Device and method for measuring expansion strain of elastomer material

Abstract

The invention discloses a device and a method for measuring the expansion strain of an elastomer material, and solves the technical problems of insufficient precision and limited application range when measuring equipment deals with complex deformation in the prior art. Comprising a measuring seat for placing a tested sample, cameras are arranged at positions close to the front surface and the side surface of the measuring seat, and the camera close to the front surface of the measuring seat is assembled on a moving mechanism; and the controller is used for controlling the running states of the moving mechanism and the camera. According to the invention, deformation data of a tested sample is measured through two groups of mutually matched cameras, arc lengths before and after deformation are calculated by adopting a path tracking method and a geometrical relationship principle, and further strain data in an experimental process are obtained; in the measuring process, the moving mechanism adjusts the position of the camera by using the tip movement amount of the tested sample, so that the camera is always kept in a fixed focus, and the measuring precision is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of extensometers, and in particular to a device and method for measuring expansion strain of an elastomeric material. Background Art

[0002] In order to measure the deformation of the tested sample with an inflatable equibiaxial testing machine, it is usually necessary to add marks on the tested sample, which can be marking points or reflective strips for measurement by measuring equipment. Currently, there are roughly two types of measuring equipment available on the market for this experiment, one is a laser extensometer and the other is a video extensometer. However, it was found during the experiment that:

[0003] Laser extensometers have the following disadvantages:

[0004] 1. Although the laser extensometer has the characteristics of high precision and non-contact, it can usually only measure in one direction. For situations where it is necessary to monitor the deformation of the sample in two or more directions at the same time, such as the three-dimensional change of the sample during inflation, the measurement data in a single direction is difficult to meet the needs of comprehensive analysis;

[0005] 2. When the sample surface has complex texture or uneven reflectivity, the laser extensometer may experience unstable or even lost signals, affecting the accuracy of the measurement results;

[0006] 3. When the sample undergoes a large deformation, resulting in a significant change in the surface curvature, the laser point may deviate from the preset path, further reducing the reliability of the test.

[0007] Video extensometers have the following disadvantages:

[0008] 1. Most video extensometers are designed as single-camera systems, which are mainly used to monitor deformation in one direction. For situations where it is necessary to monitor deformation in multiple directions of the sample at the same time, such as the three-dimensional change of the sample during inflation, the single-camera system cannot provide comprehensive data. In addition, when the deformation of the sample is too large, the focal length problem will affect the quality of the image and cause the experiment to fail.

[0009] 2. When trying to use multiple independent video extensometers to achieve multi-angle measurement, data synchronization between different devices becomes a challenge. Without a good synchronization mechanism, it is difficult to ensure that the data obtained from each angle can accurately correspond to the status at the same time point.

[0010] The use of the above two measuring devices will affect the measurement of the actual deformation, so an extensometer for measuring the deformation of the test sample is urgently needed to solve the above problem. Summary of the invention

[0011] The object of the present invention is to provide a device and method for measuring the expansion strain of an elastomeric material, so as to solve the technical problems of insufficient accuracy and limited scope of application of the measuring equipment in the prior art when dealing with complex deformations.

[0012] To achieve the above object, the present invention provides the following technical solutions:

[0013] In the first aspect, the present invention provides a device for measuring the expansion strain of an elastomeric material, comprising a test sample and a measuring seat for placing the test sample, a second camera is arranged on the front of the measuring seat, and a first camera is arranged on the side of the measuring seat, the first camera and the second camera are arranged orthogonally, and their relative positions change with the deformation of the test sample, the second camera has a digital image correlation analysis function and is assembled on a moving mechanism; and also includes a controller for controlling the operating status of the moving mechanism and the camera, the controller being electrically connected to the moving mechanism, the first camera and the second camera.

[0014] Furthermore, the moving mechanism is a linear motor.

[0015] Furthermore, it also includes a test board for fixing the entire measuring device, and the measuring seat, the moving mechanism and the first camera are all fixed on the test board.

[0016] Furthermore, the second camera calibrates the distance through a camera calibration plate, marks points on the tested sample, and the first camera obtains the arc contour between the two marking points through image processing.

[0017] Furthermore, the deformation of the tested sample is a circularly symmetrical curved surface.

[0018] In a second aspect, the present invention provides a method for measuring expansion strain of an elastomeric material, comprising the following steps:

[0019] S1. Initial data recording: record the gauge length of the tested sample before deformation, i.e., the initial gauge length, and determine the position of the tip of the tested sample before deformation, i.e., the initial tip position;

[0020] S2, layout and calibration: mark points on the test sample, which will be used for subsequent image recognition and tracking, and use the camera calibration plate to calibrate the distance of the second camera to ensure measurement accuracy;

[0021] S3, data acquisition: start the first camera and the second camera to perform synchronous data acquisition, the second camera can identify and simultaneously track the position changes of multiple marking points, the first camera obtains the arc contour between the two marking points through image processing, and preliminarily calculates the straight-line distance of the arc on the camera imaging plane;

[0022] S4, data recording after deformation: record the gauge length of the tested sample after deformation, i.e. gauge length after deformation, and determine the position of the tip of the tested sample after deformation, i.e. tip position after deformation;

[0023] S5. Calculate the arc length and strain according to the recorded data.

[0024] Furthermore, after the test sample is deformed, the focal length needs to be adjusted. Specifically, the controller determines whether the position of the second camera needs to be adjusted based on the tip movement amount transmitted by the first camera. When the tip movement amount reaches a preset threshold, the controller starts the moving mechanism to adjust the distance between the second camera and the test sample, so that the camera always maintains fixed focus to ensure measurement accuracy.

[0025] Furthermore, the second camera cooperates with the first camera using digital image correlation analysis software to analyze the acquired image sequence, automatically identifies and tracks the position changes of each feature point, and calculates the two-dimensional or three-dimensional displacement of each point on the surface of the object based on the movement of the feature points. After the experiment, a cloud map is generated to help researchers quickly understand the deformation pattern of the object.

[0026] Further, in step S5, the arc length is calculated by the integral discretization principle. The specific process is: applying the integral discretization principle, using the path tracing method to move along the curve and accumulate the step length, connecting the straight line segments between two adjacent points by the discrete point approximation method to approximate the curve, and accumulating the lengths of all line segments to obtain the total length of the entire curve, and then calculating the strain from the deformed arc length and the initial arc length, as follows:

[0027]

[0028] Among them: ε is the dependent variable; l i The straight-line distance between adjacent pixels on the deformed contour curve; ∑l i is the total arc length of the contour curve after deformation; l 0,i is the straight-line distance between adjacent pixel points on the initial state contour curve; ∑l 0，i is the total arc length of the contour curve in the initial state.

[0029] Furthermore, in step S5, the arc length is calculated by the principle of geometric relationship. The specific process is as follows: ignoring the change of the sample shape, the distance between the tip of the tested sample and the measuring seat is calculated according to the first camera as the basis of the movement of the moving mechanism, and the arc radius is calculated according to the inner hole size of the measuring seat and the distance between the tip of the tested sample and the measuring seat measured by the first camera. According to the arc radius and the chord length of the marked point tested by the second camera, the real arc length of the marked point is calculated by geometric relationship, and then the real strain value is obtained. The calculation formula is:

[0030]

[0031] Among them, ε is the strain; L' is the length of the arc after deformation; L0 is the initial gauge length; R is the radius of the tested sample after deformation; L is the gauge length after deformation.

[0032] Based on the above technical solution, the embodiments of the present invention can at least produce the following technical effects:

[0033] (1) The present invention can capture the deformation of the test sample from two different angles at the same time by using the orthogonally arranged first camera and second camera. This design enables the device to comprehensively and accurately measure the strain of the elastomeric material under the expansion stress state, solving the problem that single-direction measurement in the prior art is difficult to meet the comprehensive analysis requirements. The second camera has a digital image correlation analysis function, which can identify and track the position changes of the marking points, thereby achieving high-precision strain measurement. At the same time, the controller adjusts the position of the second camera according to the tip movement transmitted by the first camera to maintain a fixed focus state, further improving the stability and accuracy of the measurement. This adaptive adjustment mechanism ensures that clear image data can be obtained under different deformation degrees, providing a reliable basis for subsequent strain calculations.

[0034] (2) The device and method of the present invention are suitable for measuring the expansion strain of various elastomeric materials, and are particularly suitable for measuring the deformation of specimens on an inflatable equibiaxial testing machine. The high precision and stability of the device make it widely applicable in the fields of material science, aerospace, and automobile manufacturing. Moreover, after the experiment, a strain cloud map can be generated to help researchers quickly understand the deformation mode of the object. This data visualization method makes the analysis results more intuitive and easy to understand, providing strong support for scientific research and technological development. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0036] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0037] Figure 2 It is a side view of the overall structure of the present invention;

[0038] Figure 3 is a schematic diagram of the calculation radius after deformation of the present invention;

[0039] Figure 4It is a schematic diagram of calculating arc length after deformation of the present invention.

[0040] In the figure: 1. Test sample, 2. Measuring seat, 3. Moving mechanism, 4. First camera, 5. Controller, 6. Test board, 7. Second camera, 8. Marking point. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in this field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0042] Example 1

[0043] like Figure 1 and Figure 2 As shown, a device for measuring expansion strain of an elastomeric material comprises a test sample 1 and a measuring seat 2 for placing the test sample 1, a second camera 7 is arranged on the front of the measuring seat 2, and a first camera 4 is arranged on the side of the measuring seat 2, the first camera 4 and the second camera 7 are arranged orthogonally, and the relative position changes with the deformation of the test sample 1, the second camera 7 has a digital image correlation analysis function and is assembled on a moving mechanism 3, the first camera 4 and the second camera 7 are arranged to ensure that the deformation can be captured from two different angles of the side and the front of the test sample 1 at the same time, and the position of the second camera 7 relative to the test sample 1 can be adjusted by using the arranged moving mechanism 3, so as to achieve the purpose of adjusting the focal length; and also includes a controller 5 for controlling the moving mechanism 3 and the camera operation state, the controller 5 is electrically connected to the moving mechanism 3, the first camera 4 and the second camera 7.

[0044] In specific implementation, the second camera 7 arranged on the front side of the measuring seat 2 is used to collect deformation data of the test sample 1. During the data collection process, the controller 5 uses the data collected by the first camera 4 arranged on the side of the measuring seat 2 to calculate the tip movement of the test sample 1. The controller 5 then controls the moving mechanism 3 according to the tip movement of the test sample 1, and adjusts the distance between the second camera 7 arranged on the front side of the measuring seat 2 and the test sample 1, so that the second camera 7 and the test sample 1 always maintain a fixed focus, thereby ensuring the collection accuracy of the deformation data of the test sample 1.

[0045] The controller 5 has a trigger, and the first camera 4 and the second camera 7 are both started by the trigger, thereby achieving synchronous start-up of the two cameras to ensure that the time of the two video data obtained is consistent, thereby improving the measurement accuracy. Specifically, the control ends of the two cameras are connected at the same time through the trigger and shooting pulses are sent at a high frequency, so that the collected video data are at the same time.

[0046] As a preferred solution of the above embodiment, it is characterized in that the moving mechanism 3 is a linear motor.

[0047] As a preferred solution of the above embodiment, it also includes a test plate 6 for fixing the entire measuring device, and the measuring seat 2 , the moving mechanism 3 and the first camera 4 are all fixed on the test plate 6 .

[0048] As a preferred solution of the above embodiment, the second camera 7 calibrates the distance through a camera calibration plate, marks a mark point 8 on the tested sample 1, and the first camera 4 obtains the arc contour between the two mark points 8 through image processing.

[0049] As a preferred solution of the above embodiment, the deformation of the tested sample 1 is a circularly symmetrical curved surface.

[0050] Example 2

[0051] A method for measuring expansion strain of an elastomeric material, comprising the following steps:

[0052] S1. Initial data recording: record the gauge length of the tested sample before deformation, i.e., the initial gauge length L0, and determine the position of the tip of the tested sample before deformation, i.e., the initial tip position y0;

[0053] S2, layout and calibration: mark points on the test sample, which will be used for subsequent image recognition and tracking, and use the camera calibration plate to calibrate the distance of the second camera to ensure measurement accuracy;

[0054] S3, data acquisition: start the first camera and the second camera to perform synchronous data acquisition, the second camera can identify and simultaneously track the position changes of multiple marking points, the first camera obtains the arc contour between the two marking points through image processing, and preliminarily calculates the straight-line distance of the arc on the camera imaging plane;

[0055] S4, data recording after deformation: record the gauge length of the tested sample after deformation, i.e., gauge length after deformation L, and determine the position of the tip of the tested sample after deformation, i.e., tip position y after deformation;

[0056] S5. Calculate the arc length and strain according to the recorded data.

[0057] Specifically, after the second camera 7 is arranged, the distance calibration of the second camera 7 is completed through the camera calibration plate and the marking point 8 is painted on the test sample 1. In order to solve the problem of the second camera 7 shooting out of focus due to the deformation of the test sample 1, a first camera 4 is added to the side of the measuring seat 2, and a moving mechanism 3 is added below the front camera 7. The second camera 4 is used to synchronously collect the movement of the tip of the test sample 1. By setting a movement threshold, after the tip change reaches the threshold, the controller 6 starts the moving mechanism 3 to adjust the second camera 7 to move the corresponding threshold distance to ensure the quality of the second camera 7 to collect the picture for analysis by the digital image correlation analysis software. The first camera 4 calculates the actual length between the sample measurement points through image recognition technology, and uses the path tracking method to move along the curve and accumulate the step length. The OpenCV library provides the cv::arcLength() function, which can accept a contour or curve as input and return the length of the curve. The discrete point approximation method is implemented internally in this function. The discrete point approximation method is to approximate the curve by connecting the straight line segments between two adjacent points, and accumulate the lengths of all line segments to obtain the total length of the entire curve.

[0058] Furthermore, since this method may have a slight error, before using the device, numerical calibration is performed through the second camera 7 and the first camera 4. The calculation method is to obtain the initial scale distance of the marking point 8 through the second camera 7, and compare it with the straight-line distance value of the marking point 8 obtained by the first camera 4 to obtain the conversion coefficient for subsequent routine tests.

[0059] Specifically, the principle of integral discretization is applied, and the path tracing method is used to move along the curve and accumulate the step length. The curve is approximated by connecting the straight line segments between two adjacent points through the discrete point approximation method, and the length of all line segments is accumulated to obtain the total length of the entire curve. The strain is then calculated from the deformed arc length and the initial arc length, as follows:

[0060]

[0061] Among them: ε is the dependent variable; l i The straight-line distance between adjacent pixels on the deformed contour curve; ∑l i is the total arc length of the contour curve after deformation; l 0,i is the straight-line distance between adjacent pixel points on the initial state contour curve; ∑l 0，i is the total arc length of the contour curve in the initial state.

[0062] Example 3

[0063] A method for measuring expansion strain of an elastomeric material. The technical features in this embodiment are basically the same as the technical features described in Example 2. The same technical features and technical solutions will not be repeated here. Only the differences between Example 3 and Example 2 are described here, which mainly lies in the different methods used to calculate the arc length.

[0064] Specifically, ignoring the changes in the shape of the sample, the distance between the tip of the tested sample and the measuring seat is calculated by the first camera as the basis for the movement of the moving mechanism, and the arc radius is calculated based on the inner hole size of the measuring seat and the distance between the tip of the tested sample and the measuring seat measured by the first camera. According to the arc radius and the chord length of the mark point tested by the second camera, the real arc length of the mark point is calculated through geometric relationship, and then the real strain value is obtained.

[0065] Specifically, it is defined as follows:

[0066] Initial gauge length L0, gauge length of the tested sample 1 before deformation;

[0067] The gauge length after deformation L is the gauge length of the tested sample 1 after deformation;

[0068] Initial tip position y0, the position of the tip of the test sample 1 before deformation;

[0069] Tip position after deformation y, the tip position of the tested sample 1 after deformation;

[0070] The radius of the specimen base fixture is set to R0. Figure 3 As shown;

[0071] The length of the side of triangle ACD And triangle ACD is similar to triangle ABC, then

[0072] The radius of the deformed specimen

[0073] Deformed arc length reference Figure 4 As shown,

[0074] The calculation formula of the strain variable ε of the tested sample 1 is:

[0075]

[0076] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of the present invention to be protected is defined by the attached claims and their equivalents.

Claims

1. A device for measuring expansion strain of an elastomeric material, characterized in that: The invention comprises a test sample (1) and a measuring seat (2) for placing the test sample (1), wherein a second camera (7) is arranged on the front of the measuring seat (2), and a first camera (4) is arranged on the side of the measuring seat (2), wherein the first camera (4) and the second camera (7) are arranged orthogonally, and their relative positions change as the test sample (1) is deformed, and the second camera (7) has a digital image correlation analysis function and is mounted on a moving mechanism (3); and further comprises a controller (5) for controlling the operating state of the moving mechanism (3) and the camera, wherein the controller (5) is electrically connected to the moving mechanism (3), the first camera (4) and the second camera (7).

2. The device for measuring expansion strain of an elastomeric material according to claim 1, characterized in that: The moving mechanism (3) is a linear motor.

3. The device for measuring expansion strain of an elastomeric material according to claim 1, characterized in that: It also comprises a test plate (6) for fixing the entire measuring device, and the measuring seat (2), the moving mechanism (3) and the first camera (4) are all fixed on the test plate (6).

4. The device for measuring expansion strain of an elastomeric material according to claim 1, characterized in that: The second camera (7) calibrates the distance through a camera calibration plate, marks a mark point (8) on the test sample (1), and the first camera (4) obtains the arc contour between the two mark points (8) through image processing.

5. The device for measuring expansion strain of an elastomeric material according to claim 1, characterized in that: The deformation of the tested sample (1) is a circularly symmetrical curved surface.

6. A method for measuring expansion strain of an elastomeric material, using the device for measuring expansion strain of an elastomeric material according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Initial data recording: record the gauge length of the tested sample before deformation, i.e., the initial gauge length, and determine the position of the tip of the tested sample before deformation, i.e., the initial tip position; S2, layout and calibration: mark points on the test sample, which will be used for subsequent image recognition and tracking, and use the camera calibration plate to calibrate the distance of the second camera to ensure measurement accuracy; S3, data acquisition: start the first camera and the second camera to perform synchronous data acquisition, the second camera can identify and simultaneously track the position changes of multiple marking points, the first camera obtains the arc contour between the two marking points through image processing, and preliminarily calculates the straight-line distance of the arc on the camera imaging plane; S4, data recording after deformation: record the gauge length of the tested sample after deformation, i.e. gauge length after deformation, and determine the position of the tip of the tested sample after deformation, i.e. tip position after deformation; S5. Calculate the arc length and strain according to the recorded data.

7. The method for measuring expansion strain of an elastomeric material according to claim 6, characterized in that: After the test sample is deformed, the focal length needs to be adjusted. Specifically, the controller determines whether the position of the second camera needs to be adjusted based on the tip movement amount transmitted by the first camera. When the tip movement amount reaches a preset threshold, the controller starts the moving mechanism to adjust the distance between the second camera and the test sample, so that the camera always maintains a fixed focus to ensure measurement accuracy.

8. The method for measuring expansion strain of an elastomeric material according to claim 6, characterized in that: The second camera cooperates with the first camera to use digital image correlation analysis software to analyze the collected image sequence, automatically identify and track the position changes of each feature point, and calculate the two-dimensional or three-dimensional displacement of each point on the surface of the object based on the movement of the feature points. After the experiment, a cloud map is generated to help researchers quickly understand the deformation pattern of the object.

9. The method for measuring expansion strain of an elastomeric material according to claim 6, characterized in that: In step S5, the arc length is calculated by the principle of integral discretization. The specific process is: applying the principle of discrete integration, using the path tracing method to move along the curve and accumulate the step length, connecting the straight line segments between two adjacent points by the discrete point approximation method to approximate the curve, and accumulating the lengths of all line segments to obtain the total length of the entire curve, and then calculating the strain from the deformed arc length and the initial arc length, as follows: Among them: ε is the dependent variable; l i The straight-line distance between adjacent pixels on the deformed contour curve; ∑l i is the total arc length of the contour curve after deformation; l 0,i is the straight-line distance between adjacent pixel points on the initial state contour curve; ∑l 0，i is the total arc length of the contour curve in the initial state.

10. The method for measuring expansion strain of an elastomeric material according to claim 6, characterized in that: In step S5, the arc length is calculated by the principle of geometric relationship. The specific process is as follows: ignoring the change of the sample shape, the distance between the tip of the tested sample and the measuring seat is calculated according to the first camera as the basis of the movement of the moving mechanism. At the same time, the arc radius is calculated according to the inner hole size of the measuring seat and the distance between the tip of the tested sample and the measuring seat measured by the first camera. According to the arc radius and the chord length of the marked point tested by the second camera, the real arc length of the marked point is calculated by geometric relationship, and then the real strain value is obtained. The calculation formula is: Among them, ε is the strain; L' is the length of the arc after deformation; L0 is the initial gauge length; R is the radius of the tested sample after deformation; L is the gauge length after deformation.

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