JP7757652B2 - Digital image correlation method and specimen - Google Patents

Description

This disclosure relates to a digital image correlation method and a test specimen.

When conducting strength tests on materials (test specimens), such as static or dynamic tests, digital image correlation (DIC) is known, which analyzes the displacement behavior of the test specimen based on digital images taken before and after deformation (see, for example, Patent Documents 1 and 2).

In digital image correlation, a random pattern is first formed on the surface of the test piece. Then, before the test piece is deformed, a digital image of the random pattern is captured on the test piece. The test piece is then deformed, and a digital image of the random pattern is captured again. Subsets are then set in the digital image before deformation. A subset is an area of a predetermined size set on the digital image, and is used to analyze the amount of displacement at each position on the test piece. Next, a subset corresponding to the subset set in the digital image before deformation is identified in the digital image after deformation. The displacement behavior of the test piece is then analyzed by calculating the amount of displacement of the center point (representative point) of the subset before and after deformation.

Japanese Patent Application Laid-Open No. 2020-3234 International Publication No. 2007/072905

As mentioned above, digital image correlation calculates the displacement of the center point of the subset. Therefore, if a subset is set so that its center point is located at the edge of the test piece, part of the subset will not fit inside the test piece. In this case, the area of the subset that extends beyond the test piece will not contain a random pattern (will be blank).

As a result, it becomes difficult to identify the subset in the post-deformation digital image that corresponds to the subset before deformation, and the amount of displacement cannot be calculated for that subset. In other words, there is a problem in that the amount of displacement at the end of the test specimen cannot be calculated.

In light of these issues, the present disclosure aims to provide a digital image correlation method and a test specimen that can calculate the amount of displacement at the end of the test specimen.

In order to solve the above problem, a digital image correlation method according to one embodiment of the present disclosure is a digital image correlation method for measuring the displacement of a deforming test body, and includes an attachment process for adding an extension body made of a ductile material to the end of the test body in a plane perpendicular to the optical axis direction of the camera; a random pattern formation process for forming a random pattern on the surface of the test body to which the extension body is added and on the surface of the extension body after the attachment process is performed; a first imaging process for imaging the test body to which the extension body is added using a camera; a deformation process for deforming the test body to which the extension body is added; a second imaging process for imaging the test body deformed in the deformation process using a camera; a subset setting process for setting a first subset in the first image obtained in the first imaging process so that it spans both the surface of the test body and the surface of the extension body; a subset identification process for identifying a second subset corresponding to the first subset in the second image obtained in the second imaging process; and a calculation process for calculating the amount of displacement between corresponding representative points of the first subset and the second subset.

Furthermore, in the first and second imaging steps, the camera may be focused on the boundary between the test body and the expansion body.

Furthermore, the extension body added to the end of the test specimen in the above-mentioned addition process may be free-standing in a plane perpendicular to the optical axis direction of the camera.

This disclosure makes it possible to calculate the amount of displacement at the end of the test specimen.

1 is a flowchart illustrating a digital image correlation method according to an embodiment of the present disclosure. FIG. 1 is a diagram illustrating a test specimen. FIG. 10 is a process diagram illustrating an additional process. FIG. 10 is a process diagram illustrating a random pattern forming process. 10A to 10C are process diagrams illustrating a first imaging step and a second imaging step. FIG. 10 is a process diagram illustrating a subset setting process. FIG. 10 is a process diagram illustrating a subset identification process. FIG. 10 is a diagram illustrating a first image acquired by imaging a specimen of a comparative example. 1 is a diagram illustrating a first image acquired by imaging a specimen according to this embodiment. FIG. FIG. 10 is a diagram illustrating a test specimen according to a modified example.

Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Dimensions, materials, specific numerical values, etc. shown in the embodiments are merely examples for ease of understanding and, unless otherwise specified, do not limit the present disclosure. Note that in this specification and drawings, elements having substantially the same function and configuration are designated by the same reference numerals to avoid redundant explanation. Elements not directly related to the present disclosure are not shown.

[Digital Image Correlation Method]
FIG. 1 is a flowchart illustrating a digital image correlation method according to an embodiment of the present disclosure. The digital image correlation method according to this embodiment can be applied, for example, to measuring the strength of a test specimen (strength test). As shown in FIG. 1 , the digital image correlation method according to this embodiment includes a test specimen manufacturing process S110, a first imaging process S120, a deformation process S130, a second imaging process S140, a subset setting process S150, a subset identification process S160, and a calculation process S170. Each process will be described below. Here, an example will be given in which the digital image correlation method is applied to a tensile test as a strength test.

[Specimen manufacturing process S110]
The specimen manufacturing step S110 is a step of manufacturing a specimen to be used in a strength test, and includes an adding step S112 and a random pattern forming step S114.

[Additional step S112]
The adding step S112 is a step of adding an extension body to the end of the test body. Fig. 2 is a diagram illustrating the test body 10. In Fig. 2 and other figures of this embodiment, the X-axis, Y-axis, and Z-axis that intersect perpendicularly are defined as shown.

The test specimen 10 is made of, for example, a composite material such as fiber-reinforced plastics (FRP), metal, etc. As shown in Figure 2, the test specimen 10 according to this embodiment has a flat plate shape.

The test specimen 10 includes a parallel portion 12 and grip portions 14a and 14b. In Figure 2, the parallel portion 12 has a constant length in the X-axis direction, a constant length in the Y-axis direction, and a constant length in the Z-axis direction. Hereinafter, the length of the test specimen 10 in the X-axis direction may be referred to as "width." The length of the test specimen 10 in the Y-axis direction may be referred to as "thickness." The length of the test specimen 10 in the Z-axis direction may be simply referred to as "length."

Grip portions 14a, 14b are provided continuously at both longitudinal ends of parallel portion 12. Grip portions 14a, 14b have, for example, a width greater than that of parallel portion 12. Grip portions 14a, 14b also have, for example, a thickness substantially equal to that of parallel portion 12. Grip portions 14a, 14b are gripped by a strength testing machine in deformation step S130, which will be described later.

Furthermore, in the first imaging step S120 and the second imaging step S140 described below, the test object 10 is placed in front of the camera so that the optical axis of the camera is in the Y-axis direction. That is, in the first imaging step S120 and the second imaging step S140, the surface 10a of the test object 10 is imaged. In other words, the surface 10a of the test object 10 is the surface of the test object 10 facing the camera, i.e., the surface on the camera side.

In the adding step S112, an extension body is added to the test specimen 10. Figure 3 is a process diagram illustrating the adding step S112. As shown in Figure 3, in the adding step S112, an extension body 20 is added to the end of the test specimen 10 (i.e., the edge of the test specimen 10 in the image captured by the camera) within a plane 10a (the XZ plane in Figure 3) perpendicular to the optical axis direction of the camera (the Y axis direction in Figure 3). In this embodiment, the extension body 20 is added to the end 12a on both sides of the parallel portion 12 of the test specimen 10 in the left-right direction (the X direction in Figure 3). Note that the end 12a of the parallel portion 12 is the end that is not gripped by the strength testing machine. Furthermore, the end 12a of the parallel portion 12 is the end that is perpendicular to the load direction in the strength test (tensile test).

In the attachment step S112, the extension body 20 is attached to the end 12a of the parallel portion 12 so that the parallel portion 12 and the extension body 20 are flush or nearly flush with each other on the surface 10a. In other words, the extension body 20 is attached to the end 12a of the parallel portion 12 so that the surface 10a of the test body 10 and the surface 20a of the extension body are flush or nearly flush with each other. Therefore, like the surface 10a of the test body 10, the surface 20a of the extension body 20 is the surface of the extension body 20 that faces the camera, i.e., the surface on the camera side. Here, the extension body 20 is bonded to the test body 10 by its surface. Therefore, as the test body 10 deforms, the extension body 20 follows the test body 10 without separating. Note that although the term "bonded" is used, this does not necessarily mean that an intervening material such as an adhesive is used, and it also includes cases where the extension body 20 is bonded in direct contact with the test body 10.

Note that the width of the expansion body 20 in the X-axis direction in Figure 3 is, for example, several times larger than that of the first subset 220 described below.

The expandable body 20 is formed from a ductile material. That is, the expandable body 20 is formed from a ductile material such as resin, rubber, elastomer, or metal, and can be formed from, for example, silicone resin (silicone sealant). Furthermore, the expandable body 20 made from a ductile material in this embodiment is isotropic, meaning it stretches evenly in any direction, and is prone to stretching regardless of a specific direction. Therefore, the expandable body 20 can naturally stretch in any direction in response to deformation in any direction of the test specimen 10 during strength testing. However, even if the expandable body 20 has some anisotropy, it can be applied to testing in the same way as this embodiment, as long as the displacement amount described below can be calculated.

In this embodiment, the rigidity of the expandable body 20 is lower than the rigidity of the test body 10. The rigidity of the expandable body 20 is, for example, 1% or less of the rigidity of the test body 10. As a result, the rigidity of the expandable body 20 is sufficiently smaller than the rigidity of the test body 10, so the test body 10, which deforms during a strength test, is less affected by the rigidity of the expandable body 20, and this effect can be kept within the error range of the strength test. Therefore, the strength of the test body 10 measured in a strength test is not affected by the strength of the expandable body 20, making it possible to accurately measure the strength of the test body 10.

Furthermore, the extension body 20 attached to the end 12a of the test specimen 10 is free-standing within a plane 20a perpendicular to the optical axis direction of the camera. Therefore, the plane 20a of the extension body 20 extends toward the end 12a almost perpendicular to the optical axis direction of the camera. The shape and thickness of the extension body 20, as well as the ductile material forming the extension body 20, are appropriately adjusted so that the extension body 20 can be free-standing within the plane 20a.

[Random pattern formation step S114]
The random pattern forming step S114 is a step of forming a random pattern on the surface 10a (front surface) of the test body 10 to which the expansion body 20 is attached, and on the surface 20a (front surface) of the expansion body 20. Figure 4 is a process diagram illustrating the random pattern forming step S114. As shown in Figure 4, in the random pattern forming step S114, a random pattern 30 is formed on the surfaces 10a, 20a of the test body 10 to which the expansion body 20 is attached. The random pattern 30 is a random design.

As described above, in this embodiment, the extension body 20 is added to the end 12a of the parallel portion 12 so that the parallel portion 12 and the extension body 20 are flush or nearly flush with each other on the surfaces 10a, 20a. Therefore, the random pattern 30 is formed evenly from the parallel portion 12 to the extension body 20.

In the random pattern forming step S114, the random pattern 30 may be formed on the surfaces 10a and 20a by spray coating. In addition, in the random pattern forming step S114, the random pattern 30 may be formed on the surfaces 10a and 20a by applying a screen tone on which the random pattern 30 is printed.

In this way, by performing the addition step S112 and the random pattern formation step S114, a specimen 100 is manufactured, which includes a test piece 10, an extension body 20 made of a ductile material that is added to the end 12a of the test piece 10 within a plane 10a perpendicular to the optical axis direction of the camera that images the test piece 10 using the digital image correlation method, and a random pattern 30 formed on the surface 10a of the test piece 10 and the surface 20a of the extension body 20.

[First imaging step S120]
The first imaging step S120 is a step of imaging the specimen 100 with a camera. FIG. 5 is a process diagram illustrating the first imaging step S120 and the second imaging step S140. As shown in FIG. 5, in the first imaging step S120 and the second imaging step S140 described later, the specimen 100 is placed in front of the camera 50 so that the direction of the optical axis 52 of the camera 50 (the Y-axis direction in FIG. 5) is perpendicular to the surfaces 10a, 20a of the specimen 100 on which the random pattern 30 is formed. At this time, the camera 50 is focused on the surface 10a of the specimen 100.

Then, the camera 50 captures an image of the specimen 100 (random pattern 30).

[Deformation step S130]
The deformation step S130 is a step of deforming the specimen 100. In the deformation step S130 of this embodiment, a tensile test is performed by a strength tester. For example, the grip portions 14a and 14b of the specimen 100 are held by the strength tester. Then, the strength tester pulls the grip portions 14a and 14b in a direction separating them from each other.

[Second imaging step S140]
The second imaging step S140 is a step of imaging the test piece 10 (specimen 100) deformed in the deformation step S130 using the camera 50.

[Subset setting step S150]
The subset setting step S150 is a step of setting a first subset in the first image obtained in the first imaging step S120.

Figure 6 is a process diagram illustrating the subset setting process S150. Note that in Figure 6, the first subset 220 is shown relatively large to facilitate understanding. Also, in Figure 6, the random pattern 30 is shown cross-hatched.

As shown in FIG. 6, in the subset setting step S150, a first subset 220 is first set in the first image 210 (digital image) acquired in the first imaging step S120. The first subset 220 is an area of a predetermined size set on the first image 210. The first subset 220 is, for example, a square area of 21 pixels by 21 pixels.

The first subset 220 is set so that the random pattern 30 is included throughout the entire first subset 220. Furthermore, multiple first subsets 220 are set in the first image 210. The multiple first subsets 220 may be set so that they do not overlap with each other, or so that they partially overlap. The multiple first subsets 220 are set, for example, so that the entire area of the first image 210 that corresponds to the parallel portion 12 is covered.

[Subset Identification Step S160]
The subset identification step S160 is a step of identifying a second subset corresponding to the first subset 220 in the second image obtained in the second imaging step S140. Figure 7 is a process diagram illustrating the subset identification step S160. Note that in Figure 7, the second subset 230 is shown relatively large for ease of understanding. Also, in Figure 7, the random pattern 30 is shown cross-hatched.

As shown in FIG. 7, in the subset identification process S160, a second subset 230 is identified in the second image 212 (digital image) acquired in the second imaging process S140, which has the highest correlation with the random pattern 30 (design) of the first subset 220 set on the first image 210. Since various existing technologies can be applied to identify the second subset 230 corresponding to the first subset 220, detailed explanations will be omitted here.

[Calculation step S170]
The calculation step S170 is a step of calculating the amount of displacement between a corresponding representative point P of the first subset 220 and a corresponding representative point P of the second subset 230. The representative point P of the first subset 220 is, for example, the center point (center of gravity) of the first subset 220. Similarly, the representative point P of the second subset 230 is, for example, the center point (center of gravity) of the second subset 230.

Then, the displacement behavior of the entire test body 10 is analyzed based on the amount of displacement between the representative point P of each of the multiple first subsets 220 and the representative point P of each of the multiple second subsets 230 corresponding to each of the multiple first subsets 220.

[Setting of the first subset 220 in the specimen of the comparative example]
8 is a diagram illustrating a first image 250 acquired by imaging a specimen of the comparative example. The specimen of the comparative example does not include the expansion body 20. In other words, the specimen of the comparative example is composed of only the test body 10.

As described above, the first subset 220 is set so that the random pattern 30 is included throughout the entire area of the first subset 220. Therefore, as shown in Figure 8, when attempting to set the first subset 220 in the first image 250 acquired by capturing an image of the comparative example specimen (test piece 10), the first subset 220 cannot be set at a location where the representative point P is located at the end 12a of the parallel portion 12.

Specifically, if the first subset 220 is set at a location where the representative point P is located at the end 12a of the parallel portion 12, a portion of the first subset 220 will not fit within the test piece 10. In this case, the area of the first subset 220 that extends beyond the test piece 10 will not have a random pattern 30. Hereinafter, the area of the first subset 220 that does not have a random pattern 30 may be referred to as a "blank."

Therefore, if the first subset 220 is set at a location where the representative point P is located at the end 12a of the parallel portion 12, and at a location where the representative point P is located within a distance L from the end 12a of the parallel portion 12, a blank will be included within the first subset 220. Note that the distance L is half the width of the first subset 220 when the representative point P is the center point.

If there is a blank in the first subset 220, it will be impossible to identify the second subset 230 with which a correlation can be obtained in the second image acquired in the second imaging process S140.

For this reason, for the comparative example specimen (specimen 10 only), it was not possible to calculate the displacement at end 12a of parallel portion 12 and within the range of distance L from end 12a of parallel portion 12.

Figure 9 is a diagram illustrating a first image 210 obtained by imaging the specimen 100 according to this embodiment. As shown in Figure 9, the specimen 100 according to this embodiment has an extension 20 extending from the end 12a of the specimen 10.

Therefore, when the first subset 220 is set at a location where the representative point P is located at the end 12a of the parallel portion 12 and at a location where the representative point P is located within a distance L from the end 12a of the parallel portion 12, the first subset 220 is set in the first image 210 so as to span both the surface 10a of the test body 10 and the surface 20a of the extension body 20.

As described above, in the specimen 100 according to this embodiment, the random pattern 30 is formed across both the surface 10a of the specimen 10 and the surface 20a of the expansion body 20. Therefore, even if the first subset 220 is set at a location where the representative point P is located at the end 12a of the parallel portion 12 and at a location where the representative point P is located within the distance L from the end 12a of the parallel portion 12, it is possible to avoid the inclusion of blanks within the first subset 220. In other words, it is possible to fill the first subset 220 with the random pattern 30 formed on the surface 10a of the specimen 10 and the random pattern 30 formed on the surface 20a of the expansion body 20.

Therefore, when using the specimen 100 according to this embodiment, it is possible to easily identify the second subset 230 that exhibits a high correlation in the second image 212 acquired in the second imaging step S140.

Therefore, with the specimen 100 according to this embodiment, it is possible to calculate the displacement of the end 12a of the parallel portion 12 and within a distance L from the end 12a of the parallel portion 12.

As described above, the digital image correlation method according to this embodiment uses a specimen 100 comprising a test piece 10, an extension body 20, and a random pattern 30. This makes it possible for the digital image correlation method according to this embodiment to calculate the amount of displacement of the end 12a of the test piece 10.

Furthermore, as described above, the expandable body 20 is made of a ductile material. This allows the expandable body 20 to follow the deformation of the surface (face 10a) of the test body 10 and deform to the same extent as the deformation of the test body 10. Therefore, the deformation behavior of the random pattern 30 formed on the surface (face 10a) of the test body 10 and the random pattern 30 formed on the surface (face 20a) of the expandable body 20 is substantially the same. This makes it possible to obtain a high correlation between the first subset 220 of the first image 210 and the second subset 230 of the second image 212.

Furthermore, as described above, the random pattern formation process S114 is performed after the addition process S112. This allows the random pattern 30 to be formed continuously on the surface 10a of the test body 10 and the surface 20a of the expansion body 20.

Furthermore, as described above, the extension body 20 is configured to be self-supporting within a plane 20a perpendicular to the optical axis 52 direction of the camera 50. This allows the camera 50 to capture images of the random pattern 30 formed on the plane 20a of the extension body 20 with high accuracy.

Although the embodiments have been described above with reference to the accompanying drawings, it goes without saying that the present disclosure is not limited to the above-described embodiments. It is clear that a person skilled in the art could conceive of various modifications or alterations within the scope of the claims, and it is understood that these naturally fall within the technical scope of the present disclosure.

For example, in the above-described embodiment, a tensile test was used as an example of a strength test. However, the strength test may be any test that deforms the test specimen 10. For example, static tests such as compression tests and bending tests, or dynamic tests such as fatigue tests, can be suitably applied as strength tests. Furthermore, shear tests or torsion tests may also be applied as strength tests. Furthermore, the present invention is not limited to strength tests, and various methods for measuring the displacement of a deforming test specimen can also be applied.

Furthermore, in the above embodiment, the test specimen 10 has a flat plate shape. However, there are no limitations on the shape of the test specimen 10.

Figure 10 is a diagram illustrating a test body 300 according to a modified example. As shown in Figure 10, the test body 300 is cylindrical, for example. Even in this case, the extension body 20 is attached to the end 300a (edge) of the test body 300 in a plane (the XZ plane in Figure 10) perpendicular to the optical axis direction of the camera 50 (the Y axis direction in Figure 10). Note that in this modified example, the camera 50 is focused on the boundary 24 between the test body 300 and the extension body 20 in the first imaging step S120 and the second imaging step S140. This allows the displacement of the end 300a of the test body 300 to be calculated with high accuracy.

In the above embodiment, an example was given in which the expansion body 20 is attached to the entire end 12a of the parallel portion 12. However, the expansion body 20 may also be attached to a portion of the end 12a of the parallel portion 12. Furthermore, the expansion body 20 may also be attached to all of the ends around the entire circumference of the test body 10, or to a portion of the end of the test body 10.

Furthermore, in the above embodiment, an example was given in which the expansion body 20 is added so that the surface 10a of the test body 10 and the surface 20a of the expansion body 20 are flush or nearly flush with each other. However, the surface 20a of the expansion body 20 may be offset from the surface 10a of the test body 10 to the extent that the displacement of the test body 10 can be calculated.

Furthermore, the thickness of the expandable body 20 may be less than the thickness of the test body 10, or may be greater than or equal to the thickness of the test body 10. In either case, the expandable body 20 only needs to be thick enough to be able to stand on its own.

Furthermore, in the above embodiment, an example was given in which the random pattern 30 was formed after the expansion body 20 was added to the test body 10. However, it is also possible to form the random pattern 30 on the test body 10, form the random pattern 30 on the expansion body 20, and then add the expansion body 20 to the test body 10. In other words, the expansion body 20 on which the random pattern 30 was formed may be added to the test body 10 on which the random pattern 30 was formed.

In the above embodiment, the first subset 220 and the second subset 230 are square regions. However, the first subset 220 and the second subset 230 may be regions consisting of multiple pixels that are smaller than the first image 210 and the second image 212, and there are no limitations on their shapes. The first subset 220 and the second subset 230 may be, for example, rectangular, or polygonal, such as triangular, pentagonal, or hexagonal, or may be circular.

Furthermore, in the above embodiment, an example was given in which the subset setting process S150 is executed after the second imaging process S140. However, there are no limitations on the timing at which the subset setting process S150 is executed, as long as it is executed after the first imaging process S120.

Furthermore, the deformation step S130 and the second imaging step S140 may be repeated multiple times.

Furthermore, in the subset setting step S150 of the above embodiment, an example was given in which the image obtained by capturing an image of the test specimen 10 in its initial state (an undistorted test specimen 10) is used as the first image 210 to set the first subset 220. However, if the deformation step S130 and the second imaging step S140 are performed multiple times, the first subset 220 may be set using an image captured before the execution of one deformation step S130 as the first image, and the second subset 230 may be identified using an image captured after the execution of that one deformation step S130 as the second image. In other words, the first image may be obtained by capturing an image of the deformed test specimen 10.

Furthermore, when the test specimen 10 is continuously deformed, the image acquired at time t1 may be the first image, and the image acquired at time t2 after time t1 may be the second image.

This disclosure can contribute, for example, to Sustainable Development Goal (SDG) Goal 9: "Build resilient infrastructure, promote sustainable industrialization and foster innovation" and Goal 12: "Ensure sustainable consumption and production patterns."

S112 Addition step S114 Random pattern formation step S120 First imaging step S130 Deformation step S140 Second imaging step S150 Subset setting step S160 Subset identification step S170 Calculation step 10 Test body 10a Surface 12a End 20 Extension body 20a Surface 30 Random pattern 50 Camera 52 Optical axis 100 Test body 210 First image 212 Second image 220 First subset 230 Second subset 300 Test body

Claims (3)

1. A digital image correlation method for measuring displacement of a deforming specimen, comprising:
an attachment step of attaching an extension body made of a ductile material to an end of the test specimen in a plane perpendicular to the optical axis direction of the camera;
a random pattern forming step of forming a random pattern on the surface of the test body to which the expansion body has been added and on the surface of the expansion body after the addition step is performed;
a first imaging step of imaging the test body to which the extension body is attached using the camera;
a deformation step of deforming the test specimen to which the expansion body is attached;
a second imaging step of imaging the test body deformed in the deformation step with the camera;
a subset setting step of setting a first subset so as to span both the surface of the test body and the surface of the extension body in a first image obtained in the first imaging step;
a subset identifying step of identifying a second subset corresponding to the first subset in a second image obtained in the second imaging step;
a calculation step of calculating a displacement between a representative point of the first subset and a representative point of the second subset that correspond to each other;
A digital image correlation method, including: The digital image correlation method of claim 1, wherein the camera is focused on the boundary between the test object and the extension object during the first and second imaging steps. The digital image correlation method according to claim 1 or 2 , wherein the extension body added to the end of the test piece in the adding step is self-supporting in a plane perpendicular to the optical axis direction of the camera.