Strain distribution acquisition method

Abstract

To provide a method of acquiring a strain distribution generated in an object to be measured by the use of a digital image correlation method (DIC), wherein a pattern that the SIC can recognize can be formed on the object to be measured safely no matter how unexperienced a worker is.SOLUTION: A strain distribution acquisition method includes: a first process S1 of forming a pattern 2 that a digital image correlation method (DIC) can recognize on an object 1 to be measured; a second process S2 of acquiring a reference image of the object 1 to be measured, the reference image being an image of the pattern 2 before a load 7 is placed on the object 1 to be measured; a third process S3 of acquiring an image of the pattern 2 of the object 1 to be measured after the load is placed on the object 1 to be measured; and a fourth process S4 of acquiring a strain distribution of the object 1 to be measured from the reference image and the image acquired in the third process S3. In the first process S1, the pattern 2 formed on a master pattern member 3 is transferred to the object 1 to be measured so as to form the pattern 2 on the object 1 to be measured.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for acquiring a strain distribution occurring in a measurement object using a digital image correlation (DIC) method. [Background technology]

[0002] Strain gauges are commonly used to measure strains occurring in mechanical structures when evaluating the strength of railway vehicles, construction machinery, etc. Measurements using strain gauges can be completed simply by adhering the strain gauge to the surface of the object being measured, and the strain detection accuracy is 5.0 x 10 -4 ~1.0×10 -3 %, and the limiting measurable frequency is also high at around several kHz. On the other hand, measurements using strain gauges can only measure the strain at the position where the strain gauge is installed. For this reason, when measuring the strain distribution of an object, it is necessary to attach many strain gauges to the object. Measurements using strain gauges have the disadvantages of requiring the time-consuming task of attaching many strain gauges to the object, and of only being able to measure discrete strain distributions. Therefore, a technology is needed to measure strain distributions without using strain gauges.

[0003] In recent years, digital image correlation (hereinafter referred to as "DIC") has become a popular technique for measuring strain distribution. When measuring strain distribution using DIC, a pattern that can be recognized by DIC is formed on the object to be measured, and the amount of deformation of the pattern is determined from images of the object before and after a load is applied. The distribution of strain generated on the surface of the object due to the load is then derived from the amount of deformation of the pattern. Measuring strain distribution using DIC has the advantage of being able to derive the strain distribution from images of the object without having to attach numerous strain gauges to the object, thereby reducing the workload and enabling strain distribution measurement at low cost.

[0004] Examples of conventional methods for measuring strain distribution using DIC are described in Patent Documents 1 and 2. Patent Document 1 describes a displacement measurement device that includes an acquisition unit that acquires an image of a random pattern drawn on a specimen (test piece) before deformation, a calculation unit that calculates the size of spots included in the random pattern based on the image before deformation acquired by the acquisition unit, and a determination unit that determines the size of a subset to be used in analysis using digital image correlation based on the size of the spots calculated by the calculation unit. Patent Document 2 describes a degradation diagnosis method for irradiated materials that includes a load application step of applying a load to an irradiated material, which is an evaluation object, a marking pattern image capture step of capturing an image of a marking pattern applied to the surface of the evaluation object simultaneously with the load application step, and a deformation and strain calculation step of calculating the deformation and strain on the surface of the evaluation object from changes in the marking pattern image. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-3234 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-203685 Summary of the Invention [Problem to be solved by the invention]

[0006] Patterns that can be recognized by DIC (for example, random patterns) are generally formed by spraying (spray coating) paint onto the object to be measured, as described in Patent Documents 1 and 2. For example, random patterns can be formed by spraying white paint onto the object to be measured to form a white base, and then spraying black paint onto the object to form black dots, or conversely, by forming a black base and then forming white dots.

[0007] With this type of spray application, it is difficult to form dots with uniform density, and if the worker's skill level is low, a completely random pattern will not be formed on the object being measured, raising concerns that this will reduce the accuracy of pattern recognition by DIC and the accuracy of strain distribution acquisition. Furthermore, spraying paint in manufacturing and maintenance sites exposes workers to volatile organic compounds, raising concerns about health issues for the workers. For this reason, particularly in manufacturing and maintenance sites, there is a need for a method that can safely form high-quality patterns that can be recognized by DIC on objects being measured, regardless of the worker's skill level.

[0008] The object of the present invention is to provide a method for acquiring the strain distribution occurring in a measurement object using digital image correlation (DIC), which can safely form a pattern recognizable by DIC on the measurement object without depending on the skill level of the operator. [Means for solving the problem]

[0009] A strain distribution acquisition method according to the present invention includes a first step of forming a pattern recognizable by a digital image correlation method on a measurement object, a second step of acquiring a reference image of the measurement object, where the image of the pattern before a load is applied to the measurement object is a reference image, a third step of acquiring an image of the pattern on the measurement object after the load is applied to the measurement object, and a fourth step of acquiring a strain distribution of the measurement object from the reference image and the image acquired in step 3. In the first step, the pattern formed on a parent pattern member is transferred to the measurement object to form the pattern on the measurement object. [Effects of the Invention]

[0010] According to the present invention, a method for acquiring the strain distribution occurring in a measurement object using digital image correlation (DIC) can be provided, which can safely form a pattern recognizable by DIC on the measurement object without depending on the skill level of the operator. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a flowchart showing steps of a strain distribution acquisition method according to a first embodiment of the present invention. [Figure 2A] 10A and 10B are diagrams illustrating a state in which a pattern is formed on a parent pattern member in step S1 in Example 1. FIG. [Figure 2B] FIG. 2 is a diagram showing a parent pattern member on which a pattern is formed in Example 1. [Figure 3] 2 is a diagram schematically showing step S1 of forming a pattern 2 on a measurement object 1 in Example 1. FIG. [Figure 4A] 10 is a diagram schematically illustrating step S2 of acquiring a reference image of the measurement object in the first embodiment. FIG. [Figure 4B] FIG. 10 is a diagram schematically illustrating step S3 of acquiring an image of a pattern after a load is applied to the measurement object in the first embodiment. [Figure 5] FIG. 2 is a diagram schematically illustrating an example of the strain distribution generated on the surface of the measurement object, output by a computer in Example 1. [Figure 6A] FIG. 10 is a diagram schematically illustrating step S2 of acquiring a reference image of the measurement object in the second embodiment of the present invention. [Figure 6B] FIG. 10 is a diagram schematically illustrating step S3 of acquiring an image of a pattern after a load is applied to the measurement object in Example 2 of the present invention. [Figure 7] 10 is a flowchart showing steps of a strain distribution acquisition method according to a third embodiment of the present invention. [Figure 8] FIG. 11 is a diagram schematically illustrating step S1 of forming patterns on a plurality of measurement targets in Example 3. [Figure 9] FIG. 10 is a diagram schematically showing step S1 of forming a plurality of identical patterns on one measurement object in Example 3 of the present invention. [Figure 10] 10 is a flowchart showing steps of a strain distribution acquisition method and steps of a method for estimating a bolt axial force according to a fourth embodiment of the present invention. [Figure 11]FIG. 10 is a diagram schematically illustrating step S1 of forming a pattern on the top surface of the bolt in Example 4. [Figure 12A] FIG. 13 is a diagram schematically illustrating step S2 of acquiring a reference image of the bolt head surface in the fourth embodiment. [Figure 12B] FIG. 11 is a diagram schematically illustrating step S3 of acquiring an image of the pattern after a load is applied to the top surface of the bolt in Example 4. [Figure 13] FIG. 10 is a diagram schematically illustrating an example of the strain distribution generated on the bolt head surface due to axial force, output by a computer in Example 4. [Figure 14A] FIG. 10 is a diagram showing an example of a finite element model used in stress analysis to obtain strain distribution on the bolt head surface in Example 4. [Figure 14B] FIG. 11 is a diagram showing an example of an evaluation line indicating the position at which strain is displayed on the bolt head surface in Example 4. [Figure 14C] FIG. 10 is a diagram showing an example of the magnitude of distortion in an evaluation line in Example 4. [Figure 15] FIG. 10 is a diagram schematically showing the relationship between minimum point strain and bolt axial force in Example 4. [Figure 16] FIG. 10 is a diagram showing a method for calculating the axial force of a bolt in the fourth embodiment. [Figure 17] 10 is a flowchart showing steps of a strain distribution acquisition method according to a fifth embodiment of the present invention. [Figure 18] FIG. 13 is a diagram schematically illustrating step S1 of forming patterns on the top surfaces of a plurality of bolts in Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0012] The strain distribution acquisition method according to the present invention uses digital image correlation (DIC) to acquire the distribution of strain occurring in a measurement object with high accuracy and high efficiency. In the present invention, a pattern formed on a parent pattern member is transferred to the measurement object using, for example, paint. Therefore, unlike methods of forming a pattern by spraying paint onto the measurement object (spray coating), a pattern can be formed on the measurement object without relying on the skill level of the operator. Furthermore, even if there are multiple measurement objects, the same pattern can be formed on multiple measurement objects using a single parent pattern member. Furthermore, since the present invention does not use spray coating, a pattern can be formed on the measurement object safely without posing a health risk to the operator. Since the strain distribution acquisition method according to the present invention can safely form a pattern on the measurement object without relying on the skill level of the operator, it can also be used in manufacturing and maintenance sites.

[0013] A strain distribution acquisition method according to an embodiment of the present invention will be described below with reference to the drawings. In the following description, digital image correlation will be referred to as "DIC." In the following examples, patterns that can be recognized by DIC include not only random patterns but also patterns that can vary in brightness, such as irregular grid patterns and partially regular patterns. A random pattern is an irregular pattern that is drawn by changing the brightness of the object to be measured so that an image can be captured that allows the position of the surface of the object to be accurately determined. In the drawings used in this specification, identical or corresponding components are designated by the same reference numerals, and repeated description of these components may be omitted. [Example]

[0014] A strain distribution acquisition method according to a first embodiment of the present invention will be described with reference to Figs. 1 to 5. In this embodiment, an example will be described in which a pattern recognizable by DIC is formed on a measurement object, and the distribution of strain generated on the surface of the measurement object is acquired. The measurement object is an object in which strain occurs on the surface when a load is applied.

[0015] 1 is a flowchart showing the steps of the strain distribution acquisition method according to this embodiment. An overview of the steps constituting the strain distribution acquisition method according to this embodiment will be explained using FIG.

[0016] In step S1, a pattern that can be recognized by DIC is formed on the measurement object by transferring the pattern using a parent pattern member, which will be described later.

[0017] In step S2, a reference image of the object to be measured is acquired. The reference image is an image of the pattern that serves as a reference when acquiring the strain distribution using DIC, and is an image of the pattern before a load is applied to the object to be measured.

[0018] In step S3, an image of the pattern of the measurement object after the load is applied to the measurement object is acquired.

[0019] In step S4, the amount of deformation of the pattern is calculated from the reference image acquired in step S2 and the image acquired in step S3, the distribution of strain generated on the surface of the measurement object is derived, and the strain distribution of the measurement object is obtained.

[0020] In step S5, the strain distribution obtained in step S4 is output.

[0021] Each step will be described in detail below.

[0022] 2A and 2B are diagrams schematically showing a parent pattern member 3 used to transfer a pattern to a measurement object in step S1. Fig. 2A is a diagram showing how a pattern 2 is formed on the parent pattern member 3. Fig. 2B is a diagram showing the parent pattern member 3 on which the pattern 2 has been formed.

[0023] The parent pattern member 3 is a member different from the object to be measured and can be made of any member. The parent pattern member 3 has a printing surface 4 on its surface, on which the pattern 2 is formed. The printing surface 4 is made of any material that can be used to process the pattern 2, such as silicone rubber, natural rubber, synthetic rubber, polymer synthetic resin, and acrylic resin. The parent pattern member 3 can have any shape, such as a plate (thin plate or thick plate), a cylindrical shape, or a prismatic shape.

[0024] A pattern 2 is formed on the printing surface 4 of the parent pattern member 3 by processing using, for example, laser light. The pattern 2 is a pattern that can be recognized by DIC. The pattern 2 can be formed on the printing surface 4 by any method, not limited to processing using laser light. For example, the pattern 2 can be formed on the printing surface 4 by mechanical processing or the like. Furthermore, the parent pattern member 3 including the printing surface 4 can be formed all at once using, for example, a 3D printer.

[0025] 3 is a diagram schematically illustrating step S1 of forming a pattern 2 on the measurement object 1. In step S1, the pattern 2 formed on the printing surface 4 of the parent pattern member 3 is transferred to the measurement object 1, thereby forming the pattern 2 on the measurement object 1. More specifically, the paint applied to the pattern 2 formed on the printing surface 4 of the parent pattern member 3 is transferred to the measurement object 1, thereby forming the pattern 2 on the measurement object 1.

[0026] First, paint is applied to the pattern 2 formed on the printing surface 4 of the parent pattern member 3. The application of paint can be performed by any method. Next, the printing surface 4 of the parent pattern member 3 is pressed against the object 1 to be measured (for example, by pressing the parent pattern member 3 against the object 1 as if pressing a stamp), thereby transferring the pattern 2 to the object 1 to be measured. The work of applying paint to the pattern 2 of the parent pattern member 3 and the work of transferring the pattern 2 formed on the printing surface 4 of the parent pattern member 3 to the object 1 to be measured may be performed by an operator or by a device such as a robot.

[0027] Pattern 2 of parent pattern member 3 is transferred to the position on measurement object 1 where strain distribution is desired to be obtained. For example, if measurement object 1 is a bolt, pattern 2 is transferred to the top surface of the bolt, which is the position where strain distribution is desired to be obtained.

[0028] The paint to be applied to the pattern 2 of the parent pattern member 3 can be any transferable paint, such as stamp ink, ink used in printing or writing tools, paint, organic paint, pigment, dye, etc. The paint may be liquid or powder.

[0029] The paint color is preferably one color, and is preferably black or white depending on the color (or brightness) of the base of the measurement object 1. If the measurement object 1 does not have a gloss, it is sufficient to transfer a black or white pattern 2 to the measurement object 1, but if the measurement object 1 has a gloss, it is necessary to form a base on the measurement object 1. The base can be formed on the measurement object 1 by the above-mentioned transfer method, or by applying any paint with a brush or roller, etc.

[0030] 4A is a diagram schematically illustrating step S2 of acquiring a reference image of the measurement object 1. In step S2, the camera 5 captures an image of the pattern 2 before a load is applied to the measurement object 1 as a reference image. The captured reference image is stored in the computer 6.

[0031] 4B is a diagram schematically illustrating step S3 of acquiring an image of pattern 2 after load 7 is applied to measurement object 1. In step S3, camera 5 captures an image of pattern 2 after load 7 is applied to measurement object 1. The captured image is stored in computer 6.

[0032] In this embodiment, one camera 5 is used to capture images of pattern 2 in steps S2 and S3. Capturing images with one camera 5 reduces the number of devices required to obtain the strain distribution of the measurement object 1, making it possible to obtain the strain distribution at lower cost.

[0033] In steps S2 and S3, it is desirable to photograph the object 1 under fixed photographing conditions (for example, a fixed photographing distance) with one camera 5 facing directly in front of the object 1. However, even if the photographing conditions are changed, an image equivalent to that obtained under fixed photographing conditions can be obtained by having the computer 6 perform image processing according to the photographing conditions.

[0034] The camera 5 and the computer 6 may be connected to each other by wire or wirelessly. For example, the computer 6 may be placed at the site where the measurement target 1 is installed and connected to the camera 5 by wire. Also, for example, the computer 6 located away from the camera 5 may be connected to the camera 5 wirelessly.

[0035] Step S4 starts when a reference image is captured in step S2 and an image of pattern 2 is captured after load 7 is applied to measurement object 1 in step S3. In step S4, computer 6 performs image processing using DIC on these images to calculate the amount of deformation of pattern 2 (i.e., the amount of deflection on the surface of measurement object 1) and derive the distribution of strain occurring on the surface of measurement object 1. Computer 6 calculates the amount of deformation of pattern 2 using a known method using DIC and derives the strain distribution on the surface of measurement object 1.

[0036] In step S5, the computer 6 outputs the strain distribution on the surface of the measurement object 1 acquired in step S4. The computer 6 outputs the strain distribution to a display device included in the computer 6 or a display device connected to the computer 6. The computer 6 can also output the strain distribution to a storage device included in the computer 6 or a storage device connected to the computer 6.

[0037] FIG. 5 is a diagram showing a schematic example of the strain distribution generated on the surface of the measurement object 1, output by the computer 6. The upper part of FIG. 5 shows an example of the intensity distribution 20 of the strain generated on the surface of the measurement object 1, and an example of an evaluation line 8 indicating the position where the strain is displayed on the measurement object 1. The lower part of FIG. 5 shows an example of the magnitude of the strain on the evaluation line 8 (strain distribution along the evaluation line 8). In the strain intensity distribution 20, the sign of the strain (+ indicates tensile strain, - indicates compressive strain) and the magnitude of the strain can be indicated by color contours. In the example shown in FIG. 5, the magnitude of the strain is the same regardless of position, so the strain intensity distribution 20 remains a constant color and does not change.

[0038] A tensile load is applied to the measurement object 1 in the in-plane direction of the measurement object 1 as a load 7 (FIG. 4B), and tensile strain is generated by this load 7. A strain intensity distribution 20 is acquired at the position where the pattern 2 is formed. In the example shown in FIG. 5, it can be seen that a tensile strain of uniform magnitude is generated over the entire surface of the measurement object 1 at the position where the pattern 2 is formed.

[0039] By setting an evaluation line 8 as shown in FIG. 5, the relationship between the position on the measurement object 1 and the magnitude of the strain can be expressed in a graph (as shown in the lower part of FIG. 5).

[0040] As described above, in the strain distribution acquisition method according to this embodiment, by transferring the pattern 2 formed on the parent pattern member 3 to the object to be measured 1, a pattern 2 that can be recognized by DIC can be formed on the object to be measured 1 safely and without depending on the skill level of the operator. [Example]

[0041] A strain distribution acquisition method according to a second embodiment of the present invention will be described with reference to Figures 6A and 6B. In this embodiment, an example will be described in which multiple cameras 5 are used to capture images of pattern 2 in steps S2 and S3. Below, differences between the strain distribution acquisition method according to this embodiment and the strain distribution acquisition method according to the first embodiment will be mainly described.

[0042] Fig. 6A is a diagram schematically illustrating step S2 of acquiring a reference image of the measurement object 1 in this embodiment. Fig. 6B is a diagram schematically illustrating step S3 of acquiring an image of the pattern 2 after a load 7 is applied to the measurement object 1 in this embodiment. Figs. 6A and 6B show an example in which two cameras 5 are used. The two cameras 5 are fixed to a fixture 5a and capture images.

[0043] A load 7 is applied to the measurement object 1 shown in FIG. 6B, causing out-of-plane deformation (deformation occurring in a direction perpendicular to the surface) as indicated by the arrows.

[0044] When an image is captured with one camera 5 as in Example 1, only a two-dimensional image of the measurement object 1 can be acquired, and information in the depth direction (the up and down direction in FIGS. 6A and 6B ) cannot be obtained. For this reason, for example, when the measurement object 1 undergoes out-of-plane deformation and the shooting distance changes, it is necessary to measure the amount of change in the shooting distance using a laser displacement meter or the like, correct the image according to the shooting distance, and then acquire the strain distribution.

[0045] In this embodiment, two cameras 5 are used, so a three-dimensional image of the measurement object 1 can be acquired. Even if the measurement object 1 undergoes out-of-plane deformation, there is no need to measure the change in shooting distance, etc., and the strain distribution can be easily acquired using DIC.

[0046] 6A and 6B show an example in which two cameras 5 are used, but three or more cameras 5 may be used to capture images of the pattern 2. By using multiple cameras 5, a more accurate three-dimensional image of the measurement object 1 can be obtained, and a more accurate strain distribution can be obtained. [Example]

[0047] A strain distribution acquisition method according to a third embodiment of the present invention will be described with reference to Fig. 7 to Fig. 9. In this embodiment, an example will be described in which there are a plurality of measurement objects 1 and the strain distributions occurring on the surfaces of the plurality of measurement objects 1 are acquired. Below, the differences between the strain distribution acquisition method according to this embodiment and the strain distribution acquisition method according to the first embodiment will be mainly described.

[0048] FIG. 7 is a flowchart showing the steps of the strain distribution acquisition method according to this embodiment.

[0049] In step S1, a pattern 2 recognizable by DIC is formed on a plurality of measurement objects 1. As described in Example 1, the pattern 2 is formed on a plurality of measurement objects 1 by transferring it using a parent pattern member 3. The pattern 2 is transferred to a plurality of measurement objects 1 using one parent pattern member 3. Therefore, the patterns 2 formed on a plurality of measurement objects 1 are identical to each other.

[0050] In steps S2 to S5, processing similar to that in Example 1 is performed on only one of the multiple measurement objects 1, and the strain distribution generated on the surface of the measurement object 1 is acquired and output.

[0051] In step S8, the operator determines whether there are any measurement objects 1 among the multiple measurement objects 1 for which strain distribution has not been acquired. If there are no measurement objects 1 for which strain distribution has not been acquired (i.e., strain distributions have been acquired for all measurement objects 1), the process shown in FIG. 7 ends. If there are measurement objects 1 for which strain distribution has not been acquired, the process returns to step S3, and steps S3 to S5 are executed for the measurement objects 1 for which strain distribution has not been acquired, and strain distributions are acquired. At this time, in step S4, strain distributions of the multiple measurement objects 1 are acquired using one reference image acquired in step S2.

[0052] As described above, the same pattern 2 is formed on each of the multiple measurement objects 1. For this reason, the reference image obtained from the measurement object 1 whose strain distribution is to be acquired first can be used when acquiring the strain distribution of other measurement objects 1. In other words, step S2 for acquiring the reference image of the measurement object 1 only needs to be performed on the measurement object 1 whose strain distribution is to be acquired first, and for the measurement objects 1 whose strain distribution is to be acquired second or later, step S2 can be omitted and the strain distribution can be acquired.

[0053] 8 is a diagram schematically illustrating step S1 of forming patterns 2 on a plurality of measurement objects 1. In step S1, patterns 2 formed on the stamping surface 4 of the parent pattern member 3 are transferred to each of the plurality of measurement objects 1, thereby forming patterns 2 on the plurality of measurement objects 1. The same patterns 2 are formed on the plurality of measurement objects 1.

[0054] Note that the measurement object 1 may be provided with an identifier 1a for distinguishing between multiple measurement objects 1. The identifier 1a is depicted using, for example, numbers, letters, symbols, barcodes, or two-dimensional codes. Because there is a concern that the accuracy of strain detection may be reduced, it is preferable that the pattern 2 be formed on the measurement object 1 so as not to overlap with the identifier 1a. However, the pattern 2 may overlap with the identifier 1a if it is located in a position where a reduction in the accuracy of strain detection is not an issue or where it is not necessary to acquire a strain distribution.

[0055] Furthermore, the parent pattern member 3 having the pattern 2 formed on the seal surface 4 can be used repeatedly until the state of the seal surface 4 or the pattern 2 changes.

[0056] In this embodiment, patterns 2 are formed on a plurality of measurement objects 1 by transfer, so that identical patterns 2 can be efficiently formed on a plurality of measurement objects 1. Therefore, in this embodiment, even if there are a plurality of measurement objects 1, it is only necessary to acquire a reference image once, and one common reference image can be used for the plurality of measurement objects 1, so that it is not necessary to acquire a reference image for all of the measurement objects 1, and therefore the work time required to acquire the strain distribution of a plurality of measurement objects 1 can be shortened.

[0057] Furthermore, in this embodiment, in step S1 in embodiment 1 or this embodiment, a plurality of identical patterns 2 can be formed on one measurement object 1. By forming a plurality of patterns 2 on one measurement object 1, it is possible to obtain the strain distribution at a plurality of locations on the measurement object 1. Because a plurality of identical patterns 2 are formed on one measurement object 1, it is possible to obtain the strain distribution at a plurality of locations on one measurement object 1 using one reference image.

[0058] 9 is a diagram schematically illustrating step S1 of forming a plurality of identical patterns 2 on one measurement object 1. By performing transfer using the parent pattern member 3 on one measurement object 1 multiple times, a plurality of identical patterns 2 can be formed on one measurement object 1. FIG. 9 shows, as an example, an example of forming two patterns 2 on one measurement object 1.

[0059] In this embodiment, the pattern 2 is formed on the measurement object 1 by transfer, so that even when multiple patterns 2 are formed on one measurement object 1, the same patterns 2 can be formed efficiently. Step S2 for acquiring a reference image only needs to be performed for one pattern 2, and for other patterns 2, step S2 can be omitted and the strain distribution can be derived using the already acquired reference image. [Example]

[0060] A strain distribution acquisition method according to a fourth embodiment of the present invention will be described with reference to Figs. 10 to 16. In this embodiment, an example will be described in which the measurement object 1 is a bolt. By forming a pattern 2 on the top surface of the bolt (hereinafter referred to as the "bolt top surface") and acquiring the strain distribution on the bolt top surface, it is possible to estimate the axial force of the bolt fastened to a fastened object. In the following explanation, explanations of configurations common to the first embodiment may be omitted.

[0061] Fig. 10 is a flowchart showing the steps of the strain distribution acquisition method and the steps of the method for estimating the bolt axial force according to this embodiment. An overview of the steps constituting the strain distribution acquisition method and the method for estimating the bolt axial force according to this embodiment will be described using Fig. 10.

[0062] In step S1, a pattern 2 that can be recognized by DIC is formed on the top surface of the bolt head.

[0063] In step S2, a reference image of the bolt head surface is acquired.

[0064] In step S3, an image is acquired after the bolt has been fastened to the workpiece, i.e., after axial force has been generated in the bolt and pattern 2 on the top surface of the bolt has been deformed.

[0065] In step S4, the amount of deformation of pattern 2 is calculated from the reference image acquired in step S2 and the image acquired in step S3, the distribution of strain generated on the top surface of the bolt is derived, and the strain distribution on the top surface of the bolt is obtained.

[0066] In step S5, the strain distribution obtained in step S4 is output.

[0067] In step S6, the axial force of the bolt is calculated and estimated from the strain distribution obtained in step S4.

[0068] In step S7, the axial force of the bolt obtained in step S6 is output.

[0069] Each step will be described in detail below, and the description of the steps described in Example 1 may be omitted.

[0070] 11 is a diagram schematically illustrating step S1 of forming a pattern 2 on a bolt top surface 10. In step S1, the pattern 2 formed on the marking surface 4 of the parent pattern member 3 is transferred to the top surface (bolt top surface 10) of the bolt 9, which is the measurement object 1, thereby forming the pattern 2 on the bolt top surface 10.

[0071] After paint is applied to the pattern 2 formed on the printing surface 4 of the parent pattern member 3, the printing surface 4 of the parent pattern member 3 is pressed against the bolt top surface 10 to transfer the pattern 2 to the bolt top surface 10. If the bolt top surface 10 does not have a glossy finish, it is sufficient to transfer the black or white pattern 2 to the bolt top surface 10; however, if the bolt top surface 10 has a glossy finish, it is necessary to form a base on the bolt top surface 10.

[0072] 12A is a diagram schematically showing step S2 of acquiring a reference image of the bolt head surface 10. In FIG. 12A, a cross section of the bolt 9 and the fastened body 13 is shown schematically so that the structure of the fastening portion of the bolt 9 can be seen.

[0073] In step S2, the camera 5 captures an image as a reference image before the bolt 9 is fastened to the workpiece 13, i.e., before the pattern 2 on the bolt head surface 10 is deformed by the axial force. The captured reference image is stored in the computer 6.

[0074] 12B is a diagram showing a schematic diagram of step S3 for acquiring an image of pattern 2 after load 7 (reaction force) is generated on the bolt head due to axial force 11. In FIG. 12B, a cross section of bolt 9 and fastened body 13 is shown in schematic form so that the structure of the fastening portion of bolt 9 can be seen. Axial force 11 is generated in bolt 9 when bolt 9 is fastened to fastened body 13.

[0075] In step S3, the camera 5 captures an image of the bolt 9 after it has been fastened to the workpiece 13, i.e., after the pattern 2 on the bolt head surface 10 has been deformed by the axial force. The captured image is stored in the computer 6.

[0076] In steps S2 and S3, it is desirable to have the camera 5 directly facing the front of the bolt head surface 10 and to photograph it under certain photographing conditions (for example, at a certain photographing distance).

[0077] Step S4 starts when a reference image is taken in step S2 and an image after deformation of pattern 2 on the bolt head surface 10 is taken in step S3. In step S4, the computer 6 performs image processing using DIC on these images to calculate the amount of deformation of pattern 2 (i.e., the amount of deflection of the bolt head surface 10) and derive the distribution of strain generated on the bolt head surface 10.

[0078] In step S5, the computer 6 outputs the strain distribution of the bolt head surface 10 acquired in step S4.

[0079] FIG. 13 is a diagram output by the computer 6, showing a schematic example of the strain distribution generated on the bolt head surface 10 due to the axial force 11. The upper part of FIG. 13 shows an example of the strain intensity distribution 20 generated on the bolt head surface 10, and an example of an evaluation line 8 indicating the position on the bolt head surface 10 where the strain is displayed. The lower part of FIG. 13 shows an example of the magnitude of the strain on the evaluation line 8 (strain distribution along the evaluation line 8). In the strain intensity distribution 20, the sign and magnitude of the strain are indicated by color contours; for example, when expressed by a color change from blue to red, blue is a negative sign and red is a positive sign, and the change in color from blue to red represents the sign and absolute value of the strain.

[0080] In the example shown in Figure 13, a strain distribution occurs on the bolt head surface 10 such that the strain is minimum at the center of the bolt head surface 10. When expressed as a two-dimensional graph, the relationship between the positions along the evaluation line 8 on the bolt head surface 10 and the magnitude of strain at these positions, i.e., the strain distribution along the evaluation line 8, resembles a downwardly convex quadratic function. Hereinafter, this strain distribution will be referred to as the "strain distribution 12 obtained by DIC."

[0081] In step S6, the computer 6 uses a known method to calculate and estimate the axial force 11 of the bolt 9 from the strain distribution obtained in step S4 (strain distribution 12 obtained by DIC). For example, as will be described in detail later, the computer 6 estimates the axial force 11 of the bolt 9 from the strain distribution obtained in step S4 (strain distribution 12 obtained by DIC) using the relationship between the strain distribution of the bolt head surface 10 obtained by stress analysis and the axial force 11 of the bolt 9.

[0082] FIG. 14A is a diagram showing an example of a finite element model (hereinafter referred to as an FE (Finite Element) model) used in a stress analysis for determining the strain distribution on the bolt head surface 10. As shown in FIG.

[0083] The FE model simulates the actual structure of the fastening part of the bolt 9, and is composed of models of the bolt 9, the fastened body 13, the washer 14, and the nut 15. In the stress analysis, this FE model was used to simulate an actual bolt fastening body, the side of the fastened body 13 was restrained, a unit axial force of 1 kN was applied to the bolt 9, and the strain distribution on the bolt head surface 10 for the applied unit axial force was calculated.

[0084] FIG. 14B is a diagram showing an example of an evaluation line 8 indicating the position where strain is displayed on the bolt head surface 10.

[0085] FIG. 14C is a diagram showing an example of the magnitude of strain on evaluation line 8 (strain distribution along evaluation line 8). When expressed as a two-dimensional graph, the strain distribution along evaluation line 8 resembles a downwardly convex quadratic function, with the strain being minimum at the center of bolt head surface 10. Hereinafter, this strain distribution will be referred to as "strain distribution 16 obtained by stress analysis," and the minimum strain will be referred to as "minimum point strain 17."

[0086] Note that the axial force 11 generated by tightening the bolt 9 causes a depression in the center of the bolt head surface 10, which changes the shooting distance of the camera 5. Therefore, when images are captured using a single camera 5 as in this embodiment, the strain distribution 12 determined by DIC includes an apparent strain distribution that occurs due to changes in the shooting distance. Therefore, it is preferable to correct the strain distribution 16 determined by stress analysis to take into account the apparent strain distribution that occurs due to changes in the shooting distance. However, as described in Example 2, when images are captured using multiple cameras 5, correction due to changes in the shooting distance is not necessary.

[0087] FIG. 15 is a diagram showing a schematic diagram of the relationship between minimum point strain 17 and axial force 11 of bolt 9. By performing stress analysis while changing the set axial force using the FE model described in FIG. 14A, the relationship between minimum point strain 17 and axial force 11 of bolt 9 can be determined, and it can be deduced that this relationship is roughly proportional. As is clear from this relationship, there is a proportional relationship between axial force 11 and the absolute value of the strain distribution occurring on the bolt head surface 10. This can be expressed as the following relational equation:

[0088] Strain distribution determined by DIC = Axial force × Strain distribution per unit axial force obtained by stress analysis FIG. 16 is a diagram showing a method for calculating the axial force 11 of the bolt 9 using the above relational expression.

[0089] The computer 6 calculates the strain distribution on the bolt head surface 10 when a unit axial force of 1 kN is applied to the bolt 9 by stress analysis, and estimates the axial force 11 by fitting the strain distribution 16 obtained by this stress analysis to the strain distribution 12 obtained by DIC while changing the axial force 11 as a parameter.

[0090] For the purpose of explanation, Figure 16 shows an example in which strain distribution 16a determined by stress analysis is fitted to strain distribution 12 determined by DIC as indicated by arrow 18a, and an example in which strain distribution 16b determined by stress analysis is fitted to strain distribution 12 determined by DIC as indicated by arrow 18b. When fitting is performed as indicated by arrow 18a, it can be seen that the axial force is smaller than 1 kN, and when fitting is performed as indicated by arrow 18b, it can be seen that the axial force is larger than 1 kN.

[0091] In step S7, the computer 6 outputs the axial force 11 of the bolt 9 acquired in step S6. The computer 6 outputs the axial force 11 of the bolt 9 to a display device included in the computer 6 or a display device connected to the computer 6. The computer 6 can also output the axial force 11 of the bolt 9 to a storage device included in the computer 6 or a storage device connected to the computer 6.

[0092] As described above, in the strain distribution acquisition method according to this embodiment, when the measurement object 1 is a bolt 9, the strain distribution of the bolt head surface 10 can be acquired, and the axial force 11 of the bolt 9 fastened to the fastened body 13 can be estimated. In this embodiment as well, by transferring the pattern 2 formed on the parent pattern member 3 to the bolt head surface 10, the pattern 2 that can be recognized by DIC can be formed on the bolt head surface 10 safely and independently of the skill level of the worker. [Example]

[0093] A strain distribution acquisition method according to a fifth embodiment of the present invention will be described with reference to Figures 17 and 18. Like the fourth embodiment, this embodiment is an example in which the strain distribution generated on the bolt head surface 10 is acquired to estimate the axial force 11 of a bolt 9 fastened to a fastened body 13, but in this embodiment, an example in which the strain distribution on the bolt head surface 10 is acquired for a plurality of bolts 9 to estimate the axial force 11 will be described. Below, the differences between the strain distribution acquisition method according to this embodiment and the strain distribution acquisition method according to the fourth embodiment will be mainly described.

[0094] FIG. 17 is a flowchart showing the steps of the strain distribution acquisition method according to this embodiment.

[0095] In step S1, a pattern 2 that can be recognized by DIC is formed on the bolt head surfaces 10 of a plurality of bolts 9. The pattern 2 is formed on the plurality of bolt head surfaces 10 by transferring it using a parent pattern member 3. Therefore, the patterns 2 formed on the plurality of bolt head surfaces 10 are identical to one another.

[0096] In steps S2 to S7, processing similar to that in Example 4 is performed on only one of the multiple bolts 9, the strain distribution generated on the surface of the bolt head surface 10 is acquired and output, and the axial force 11 of the bolt 9 is calculated and output.

[0097] In step S8, the worker determines whether there are any bolts 9 among the multiple bolts 9 for which the strain distribution of the bolt head surfaces 10 has not been acquired. If there are no bolts 9 for which the strain distribution has not been acquired (i.e., if the strain distribution of all the bolt head surfaces 10 has been acquired), the process shown in Fig. 17 ends. If there are any bolts 9 for which the strain distribution has not been acquired, the process returns to step S3, and steps S3 to S7 are performed for the bolts 9 for which the strain distribution has not been acquired, to acquire the strain distribution and calculate the axial force 11.

[0098] As described above, the same pattern 2 is formed on each of the multiple bolt top surfaces 10. For this reason, the reference image obtained from the bolt top surface 10 whose strain distribution is acquired first can be used when acquiring the strain distributions of the other bolt top surfaces 10. In other words, step S2, for acquiring the reference image of the bolt top surface 10, needs to be performed only on the bolt top surface 10 whose strain distribution is acquired first; for the bolt top surfaces 10 whose strain distributions are acquired second or later, step S2 can be omitted, and the axial force 11 can be estimated by acquiring the strain distributions.

[0099] 18 is a diagram schematically illustrating step S1 of forming patterns 2 on a plurality of bolt top surfaces 10. In step S1, the patterns 2 formed on the marking surface 4 of the parent pattern member 3 are transferred to each of the plurality of bolt top surfaces 10, thereby forming the patterns 2 on the plurality of bolt top surfaces 10. The same patterns 2 are formed on the plurality of bolt top surfaces 10.

[0100] Note that an identifier 1a (e.g., an engraving) may be attached to the bolt head surface 10 to distinguish multiple bolts 9 from one another. Pattern 2 is preferably formed on the bolt head surface 10 so as not to overlap with identifier 1a, as there is a concern that this may reduce the accuracy of strain detection. For example, if identifier 1a is drawn on the periphery of the bolt head surface 10, pattern 2 is preferably transferred to the center of the bolt head surface 10. However, pattern 2 may overlap with identifier 1a if it is in a position where a reduction in strain detection accuracy is not an issue or where it is not necessary to acquire strain distribution.

[0101] In this embodiment, the pattern 2 is formed on a plurality of bolt top surfaces 10 by transfer, so that the same pattern 2 can be efficiently formed on the plurality of bolt top surfaces 10. Therefore, in this embodiment, even if there are a plurality of bolts 9, it is only necessary to acquire a reference image once, and one common reference image can be used for the plurality of bolts 9, so that it is not necessary to acquire a reference image for all of the bolts 9, thereby shortening the work time required to acquire the strain distribution of a plurality of bolt top surfaces 10.

[0102] The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to embodiments including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment. It is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to delete part of the configuration of each embodiment, or to add or replace other configurations. [Explanation of symbols]

[0103] 1...measurement object, 1a...identifier, 2...pattern, 3...parent pattern component, 4...printing surface, 5...camera, 5a...fixing device, 6...computer, 7...load, 8...evaluation line, 9...bolt, 10...bolt head surface, 11...axial force, 12...strain distribution obtained by DIC, 13...fastened body, 14...washer, 15...nut, 16, 16a, 16b...strain distribution obtained by stress analysis, 17...minimum point strain, 18a, 18b...arrows, 20...strain intensity distribution.

Claims

1. a first step of forming a pattern recognizable by a digital image correlation method on a measurement object; a second step of acquiring a reference image of the measurement object, the reference image being an image of the pattern before a load is applied to the measurement object; a third step of acquiring an image of the pattern on the measurement object after the load is applied to the measurement object; a fourth step of acquiring a strain distribution of the measurement object from the reference image and the image acquired in the third step; and In the first step, the pattern formed on the parent pattern member is transferred onto the measurement object to form the pattern on the measurement object. A strain distribution acquisition method characterized by:

2. In the first step, the pattern is formed on the object to be measured by transferring paint applied to the pattern formed on the parent pattern member onto the object to be measured. The strain distribution acquisition method according to claim 1 .

3. There are a plurality of measurement objects, In the first step, the pattern is transferred onto the plurality of measurement objects using one of the parent pattern members, thereby forming the pattern on the plurality of measurement objects. The strain distribution acquisition method according to claim 1 .

4. In the fourth step, strain distributions of the plurality of measurement objects are acquired using one of the reference images acquired in the second step. The strain distribution acquisition method according to claim 3 .

5. The pattern is a random pattern. The strain distribution acquisition method according to claim 1 .

6. the measurement object is a bolt, In the fourth step, a strain distribution on the top surface of the bolt is acquired. The strain distribution acquisition method according to claim 1 .

7. a fifth step of estimating the axial force of the bolt from the strain distribution acquired in the fourth step; In the fifth step, the axial force of the bolt is estimated from the strain distribution obtained in the fourth step using the relationship between the strain distribution on the top surface of the bolt obtained by stress analysis and the axial force of the bolt. The strain distribution acquisition method according to claim 6 .

8. The parent pattern member has a stamp surface made of silicone rubber, and the pattern is formed on the stamp surface. The strain distribution acquisition method according to claim 1 .

9. The measurement object is assigned an identifier, In the first step, the pattern is formed on the measurement object so as not to overlap the identifier or so as to overlap the identifier. The strain distribution acquisition method according to claim 1 .

10. In the second step, the reference image is captured and acquired by one camera; In the third step, an image of the pattern of the measurement object is captured and acquired by one of the cameras. The strain distribution acquisition method according to claim 1 .

11. In the first step, the pattern is transferred to a position on the measurement object where a strain distribution is to be acquired. The strain distribution acquisition method according to claim 1 .

12. In the first step, the pattern is transferred to a position where a strain distribution of the bolt is to be acquired. The strain distribution acquisition method according to claim 6 .

Images