JP6958494B2 - Displacement amount measuring device, displacement amount measuring method and displacement amount measuring program - Google Patents

Description

The present invention relates to a displacement amount measuring device, a displacement amount measuring method, and a displacement amount measuring program.

A digital image that measures the amount of displacement based on the digital images before and after deformation of the specimen that receives loads such as tension and compression when performing material tests such as tensile tests on the test piece that is the specimen with a material tester. A method called a correlation method (DIC, Digital Image Correlation) is known (see, for example, Patent Document 1). In the digital image correlation method, based on the digital images before and after the deformation of the specimen, it is possible to grasp to which position any measurement point set on the specimen has moved after being loaded. I'm getting information about deformation.

Japanese Unexamined Patent Publication No. 2007-078659

In the digital image correlation method, the images before and after the deformation of the random pattern drawn on the object surface are compared, and the positions before and after the deformation of the analysis area consisting of a plurality of pixels called a subset are obtained. In order to measure the displacement amount with high accuracy in the digital image correlation method, it is desired to set an appropriate subset.

According to the first aspect of the present invention, the displacement amount measuring device that measures the displacement amount of the specimen by performing the analysis by the digital image correlation method acquires an image of the random pattern drawn on the specimen before deformation. Based on the acquisition unit, the calculation unit that calculates the size of the spots included in the random pattern based on the image before the deformation acquired by the acquisition unit, and the spot size calculated by the calculation unit, the said It includes a determination unit that determines the size of the subset used in the analysis of the digital image correlation method.
According to the second aspect of the present invention, it is a displacement amount measuring method in which the displacement amount of the specimen is measured by performing the analysis by the digital image correlation method, and the image before deformation of the random pattern drawn on the specimen is obtained. Acquired, the size of the spots included in the random pattern is calculated based on the acquired image before deformation, and the subset used for the analysis of the digital image correlation method is calculated based on the calculated spot size. Determine the size.
According to the third aspect of the present invention, the displacement amount measurement program for measuring the displacement amount of the specimen by performing the analysis by the digital image correlation method uses the image before deformation of the random pattern drawn on the specimen. Based on the acquisition step to be acquired, the calculation step of calculating the size of the spots included in the random pattern based on the image before the deformation acquired in the acquisition step, and the size of the spots calculated in the calculation step. , The computer is made to perform a determination step of determining the size of the subset used in the analysis of the digital image correlation method.

According to the present invention, the size of the subset used for the analysis of the digital image correlation method can be appropriately set.

FIG. 1 is a schematic view of a material testing machine used for carrying out displacement amount measurement according to an embodiment of the present invention. 2 (a) and 2 (b) are diagrams for explaining an example of a random pattern drawn on a specimen and the size of spots included in the random pattern. FIG. 3 is a flowchart illustrating the displacement amount measurement. FIG. 4 is a flowchart illustrating a process of determining analysis conditions for DIC analysis and measuring a displacement amount according to an embodiment of the present invention. FIG. 5 is a stress-strain diagram based on the displacement amount of the specimen. FIG. 6 is a diagram showing how the program is provided. FIG. 7 is a histogram of the ferret diameter of the spots of the random pattern applied to the test piece used in the example. FIG. 8 is a stress-strain diagram in the microdeformation region based on the test results of the examples. FIG. 9 is a table showing the calculation results of the elastic modulus in the minute deformation region according to the examples.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view of a material testing machine according to an embodiment of the present invention. As shown in FIG. 1, the material testing machine 1 according to the present embodiment is arranged on a base 11, a pair of left and right screw rods 12 erected on the base 11, and above the screw rods 12. The yoke 13 and the crosshead 15 are provided. The crosshead 15 is provided with a nut portion that is screwed with a pair of left and right screw rods 12, and moves up and down with respect to the screw rods 12.

Below the base 11, a motor 20 for compression and tension load, a synchronous pulley 21 rotated by driving the motor 20, and a synchronous pulley 22 arranged at the lower ends of a pair of screw rods 12 are synchronized. The belt 23 and the belt 23 are arranged. The synchronous pulley 21 and the synchronous pulley 22 are respectively engaged with the synchronous belt 23. The synchronous pulley 21 and the synchronous pulley 22 are rotated by the drive of the motor 23, and the pair of screw rods 12 are rotated in synchronization with the rotation. The rotation of the pair of screw rods 12 causes the crosshead 15 to move up and down in the axial direction of the screw rods 12.

An upper gripper 31 for gripping the upper end portion of the specimen (test piece) TP is attached to the crosshead 15 via the load cell 16. A lower grip 32 for gripping the lower end of the specimen TP is attached to the base 11. When performing a tensile test, a test force (tensile load) is applied to the specimen TP by raising the crosshead 15 while gripping both ends of the specimen TP with the upper grip 31 and the lower grip 32. Give.

A digital camera 35 is installed on a support column (not shown) covering one of the screw rods 12, and an image of the specimen TP gripped by the upper gripping tool 31 and the lower gripping tool 32 is taken. The digital camera 35 includes an image sensor such as a CCD sensor, and can shoot a still image or a moving image. The position of the digital camera 35 is adjustable, and the position is adjusted so that the area required for measuring the displacement amount in the specimen TP can be photographed. The image data of the still image or the moving image acquired by the digital camera 35 is input to the processing device 40. Further, the test force acting on the specimen TP detected by the load cell 16 is also input to the processing device 40.

The processing device 40 performs an analysis (hereinafter referred to as DIC analysis) by a digital image correlation method (DIC) based on a signal such as image data input from the material testing machine 1 in the specimen TP. Calculate the amount of displacement. The processing device 40 is composed of a computer having a CPU for executing a logical operation, a ROM in which an operation program necessary for the operation is stored, and a storage device such as a RAM in which data or the like is temporarily stored at the time of the operation. An input unit 41 such as a keyboard and a display unit 42 such as a liquid crystal display device are connected to the processing device 40. Further, a control device 36 that controls the operation of the material testing machine 1 is also connected to the processing device 40.

When measuring the displacement of the specimen TP by DIC analysis, a random pattern (also called a random pattern or speckle pattern) as shown in FIG. 2A is drawn on the surface of the specimen TP, and deformation by a tensile test is performed. The displacement amount (deformation amount) of the specimen TP is detected by comparing the images before and after. FIG. 2A is an example of a random pattern in which a white paint is applied to the surface of a specimen TP made of carbon fiber reinforced plastic (thermoplastic CFRP) using a black thermoplastic resin by spray coating.

In DIC analysis, an analysis area consisting of a plurality of pixels called a subset is set in the image before deformation, and the displacement amount in the object plane is obtained by performing matching processing or the like to obtain the position of the subset in the image after deformation. be able to. DIC analysis does not require a sensor such as a strain gauge to come into contact with the surface of the specimen TP, and does not require a complicated optical system. Further, the DIC analysis is highly versatile because it is possible to analyze the displacement amount at an arbitrary position of the specimen TP after the tensile test based on the acquired image data.

The analysis accuracy when measuring the displacement amount by DIC analysis is related to the size of the spots (speckles) of the random pattern drawn on the surface of the object described above and the size of the subset. For example, if the size of the subset is reduced, the local strain that occurs in the object due to large deformation, so-called fracture strain, can be measured accurately, and the strain distribution resolution that visualizes the concentration of strain can be increased. can. However, if the subset size is small, the noise level rises, so it is not possible to accurately measure the elastic modulus, which requires minute strain measurement. On the other hand, when the subset size is increased, the measured values of the strain are averaged, so that the accuracy of the elastic modulus measurement can be improved. However, if the subset size is large, the fracture strain that occurs locally cannot be measured accurately.

It is desirable that the measurement accuracy required for the measurement of the elastic modulus of an object by the DIC analysis and the measurement of the fracture strain is such that the relative error with respect to the measurement result by the strain gauge is smaller than about 1%. The absolute measurement accuracy by DIC analysis differs depending on the magnitude of strain. Specifically, for elastic modulus measurement that requires measurement in a region where strain is minute (micro deformation region), strict accuracy is required in order to prevent variations in measured values. On the other hand, for fracture strain measurement, which requires measurement in a region with large strain (large deformation region), a sufficient accuracy is allowed.

Conventionally, the performer of the DIC analysis in the tensile test compares the output value of the contact strain gauge with the measured value by the DIC analysis to obtain the physical property value of the object to be tested and the required absolute accuracy by trial and error. Work had to be done to determine the size of a suitable subset. Since this work takes a lot of time, it takes a lot of time to measure the strain by DIC analysis. Further, when the determination of the subset size is left to the discretion of the performer of the DIC analysis, the measured value of the strain by the DIC analysis may vary depending on the analyst, and the reliability of the measured value may not be guaranteed. ..

Therefore, in one embodiment of the present invention, in order to measure the strain with high accuracy, the size of the subset used for the DIC analysis, that is, the analysis condition for the DIC analysis is automatically set to an appropriate value.

Hereinafter, how to determine the analysis conditions for DIC analysis in one embodiment of the present invention will be described. First, the flow of displacement amount measurement will be described with reference to the flowchart of FIG. In the present embodiment, for example, a test force (tensile load) is applied to the specimen TP on which the random pattern as shown in FIG. 2A is drawn, and the displacement amount is obtained by DIC analysis.

In step S10, the specimen TP gripped by the upper gripping tool 31 and the lower gripping tool 32 of the material testing machine 1 is imaged by the digital camera 35 before applying the test force. Here, the image of the specimen TP imaged by the digital camera 35 is an image of a random pattern drawn on the specimen TP before being deformed by being subjected to a test force.

In step S20, the crosshead 15 of the material testing machine 1 is raised to apply a test force (tensile load) to the specimen TP. In step S30, the specimen TP is imaged by the digital camera 35 while the test force is applied. Here, the image of the specimen TP imaged by the digital camera 35 is an image of a random pattern of the specimen TP deformed by being subjected to a test force. The digital camera 35 may capture a moving image of the specimen TP while the test force is being applied, or may sequentially capture still images while the test force is being applied.

In step S40, as will be described later, the processing device 40 performs DIC analysis based on the images of the random patterns before and after the deformation, and measures the displacement amount due to the deformation of the specimen TP. As a result, the processing of the flowchart shown in FIG. 3 is completed.

Next, the process of determining the analysis conditions of the DIC analysis and measuring the displacement amount executed in the processing device 40 will be described with reference to the flowchart of FIG. The process shown in the flowchart of FIG. 4 is performed by executing the program stored in the ROM in the processing device 40.

Information on the pixel resolution based on the relationship between the number of pixels of the digital camera 35 and the field of view size is input to the processing device 40 in advance. The field of view size is the length of the range imaged by the digital camera 35 in a predetermined direction, and depends on the lens used in the digital camera 35. The value obtained by dividing the field of view size by the number of pixels is the pixel resolution.

First, in step S401, the processing device 40 acquires an image of the random pattern before deformation captured in step S10 of the flowchart of FIG. In step S402, the processing device 40 acquires an image of the deformed random pattern captured in step S30 of the flowchart of FIG.

In step S403, the processing apparatus 40 calculates the size of the spots included in the random pattern based on the image of the random pattern before deformation acquired in step S401. The shape of the spots included in the random pattern is not uniform, and has a distorted shape as shown in FIG. 2 (b), for example. Therefore, the spot size is calculated based on the spot ferret diameter. The ferret diameter is the length of a rectangle circumscribing an object in the vertical and horizontal directions.

In the present embodiment, the vertical ferret diameter is calculated for a large number of spots included in the random pattern by image analysis as shown in FIG. 2 (b). The vertical direction is a direction along the direction of the test force applied to the specimen TP. The processing device 40 obtains the calculated histogram of the ferret diameter, and determines the mode value as the spot size.

In step S404, the processing apparatus 40 determines the size of the subset, which is the analysis condition used for the DIC analysis, based on the size of the spots of the random pattern determined in step S403. As mentioned above, the accuracy of the DIC analysis is related to the size of the spots in the random pattern and the size of the subset. A large subset size is desirable for measuring the elastic modulus of an object, and a small subset size is desirable for measuring fracture strain.

In this embodiment, the spot size is focused on to automatically determine the appropriate subset size, and the subset size for elastic modulus measurement and the smaller fracture strain measurement are based on the spot size. Calculate the size of each subset of. Specifically, a value larger than 6 times the size of the spot is determined as the size of the subset for elastic modulus measurement. On the other hand, a value of about 1 to 6 times the spot size is determined as the size of the subset for fracture strain measurement.

In step S405, DIC analysis is performed using the subset sized in step S404, and the displacement amount of the specimen TP is measured.
In step S406, a stress-strain diagram is created based on the displacement amount of the specimen TP measured in step S405. FIG. 5 shows an example of a stress-strain diagram based on the displacement amount of the specimen. Specifically, the load data (stress data) of the tensile test carried out in step S20 of the flowchart of FIG. 3 is acquired and combined with the strain value data based on the displacement amount of the specimen TP measured in step S405. In the stress-strain diagram of FIG. 5, the minute deformation region is the strain value data obtained by performing DIC analysis using the subset for elastic modulus measurement, and the large deformation region uses the subset for fracture strain measurement. The strain value data obtained by performing DIC analysis is used.

As a result, the process shown in FIG. 4 is completed.

According to the above-described embodiment, the following effects can be obtained.
(1) The processing device 40 measures the displacement amount of the specimen TP by performing an analysis by a digital image correlation method. The processing device 40 calculates the size of the spots included in the random pattern based on the acquisition unit that acquires the image before deformation of the random pattern drawn on the specimen TP and the image before deformation acquired by the acquisition unit. It functionally has a calculation unit and a determination unit that determines the size of the subset used in the analysis of the digital image correlation method based on the spot size calculated by the calculation unit. In this way, by automatically determining the size of the subset based on the size of the spots of the random pattern, it is not necessary for the performer of the DIC analysis to determine the size of the subset over a long period of time, and the DIC analysis is performed. The displacement amount can be measured efficiently. In addition, there is no variation in the measured values due to the difference in the method of determining the subset size depending on the practitioner, and it is possible to ensure the reliability of the measured values regardless of the practitioner.

(2) The processing unit 40 further acquires an image of a random pattern of the specimen TP deformed by applying a tensile load, and based on the size of the spots, a subset for measuring the elastic modulus and a subset for measuring the elastic modulus are used. A subset for fracture strain measurement, which is different in size from the subset, is determined. The processing unit 40 further has an analysis unit that analyzes the digital image correlation method using the subset for measuring the elastic modulus or the subset for measuring the fracture strain and measures the displacement amount of the specimen. By determining a subset for elastic modulus measurement and a subset for fracture strain measurement whose size is different from that for elastic modulus measurement, and performing DIC analysis using these, the measured values according to the purpose of the tensile test. Can be calculated accurately.

(3) The size of the subset for elastic modulus measurement is larger than the size of the subset for fracture strain measurement. As a result, the accuracy of elastic modulus measurement can be improved by DIC analysis using a subset for elastic modulus measurement having a large size. In addition, by performing DIC analysis using a subset for measuring fracture strain with a small size, fracture strain can be measured accurately and the strain distribution resolution can be increased.

The following modifications are also within the scope of the present invention, and one or more of the modifications can be combined with the above-described embodiment.
(1) In the above-described embodiment, a subset for elastic modulus measurement and a subset for fracture strain measurement were determined as analysis conditions for DIC analysis. However, this is not limited to this, and if a subset of an appropriate size can be set according to the purpose of the tensile test, only one of the subset for elastic modulus measurement and the subset for fracture strain measurement can be set. May be good.

(2) In the above-described embodiment, as shown in the flowchart of FIG. 4, analysis conditions for DIC analysis are set (step S404), and DIC analysis is performed based on the set analysis conditions to measure the displacement amount. (Step S405), but is not limited to this. For example, it may be configured to determine only the analysis conditions of the DIC analysis based on the image of the random pattern before deformation. Further, the creation of the stress-strain diagram (step S406) may be omitted.

(3) In the above-described embodiment, it has been described that the processing is performed according to the flowchart of FIG. 4, but the order of the processing is not limited to that shown in FIG. For example, the process of acquiring the random pattern after the transformation performed in step S402 may be performed after the process of determining the subset size performed in step S404.

(4) Further, the program relating to the determination of the analysis conditions and the measurement of the displacement amount described above can be provided through a recording medium such as a CD-ROM or a data signal such as the Internet. FIG. 6 is a diagram showing the situation. The personal computer 300 receives the program provided via the CD-ROM 304. Further, the personal computer 300 has a connection function with the communication line 301. The computer 302 is a server computer that provides the above program, and stores the program in a recording medium such as a hard disk 303. The communication line 301 is a communication line such as the Internet or personal computer communication, or a dedicated communication line. The computer 302 uses the hard disk 303 to read the program and transmits the program to the personal computer 300 via the communication line 301. That is, the program is embodyed on a carrier wave as a data signal and transmitted via the communication line 301. As described above, the program can be supplied as a computer-readable computer program product in various forms such as a recording medium and a carrier wave.

(5) In the above-described embodiment, as shown in FIG. 2 (a), as an example of the random pattern, a white paint spray-coated on the surface of an object made of black thermoplastic CFRP has been described. The pattern is not limited to this. A test piece other than black or a paint other than white may be used, or a random pattern may be drawn by a method other than spray application.
(6) In the above-described embodiment, the ferret diameter of the spots included in the random pattern was detected, and the mode value thereof was determined as the spot size. However, the present invention is not limited to this, and for example, the median value or the average value of the ferret diameter may be adopted as the spot size. Further, the spot size may be detected by using an index other than the ferret diameter.

Although various embodiments and modifications have been described above, the present invention is not limited to these contents. Other aspects conceivable within the scope of the technical idea of the present invention are also included within the scope of the present invention.

In the following examples, the test results of performing a tensile test on an ISO527B type test piece made of PE will be described. The present invention is not limited to the conditions shown in the following examples.

The test piece is an ISO527B type test piece, and the distance between marked lines GL is 1 mm. The number of pixels of the digital camera that captured the test piece is 5 million pixels, and the pixel resolution is about 17 μm.

FIG. 7 shows a histogram of the ferret diameter of the spots of the random pattern applied to the test piece used in this example. As shown in FIG. 7, since the mode of the ferret diameter is 50 μm to 100 μm, the spot size is set to about 75 μm.

DIC analysis was performed using subsets of three sizes: 25 × 25 pixels (subset 1), 51 × 51 pixels (subset 2), and 101 × 101 pixels (subset 3). The size of Subset 1 (425 x 425 μm) is about 5.6 times the size of the spots, the size of Subset 2 (867 x 867 μm) is about 11.56 times the size of the spots, and the size of Subset 3 (1717 x 1717 μm) is about 11.56 times the size of the spots. It is about 22.9 times the size of the spot.

Further, in the same test, measurement was performed using a strain gauge with a distance between marked lines of GL = 1 mm and an extensometer with a distance between marked lines of GL = 50 mm.

FIG. 8 shows a stress-strain diagram in a microdeformation region based on the test results of this example. As shown in FIG. 8, the test results of Subset 2 and Subset 3 have low noise, and the error when compared with the strain gauge is about 0.5 μm, which is good within the allowable range of absolute accuracy in the minute deformation region. The measurement result was obtained. The test result of Subset 1 is noisy, but the relative error when compared with the output of the strain gauge is about 2 μm, which is an error that is acceptable for the measurement of fracture strain.

FIG. 9 shows the calculation result of the elastic modulus in the minute deformation region of the strain of 0.05 to 0.25% according to this example. As shown in FIG. 9, it was found that the elastic modulus calculation value in the case of subset 1 has a large error with respect to the calculation results of the strain gauge and the extensometer, and is not suitable for the measurement of the elastic modulus in the minute deformation region. In the case of Subset 2 and Subset 3, good calculation results were obtained.

1: Material tester, 31: Upper gripper, 32: Lower gripper, 40: Processing device, TP: Specimen

Claims (9)

It is a displacement amount measuring device that measures the displacement amount of the specimen by performing analysis by the digital image correlation method.
An acquisition unit that acquires an image of the random pattern drawn on the specimen before deformation, and
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 calculation unit.
A displacement amount measuring device including a determination unit for determining the size of a subset used in the analysis of the digital image correlation method based on the spot size calculated by the calculation unit. In the displacement amount measuring device according to claim 1,
The acquisition unit further acquires an image of the random pattern of the specimen that has been deformed by applying a tensile load.
Based on the size of the spot, the determination unit determines a subset for elastic modulus measurement and a subset for fracture strain measurement whose size is different from that of the elastic modulus measurement subset, respectively.
The displacement amount measuring device is
A displacement amount measuring device further comprising an analysis unit that analyzes the digital image correlation method using the subset for measuring elastic modulus or the subset for measuring fracture strain and measures the amount of displacement of the specimen. In the displacement amount measuring device according to claim 2,
A displacement amount measuring device in which the size of the subset for measuring elastic modulus is larger than the size of the subset for measuring fracture strain. It is a displacement amount measurement method that measures the displacement amount of the specimen by performing analysis by the digital image correlation method.
An image of the random pattern drawn on the specimen before deformation is acquired, and the image is obtained.
Based on the acquired image before deformation, the size of the spots included in the random pattern was calculated.
A displacement amount measuring method for determining the size of a subset used in the analysis of the digital image correlation method based on the calculated spot size. In the displacement amount measuring method according to claim 4,
An image of the random pattern of the specimen deformed by applying a tensile load was acquired.
Based on the size of the spots, a subset for elastic modulus measurement and a subset for fracture strain measurement having a size different from that of the elastic modulus measurement are determined.
A displacement amount measuring method in which the digital image correlation method is analyzed using the subset for measuring elastic modulus or the subset for measuring fracture strain, and the displacement amount of the specimen is measured. In the displacement amount measuring method according to claim 5,
A method for measuring a displacement amount in which the size of the subset for measuring the elastic modulus is larger than the size of the subset for measuring the fracture strain. It is a displacement amount measurement program for measuring the displacement amount of the specimen by performing analysis by the digital image correlation method.
The acquisition step of acquiring the image before deformation of the random pattern drawn on the specimen, and
A calculation step of calculating the size of spots included in the random pattern based on the image before deformation acquired in the acquisition step, and a calculation step.
A displacement amount measurement program that causes a computer to perform a determination step of determining the size of a subset used in the analysis of the digital image correlation method based on the spot size calculated in the calculation step. In the displacement amount measuring program according to claim 7.
In the acquisition step, an image of the random pattern of the specimen deformed by applying a tensile load is further acquired.
In the determination step, a subset for elastic modulus measurement and a subset for fracture strain measurement having a size different from the modulus for elastic modulus measurement are determined based on the size of the spots, respectively.
A displacement amount measurement program further comprising an analysis step of analyzing the digital image correlation method using the subset for measuring elastic modulus or the subset for measuring fracture strain and measuring the amount of displacement of the specimen. In the displacement amount measuring program according to claim 8,
A displacement amount measurement program in which the size of the subset for measuring elastic modulus is larger than the size of the subset for measuring fracture strain.

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