Strain measurement method and strain measurement apparatus

Abstract

To correct an error in a distortion due to a change in a distance between an imaging device and a measurement object from a distortion detected by using one imaging device.SOLUTION: A distortion measurement method for measuring a distortion of a measurement object comprises steps of: extracting a distortion feature amount of the measurement object; acquiring a first image of the measurement object and a second image after the force is applied to the measurement object; calculating a distortion by applying the digital image correlation method to the first and second images; acquiring an error correction equation for removing an error included in the distortion on the basis of the distortion feature amount and the calculated distortion; removing an error that occurs due to a change in relative positions between the imaging device in the acquisition of the first and second images and the measurement object from the distortion calculated by using the error correction equation; and outputting the distortion from which the error is removed.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a strain measurement method and a strain measurement device for measuring the strain on the surface of a measurement object.

Background Art

[0002] Conventionally, methods for measuring the strain generated on the surface of a measurement object using imaging devices such as cameras and video cameras, or strain gauges, are known.

[0003] As a strain measurement method using an imaging device, for example, measurement by a digital image correlation method capable of calculating the deformation amount, that is, the strain, from the luminance value distributions of an image before deformation and an image after deformation of a measurement object is known.

[0004] However, when measuring strain using the digital image correlation method, at least two captured images, that is, captured images before and after deformation of the measurement object, are required. However, at the time of capturing the image before deformation of the measurement object and at the time of capturing the image after deformation, the positions of the imaging device and the measurement object do not exactly match, and the imaging device and the measurement object may move, resulting in a problem that the measured strain includes an error.

[0005] In consideration of such an error, conventionally, those described in Japanese Patent Application Laid-Open No. 2012-132786 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2013-170829 (Patent Document 2) have been proposed.

[0006] In the case of the above Patent Document 1, it is proposed to correct an error due to a change in the relative distance between the first imaging device and the measurement object before and after deformation of the measurement object by capturing the surface of the measurement object with the first imaging device and measuring the distance between the first imaging device and the measurement object with the second imaging device.

[0007] Also, in the case of the above Patent Document 2, it is proposed to correct an error due to a change in the distance between the imaging device and the measurement object before and after deformation of the measurement object by using two imaging devices and measuring the distance between the imaging device and the measurement object by triangulation. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2012-132786 [Patent Document 2] Japanese Patent Publication No. 2013-170829 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, in the above-mentioned Patent Documents 1 and 2, it is necessary to correct for errors caused by changes in the distance between the imaging device and the object being measured before and after deformation of the object being measured. For this reason, it was necessary to use two imaging devices.

[0010] For strain measurement using imaging devices, it would be desirable from a convenience standpoint if strain measurement could be performed using a single general-purpose imaging device, such as a digital camera or a smart device with imaging capabilities.

[0011] The object of the present invention is to provide a strain detection method and strain detection device that can correct for strain errors caused by changes in the distance between the imaging device and the object being measured, for strain detected using a single imaging device. [Means for solving the problem]

[0012] To achieve the above objective, the present invention provides a strain measurement method for measuring the strain of an object to be measured, comprising: a strain feature extraction step of pre-extracting strain feature quantities of the object to be measured obtained by applying force to the object to be measured; a first image acquisition step of acquiring an image of a predetermined region of the object to be measured by photographing the predetermined region with an imaging device; a second image acquisition step of acquiring an image of the predetermined region by photographing the predetermined region with an imaging device after force has been applied to the object to be measured; and a strain calculation step of applying a digital image correlation method to a plurality of images of the predetermined region acquired in the first image acquisition step and the second image acquisition step, respectively, and calculating the strain in the predetermined region. The system includes: an error correction formula acquisition step which obtains an error correction formula for removing error strain, which is an error included in the strain calculated in the strain calculation step, based on the strain features extracted in the strain feature extraction step and the strain calculated in the strain calculation step; an error correction step which removes error strain caused by changes in the relative position between the imaging device and the predetermined region in the first image acquisition step and the relative position between the imaging device and the predetermined region in the second image acquisition step, from the strain calculated in the strain calculation step using the error correction formula; and a strain output step which outputs the strain from which the error has been removed in the error correction step. The object to be measured is a bolt, the predetermined region is the top surface of the bolt, the force applied to the bolt in the strain feature extraction step is an axial force, and the strain feature extraction step obtains the change in radial strain with respect to the distance from the center position of the bolt when axial forces of different magnitudes are applied to the bolt, and extracts in advance as a feature a specific position in the predetermined region for calculating strain where the magnitude of radial strain does not change regardless of the magnitude of the axial force applied to the bolt. It is characterized by the following.

[0013] Other features of the present invention are The above-described strain measurement method, wherein there are multiple specific positions in the predetermined region as feature quantities, and in the error correction formula acquisition step, the strain of the object to be measured calculated by the digital image correlation method in the strain calculation step is acquired as a strain curve showing the magnitude of strain with respect to the position in the predetermined region of the object to be measured, points corresponding to the multiple specific positions as feature quantities on this strain curve are identified, and the error correction formula is acquired by deriving an approximate formula from a straight line connecting these multiple points and the strain value at the center position of the bolt on the straight line, and the error correction step uses the error correction formula to remove error strain from the strain calculated in the strain calculation step. It is characterized by the following:

[0014] A further feature of the present invention is a strain measuring device for measuring the strain of an object to be measured, comprising: an image acquisition unit that captures an image of a predetermined region of the object to be measured using a camera; a strain calculation unit that applies a digital image correlation method to a plurality of images, including a first image acquired by the image acquisition unit and a second image of the predetermined region after force has been applied to the object to be measured, and calculates the strain in the predetermined region; and a strain feature extraction unit that extracts characteristic quantities of the strain of the object to be measured obtained by applying force to the object to be measured. The strain calculation unit includes: an error correction formula acquisition unit that acquires an error correction formula for removing error strain, which is an error in strain caused by the change in the relative position between the imaging device and the predetermined area when acquiring the first image and the change in the relative position between the imaging device and the predetermined area when acquiring the second image, from the strain calculated by the strain calculation unit; an error correction unit that removes error strain from the strain calculated by the strain calculation unit using the error correction formula; and a strain output unit that outputs the strain from which the error has been removed by the error correction unit. The object to be measured is a bolt, the predetermined region is the top surface of the bolt, the force applied to the bolt by the strain feature extraction unit is an axial force, the strain feature extraction unit extracts specific positions in the predetermined region where the magnitude of radial strain does not change even when the axial force applied to the bolt is changed, based on the change in radial strain with respect to the distance from the center position of the bolt when axial forces of different magnitudes are applied to the bolt, and the error correction formula acquisition unit acquires the error correction formula for calculating the error strain based on the feature extracted by the strain feature extraction unit and the strain calculated by the strain calculation unit. It is characterized by the following:

[0015] Another feature of the present invention is The strain measuring device described above includes: the strain feature extraction unit extracts multiple specific positions in the predetermined region as feature quantities; the error correction formula acquisition unit acquires the strain of the object to be measured calculated by the digital image correlation method as a strain curve showing the magnitude of strain relative to the position in the predetermined region of the object to be measured; identifies points on this strain curve corresponding to the multiple specific positions as feature quantities; and obtains the error correction formula by deriving an approximate formula from a straight line connecting these multiple points and the strain value at the center position of the bolt on the straight line; and the error correction unit corrects the strain calculated by the strain calculation unit using the error correction formula. It is characterized by the following: [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a strain detection method and strain detection device that can correct for strain errors caused by changes in the distance between the imaging device and the object being measured, for strain detected using a single imaging device. [Brief explanation of the drawing]

[0017] [Figure 1] This figure illustrates Embodiment 1 of the present invention, and is a block diagram illustrating an example of a strain measuring device. [Figure 2A] A cross-sectional view of a bolt illustrating a specific example of the strain feature extraction unit in Figure 1. [Figure 2B] Figure 2A shows the top surface of the bolt. [Figure 2C] Figure 2B shows a diagram illustrating the radial strain in one cross-section of the bolt. [Figure 3A] A diagram showing an image of a rectangular plate for explaining a specific example of an error correction formula acquisition unit in FIG. 1. [Figure 3B] A diagram showing an image taken after horizontally moving the rectangular plate shown in FIG. 3A. [Figure 3C] A diagram explaining the change in error strain in one cross-section of the rectangular plate by horizontally moving the rectangular plate. [Figure 3D] A diagram showing curves of normalized error strain for each curve showing error strain shown in FIG. 3C. [Figure 4] A diagram explaining specific examples of the error correction formula acquisition unit and the error correction unit shown in FIG. 1. [Figure 5] A flowchart explaining an example of the strain measurement method of the present invention. [Figure 6] A diagram explaining Example 2 of the present invention, corresponding to FIG. 2B. [Figure 7] A diagram explaining Example 3 of the present invention, explaining the axial displacement on the top surface of the bolt when an axial force is applied to the bolt. [Figure 8] A diagram explaining the relationship between the strain and error generated in the bolt shown in FIG. 7.

Mode for Carrying Out the Invention

[0020] The image acquisition unit 3 acquires images of the object to be measured 1 before and after deformation. Specifically, it acquires a first image of a predetermined region of the object to be measured 1 before deformation, and a second image of the same predetermined region after force is applied to the object to be measured 1 and it is deformed. In this embodiment, a digital camera or a smart device with a shooting function can be used as the shooting device for acquiring these images.

[0021] The strain calculation unit 4 calculates the strain using the images before and after deformation acquired by the image acquisition unit 3, i.e., the first image before deformation and the second image after deformation. Specifically, the strain calculation unit 4 applies the digital image correlation method to multiple images, including the first and second images acquired by the image acquisition unit 3, to calculate the strain in the predetermined region.

[0022] Furthermore, the strain measuring device 2 includes a strain feature extraction unit 7 that extracts characteristic amounts of strain generated by applying force to the object to be measured 1, and an error correction formula acquisition unit 8 that acquires an error correction formula for correcting the strain calculated by the strain calculation unit, which is caused by the change in the relative position between the imaging device and the predetermined area when acquiring the first and second images (error strain).

[0023] In the error correction formula acquisition unit 8, an error correction formula for removing the error strain included in the calculated strain is acquired based on the strain feature amount extracted in advance by the strain feature amount extraction unit 7 and the strain calculated by the strain calculation unit 4.

[0024] The strain calculated by the strain calculation unit 4 includes an error due to a change in the relative position between the imaging device and the measurement object 1 during imaging before and after the deformation of the measurement object 1. Therefore, for the strain calculated by the strain calculation unit 4, in the error correction unit 5, the error strain of the strain calculated using the error correction formula is subtracted. The strain from which the error strain has been removed in the error correction unit 5 is output from the strain output unit 6.

[0025] A specific example of the strain feature amount extraction unit 7 in FIG. 1 will be described using FIGS. 2A to 2C. FIG. 2A is a cross-sectional view of a bolt for explaining a specific example of the strain feature amount extraction unit in FIG. 1, FIG. 2B is a view showing the top surface of the bolt shown in FIG. 2A, and FIG. 2C is a diagram showing the radial strain in one cross-section of the bolt shown in FIG. 2B. Here, as a specific example, the strain feature amount when the measurement object 1 is the bolt 10 will be described.

[0026] As shown in FIG. 2A, when an axial force F is applied to the tightening bolt 10 by the bolt 10 and the nut 11 to the fastened body 12, as the radial strain in one cross-section 10a of the top surface of the bolt 10 in FIG. 2B, a tendency as shown in FIG. 2C is obtained. FIG. 2 <00001,36>In this figure, the horizontal axis represents the distance from the center position 0 of the bolt top surface in the cross-section 10a of the bolt top surface, that is, the distance in the +x direction and the -x direction. The vertical axis represents the magnitude of the radial strain with respect to the distance from the bolt center position.

[0027] Also, the three curves shown in FIG. 2C show the change in the radial strain with respect to the distance from the bolt center position 0 of the cross-section `10a` when the axial force F shown in FIG. 2A is applied to the bolt 10. That is, it shows the change in the radial strain when different forces f1, f2, f3 (f1 < f2 < f3) are applied as the axial force F.

[0028] As shown in Figure 2C, the radial strain increases as the applied axial force F increases, but it is known that the radial strain at distances x1 and -x1 from the bolt center position 0 is constant ε0 regardless of the magnitude of the axial force F.

[0029] In this embodiment, the strain feature extraction unit 7 shown in Figure 1 extracts the distances x1 and -x1 from the bolt center position where the radial strain becomes a constant value ε0, and the constant value ε0, as strain features.

[0030] Although Figures 2A to 2C illustrate an example where the object to be measured 1 is a bolt 10, the present invention can be similarly applied to any structure in which the strain at a specific position of the object to be measured 1 remains constant regardless of the load. For example, the present invention can be similarly applied to structures in which a bending load is applied while one end is fixed, such as wind turbine blades, or to structures in which a bending load is applied while suspended, such as bridges. Furthermore, the extraction of strain characteristics of the object to be measured (structure) 1 can be done by using methods such as calculations based on formulas of material mechanics or numerical calculations.

[0031] Next, in order to detect the strain of the object to be measured 1, we will explain, using Figures 3A to 3D, how to obtain an error correction formula for calculating the error strain, i.e., the error strain, when acquiring multiple images of the object to be measured 1 before and after deformation using a single imaging device. Figure 3A is a diagram illustrating a specific example of the error correction formula acquisition unit 8 in Figure 1, showing an image of a rectangular plate; Figure 3B is a diagram showing an image taken after horizontally moving the rectangular plate shown in Figure 3A; Figure 3C is a diagram illustrating the change in error strain in one cross-section of the rectangular plate due to horizontal movement of the rectangular plate; and Figure 3D is a diagram showing the error strain curves normalized from each of the error strain curves shown in Figure 3C.

[0032] As shown in FIG. 3A, using the image 13a obtained by photographing the object to be measured 1 as a rectangular plate 14 and the image 13b obtained by photographing the rectangular plate 14 after horizontally moving it by a distance D from the state of FIG. 3A, as shown in FIG. 3B, when the strain in one cross-section 14a of the rectangular plate 14 is calculated by the digital image correlation method, a strain curve as shown in FIG. 3C is obtained.

[0033] In FIG. 3C, the horizontal axis represents the distance from one end side to the other end side in the cross-section 14a, that is, the position in the cross-section 14a. The vertical axis represents the magnitude of the strain. Also, the three curves shown in FIG. 3C show the change in strain at each position of the cross-section 14a due to the movement of the rectangular plate 14 by a distance D as shown in FIG. 3B. That is, it shows the change in strain when the rectangular plate 14 is moved by different distances d1, d2, d3 (d1 < d2 < d3) as the distance D.

[0034] This strain is the error strain associated with the horizontal movement (movement distance D) of the rectangular plate 14 and the distortion aberration of the lens of the imaging device. The magnitude of the error strain changes with the movement distance D, and the error strain increases as the movement distance D increases.

[0035] When the three curves shown in FIG. 3C are normalized, a normalized error strain curve with almost the same slope as shown in FIG. 3D is obtained.

[0036] A straight line or an approximate line 15 of a curve that approximates the normalized error strain curve is fitted. In the example shown in FIG. 3D, an approximate straight line is fitted as the approximate line 15, but in some cases, it may be preferable to fit an approximate curve. An error correction formula is obtained based on the fitted approximate line 15.

[0037] In this embodiment, since an approximate straight line is fitted as the approximate line 15, the error correction formula may be, for example, the following (Equation 1). This (Equation 1) is an example and is not limited thereto. For example, when the approximate line 15 is an approximate curve, a quadratic equation or the like may be fitted.

[0038] εe = Ax + B …(Equation 1) Here, εe is the error strain, x is the distance from a specific location (in this example, the center of the bolt), and A and B are coefficients.

[0039] Although the examples shown in Figures 3A to 3D describe the case where the object to be measured is moved horizontally, the same methods can be applied to cases where the object to be measured is moved by rotation, or where the movement involves both horizontal and rotational motion.

[0040] Figure 4 is a diagram illustrating how the coefficients of the error correction formula are determined in the error correction formula acquisition unit 8 shown in Figure 1, and a specific example of how the error correction unit 5 uses the obtained error correction formula (Equation 1) to obtain a strain curve 17 from which the error has been removed, from the strain curve 16 including the error calculated by the strain calculation unit 4.

[0041] In the strain calculation unit 4 shown in Figure 1, a strain curve 16 is calculated using the digital image correlation method from multiple images of the object being measured before and after deformation. However, this calculated strain curve contains error strain. Therefore, the feature quantities obtained by the strain feature extraction unit 7, namely the distances x1 and -x1 from the bolt center position and the radial strain ε0 at that position, are applied to the diagram of the error-laden strain curve 16, as shown in Figure 4.

[0042] Next, the point of strain ε1 at distance x1 and the point of strain ε2 at distance -x1 in the strain curve 16 including errors are connected by a straight line 15a. This straight line 15a corresponds to (Equation 1), and the slope of this straight line 15a becomes the coefficient A of (Equation 1). Also, the difference B between the value at the bolt center position on this straight line 15a (the value corresponding to radial strain) and the radial strain ε0 as a feature quantity becomes the coefficient B of (Equation 1).

[0043] The error correction unit 5 uses the error correction formula (Equation 1), to which the values ​​of coefficients A and B are given, to determine the value of the error strain εe at each position from the bolt center position 0, and obtains a strain curve 17 by subtracting the error from the strain curve 16 including the error. This strain curve 17 with the error subtracted is output from the strain output unit 6 shown in Figure 1.

[0044] According to this embodiment, it is possible to measure strain with high accuracy by eliminating the change in the horizontal positional relationship between the imaging device and the object being measured before and after deformation of the object being measured, as well as error distortion caused by lens distortion.

[0045] Next, a strain measurement method for measuring the strain of an object using the strain measuring device described above will be explained with reference to Figure 5. Figure 5 is a flowchart illustrating an example of the strain measurement method of the present invention.

[0046] First, in step S1, the strain characteristics of the object to be measured 1 obtained by applying force to the object to be measured 1 are extracted in advance (strain characteristic extraction step). That is, as explained in Figure 2C, the value of ε0 at which the magnitude of strain remains constant regardless of the magnitude of the force applied to the object to be measured, and the positions (specific positions in the predetermined region, for example, the distance from the center of the bolt) x1 and -x1 in the predetermined region where the strain is constant ε0 are extracted.

[0047] In step S2, a predetermined area of ​​the object to be measured 1 is photographed with an imaging device such as a digital camera, and a first image of the predetermined area is acquired (first image acquisition step).

[0048] In step S3, the predetermined region after force is applied to the object to be measured 1 is photographed with an imaging device to acquire a second image of the predetermined region (second image acquisition step).

[0049] In step S4, the digital image correlation method is applied to the multiple images of the predetermined region acquired in the first image acquisition step S2 and the second image acquisition step S3, respectively, to calculate the strain in the predetermined region (strain calculation step).

[0050] In step S5, an error correction formula for calculating the error in the strain is obtained based on the strain features extracted in step S1 and the strain calculated in step S4 (error correction formula acquisition step). That is, using the same method as described in Figures 3A to 3C and Figure 4, an error correction formula and coefficients of the error correction formula are obtained, for example, as shown in (Equation 1).

[0051] In step S6, the strain calculated in strain calculation step S4 is corrected for the strain error caused by the change in the relative position between the imaging device and the predetermined area in the first image acquisition step S2 and the change in the relative position between the imaging device and the predetermined area in the second image acquisition step S3, using the error correction formula acquired in error correction formula acquisition step S5 (error correction step).

[0052] In step S7, the error correction step S6 outputs the strain from which the error has been removed in the strain calculation step S4 (strain output step).

[0053] In this embodiment, the specific positions in the predetermined region that serve as features in the strain feature extraction step S1 are multiple (in this example, x1 and -x1). In the error correction formula acquisition step S5, the strain of the object to be measured 1 calculated using the digital image correlation method in the strain calculation step S4 is obtained as a strain curve showing the magnitude of strain for each position in the predetermined region of the object to be measured 1, and points corresponding to the multiple specific positions in the predetermined region that serve as features are identified on this strain curve. Next, the error correction formula is obtained by deriving an approximation formula from the lines connecting these multiple points. In the error correction step S6, the error in the calculated strain is corrected using this error correction formula.

[0054] As described above, according to Embodiment 1 of the present invention, the effect is obtained that strain can be easily measured with high accuracy using a single imaging device, taking into account errors due to changes in the distance between the imaging device and the object being measured before and after deformation of the object being measured. [Examples]

[0055] Embodiment 2 of the present invention will be described with reference to Figure 1 and Figure 6. Figure 6 corresponds to Figure 2B and illustrates a different example of the feature extraction unit 7 in Figure 1 from Embodiment 1. The overall configuration of the strain measurement device in Embodiment 2 is the same as in Figure 1, and the flow of the strain measurement method is also the same as that explained in Figure 5. Therefore, the explanation of parts that are the same as in Embodiment 1 will be omitted, and the explanation will focus on the differences from Embodiment 1.

[0056] When the object to be measured is a bolt 10, similar to Example 1, the distance x1 from the center position of the bolt 10 at which the radial strain ε0 is constant regardless of the axial force F is determined in advance. In this Example 2 as well, the strain feature extraction unit 7 in Figure 1 extracts the radial strain ε0 at the distance x1 radially from the center position of the bolt 10 as a feature.

[0057] The difference between this embodiment 2 and embodiment 1 is that in the error correction formula acquisition unit 8 in Figure 1, error distortions due to horizontal and rotational movement of the object to be measured 1 in the xy plane (plane coordinates), and lens distortion, are acquired as an error correction formula for plane coordinates, for example, as shown in (Equation 2) below.

[0058] εe = Gx + Hy + I …(Math 2) Here, εe is the error strain, x and y are the distances in the x and y directions from a specific position (in this example, the center of the bolt), and G, H, and I are coefficients.

[0059] In this second embodiment, the coefficient G of (Equation 2) in the x-plane and the coefficient H of (Equation 2) in the y-plane are determined using the same method as described in the first embodiment. The coefficient I in both the x and y planes is also determined.

[0060] In the error correction unit 5 shown in Figure 1, the error strain εe value at each position from the bolt center position 0 is determined using the error correction formula (Equation 2) to which the coefficients G, H, and I are given. A strain curve is then obtained by subtracting the error from the strain curve including the error calculated by the strain calculation unit 4 shown in Figure 1. This strain curve with the error removed is output from the strain output unit 6 shown in Figure 1.

[0061] Thus, in this embodiment 2, the error strain in the x-axis direction and the error strain in the y-axis direction are determined for the error strain in the xy-plane of the object to be measured. By doing so, the error strain in the xy-plane can be determined, and the strain in the two-dimensional plane can be obtained by removing the error strain in the xy-plane from the strain calculated by the strain calculation unit 4 in Figure 1 and the strain calculation step S4 in Figure 5. Therefore, this embodiment 2 provides the same effects as in embodiment 1, as well as the ability to measure the strain in the two-dimensional plane of the object to be measured 1 with high accuracy. [Examples]

[0062] Embodiment 3 of the present invention will be described with reference to Figures 7 and 8. Figure 7 is a diagram illustrating the axial displacement at the top surface of a bolt when an axial force is applied to the bolt, and Figure 8 is a diagram illustrating the relationship between strain and error occurring in the bolt shown in Figure 7. The overall configuration of the strain measuring device in Embodiment 3 is the same as in Figure 1, and the flow of the strain measuring method is also the same as that explained in Figure 5. Therefore, the explanation of parts that are the same as in Embodiment 1 will be omitted, and the explanation will focus on the differences from Embodiment 1.

[0063] Figure 7 shows a bolt 10 as the object to be measured. When an axial force F is applied to this bolt 10, an axial displacement occurs at the bolt top surface 10b. This displacement also causes errors. This embodiment 3 removes the error caused by the axial deformation ΔL at the bolt top surface 10b when an axial force F is applied to the bolt 10 from the strain calculated by the strain calculation unit 4 in Figure 1.

[0064] In Figure 7, if L is the distance between the top surface 10b of the bolt 10 and the imaging device 20 before the axial force F is applied, and ΔL is the axial deformation at the top surface 10b of the bolt 10 when the axial force F is applied to the bolt 10, then the apparent strain εez generated by the axial deformation ΔL with respect to the distance L is given by the following equation (Equation 3).

[0065] εez = L / (L+ΔL) …(Math 3) In Figure 8, 18 is the curve of radial strain εr per unit axial force (strain curve), and 19 is the curve of strain (εr+εez) obtained by adding the radial strain εr and the apparent strain εez shown in (Equation 3) above, which is generated by the axial deformation. Furthermore, it has been found that the radial strain εr and the axial deformation ΔL of the bolt head have a roughly linear relationship with respect to the axial force F in Figure 7, and the ratio of the radial strain εr per unit axial force in Figure 8 to the strain (εr+εez) obtained by adding this radial strain εr and the apparent strain εez generated by the axial deformation ΔL is roughly the same regardless of the magnitude of the axial force F.

[0066] Therefore, the strain feature extraction unit 7 shown in Figure 1 extracts the strain (εr+εez), which is the sum of the radial strain εr per unit axial force and the apparent strain εez generated by the radial strain εr and the axial deformation amount ΔL per unit axial force, as a strain feature. Furthermore, the strain feature extraction unit 7 extracts at least the radial strain εr and the axial displacement ΔL or the apparent strain εez as strain features, and if necessary, the strain (εr+εez), which is the sum of the radial strain εr and the apparent strain εez generated by the axial deformation ΔL, may be acquired by the strain feature extraction unit 7 or the error correction formula acquisition unit 8, which will be described next.

[0067] In the error correction formula acquisition unit 8 shown in Figure 1, the ratio Re(x) of the radial strain εr(x) per unit axial force and the apparent strain εez(x) generated by the radial strain εr(x) and the axial deformation amount ΔL per unit axial force is acquired as the error correction formula shown in (Equation 4) below.

[0068] Re(x)=εr(x) / (εr(x)+εez(x)) …(Math 4) Here, εr(x) is the radial strain per unit axial force at a radial distance x from the bolt center, and εez(x) is the apparent strain associated with the axial deformation ΔL of the bolt top surface per unit axial force at a radial distance x from the bolt center.

[0069] The error correction unit 5 obtains a strain and strain curve from which the error has been removed by integrating (multiplying) the ratio Re with the strain that includes the error due to the apparent strain εez associated with the axial deformation amount ΔL calculated by the strain calculation unit 4. This strain and strain curve from which the error has been removed is output from the strain output unit 6 shown in Figure 1.

[0070] According to this embodiment 2, the same effects as in embodiment 1 can be obtained, and strain and strain curves can be obtained that also eliminate apparent strain caused by the axial deformation amount ΔL when an axial force F is applied to the object being measured.

[0071] In the description of this embodiment 3, the case where the object to be measured 1 is a bolt was explained, but it is not limited to bolts. It can be applied to any structure in which the strain generated in the object to be measured and the amount of deformation of the object in the direction of photography have a roughly linear relationship with respect to the magnitude of the load applied to the object to be measured, or to any structure in which the ratio of the strain generated in the object to the apparent strain due to the deformation of the object in the direction of photography remains roughly constant regardless of the magnitude of the load.

[0072] Although Examples 1 to 3 of the present invention have been described above, the present invention is not limited to the above-described examples and includes various modifications. For example, although the above examples described an example in which the object to be measured was a bolt, it is not limited to bolts and, as mentioned above, can be similarly applied to structures in which a bending load is applied while one end is fixed, or structures in which a bending load is applied while suspended. Furthermore, it is also possible to remove errors included in the strain calculated by the strain calculation unit by using both the strain measurement method described in Example 1 and the strain measurement method described in Example 3. Furthermore, it is possible to replace parts of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Moreover, the embodiments described above are explained in detail in order to make the present invention easy to understand, and are not necessarily limited to those comprising all the configurations and steps described. [Explanation of symbols]

[0073] 1: Object to be measured (structure), 2: Strain measurement unit, 3: Image acquisition unit, 4: Strain calculation unit, 5: Error correction unit, 6: Strain output unit, 7: Strain Feature Extraction Unit 8: Error correction formula acquisition part 10: Bolt, 10a: Cross-section, 10b: Bolt top surface, 11: Nut, 12: Fastened object, 13a, 13b: Image, 14: Rectangular plate, 14a: Cross section, 15: Approximate line, 16: Strain curve including error, 17: Strain curve with error removed, 18: Curve of radial strain per unit axial force, 19: A strain curve obtained by adding radial strain and apparent strain per unit axial force. 20: Imaging device.

Claims

1. A strain measurement method for measuring the strain of an object to be measured, A strain feature extraction step involves pre-extracting strain feature quantities of the object to be measured by applying force to the object to be measured, A first image acquisition step involves capturing an image of a predetermined region of the object to be measured using a camera to obtain an image of the predetermined region, A second image acquisition step involves taking a photograph of the predetermined region after a force has been applied to the object to be measured using a photographing device to acquire an image of the predetermined region, A strain calculation step which involves applying a digital image correlation method to a plurality of images of the predetermined region acquired in the first image acquisition step and the second image acquisition step, respectively, and calculating the strain in the predetermined region, An error correction formula acquisition step is to obtain an error correction formula for removing error strain, which is an error included in the strain calculated in the strain calculation step, based on the strain features extracted in the strain feature extraction step and the strain calculated in the strain calculation step. An error correction step in which error strain generated by the change in the relative position between the imaging device and the predetermined region in the first image acquisition step and the change in the relative position between the imaging device and the predetermined region in the second image acquisition step is removed from the strain calculated in the strain calculation step using the error correction formula, The system includes a strain output step that outputs the strain from which the error has been removed in the error correction step, The object to be measured is a bolt, the predetermined region is the top surface of the bolt, and the force applied to the bolt in the strain feature extraction step is an axial force. The strain measurement method is characterized in that the strain feature extraction step involves obtaining the change in radial strain with respect to the distance from the center position of the bolt when axial forces of different magnitudes are applied to the bolt, and pre-extracting as feature quantities specific positions in the predetermined region for calculating strain where the magnitude of radial strain does not change regardless of the magnitude of the axial force applied to the bolt.

2. A strain measurement method according to claim 1, The aforementioned feature quantities include multiple specific locations within the predetermined region. In the error correction formula acquisition step, the strain of the object to be measured calculated by the digital image correlation method in the strain calculation step is acquired as a strain curve showing the magnitude of strain with respect to a predetermined region of the object to be measured, a plurality of points for specific positions as feature quantities are identified on this strain curve, and the error correction formula is acquired by deriving an approximate formula from a straight line connecting these plurality of points and the strain value at the center position of the bolt on the straight line. The strain measurement method is characterized in that the error correction step removes error strain from the strain calculated in the strain calculation step using the error correction formula.

3. A strain measurement method according to claim 1, A strain measurement method characterized by determining the error strain in the x-axis direction and the error strain in the y-axis direction for the error strain in the xy-plane of the object to be measured, thereby determining the error strain in the xy-plane, and obtaining the strain in a two-dimensional plane by removing the error strain in the xy-plane from the strain calculated in the strain calculation step.

4. A strain measuring device for measuring the strain of an object to be measured, An image acquisition unit that captures an image of a predetermined region of the object to be measured by a camera, A strain calculation unit applies a digital image correlation method to multiple images of a first image acquired by the image acquisition unit and a second image of the predetermined region after force has been applied to the object to be measured, and calculates the strain in the predetermined region. A strain feature extraction unit extracts characteristic quantities of the strain of the object to be measured, obtained by applying force to the object to be measured. An error correction formula acquisition unit acquires an error correction formula for removing error strain, which is an error in strain caused by the change in the relative position between the imaging device and the predetermined region when acquiring the first image and the change in the relative position between the imaging device and the predetermined region when acquiring the second image, from the strain calculated by the strain calculation unit. An error correction unit removes error strain from the strain calculated by the strain calculation unit using the aforementioned error correction formula, The system includes a strain output unit that outputs strain from which the error has been removed by the error correction unit, The object to be measured is a bolt, the predetermined region is the top surface of the bolt, and the force applied to the bolt by the strain feature extraction unit is an axial force. The strain feature extraction unit extracts specific locations within the predetermined region for calculating strain as feature quantities, where the magnitude of the radial strain does not change even when the axial force applied to the bolt is changed, based on the change in radial strain with respect to the distance from the center position of the bolt when axial forces of different magnitudes are applied to the bolt. The strain measuring device is characterized in that the error correction formula acquisition unit acquires the error correction formula for calculating the error strain based on the feature quantities extracted by the strain feature quantity extraction unit and the strain calculated by the strain calculation unit.

5. A strain measuring device according to claim 4, The strain feature extraction unit extracts multiple specific positions in the predetermined region as feature quantities, The error correction formula acquisition unit acquires the strain of the object to be measured, calculated by the digital image correlation method, as a strain curve showing the magnitude of the strain at a predetermined location in the object to be measured, identifies multiple points at specific locations as feature quantities on this strain curve, and obtains the error correction formula by deriving an approximate formula from a straight line connecting these multiple points and the strain value at the center position of the bolt on the straight line. The strain measuring device is characterized in that the error correction unit corrects the strain calculated by the strain calculation unit using the error correction formula.

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