Method and device for measuring stress and strain of concrete material through double-machine-position DIC

Abstract

The invention relates to the technical field of material science and engineering, in particular to a method and device for measuring stress and strain of a concrete material through a double-machine-position DIC. The device comprises a tensile test frame, a first mechanical clamp, a second mechanical clamp, a third mechanical clamp, a standard test piece, a camera mounting frame, a first CCD industrial camera and a second CCD industrial camera, a standard test piece is connected between the first mechanical clamp and the second mechanical clamp, a concrete piece to be tested is connected between the second mechanical clamp and the third mechanical clamp, and the other end of the third mechanical clamp can move vertically. The method comprises the following steps: capturing and calculating the strain of a standard test piece, deducing the stress of the standard test piece by using the elastic modulus of the standard test piece, calculating and measuring the surface strain field of a to-be-tested concrete piece by using DIC, and finally obtaining a stress-strain curve and a 2D stress-strain cloud picture of the to-be-tested concrete piece. According to the invention, non-contact, high-precision and low-cost measurement of the mechanical properties of the concrete material is realized without a force sensor.

Description

Technical Field

[0001] The invention relates to the field of material science and engineering technology, and in particular to a method and a device for measuring stress and strain of concrete materials using dual-position DIC. Background Art

[0002] As a widely used building material, the mechanical properties of concrete are crucial to the safety and durability of engineering structures. The stress-strain relationship of concrete is the basis for evaluating its mechanical properties, and is of great significance for predicting the behavior of materials under stress, optimizing structural design, and guiding construction.

[0003] Traditionally, stress-strain characteristics of concrete materials are usually measured through tensile, compression, bending and other tests to evaluate the tensile strength, compressive strength, flexural strength and deformation capacity of concrete, which is a destructive test. For this reason, modern non-destructive technologies have emerged, such as ultrasonic testing, coring, strain gauges and sensor technology. These non-destructive technologies are relatively convenient, but they also have many limitations, such as complex sensor installation, susceptibility to environmental influences, and cumbersome calibration. When using them, various factors need to be considered comprehensively to ensure the accuracy of the test results. In addition, the high cost limits their application in complex loading conditions and extreme environments.

[0004] As a non-contact, full-field deformation measurement method, Digital Image Correlation (DIC) technology has received extensive attention and application in material mechanical properties testing in recent years. DIC technology tracks the displacement changes of the surface or internal speckle pattern of the specimen during loading, and calculates the strain field of the specimen in combination with appropriate algorithms. It has the advantages of high measurement accuracy, strong adaptability, and low environmental requirements. However, using DIC technology alone to directly measure the stress of concrete materials still faces challenges, because as a composite material, the stress distribution of concrete is difficult to accurately infer directly through surface deformation.

[0005] To this end, this application is filed. Summary of the invention

[0006] In view of the above-mentioned shortcomings of the prior art, the present invention provides a method and device for measuring stress and strain of concrete materials using a dual-position DIC.

[0007] On the one hand, the present invention provides a dual-camera DIC device for measuring stress and strain of concrete materials, which is used to test a concrete test piece, including: a tensile test frame, a first mechanical clamp, a second mechanical clamp, a third mechanical clamp, a standard test piece, a camera mounting frame, a first CCD industrial camera, and a second CCD industrial camera;

[0008] The tensile test frame is provided with a first crossbeam and a second crossbeam opposite to each other, a first mechanical clamp and a third mechanical clamp are respectively installed on the first crossbeam and the second crossbeam, the second mechanical clamp is located between the first mechanical clamp and the third mechanical clamp, the first mechanical clamp and the second mechanical clamp are used to rotatably connect the standard test piece, the second mechanical clamp and the third mechanical clamp are used to rotatably connect the concrete test piece, and the second crossbeam is also connected with a tensile force loading mechanism for realizing the vertical movement of the second crossbeam;

[0009] The first CCD industrial camera and the second CCD industrial camera are installed on the camera mounting frame from low to high. The positions of the first CCD industrial camera and the second CCD industrial camera correspond to the concrete test piece and the standard test piece respectively.

[0010] During the test, the standard test piece is first rotatably connected between the first mechanical clamp and the second mechanical clamp, and the concrete test piece is rotatably connected between the second mechanical clamp and the third mechanical clamp; then the positions of the first CCD industrial camera and the second CCD industrial camera are adjusted so that their fields of view are respectively facing the concrete test piece and the standard test piece; then, the tensile force is applied through the tensile force loading mechanism until the concrete test piece breaks.

[0011] Preferably, the elastic modulus of the standard test piece is 50-250 GPa.

[0012] In order to ensure that the standard test piece has a certain tensile capacity and strength, the elastic modulus of the standard test piece is preferably 50-250GPa, such as aluminum alloy with an elastic modulus of about 70GPa, titanium alloy with an elastic modulus of about 118GPa, stainless steel with an elastic modulus of about 200GPa, carbon steel with an elastic modulus of about 206GPa, etc.

[0013] Preferably, the tensile test frame is also provided with a main power switch for controlling the on and off of the circuit and protecting the safe operation of the tensile force loading mechanism, as well as an emergency stop switch for quickly cutting off the power supply in an emergency to protect the safety of people and equipment. The first CCD industrial camera and the second CCD industrial camera are connected by a hot shoe support rod, and the rotatable connection is achieved by a universal joint.

[0014] On the other hand, the present invention proposes a method for measuring stress and strain of concrete materials using a dual-camera DIC. The specific operating steps of the measurement are as follows:

[0015] S1. Prepare a concrete test piece and a standard test piece with a known elastic modulus of the same size, and evenly set speckles on the surfaces of the concrete test piece and the standard test piece;

[0016] S2. Install the standard test piece and the concrete test piece and use the preload force to tension them to ensure that the concrete test piece and the standard test piece are always in a straight line during the stretching process; turn on the first CCD industrial camera and the second CCD industrial camera and preheat them for a period of time, and ensure that the fields of view of the first CCD industrial camera and the second CCD industrial camera are respectively perpendicular to the surfaces of the concrete test piece and the standard test piece;

[0017] S3, firstly collect images of the concrete test piece and the standard test piece before the tensile test by the first CCD industrial camera and the second CCD industrial camera respectively, and use them as reference images, then gradually load the tensile force to perform the tensile test until the concrete test piece breaks, the first CCD industrial camera and the second CCD industrial camera shoot the process of the tensile test, the image taken during the tensile force loading process is the target image, and the data is recorded and statistically analyzed;

[0018] S4. Use open source software Ncorr for data processing, select the areas of interest in the standard test piece and the concrete test piece respectively, use the algorithm to obtain the real displacement field distribution in the x-direction and y-direction of the area of ​​interest, and further calculate the strain distribution; the elastic modulus E of the standard test piece is known, and the deformation Δl of the standard test piece is measured by DIC technology. According to Hooke's law F=EAΔl / L, the force value F of the standard test piece in the tensile stage is obtained, and then according to the cross-sectional area A of the standard test piece, the corresponding stress can be converted; the strain of the concrete test piece is directly measured according to the DIC technology; after integration, the stress-strain curve and the 2D stress-strain cloud map are obtained.

[0019] Digital Image Correlation (DIC) technology, referred to as DIC technology, tracks the displacement changes of the surface or internal speckle pattern of the specimen during loading, and calculates the strain field of the specimen with appropriate algorithms. It has the advantages of high measurement accuracy, strong adaptability, and low environmental requirements. However, since concrete is a composite material, its stress distribution is difficult to accurately infer directly through surface deformation, so using DIC technology alone to directly measure the stress of concrete materials faces challenges.

[0020] The present invention uses a second CCD industrial camera as an upper camera position, which captures and calculates the strain of a standard test piece that has the same size as the concrete test piece and is subjected to the same force, and indirectly derives the stress of the standard test piece using the known elastic modulus of the standard test piece, and the stress can be regarded as the tensile stress of the concrete test piece at the corresponding position (assuming that there is no slip on the interface between the two); uses a first CCD industrial camera as a lower camera position, which directly uses DIC to calculate and measure the strain field on the surface of the concrete test piece; the upper camera position and the lower camera position work together to obtain the stress-strain curve of the concrete test piece through the DIC technology.

[0021] Compared with the prior art, the method proposed in the present invention cleverly combines the non-contact measurement of DIC technology with the stability of standard test pieces of known materials as reference materials. Through the synergistic effect of CCD industrial cameras at the upper and lower positions, the precise measurement of the stress-strain process in the entire field during the tensile process of concrete materials is achieved. The present invention not only retains the advantages of non-contact measurement of DIC technology, but also effectively solves the difficulties faced by traditional stress measurement technology, providing a more efficient, accurate and economical solution for the study of the mechanical properties of concrete materials.

[0022] Preferably, in step S1, the material of the standard test piece is Q235 low carbon steel.

[0023] The tensile capacity of metal standard test pieces is closely related to their elastic modulus. The higher the elastic modulus of a material, the less likely it is to undergo elastic deformation under external force, which may cause the metal standard test piece to break more easily during the stretching process, and the elongation will decrease accordingly. The lower the elastic modulus of a material, the easier it is to undergo elastic deformation under external force, thus showing greater elongation and better flexibility during stretching. Therefore, the elastic modulus of a standard test piece should not be too large or too small. If it is too large, its elastic deformation will be small and difficult to capture, affecting the measurement effect. If it is too small, its elastic deformation will be large, but it cannot guarantee sufficient strength when subjected to force.

[0024] The present invention has been proved by creative labor that when the elastic modulus of the standard test piece is 50-250GPa, it can maintain sufficient strength during the stretching process, and can also show good elongation and flexibility, and its deformation is easily captured by a CCD industrial camera. Therefore, in the present embodiment, the material of the standard test piece can be carbon steel, low carbon steel, stainless steel, aluminum alloy, titanium alloy, etc., and Q235 low carbon steel is preferred in consideration of cost and mechanical properties. Q235 low carbon steel is a common ordinary carbon structural steel with low cost and moderate elastic modulus, and its elastic modulus is generally 200GPa, which can meet the use requirements of the present invention.

[0025] Preferably, in step S1, the setting of the speckle includes uniformly spraying the speckle and using commercial speckle, and the physical size of the speckle is preferably maintained at 3-6 pixels in the digital image recorded in the test system.

[0026] In a two-dimensional DIC test system, the quality of the speckle pattern is crucial to the accuracy of the measurement results. Generally speaking, it is ideal for the speckle to be maintained within the range of 3-6 pixels in the digital image. If the pixel is too small, the speckle pattern will become blurred in the image, increasing the error in the image matching process, thereby increasing the measurement error; at the same time, small-sized speckles are more prone to distortion, further affecting the accuracy of the measurement; in addition, the signal-to-noise ratio will be reduced, making the noise in the image have a greater impact on the measurement results. If the pixel is too large, the spatial resolution of the displacement and strain measurement will be reduced, because in order to match the large-sized speckle pattern, a larger subset area is required, thereby reducing the fineness of the measurement; large-sized speckles may also fail to accurately capture the tiny deformations on the surface of the object, because these deformations may not be clearly distinguished in the speckle pattern; in addition, in order to handle the matching of large-sized speckles, the computational burden and algorithm complexity may also increase.

[0027] Preferably, in step S2, the preheating time of the first CCD industrial camera and the second CCD industrial camera is not less than 30 minutes.

[0028] In this embodiment, preheating can not only make each component of the CCD industrial camera reach the best working state, thereby improving its sensitivity and shooting quality, but also reduce the interference of heat waves during the shooting process of the CCD industrial camera to obtain the best target image.

[0029] Preferably, in step S1, the concrete test piece and the standard test piece are rectangular structures of the same size, and the length, width and height of the rectangular structure are 38 mm, 13 mm and 250 mm respectively; in step S3, the maximum value of the tensile force is 0.494-1.482 KN.

[0030] It should be noted that the size of the above-mentioned rectangular structure is only the size adopted by the present invention. When the concrete test piece is subjected to tensile test by a universal testing machine, the tensile force is continuously loaded. When the maximum value of the tensile force is 0.494-1.482KN, the concrete test piece can be broken. Continuously loading the tensile force according to the above requirements can ensure that the concrete test piece goes through the elastic stage, the yield stage, the plastic deformation stage and the fracture, which helps the universal testing machine to effectively apply and record the mechanical response of the concrete test piece, and ensure the accuracy and repeatability of the experimental data. In actual operation, those skilled in the art can flexibly select the size of the concrete test piece and the standard test piece according to the specific tensile test device, and further determine the maximum value range of the tensile force. Preferably, in step S3, the shooting intervals of the first CCD industrial camera and the second CCD industrial camera are both 2 seconds.

[0031] Preferably, in step S4, the sub-region domain radius of the region of interest is set to 21 pixels, the step length is 5 pixels, the strain calculation window is selected to be 5 pixels, and the strain distribution is calculated.

[0032] The size of the sub-region directly affects the accuracy of strain measurement. Smaller sub-regions can provide higher spatial resolution, but may increase noise and errors. Larger sub-regions can reduce noise, but may reduce spatial resolution. In addition, the size of the sub-region will also affect the demand for computing resources and time efficiency. Larger sub-regions may require more computing resources and time to complete strain measurement. Taking into account the accuracy and efficiency of strain measurement, the present invention determines that the sub-region radius of the region of interest is set to 21 pixels, the step size is 5 pixels, and the strain calculation window is selected as 5 pixels. Under these technical parameters, both the accuracy and efficiency of measurement can be taken into account.

[0033] The present invention measures the stress and strain of concrete materials based on the DIC technology, replacing the contact displacement sensor based on the electromagnetic induction principle. Compared with the prior art, the present invention has at least the following beneficial effects:

[0034] 1. Stress and strain are measured based on DIC technology. The test process is intuitive and clear, and no physical contact with the concrete test piece is required. It not only avoids the measurement errors caused by friction, contact deformation, etc. in traditional contact measurement, but also expands the scope of application. It has very low requirements on the environment and materials, and is applicable to high temperature, high pressure, and corrosive environments. Whether it is metal, non-metal or composite material, whether it is a flat surface, curved surface or irregular surface, as long as the surface can be imaged, it is applicable.

[0035] 2. Stress and strain are measured based on DIC technology, which does not involve complicated installation and calibration. It has high measurement efficiency and high measurement accuracy, and the accuracy can reach sub-micron level, thus meeting the needs of high-precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0037] Figure 1 This is a schematic structural diagram of a dual-position DIC device for measuring stress and strain of concrete materials proposed by the present invention.

[0038] Figure 2 This is a photo of the concrete test piece prepared according to the present invention.

[0039] Figure 3 Schematic diagram of spraying speckle on the concrete test piece.

[0040] Figure 4 This is the operation interface for selecting the region of interest (ROI) using the open source Ncorr software;

[0041] Figure 5 For Figure 4 The operation interface for setting the sub-area radius and step size based on the

[0042] Figure 6 For Figure 5 Based on the algorithm, the real displacement field distribution in the x-direction and y-direction of the image sequence is obtained;

[0043] Figure 7 For Figure 6 The strain distribution results calculated when the strain calculation window is 5 pixels are selected on the basis of

[0044] Figure 8 DIC test results of the concrete specimen to be tested;

[0045] Fig. 9 It is the stress-strain curve of the concrete specimen.

[0046] In the figure: 1. tensile test frame; 11. first crossbeam; 12. second crossbeam; 13. main power switch; 14. emergency stop switch; 2. first mechanical clamp; 3. second mechanical clamp; 4. third mechanical clamp; 5. standard test piece; 6. concrete test piece; 7. camera mounting frame; 8. first CCD industrial camera; 9. hot shoe support rod; 10. second CCD industrial camera. DETAILED DESCRIPTION

[0047] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0048] In the following embodiments, the first CCD industrial camera and the second CCD industrial camera are both Hikvision MV-CH250-90YC-C-NN V2, the hot shoe support rod is commercially purchased, the speckle pattern is a commercially purchased transfer speckle patch, and the open source Ncorr software is purchased commercial software.

[0049] like Figure 1As shown: The present invention proposes a dual-camera DIC device for measuring stress and strain of concrete materials, which is used to test a concrete test piece 6, including: a tensile test frame 1, a first mechanical clamp 2, a second mechanical clamp 3, a third mechanical clamp 4, a standard test piece 5, a camera mounting frame 7, a first CCD industrial camera 8 and a second CCD industrial camera 10;

[0050] The tensile test frame 1 is provided with a first crossbeam 11 and a second crossbeam 12 opposite to each other, the first mechanical clamp 2 and the third mechanical clamp 4 are respectively installed on the first crossbeam 11 and the second crossbeam 12, the second mechanical clamp 3 is located between the first mechanical clamp 2 and the third mechanical clamp 4, the first mechanical clamp 2 and the second mechanical clamp 3 are rotatably connected to the standard test piece 5 through a universal joint, the second mechanical clamp 3 and the third mechanical clamp 4 are rotatably connected to the concrete test piece 6 through a universal joint, and the second crossbeam 12 is also connected to a tensile force loading mechanism, under the action of the tensile force loading mechanism, the second crossbeam 12 can move downward to apply a tensile force to the concrete test piece 6; in order to facilitate operation and ensure the safety of operation, the tensile test frame 1 is also provided with a main power switch 13 and an emergency stop switch 14;

[0051] The first CCD industrial camera 8 and the second CCD industrial camera 10 are installed on the camera mounting frame 7 from low to high. The first CCD industrial camera 8 and the second CCD industrial camera 10 are connected by a hot shoe support rod 9. The positions of the first CCD industrial camera 8 and the second CCD industrial camera 10 correspond to the concrete test piece 6 and the standard test piece 5 respectively.

[0052] The present invention also proposes a method of utilizing Figure 1 The method for measuring the stress and strain of concrete materials by the device shown in the figure, the specific operating steps of the measurement are as follows:

[0053] S1,

[0054] First, add cement, fly ash, sand, and fiber into a mixer and stir for 1-2 minutes. Then add water and water reducing agent and mix for 5 minutes. Pour into a mold and let it stand in the air for 24 hours. Figure 2 Finally, the sample was taken out of the mold and cured in a humid cabinet at 24°C and 95% humidity for 7 days to obtain the following Figure 2 The concrete test piece shown in Figure b is a rectangular structure with a length, width and height of 38 mm, 13 mm and 250 mm respectively;

[0055] Make a standard test piece 5 made of Q235 low carbon steel with the same size as the concrete test piece;

[0056] The speckle is evenly arranged on the surface of the concrete test piece 6 and the standard test piece 5, and the physical size of the speckle is preferably maintained at 3-6 pixels in the digital image recorded in the test system;

[0057] S2,

[0058] Install the standard test piece 5 and the concrete test piece 6 and use the pre-tightening force to tension them to ensure that the concrete test piece 6 and the standard test piece 5 are always in a straight line during the stretching process. Specifically, the two ends of the standard test piece 5 and the concrete test piece 6 are clamped by clamps respectively, and then connected to each mechanical clamp through the clamps. After the connection, the loading force of the tensile force loading mechanism is gradually increased until the standard test piece 5 and the concrete test piece 6 remain stable and will not move due to slight external force;

[0059] Turn on the first CCD industrial camera 8 and the second CCD industrial camera 10 and preheat them for 30 minutes, and ensure that the fields of view of the first CCD industrial camera 8 and the second CCD industrial camera 10 are perpendicular to the surfaces of the concrete test piece 6 and the standard test piece 5, respectively;

[0060] S3,

[0061] First, the images of the concrete test piece 6 and the standard test piece 5 before the tensile test are collected by the first CCD industrial camera 8 and the second CCD industrial camera 10 respectively, and used as reference images, and then the tensile force is gradually loaded to perform the tensile test. When the tensile force is loaded to 1.35KN, the concrete test piece 6 breaks. The first CCD industrial camera 8 and the second CCD industrial camera 10 shoot the process of the tensile test, and the shooting interval is 2 seconds. The images taken during the tensile force loading process are the target images, and the data are recorded and statistically analyzed;

[0062] S4,

[0063] The open source software Ncorr was used for data processing. Figure 4 As shown: select the region of interest (ROI) in the standard test piece 5 and the concrete test piece 6 respectively; secondly, take the concrete test piece 6 as an example, as Figure 5 As shown: Set the sub-area radius to 21 pixels and the step size to 5 pixels, and use the algorithm to get the following Figure 6 The image sequence shown shows the real displacement field distribution in the x and y directions (unit: mm). The strain calculation window is selected as 5 pixels. The calculated strain distribution results are as follows: Figure 7 As shown, the DIC test results are as follows Figure 8 As shown; again, the strain value of the standard test piece 5 is calculated according to the similar operation as above; finally, the strain value of the standard test piece 5 is combined with the strain value of the concrete test piece 6 to obtain the following Fig. 9 The stress-strain curve is shown.

[0064] The second CCD industrial camera 10 at the upper position selects the fixed elastic modulus E of Q235 low-carbon steel and uses the DIC technology to measure the deformation Δl of the steel. According to Hooke's law, Δl=FL / (EA), L is the length of the region of interest, the elastic modulus E is 200GPa, and A is the cross-sectional area (in this embodiment, specifically 38mm×13mm). The force value F of the material in the tensile stage is then obtained, and the corresponding stress can be converted according to the cross-sectional area A of the specimen. The first CCD industrial camera 8 at the lower position directly obtains the strain of the concrete material according to the DIC testing technology. The upper and lower positions jointly obtain the stress-strain curve of this material, as shown in FIG. Fig. 9 shown.

[0065] In summary: the principle of the present invention is reliable, which not only avoids the problem of calibration of contact sensors in the prior art, but also reduces the testing cost of the laboratory, improves the test accuracy, and can also test the stress-strain process of a certain area of ​​the concrete material in the tensile stage. The test device has a simple structure and is easy to operate. The test method results are accurate, and it has high practical value and good promotion value.

[0066] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Those skilled in the art may change, modify, replace and deform the above embodiments within the scope of the present invention. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples without contradiction.

Claims

1. A dual-station DIC device for measuring stress and strain of concrete materials, used for testing a concrete test piece (6), characterized in that: include: A tensile test frame (1), a first mechanical clamp (2), a second mechanical clamp (3), a third mechanical clamp (4), a standard test piece (5), a camera mounting frame (7), a first CCD industrial camera (8) and a second CCD industrial camera (10); A tensile test frame (1) is provided with a first crossbeam (11) and a second crossbeam (12) which are opposite to each other; the second crossbeam (12) is vertically movable; a first mechanical clamp (2) and a third mechanical clamp (4) are respectively mounted on the first crossbeam (11) and the second crossbeam (12); the second mechanical clamp (3) is located between the first mechanical clamp (2) and the third mechanical clamp (4); a standard test piece (5) is rotatably connected between the first mechanical clamp (2) and the second mechanical clamp (3); a concrete test piece (6) is rotatably connected between the second mechanical clamp (3) and the third mechanical clamp (4); and a tensile force loading mechanism for realizing its vertical movement is also connected to the second crossbeam (12); A first CCD industrial camera (8) and a second CCD industrial camera (10) are mounted on a camera mounting frame (7) from low to high, and the positions of the first CCD industrial camera (8) and the second CCD industrial camera (10) correspond to the concrete test piece (6) and the standard test piece (5), respectively.

2. The dual-position DIC device for measuring stress and strain of concrete materials according to claim 1 is characterized in that: The elastic modulus of the standard test piece (5) is 50-250 GPa.

3. The dual-position DIC device for measuring stress and strain of concrete materials according to claim 1 is characterized in that: The tensile test frame (1) is also provided with a main power switch (13) and an emergency stop switch (14); the first CCD industrial camera (8) and the second CCD industrial camera (10) are connected via a hot shoe support rod (9); and the rotatable connection is achieved via a universal joint.

4. A method for measuring stress and strain of concrete materials using dual-position DIC, characterized in that: The measurement is performed using the device described in any one of claims 1 to 3, and the specific operation steps of the measurement are as follows: S1, preparing a concrete test piece (6) of the same size and a standard test piece (5) with a known elastic modulus, and evenly setting speckles on the surfaces of the concrete test piece (6) and the standard test piece (5); S2, installing the standard test piece (5) and the concrete test piece (6) and tensioning them with a preload force to ensure that the concrete test piece (6) and the standard test piece (5) are always in a straight line during the stretching process; turning on the first CCD industrial camera (8) and the second CCD industrial camera (10) and preheating them for a period of time, and ensuring that the fields of view of the first CCD industrial camera (8) and the second CCD industrial camera (10) are respectively perpendicular to the surfaces of the concrete test piece (6) and the standard test piece (5); S3, firstly, using a first CCD industrial camera (8) and a second CCD industrial camera (10) to respectively collect images of the concrete test piece (6) and the standard test piece (5) before the tensile test, and use them as reference images, then gradually apply tensile force to perform the tensile test until the concrete test piece (6) breaks, and the first CCD industrial camera (8) and the second CCD industrial camera (10) take pictures of the tensile test process, and the images taken during the tensile force application process are target images, and the data are recorded and statistically analyzed; S4. Use open source software Ncorr to process data, select the selected areas of interest in the standard test piece (5) and the concrete test piece (6), respectively, use the algorithm to obtain the real displacement field distribution in the x-direction and y-direction of the area of ​​interest, and further calculate the strain distribution; use DIC technology to measure the deformation Δl of the standard test piece (5), and obtain the force value F of the standard test piece (5) in the tensile stage according to Hooke's law F=EAΔl / L, and then convert the corresponding stress according to the cross-sectional area A of the standard test piece (5); The strain of the concrete test piece (6) is directly measured according to the DIC technology; after integration, a stress-strain curve and a 2D stress-strain cloud diagram are obtained.

5. The method for measuring stress and strain of concrete material by using dual-position DIC according to claim 4 is characterized in that: In step S1, the material of the standard test piece (5) is Q235 low carbon steel.

6. The method for measuring stress and strain of concrete material by using dual-position DIC according to claim 4 is characterized in that: In step S1, the setting of speckle includes uniformly spraying speckle and using commercial speckle, and the physical size of the speckle is preferably maintained at 3-6 pixels in the digital image recorded in the test system.

7. The method for measuring stress and strain of concrete material by using dual-position DIC according to claim 4 is characterized in that: In step S2, the preheating time of the first CCD industrial camera (8) and the second CCD industrial camera (10) is not less than 30 minutes.

8. The method for measuring stress and strain of concrete material by using dual-position DIC according to claim 4 is characterized in that: In step S1, the concrete test piece (6) and the standard test piece (5) are rectangular structures of the same size, and the length, width and height of the rectangular structure are 38 mm, 13 mm and 250 mm respectively; in step S3, the maximum value of the tensile force is 0.494-1.482 KN.

9. The method for measuring stress and strain of concrete material by using dual-position DIC according to claim 4, characterized in that: In step S3, the shooting intervals of the first CCD industrial camera (8) and the second CCD industrial camera (10) are both 2 seconds.

10. The method for measuring stress and strain of concrete material by using dual-position DIC according to claim 4, characterized in that: In step S4, the sub-region radius of the region of interest is set to 21 pixels, the step length is 5 pixels, the strain calculation window is selected to be 5 pixels, and the strain distribution is calculated.

Images