame 44 in FIG. 4 indicates a position in the vicinity of the contour 30 where the subset 42 should not be set.

In order to set the outline of the test piece so that it does not pass through the subset, it is preferable to set it as follows in one embodiment. For example, when the subset is a rectangular area centered on the analysis point, when setting the analysis area, the outline of the specimen should be separated from the outline by more than half the subset size. Set analysis points. More specifically, it is preferable to set the analysis points in the contour portion of the test piece with a gap of at least half the subset size and less than the subset size from the contour.

Specifically, as shown in FIG. 5(A), a case will be considered in which a rectangular subset 42 is set for the contour 30 that extends obliquely with respect to the grids 38 and 40. In this case, half of the subset size is a range 42A that bisects the subset 42 in the X direction, and a range 42B that bisects the subset 42 in the Y direction. Regarding the lattice points 46 indicated by black circles in the figure, there is a gap (that is, a margin) between them and the contour 30 in the X direction by a range 42A or more, which is half the subset size. Specifically, the range 42A is half the subset size or more, and the distance is less than one subset size. Furthermore, there is a gap (that is, a margin) between the contour 30 and the contour 30 in the Y direction as well, which is equal to or larger than the half range 42B of the subset size. Specifically, the range is equal to or larger than half the subset size 42B, and there is a gap of less than one subset size. Therefore, an analysis point 36 is set for this grid point 46, and a subset 42 centered around the analysis point 36 is set.

On the other hand, regarding the grid point 48 adjacent to the right side of the grid point 46 and the grid point 50 adjacent to the bottom side, there is a gap of half the subset size 42A and 42B in the X direction and the Y direction, respectively, from the contour 30. There is no. Therefore, the analysis points 36 and the subsets 42 are not set at these grid points 48 and 50.

As shown in FIG. 5(B), when setting a rectangular subset 42 for the contour 30 extending parallel to the grid grid 40 in the X direction, the grid points 52 indicated by black circles are There is a gap of half the subset size 42B not only in the X direction but also in the Y direction. Specifically, in the Y direction, there is a gap of more than half the subset size 42B and less than one subset size. Therefore, an analysis point 36 is set for this grid point 52, and a subset 42 centered around the analysis point 36 is set. On the other hand, the grid point 54 adjacent to the lower side of the grid point 52 is not separated from the contour 30 by the range 42B, which is half the subset size, in the Y direction. Therefore, the analysis point 36 is not set at the grid point 54, nor is the subset 42 set. The same applies to the contour 30 extending parallel to the grid 38 in the Y direction, and the same concept may be applied to contours having other shapes or positional relationships.

After setting the analysis region as described above, in the deformation analysis method according to the embodiment, in step (e), the displacement of each analysis point is calculated with respect to the analysis region by digital image correlation method (DIC) (step S5). .

The digital image correlation method is a method for analyzing the deformation of an object by comparing images before and after the deformation of a random pattern applied to the object, and uses movement tracking of a brightness value pattern as the measurement principle. Specifically, for the images at the first time t=t ₀ and the second time t=t ₀ +dt, the subsets showing a high correlation with the luminance ground distribution of the subset in the first time image are selected from the ones in the second time image. Search for arbitrary subsets using numerical analysis and identify corresponding subsets. Thereby, it is possible to calculate the movement direction and movement amount of the analysis point in the transformation from the first time to the second time, that is, the displacement for the subset. The image correlation method itself is described on pages 31 to 46 of "Visualization Information Library 4 PIV and Image Analysis Technology" (published by Asakura Shoten Co., Ltd., edited by the Visualization Information Society, April 25, 2012). It can be carried out using the method described above, or it can also be carried out using commercially available software.

In this example, the displacement of each analysis point is calculated from the initial image of the deformation to be analyzed (also referred to as a reference image; for example, the image before deformation) toward the final image (target image in deformation analysis) by a digital image correlation method. That is, calculations are performed while sequentially updating reference images from the initial image to the final image. This provides the displacement from the initial image to the final image for each analysis point within the analysis area. For example, when applying the elongation strain shown in Fig. 2, the displacements in two orthogonal directions such as the X direction displacement (displacement in the direction perpendicular to the elongation direction) and the Y direction displacement (displacement in the elongation direction) are calculated. Good too.

In the deformation analysis method according to the embodiment, next, in step (f), distortion in the analysis region is calculated from the displacement of each analysis point obtained by the above calculation (step S6). Specifically, distortion in the final image is calculated for each analysis point within the analysis region.

Strain can be obtained by differentiating the distribution of displacement. For example, when applying the elongation strain shown in Fig. 2, the strain in the Strain in two orthogonal directions, such as (distortion), may be determined, or shear strain may be determined. The shear strain is obtained by adding the value obtained by differentiating the X-direction displacement with respect to the Y-direction coordinate and the value obtained by differentiating the Y-direction displacement with respect to the X-direction coordinate.

As described above, in this embodiment, it is possible to perform deformation analysis using the digital image correlation method for mechanical testing of rubber materials, and to stably perform calculations using the digital image correlation method along the contour of the test piece. I can do it.

In detail, when setting the analysis area, each subset is set so that the outline of the test piece does not pass through, so the background air area is not included in the subset, making the analysis stable. You can increase your sexuality. In particular, since each analysis point is set with a gap of more than half the subset size from the contour, it is more certain that air regions are not included in the subset for those with contour shapes of irregular parts. It is possible to define an analysis region that can be stably calculated near the contour.

Furthermore, by binarizing the image, the outline of the test piece can be obtained, and by using the obtained outline to set analysis points and subsets, a more accurate analysis region can be set.

Further, by obtaining a transmission image by imaging using radiation such as X-rays, contour information can be easily extracted when creating a binarized image and extracting the contour. Specifically, in the case of visible light, the contrast difference between the rubber and the background is large, but the contrast difference between the marker and the rubber also becomes large. Therefore, errors may occur and the effort required for correction may increase. On the other hand, in the case of a radiographic image, the contrast difference between the rubber and the background is large, and the contrast difference between the rubber and the marker can be made small, so the outline can be extracted more easily than in the case of visible light.

As described above, according to the present embodiment, calculation errors are unlikely to occur even for a test piece with an irregularly shaped part such as a notch, and the digital image correlation method can be used stably along the outline of the test piece. Able to perform calculations. Therefore, it can be useful for developing materials for rubber products.

Examples of the present invention will be shown below, but the present invention is not limited to these Examples.

Using a Banbury mixer, 100 parts by mass of styrene-butadiene rubber ("JSR1502" manufactured by JSR Corporation), 50 parts by mass of carbon black ("SEAST 3" manufactured by Tokai Carbon Co., Ltd.) and zinc white (Mitsui Metal Mining Co., Ltd.) were added. 2 parts by mass of "Zinc White Type 1" manufactured by Kao Corporation), 1 part by mass of stearic acid ("Lunac S-20" manufactured by Kao Corporation), and 1 part by mass of sulfur ("Sulfur Powder for Rubber 150" manufactured by Hosoi Chemical Industry Co., Ltd.). 2 parts by mass of "Mesh") and 1 part by mass of a vulcanization accelerator ("Noxela CZ" manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd.) were added and kneaded. Next, 4 parts by mass of marker particles (product number 327077 Silver flakes 10μ product manufactured by Sigma-Aldrich) were roll-mixed at a roll surface temperature of 60° C. to prepare an unvulcanized rubber composition.

The obtained unvulcanized rubber composition was formed into an unvulcanized rubber sheet with a thickness of 1.00 mm using a roll, and press-processed with a metal mold (160° C., 30 minutes) to a thickness of 1.00 mm. A rubber sheet with a thickness of 00 mm was produced. The test piece 10 shown in FIG. 2 was prepared by punching out the obtained rubber sheet into a strip shape of 9 mm width x 30 mm length, and making a cut of 1.42 mm length in the center thereof.

Using the obtained test piece 10, as shown in FIG. 2, the measurement target region of the test piece 10 was photographed while repeatedly applying elongation strain, that is, the image was acquired by an X-ray imaging method. The repeated elongation strain was a dynamic strain of 25% (the distance between the grips of the tensile tester was extended from the unstretched state (0%) of the test piece to the 25% stretched state) and a frequency of 0.34 Hz. The measurement target site was the tip of the notch 12. The X-ray conditions and photographing conditions were: frame rate: 50 fsp, X-ray exposure time: 10 ms, X-ray energy: 15 keV, and resolution of photographed image: 7 μm/px.

Next, among the plurality of images obtained, the initial image before deformation (see FIG. 3(A)) is binarized using the luminance value difference to create a binarized image (see FIG. 3(B)). The outline of the specimen was determined by creating an analysis area, and the analysis area was set by setting analysis points and subsets. The subset was a square area of 21×21 pixels, and the analysis point was set at the center. At that time, as mentioned above, by setting each analysis point with a gap of at least half the subset size between the outline of the test piece and the relevant outline, the outline of the test piece does not pass through each subset. I made it.

Next, the X-direction displacement (displacement in the direction perpendicular to the stretching direction) and Y-direction displacement (displacement in the stretching direction) of each analysis point is calculated by applying the digital image correlation method from the initial image to the final image. did. Then, by calculating the X-direction strain and Y-direction strain at each analysis point in the final image from the obtained displacement, the distribution of strain in the measurement target site was determined. In this case, according to the example, the displacement could be stably calculated by the digital image correlation method without calculating an abnormal value.

On the other hand, as a comparative example, when setting the analysis area, the analysis points were set without a gap of more than half the subset size in the outline of the test piece, and the other analysis was performed in the same manner as in the example. Abnormal values were calculated in the digital image correlation method, making stable calculations impossible.

10...Test piece, 12...Notch, 28...Analysis area, 30...Contour, 36...Analysis point, 42...Subset, 42A, 42B...Half range of subset size

Claims (4)

imparting a random pattern to a specimen made of rubber material;
obtaining a plurality of images by photographing the test piece while deforming it;
Obtaining the outline of the test piece from the reference image by using an initial image before deformation among the plurality of images as a reference image;
setting a plurality of analysis points and subsets in the reference image to set an analysis region; at this time, setting each subset so that the contour does not pass through;
calculating a displacement of each analysis point from the reference image toward a final image serving as a target in deformation analysis with respect to the analysis region by a digital image correlation method;
including;
The test piece has an irregularly shaped part selected from a cut or notch on the edge of the test piece, or a slit or hole inside the test piece, and the plurality of images are taken with the part including the irregularly shaped part as the measurement target part. Acquired,
The subset is a rectangular area centered on the analysis point, and when setting the analysis area, there is a gap between the outline of the test piece and the outline that is more than half the subset size and less than the subset size. A method for analyzing deformation of a rubber material , in which the analysis points are set at intervals . The deformation analysis method according to claim 1 , wherein the contour is obtained by binarizing the reference image. 3. The deformation analysis method according to claim 1 , further comprising calculating a strain in the analysis area from the displacement of each analysis point obtained by the calculation. The deformation analysis method according to any one of claims 1 to 3 , wherein the imaging is to obtain a transmission image using radiation, and the random pattern is made of particles containing metal elements.