An apparatus and method for measuring the swelling strain of an elastomeric material

Abstract

The present invention discloses a device and method for measuring the expansion strain of an elastomeric material, which solves the technical problems of insufficient accuracy and limited scope of application of the existing measuring equipment when dealing with complex deformations. It includes a measuring seat for placing the test sample, cameras are arranged at the positions close to the front and side of the measuring seat, and the camera close to the front of the measuring seat is assembled on a moving mechanism; it also includes a controller for controlling the operating status of the moving mechanism and the camera. The present invention uses two groups of cameras that cooperate with each other to measure the deformation data of the test sample, adopts the path tracking method and the principle of geometric relationship to calculate the arc length before and after deformation, and then obtains the strain data of the experimental process; the moving mechanism set during the measurement process uses the movement of the tip of the test sample to adjust the position of the camera, so that the camera always keeps a fixed focus, thereby ensuring the measurement accuracy.

Description

Technical Field

[0001] The present 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 use an inflatable equibiaxial testing machine to measure the deformation of the test sample, it is usually necessary to add marks on the test 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 that can be used for this experiment: a laser extensometer and a video extensometer. However, during the experiment, it was found that:

[0003] Laser extensometers have the following disadvantages:

[0004] 1. Although laser extensometers are highly precise and non-contact, they can usually only measure in one direction. For situations where it is necessary to monitor the deformation of a specimen in two or more directions simultaneously, such as the three-dimensional changes of a specimen during inflation, measurement data from a single direction cannot meet the needs of comprehensive analysis.

[0005] 2. When the sample surface has complex textures or uneven reflectivity, the laser extensometer may experience signal instability or even loss, affecting the accuracy of the measurement results;

[0006] 3. When the specimen undergoes large deformation, resulting in a significant change in 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, primarily for monitoring deformation in one direction. For situations where it is necessary to monitor deformation in multiple directions simultaneously, such as the three-dimensional changes of a specimen during inflation, a single-camera system cannot provide comprehensive data. Furthermore, if the specimen deformation is excessive, the focal length issue can affect the image quality, leading to experimental failure.

[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 point in time.

[0010] The use of the above two measuring devices will affect the measurement of actual deformation. Therefore, 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 measuring equipment in the prior art when dealing with complex deformations.

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

[0013] In a first aspect, the present invention provides a device for measuring the expansion strain of an elastomeric material, comprising a test sample and a measuring base for placing the test sample, a second camera being arranged on the front of the measuring base, and a first camera being arranged on the side of the measuring base, the first camera and the second camera being arranged orthogonally, and their relative positions changing with the deformation of the test sample, the second camera having a digital image correlation analysis function and being assembled on a moving mechanism; and further comprising 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 base, 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 outline 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. These points will be used for subsequent image recognition and tracking. Use a camera calibration plate to calibrate the distance of the second camera to ensure measurement accuracy.

[0021] S3, Data Collection: Start the first and second cameras to perform synchronous data collection. The second camera can identify and simultaneously track the position changes of multiple markers. The first camera obtains the arc outline between the two markers 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 based on 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 and adjusts the distance between the second camera and the test sample, so that the camera always maintains a fixed focus to ensure measurement accuracy.

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

[0026] Furthermore, in step S5, the arc length is calculated by the integral discretization principle. The specific process is as follows: the integral discretization principle is applied, 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 using the discrete point approximation method, and the lengths of all line segments are 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:

[0027]

[0028] Where: 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 based on the principle of geometric relationship. The specific process is as follows: ignoring the changes in the shape of the sample, the distance between the tip of the tested sample and the measuring seat is calculated based on the first camera, which is used as the basis of the movement of the moving mechanism. At the same time, 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. Based on the arc radius and the chord length of the mark point tested by the second camera, the true arc length of the mark point is calculated based on the geometric relationship, and then the true strain value is obtained. The calculation formula is:

[0030]

[0031] Where, 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; and L is the gauge length after deformation.

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

[0033] (1) The present invention can capture the deformation of the test sample from two different angles simultaneously by using the first camera and the second camera arranged orthogonally. This design enables the device to comprehensively and accurately measure the strain of the elastomeric material under the expansion stress state, solving the problem in the prior art that single-direction measurement cannot 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 marker 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 inflatable biaxial testing machines. The high precision and stability of the device make it widely applicable in the fields of materials science, aerospace, and automobile manufacturing. Moreover, after the experiment, a strain cloud map can be generated to help researchers quickly understand the deformation pattern 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 following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 any 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 This 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 base, 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 of the present invention, all other embodiments obtained by ordinary technicians in this field without making 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 deemed 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 includes a test sample 1 and a measuring base 2 for placing the test sample 1, a second camera 7 is arranged on the front of the measuring base 2, and a first camera 4 is arranged on the side of the measuring base 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, and the first camera 4 and the second camera 7 are arranged to ensure that the deformation can be captured simultaneously from two different angles, the side and the front of the test sample 1, and the position of the second camera 7 relative to the test sample 1 can be adjusted by using the arranged moving mechanism 3, thereby achieving the purpose of adjusting the focal length; and also includes a controller 5 for controlling the operating status of the moving mechanism 3 and the camera, and the controller 5 is electrically connected to the moving mechanism 3, the first camera 4 and the second camera 7.

[0044] During specific implementation, the second camera 7 arranged on the front of the measuring base 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 base 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, adjusts the distance between the second camera 7 arranged on the front of the measuring base 2 and the test sample 1, and thus makes the second camera 7 and the test sample 1 always maintain a fixed focus, thereby ensuring the accuracy of collecting 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 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, a test plate 6 for fixing the entire measuring device is further included, and the measuring base 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 8 on the test 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 test 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. These points will be used for subsequent image recognition and tracking. Use a camera calibration plate to calibrate the distance of the second camera to ensure measurement accuracy.

[0054] S3, Data Collection: Start the first and second cameras to perform synchronous data collection. The second camera can identify and simultaneously track the position changes of multiple markers. The first camera obtains the arc outline between the two markers 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., the gauge length after deformation L, and determine the position of the tip of the tested sample after deformation, i.e., the tip position y after deformation;

[0056] S5. Calculate the arc length and strain based on the recorded data.

[0057] Specifically, after the second camera 7 is deployed, the distance calibration of the second camera 7 is completed using a camera calibration plate, and marking points 8 are applied to the test sample 1. To address the issue of out-of-focus images captured by the second camera 7 due to deformation of the test sample 1, a first camera 4 is added to the side of the measuring base 2, and a moving mechanism 3 is added below the front camera 7. The second camera 4 synchronously captures the movement of the tip of the test sample 1. A movement threshold is set. When the tip change reaches this threshold, the controller 6 activates the moving mechanism 3 to adjust the movement of the second camera 7 to the corresponding threshold distance to ensure the quality of the images captured by the second camera 7 for analysis by the digital image correlation analysis software. The first camera 4 calculates the actual length between the measurement points on the sample using image recognition technology, moving along the curve using a path tracing method and accumulating step lengths. The OpenCV library provides the cv::arcLength() function, which accepts a contour or curve as input and returns the length of the curve. This function internally implements the discrete point approximation method, which approximates the curve by connecting straight line segments between two adjacent points and accumulating 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 correction 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] Where: 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 the 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 any changes in specimen shape, the distance between the tip of the test specimen and the measuring base is calculated using the first camera, serving as the basis for the movement of the moving mechanism. The arc radius is also calculated based on the inner hole size of the measuring base and the distance between the tip of the test specimen and the measuring base as measured by the first camera. Based on the arc radius and the chord length of the marked point measured by the second camera, the true arc length of the marked point is calculated through geometric relationships, thereby obtaining the true strain value.

[0065] Specifically, it is defined as follows:

[0066] Initial gauge length L0, the gauge length of the test specimen 1 before deformation;

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

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

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

[0070] The radius of the sample base fixture is set to R0. After the sample is deformed, 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 for the strain variable ε of the tested sample 1 is:

[0075]

[0076] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring expansion strain of an elastomeric material, the method being implemented by a device for measuring expansion strain of an elastomeric material, characterized in that: The above-mentioned device 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 the moving mechanism (3); and also includes a controller (5) for controlling the operating state of the moving mechanism (3) and the camera, and the controller (5) is electrically connected to the moving mechanism (3), the first camera (4) and the second camera (7); The method comprises the following steps: 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. These points will be used for subsequent image recognition and tracking. Use a camera calibration plate to calibrate the distance of the second camera to ensure measurement accuracy. S3, Data Collection: Start the first and second cameras to perform synchronous data collection. The second camera can identify and simultaneously track the position changes of multiple markers. The first camera obtains the arc outline between the two markers 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; After the test sample is deformed, the focus needs to be adjusted. Specifically, the controller determines whether the position of the second camera needs to be adjusted based on the tip movement transmitted by the first camera. When the tip movement reaches a preset threshold, the controller activates the movement mechanism to adjust the distance between the second camera and the test sample, thereby ensuring that the camera always maintains a fixed focus to ensure measurement accuracy. In step S5, the arc length is calculated based on the principle of geometric relationship. The specific process is as follows: ignoring the change in the shape of the sample, the distance between the tip of the test sample and the measuring seat is calculated by the first camera, which is used as the basis of the movement of the moving mechanism. At the same time, the arc radius is calculated based on the inner hole size of the measuring seat and the distance between the tip of the test sample and the measuring seat measured by the first camera. Based on the arc radius and the chord length of the mark point measured by the second camera, the true arc length of the mark point is calculated through geometric relationship, and then the true strain value is obtained. The calculation formula is: ； Where, 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; and L is the gauge length after deformation.

2. The method 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 method for measuring expansion strain of an elastomeric material according to claim 1, characterized in that: 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).

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

5. The method 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. The method for measuring expansion strain of an elastomeric material according to claim 1, characterized in that: The second camera, in conjunction with the first camera, uses digital image correlation analysis software to analyze the acquired image sequence, automatically identifying and tracking the position changes of each feature point. Based on the movement of the feature points, the two-dimensional or three-dimensional displacement of each point on the surface of the object is calculated. After the experiment, a cloud map is generated to help researchers quickly understand the deformation pattern of the object.

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