Rubber Strain Correction Method and Measuring Device Based on Two-Dimensional Digital Image Correlation Method

Abstract

The present invention discloses a rubber strain correction method and device based on the two-dimensional digital image correlation method. In the method, a measurement area is demarcated on the front surface of the gauge length area of a rubber specimen, and correction areas are arranged on both sides of the measurement area; thin sheets are adhered in the correction areas through grease; the hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected such that the thin sheets can generate relative sliding with the surface of the rubber specimen during the stretching process; stoppers are pasted on the side surface of the gauge length area corresponding to the positions of the thin sheets; speckle patterns are prepared on both the measurement area and the thin sheets; during the stretching experiment, the deformation images of the rubber specimen under loading are recorded, and the strain data of the measurement area and two correction areas are obtained by using the digital image correlation method, and the strain data of the measurement area are corrected by using the strain data of the correction areas. The present invention can eliminate the influence of out-of-plane motion caused by Poisson effect and thermal expansion on rubber strain measurement, improve the strain measurement accuracy, and significantly reduce the cost.

Description

Technical Field

[0001] The present invention belongs to the fields of optical measurement mechanics and material mechanics property testing, and particularly relates to a rubber strain correction method and a rubber strain measurement device based on the two-dimensional digital image correlation method. Background Art

[0002] Currently, in the measurement of the mechanical properties of rubber materials, a clip-on mechanical extensometer is generally used to measure the axial strain of rubber materials in the uniaxial tensile state. However, since the clip-on mechanical extensometer directly contacts the rubber surface, to a certain extent, it will cause local stress concentration, and when the rubber material undergoes large deformation, relative slip may occur between the clamping end of the clip-on mechanical extensometer and the rubber surface. Therefore, the traditional clip-on mechanical extensometer will have a certain strain measurement error, and a clip-on mechanical extensometer can only measure unidirectional strain (generally axial strain). Therefore, how to more accurately measure the axial strain or even the full-field strain of rubber materials under tensile deformation has certain research significance.

[0003] Due to the advantages of simple equipment and non-contact full-field strain measurement of the two-dimensional digital image correlation method, the two-dimensional digital image correlation method has gradually replaced the traditional clip-on mechanical extensometer and is widely used in rubber strain measurement. However, when using the two-dimensional digital image correlation method to measure the strain of rubber during the tensile process, due to the Poisson effect, the rubber specimen will undergo lateral contraction, that is, the test surface of the rubber will move away from the camera during tensile, resulting in out-of-plane movement, thereby generating strain error. In addition, due to the thermal expansion of the camera itself, it will also cause slight out-of-plane movement of the internal sensor and lens of the camera, resulting in strain error. Therefore, in order to eliminate the influence of out-of-plane movement, for the two-dimensional digital image correlation method to measure rubber strain, generally, expensive telecentric lenses or long focal length lenses need to be equipped, and using a bilateral telecentric lens can effectively reduce the influence of out-of-plane movement caused by the Poisson effect and thermal expansion on strain measurement. However, such expensive telecentric lenses or long focal length lenses increase the equipment cost and are not conducive to the two-dimensional digital image correlation method for rubber strain measurement in industry. Summary of the Invention

[0004] In view of this, the present invention provides a rubber strain correction method and a rubber strain measurement device based on the two-dimensional digital image correlation method. This solution can eliminate the influence of out-of-plane movement caused by the Poisson effect and thermal expansion on rubber strain measurement, so as to obtain a higher-precision strain value of rubber during the deformation process, and this method can use ordinary lenses without the need for expensive telecentric lenses or long focal length lenses, thereby significantly reducing the equipment cost.

[0005] To solve the above technical problems, the present invention is implemented as follows.

[0006] A rubber strain correction method based on the two-dimensional digital image correlation method, comprising the following steps:

[0007] Divide a measurement area on the front surface of the gauge length area of the rubber specimen, with correction areas on both sides of the measurement area; adhere 2n thin sheets in the correction areas, where n is a positive integer, and the positions of the thin sheets are symmetric about the measurement area; the hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected such that the thin sheets can slide relative to the surface of the rubber specimen during the stretching process; paste retaining sheets at the positions corresponding to the thin sheets on the side surface of the gauge length area; speckle patterns are prepared on both the measurement area and the thin sheets;

[0008] The tensile machine grips the rubber specimen and completes the tensile experiment. During the experiment, record the deformation image of the rubber specimen under loading, obtain the strain data of the measurement area and the two correction areas using the digital image correlation method, and correct the strain data of the measurement area using the strain data of the correction areas to obtain the strain value during the deformation process of the rubber specimen.

[0009] Preferably, the method further includes: before stretching the rubber specimen, obtain the camera internal parameter matrix and the radial distortion coefficient using the Zhang-Zhengyou calibration method:

[0010] After recording the deformation image of the rubber specimen during the experiment, perform distortion correction on the deformation image using the obtained camera internal parameter matrix and the radial distortion coefficient.

[0011] Preferably, the correction of the strain data of the measurement area using the strain data of the correction areas is: subtract the mean value of all the thin sheet strain data from the strain data of the measurement area.

[0012] Preferably, the preparation method of the speckle patterns on the measurement area and the thin sheets is: the thin sheets use matte hard sheets, adhere the thin sheets on both sides of the measurement area, coat silicone oil on the matte surface of the thin sheets and the measurement area, and then evenly sprinkle black powder to form the speckle patterns.

[0013] Preferably, there is a certain gap between the side of the thin sheet close to the retaining sheet and the plane where the retaining sheet is located; the side of the retaining sheet close to the thin sheet extends beyond the plane where the thin sheet is located by a certain distance; the retaining sheet can limit the in-plane rotation of the thin sheet during the stretching of the specimen.

[0014] The present invention also provides a rubber strain measurement device based on the two-dimensional digital image correlation method. The device includes a rubber specimen (1), two thin sheets, two retaining sheets, a camera, and a processing module;

[0015] Divide a measurement area on the front surface of the gauge length area of the rubber specimen, with correction areas on both sides of the measurement area;

[0016] Two thin sheets are adhered to the correction area by grease; the positions of the thin sheets are symmetrical about the measurement area; the hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected such that the thin sheets can slide relative to the surface of the rubber specimen during the stretching process; the stop is attached to the side surface of the gauge length area, corresponding to the position of the thin sheets; speckle patterns are prepared on both the measurement area and the thin sheets;

[0017] The camera is used to record the deformation image of the rubber specimen under loading during the tensile test of the rubber specimen clamped by the tensile machine and send it to the processing module;

[0018] The processing module is used to obtain the strain data of the measurement area and the two thin sheets by using the digital image correlation method according to the deformation image, and subtract the mean value of the strain data of the two thin sheets from the strain data of the measurement area to obtain the corrected strain value of the rubber specimen.

[0019] Preferably, the thin sheets are made of matte hard sheets. After the thin sheets are adhered to both sides of the measurement area, silicone oil is coated on the matte surface of the thin sheets and the measurement area, and then black powder is evenly sprinkled to obtain the speckle pattern.

[0020] Preferably, there is a certain gap between the side of the thin sheet close to the stop and the plane where the stop is located; the side of the stop close to the thin sheet extends beyond the plane where the thin sheet is located by a certain distance; the stop can limit the in-plane rotation of the thin sheet during the stretching of the specimen.

[0021] Preferably, the stop is made of the same material and structure as the thin sheet.

[0022] Preferably, the thickness of the thin sheet is less than 0.5 mm; the elastic modulus of the thin sheet is Esheet≥1 GPa; the grease is a grease with a consistency grade greater than or equal to 2, presenting a solid or semi-solid state.

[0023] Beneficial effects:

[0024] (1) Considering that rigid connection will significantly limit the deformation of rubber-like materials and cause stress concentration, thus affecting the strain measurement results of rubber-like materials. Based on this, the present invention adheres thin sheets to the rubber surface by grease. Due to the non-rigid connection of the grease and the relative sliding between the thin sheets and the surface of the rubber specimen under the action of a small viscous force, the influence of the existence of the thin sheets on the rubber deformation can be ignored. And here the grease has a certain viscosity and can drive the thin sheets to move along with the surface of the rubber specimen during the rubber deformation process, thus avoiding stress concentration.

[0025] (2) The thin sheet of the present invention is a hard material compared to the rubber specimen. That is, in the tensile experiment, the thin sheet will not produce any deformation. Therefore, the strain measured on the thin sheet by the two-dimensional digital image correlation method is the strain error caused by the out-of-plane movement due to Poisson's effect and thermal expansion. Here, subtracting the average value of the strain measured on the upper and lower thin sheets from the strain measured on the surface of the rubber specimen effectively eliminates the influence of the out-of-plane movement caused by Poisson's effect and thermal expansion on the measurement of rubber strain by the two-dimensional digital image correlation method, and thus obtains a more accurate strain value during the rubber deformation process.

[0026] (3) In a preferred embodiment of the present invention, there is a gap between the retaining piece and the thin sheet, so that the thin sheet will not lift the retaining piece during rotation. Otherwise, the lifted retaining piece will reduce the adhesion area with the rubber specimen and significantly reduce its adhesion force with the surface of the rubber specimen, thus failing to play a role in preventing the in-plane rotation of the thin sheet.

[0027] (4) During the process of measuring rubber strain using the two-dimensional digital image correlation method in this experiment, expensive telecentric lenses or long focal length lenses are not required, and relatively accurate strain data can be obtained using ordinary lenses, thus significantly reducing the equipment cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic structural diagram of the present invention;

[0029] Figure 2 is the chessboard image of the grid calibration plate at different positions and angles;

[0030] Figure 3 is the strain error diagram of the rubber specimen before and after strain correction in the static experiment;

[0031] Figure 4 is the deformation image of the rubber specimen when measuring strain using the two-dimensional digital image correlation method and the clip-on mechanical extensometer simultaneously;

[0032] Figure 5 is the strain-time diagram of the rubber specimen when measuring strain using the two-dimensional digital image correlation method and the clip-on mechanical extensometer simultaneously;

[0033] Wherein, 1 - rubber specimen, 2 - thin sheet, 3 - retaining piece, 4 - camera, 5 - computer (processing module). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The present invention provides a rubber strain correction solution based on the two-dimensional digital image correlation method. The core idea is as follows: a measurement area is divided on the front surface of the gauge length area of the rubber specimen, and the two sides of the measurement area are correction areas; 2n thin sheets are adhered in the correction areas by grease, where n is a positive integer, and the positions of the thin sheets are symmetric about the measurement area; the hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected such that the thin sheets can have relative sliding with the surface of the rubber specimen during the stretching process; a retaining piece is pasted on the side surface of the gauge length area corresponding to the position of the thin sheet; speckle patterns are prepared on both the measurement area and the thin sheets.

[0035] During the experiment, the stretching machine grips the rubber specimen and completes the stretching experiment. During the experiment, the deformation images of the rubber specimen under uniaxial stretching are recorded, and the strain data of the measurement area and the two correction areas are obtained by using the digital image correlation method. The strain data of the measurement area is corrected by using the strain data of the correction areas to obtain the strain value during the deformation process of the rubber specimen.

[0036] It can be seen that the present invention adheres thin sheets on the rubber surface by using grease, which can eliminate the influence of out-of-plane motion caused by Poisson effect and thermal expansion on the rubber strain measurement, so as to obtain a higher-precision strain value during the deformation process of the rubber. Secondly, this method can use ordinary lenses and does not require expensive telecentric lenses or long focal length lenses, thus significantly reducing the equipment cost.

[0037] For the selection of the thin sheet thickness, it is assumed that the rubber is incompressible and isotropic during the deformation process. And since the out-of-plane translation is the main factor affecting the measurement of rubber strain by the two-dimensional digital image correlation method, therefore, for the convenience of selecting the thin sheet thickness, only the influence of out-of-plane translation is considered here. Then, combined with the principle of pinhole imaging, the strain error Δε(t) determined by different thin sheet thicknesses during the deformation process of the rubber is obtained as shown in the following formula:

[0038]

[0039] Here, a is the initial object distance, b is the initial side width of the rubber specimen, ε y is the current longitudinal strain of the rubber specimen, and t is the thickness of the thin sheet.

[0040] Here, the thin sheet with a thickness of 0 should have the same strain error as the rubber surface. Therefore, taking the strain error determined by the thin sheet with a thickness of 0 during the deformation process of the rubber as the standard, and then using the following formula to determine the relative strain error between different thin sheet thicknesses and the thin sheet with a thickness of 0:

[0041]

[0042] Here, substituting the common parameters, a = 100 mm, b = 2 mm, ε y= 3, it can be found that when t < 0.5 mm, the strain error of different sheet thicknesses relative to a sheet thickness of 0 will be less than 21 με. That is, in order to obtain strain measurement with higher precision, the thickness of the sheet is generally less than 0.5 mm here.

[0043] Preferably, the elastic modulus of the sheet is selected as E sheet ≥ 1 GPa, so that when initially adhering the sheet, the sheet will not produce initial buckling but adhere to the surface of the rubber specimen in a flat state. And due to the small viscous force generated by the selected grease during subsequent movement, the sheet will not produce any deformation.

[0044] For the grease, select a grease with a consistency grade greater than or equal to 2 and presenting a solid or semi-solid state, so as to ensure that the sheet can stably adhere to the surface of the rubber specimen during stretching and move with it; and it can also ensure relative sliding between the sheet and the surface of the rubber specimen under the action of a small viscous force.

[0045] The present invention will be described in detail below with reference to the accompanying drawings and by way of examples.

[0046] Example 1:

[0047] This example provides a rubber strain correction method based on the two-dimensional digital image correlation method, which uses 2 sheets to achieve strain correction.

[0048] This method includes the following steps:

[0049] Step 1. Fabricate rubber specimen 1.

[0050] The rubber specimen in this example is in the shape of a "dumbbell". The specific preparation method is: Mix the commercially available imported two-component silicone rubber (Sylgard 184) evenly according to the curing ratio A:B = 10:1, pour it into a polytetrafluoroethylene mold after evacuating, and cure it at 70 degrees for 3 hours to form a type 2 dumbbell-shaped rubber specimen 1 (meeting the international standard: ISO37-2017), that is, the gauge size of the rubber specimen 1 is 6 mm wide and 2 mm thick. The present invention has no requirements on the rubber material, and NR, SBR, etc. can also be selected.

[0051] Divide the gauge area of the rubber specimen 1 into a measurement area and a correction area. As shown in the figure, the middle of the gauge area is the measurement area, and both sides are the correction areas.

[0052] Step 2. Prepare sheet 2 and stop piece 3.

[0053] In this embodiment, the speckle patterns of the thin sheet and the measurement area are achieved by sprinkling black powder. Therefore, a matte hard sheet is used as the substrate of the thin sheet. The matte hard sheet is cut into thin sheets 2 of the required size. In practice, the speckle patterns of the thin sheet and the measurement area can also be achieved by spraying two kinds of paints, black and white. In this way, it is not necessary to limit that the thin sheet must be made of a matte hard sheet, but a hard sheet that meets the requirements can be used.

[0054] In this embodiment, the baffle is also prepared from a matte hard sheet. Using the same material to prepare the baffle and the thin sheet can save materials. However, the baffle is only used to block the rotation of the thin sheet, so its thickness does not necessarily need to be the same as that of the thin sheet. In this embodiment, the thicknesses of the thin sheet and the baffle are 0.17 mm.

[0055] According to the above gauge length dimensions, a matte white polyvinyl chloride (PVC) thin sheet is cut into thin sheets 2 of 6.2 mm × 6.2 mm and baffles 3 of 2.2 mm × 6.2 mm.

[0056] Step 3: Apply a thin layer of grease on the surface of the prepared rubber specimen 1, the surface of the thin sheet 2, and any one side surface of the baffle 3. Adhere the thin sheet 2 to the front surface (wide surface) of the rubber specimen 1, and adhere the baffle 3 to the side surface (narrow surface) of the rubber specimen 1.

[0057] The grease has a certain viscosity. Here, high-vacuum silicone grease can be selected, and in practice, white lithium-based grease can also be selected. Then, in this embodiment, a thin layer of high-vacuum silicone grease is applied on the surface of the rubber specimen 1 to be measured, the surface of the thin sheet 2, and any one side surface of the baffle 3. Then, the thin sheet 2 is adhered to the wide surface of the rubber specimen 1, and the baffle 3 is adhered to the narrow surface of the rubber specimen 1.

[0058] As Figure 1 shown, the positional relationship between the thin sheet and the baffle is that there is a gap between the side of the thin sheet 2 close to the baffle 3 and the plane where the baffle 3 is located. This gap can be 0.5 mm. The side of the baffle 3 close to the thin sheet 2 exceeds the plane where the thin sheet 2 is located by a certain distance. This distance can be 0.5 mm. Having a gap between the baffle and the thin sheet enables the thin sheet not to lift the baffle during rotation. Otherwise, the lifted baffle will reduce the adhesion area with the rubber specimen and significantly reduce its adhesion force with the surface of the rubber specimen, thus failing to play the role of preventing the in-plane rotation of the thin sheet.

[0059] Step 4: Prepare speckle patterns on the measurement area and the thin sheet.

[0060] In this step, black iron powder (which can be black iron powder of about 100 microns) is evenly sprinkled on the wide surface of the rubber specimen 1 and the surface of the thin sheet 2, and the black iron powder is adhered to the above surfaces by using high-vacuum silicone grease, thereby forming a good-quality speckle pattern.

[0061] Step 5: Before stretching the rubber specimen, first obtain the camera internal parameter matrix and radial distortion coefficients using the Zhang-Zhengyou calibration method.

[0062] In this step, first use the camera 4 (which can be a CMOS camera or a CCD camera) to take a series of images of the checkerboard calibration plate at different positions and angles, as Figure 2 shown. Then input the obtained series of images into the camera calibration toolbox of MATLAB, select the images with a projection error less than 0.5 pixels, and then correct and obtain the camera internal parameter matrix and radial distortion coefficients. The radial distortion coefficients take the first two orders.

[0063] Step 6: Conduct a stretching experiment, record the deformation images of the rubber specimen 1, obtain the strain data of the measurement area and two correction areas using the digital image correlation method, and correct the strain data of the measurement area using the strain data of the correction areas to obtain the strain values during the deformation process of the rubber specimen.

[0064] In this step, use a tensile machine to clamp the specimen and complete the stretching experiment.

[0065] During the experiment, use the camera 4 to record the deformation images of the prepared sample under uniaxial tension. Then, using the obtained camera internal parameter matrix and radial distortion coefficients, write the corresponding MATLAB image distortion correction code to correct the deformation images. Then use the processing module 5 (which can be a computer) to analyze and process the distorted corrected images, that is, use the digital image correlation method to match the gray-scale correlation of the initial image and the subset of the deformed image, so as to obtain the strain field of the corresponding area, and then take the average of the strains in the strain field to determine the measured strain of the rubber specimen 1: The measured strain of the upper thin sheet 1 (2): And the measured strain of the lower thin sheet 1 (2):

[0066] And due to the presence of high-vacuum silicone grease, the thin sheet 2 can slide on the surface of the rubber specimen 1 and will not affect the strain measurement of the rubber specimen 1. Use the baffle 3 to prevent the thin sheet 1-2 from rotating in the plane. And here the PVC thin sheet is a hard plastic compared to the rubber specimen 1, that is, in the stretching experiment, the thin sheet 2 will not produce any deformation. Therefore, the strain measured on the thin sheet 2 by the two-dimensional digital image correlation method is the strain error caused by the out-of-plane movement due to Poisson's effect and thermal expansion. Therefore, the following relationship exists between the above-mentioned measured strain and the true strain:

[0067]

[0068]

[0069]

[0070] Among them, represents the true strain of rubber specimen 1, represents the strain error of rubber specimen 1, represents the strain error of one of the thin sheets, represents the strain error of the other thin sheet. The subscript x indicates that the above data are strain data in the x direction, and x represents the transverse direction.

[0071] Since thin sheet 2 is very thin, the measurement surfaces of thin sheet 2 and rubber specimen 1 can be regarded as being in the same plane, that is, the strain errors caused by out-of-plane motion for both can be regarded as the same, that is

[0072]

[0073] Therefore, using the aforementioned formula, the true transverse strain of rubber specimen (1) during the tensile process can be determined, that is

[0074]

[0075] Similarly, the true longitudinal strain of rubber specimen (1) during the tensile process can be determined simultaneously, that is

[0076]

[0077] Here, since the influence of out-of-plane motion caused by lens distortion, Poisson effect, and thermal expansion on the measurement of rubber strain by the two-dimensional digital image correlation method is eliminated, relatively accurate strain values during the deformation process of rubber specimen 1 can be obtained.

[0078] To illustrate the rationality of the above strain correction process, a static experiment is carried out below. That is, when rubber specimen 1 is in a stationary state, a series of pictures are taken using CMOS camera 4. Here, since the rubber is not stretched, no deformation will occur on the surface of rubber specimen 1. Therefore, the strain on the surface of rubber specimen 1 obtained by the two-dimensional digital image correlation method is completely due to the strain error caused by the slight out-of-plane motion caused by the thermal expansion of the camera itself. And the Figure 3 gives the strain error data on the surface of rubber specimen 1 before and after correction. It can be found in the Figure 3 that the strain error after correction is about 30 με, which is much smaller than the strain error before correction. That is, the above rubber strain correction method can indeed improve the rubber strain measurement accuracy. And here, a Hikvision industrial camera with a resolution of only six million pixels is used. Since the resolution is also a major factor affecting the strain accuracy, if a Hikvision industrial camera with a higher resolution is used, the strain error can be further reduced and the rubber strain measurement accuracy can be improved.

[0079] Figure 4It is a physical diagram for simultaneously measuring the strain of a rubber specimen 1 by using the two-dimensional digital image correlation method and a clip-on mechanical extensometer. Figure 5 The strain results measured by the two-dimensional digital image correlation method and the clip-on mechanical extensometer were compared, and the results showed good consistency. The main reason for the slight difference was that there might be slippage at the clamping end of the extensometer, resulting in a smaller measured strain. In Figure 5 the strain measurement results of the two-dimensional digital image correlation method before and after correction were also compared. It can be seen from the results that the influence of out-of-plane motion on strain measurement was very small. That is, when the material tensile ratio was 1.85, the strain results before and after correction differed by about 0.006. Here, because the digital image correlation method was used, the longitudinal strain and transverse strain of the rubber under uniaxial tension could be obtained simultaneously. Then, combined with the isotropic assumption, the volume strain of the rubber could be obtained. However, when measuring the volume strain of the rubber under uniaxial tension, this small strain error caused by out-of-plane motion would result in an obvious volume strain error. Therefore, in some occasions where higher strain accuracy was required, the experimental equipment of the present invention was simple and low-cost, that is, a relatively high-precision strain value of the rubber during deformation could be obtained by using an ordinary lens.

[0080] Example 2:

[0081] This example provides a rubber strain measurement device based on the two-dimensional digital image correlation method. Still referring to Figure 1 it, the device includes a rubber specimen 1, two thin sheets 2, two retaining plates 3, a camera 4, and a processing module 5.

[0082] The front surface of the gauge section of the rubber specimen 1 is divided into a measurement area, and the two sides of the measurement area are correction areas;

[0083] The two thin sheets 2 are adhered to the correction areas through grease; the positions of the thin sheets are symmetric about the measurement area; the hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected such that the thin sheet 2 can have relative sliding with the surface of the rubber specimen 1 during the stretching process; the retaining plates 3 are attached to the side surfaces of the gauge section and correspond to the positions of the thin sheets 2; speckle patterns are prepared on both the measurement area and the thin sheets 2.

[0084] The camera is used to record the deformation image of the rubber specimen 1 under uniaxial tension during the stretching experiment of the rubber specimen (1) clamped by the tensile machine and send it to the processing module;

[0085] The processing module is used to obtain the strain data of the measurement area and the two thin sheets according to the deformation image by using the digital image correlation method, and subtract the mean value of the strain data of the two thin sheets 2 from the strain data of the measurement area to obtain the corrected strain value of the rubber specimen.

[0086] The preparation method of the thin sheet 2 and the speckle pattern of the measurement area, as well as the installation positions of the thin sheet and the baffle, are the same as those in Embodiment 1, and will not be elaborated here.

[0087] In the above experiment, the equipment for measuring rubber strain by the two-dimensional digital image correlation method includes a six-million-pixel Hikvision industrial camera, an ordinary lens, and devices such as wires and cables, which only cost about 1,800 yuan. It can be seen that the present invention can indeed use an ordinary lens without the need for an expensive telecentric lens or a long-focus lens, thus significantly reducing the equipment cost.

[0088] The above specific embodiments only describe the design principle of the present invention. The shapes and names of the components in this description can be different and are not restricted. Therefore, those skilled in the art of the present invention can modify or equivalently replace the technical solutions recorded in the foregoing embodiments; and these modifications and replacements do not depart from the purpose and technical solutions of the present invention, and shall all fall within the protection scope of the present invention.

Claims

1. A rubber strain correction method based on the two-dimensional digital image correlation method, characterized in that: The method includes the following steps: Divide a measurement area on the front surface of the gauge section of the rubber specimen (1). The two sides of the measurement area are correction areas. Adhere 2n thin sheets (2) to the correction areas through grease, where n is a positive integer, and the positions of the thin sheets are symmetrical about the measurement area. The hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected to enable the thin sheets (2) to slide relative to the surface of the rubber specimen (1) during the stretching process; Paste a retaining piece (3) on the side surface of the gauge section corresponding to the position of the thin sheet; Speckle patterns are prepared on both the measurement area and the thin sheet (2); The tensile machine grips the rubber specimen (1) and completes the tensile experiment. During the experiment, record the deformation image of the rubber specimen (1) under loading, obtain the strain data of the measurement area and the two correction areas by using the digital image correlation method, and correct the strain data of the measurement area by using the strain data of the correction areas to obtain the strain value during the deformation process of the rubber specimen.

2. The method according to claim 1, wherein This method further includes: before stretching the rubber specimen (1), obtaining the camera internal parameter matrix and the radial distortion coefficient by using the Zhang Zhengyou calibration method; After recording the deformation image of the rubber specimen (1) during the experiment, correct the deformation image by using the obtained camera internal parameter matrix and the radial distortion coefficient.

3. The method according to claim 1, wherein The correction of the strain data of the measurement area by using the strain data of the correction area is: subtracting the mean value of the strain data of all the thin sheets (2) from the strain data of the measurement area.

4. The method according to claim 1, wherein The preparation method of the speckle patterns on the measurement area and the thin sheet (2) is: the thin sheet (2) uses a matte hard sheet, adhere the thin sheet (2) to both sides of the measurement area, coat silicone oil on the matte surface of the thin sheet (2) and the measurement area, and then evenly sprinkle black powder to form a speckle pattern.

5. The method according to any one of claims 1-4, characterized in that, There is a certain gap between the side of the thin sheet (2) close to the retaining piece (3) and the plane where the retaining piece (3) is located; The side of the retaining piece (3) close to the thin sheet (2) extends beyond the plane where the thin sheet (2) is located by a certain distance; The retaining piece (3) can limit the in-plane rotation of the thin sheet (2) during the stretching of the specimen.

6. A rubber strain measurement device based on the two-dimensional digital image correlation method, characterized in that, This device includes a rubber specimen (1), two thin sheets (2), two retaining pieces (3), a camera (4) and a processing module (5); Divide a measurement area on the front surface of the gauge section of the rubber specimen (1). The two sides of the measurement area are correction areas; Two thin sheets (2) are adhered to the correction areas through grease; The positions of the thin sheets are symmetrical about the measurement area; The hardness of the thin sheets is higher than that of the rubber specimen, and the grease is selected to enable the thin sheets (2) to slide relative to the surface of the rubber specimen (1) during the stretching process; The retaining piece (3) is attached to the side surface of the gauge section and corresponds to the position of the thin sheet (2); Speckle patterns are prepared on both the measurement area and the thin sheet (2); The camera is used to record the deformation image of the rubber specimen (1) under loading during the tensile experiment when the tensile machine grips the rubber specimen (1) and send it to the processing module; The processing module is used to obtain the strain data of the measurement area and the two thin sheets according to the deformation image by using the digital image correlation method, and subtract the mean value of the strain data of the two thin sheets (2) from the strain data of the measurement area to obtain the corrected strain value of the rubber specimen.

7. The device according to claim 6, characterized in that, The thin sheet (2) is made of a matte hard sheet. After the thin sheet (2) is adhered to both sides of the measurement area, silicone oil is coated on the matte surface of the thin sheet (2) and the measurement area, and then black powder is evenly sprinkled to obtain the speckle pattern.

8. The device according to claim 6 or 7, characterized in that There is a certain gap between the side of the thin sheet (2) close to the baffle (3) and the plane where the baffle (3) is located; the side of the baffle (3) close to the thin sheet (2) extends beyond the plane where the thin sheet (2) is located by a certain distance; the baffle (3) can limit the in-plane rotation of the thin sheet (2) during the tensile process of the specimen.

9. The device according to claim 8, wherein, The baffle (3) is made of the same material and has the same structure as the thin sheet (2).

10. The device according to claim 9, characterized in that, The thickness of the thin sheet is less than 0.5 mm; the elastic modulus of the thin sheet is E sheet ≥ 1 GPa; a grease that is solid or semi-solid and has a grease consistency grade greater than or equal to 2.

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