An optical extensometer based on lens imaging and dual prism reflection and uniform strain testing method

Abstract

The invention discloses an optical extensometer based on lens imaging and double prism reflection and a uniform strain testing method. The optical extensometer comprises a digital camera, an imaging lens, a first rhombus prism, a second rhombus prism and a data processing device; the two rhombus prisms are arranged between the imaging lens and a tested sample, a reflection surface of each rhombus prism faces the lens direction of the digital camera, the digital camera is connected to the data processing device, and a uniform strain testing method based on lens imaging and double prism reflection is provided. The optical extensometer of the invention makes hardware construction more convenient, the calibration of the gauge length in the strain testing method is more accurate, and the optical extensometer is hardly affected by the off-plane displacement of the sample, so that high-resolution and high-precision strain measurement based on single ordinary lens imaging is realized.

Description

Technical Field

[0001] The invention relates to a uniform strain testing method of an optical extensometer, in particular to a uniform strain testing method of an optical extensometer based on lens imaging and double prism reflection. Background Art

[0002] Strain measurement is of great significance for the determination of mechanical properties of various materials and the design of structures. The commonly used strain measurement methods, such as resistance strain gauges, electronic extensometers or optical fiber strain gauges, need to install components on the test sample during measurement, which is easy to cause damage, making these methods unsuitable for the detection of small size, large deformation, especially flexible materials, which are precisely the areas where non-contact measurement methods can be used. Non-contact strain measurement methods are mainly represented by optical (video) extensometers. At present, internationally renowned testing machine manufacturers such as MTS and Instron have launched their own video extensometer products. However, the strain measurement accuracy of optical extensometers is often not high, which limits their application. There are two main reasons: First, under actual test conditions, the sample will inevitably produce a certain amount of off-plane displacement. According to the pinhole camera model, false displacement and false strain will be generated on the image plane of the imaging system, thereby greatly reducing the strain measurement accuracy and resolution; second, due to the limitation of camera resolution, the extensometer gauge length usually cannot exceed the camera resolution.

[0003] The current optical extensometer is not convenient enough when adjusting the optical path. The use of telecentric lens for imaging has the disadvantages of high cost and constant magnification, and it cannot be flexibly adjusted and is not universal. The use of ordinary lens for imaging requires a rigid sheet to be pasted on the surface of the sample to be tested, which is still relatively cumbersome in operation. Summary of the invention

[0004] Purpose of the invention: The first purpose of the present invention is to provide an optical extensometer based on lens imaging and bi-prism reflection, which has a more accurate calibration of the gauge length, is not affected by the off-plane displacement of the sample, has high resolution and high precision. The second purpose of the present invention is to provide a uniform strain testing method based on lens imaging and bi-prism reflection.

[0005] Technical solution: The optical extensometer based on lens imaging and double prism reflection of the present invention comprises a digital camera, an imaging lens, a first rhombus prism, a second rhombus prism and a data processing device;

[0006] Two rhombic prisms are placed between the imaging lens and the sample being tested. One reflective surface of each rhombic prism faces the direction of the lens of the digital camera.

[0007] The upper target point on the surface of the tested sample is reflected twice by the upper rhombus prism, and the lower target point is reflected twice by the lower rhombus prism, and then incident on the left and right halves of the ordinary lens respectively; the ordinary lens simultaneously images the upper and lower target areas on the surface of the tested sample after reflection, and forms a digital image on the target surface of the digital camera; the digital camera simultaneously photographs the two target areas to obtain a digital image, and transmits the photographed digital image to the data processing module, and the information of each target area occupies half of the frame of the target surface of the digital camera, so that a single ordinary camera can simultaneously collect two separated target images; the two target points of the optical extensometer are respectively taken on the horizontal midlines of the left and right areas of a digital image.

[0008] Furthermore, the two rhombus prisms have the same size. The two 45-degree inclined surfaces of each rhombus prism are coated surfaces to increase the reflection performance.

[0009] Two rhombus prisms form an X shape and are located at the same height. The imaging lens is a fixed focus lens or a zoom lens. The data processing device includes a calibration module, a calculation module and a post-processing module. The calibration module is used to calibrate the pixel distance between the first and second target points in the separation field of view. The calculation module obtains the displacement information of the first and second target points along the measurement direction according to the received digital image. The post-processing module uses the displacement information obtained by the calculation module and combines it with the pixel distance between the first and second target points to obtain the uniform strain information of the surface of the tested sample.

[0010] The uniform strain testing method of the optical extensometer based on lens imaging and double rhombus prism reflection of the present invention comprises the following steps:

[0011] (a) Place a ruler on the surface of the sample to be tested, take a digital image, and calculate the pixel distance produced by the field of view separation based on the scale line readings of the digital image;

[0012] (b) using the speckle image of the surface of the sample under test; taking the first target point and the second target point on the horizontal midline of the left and right regions of the digital image respectively, and calculating the initial spacing s between the two target points in combination with the pixel distance generated by the field of view separation;

[0013] (c) Using the digital image correlation method, the vertical displacements y1 and y2 of the two target points during loading are obtained. Combined with the initial spacing s between the two target points, the uniform strain of the measured surface is calculated as (y2-y1) / s.

[0014] In step (b), the scales at the upper and lower boundaries on the left side of the digital image are read out and marked as K1 and K2, the scale at the lower boundary in the right image is marked as K3, and the image pixel distance l between K1 and K3 is calculated. 13 :

[0015]

[0016] In the formula, l 12 is the pixel distance between scale lines K1 and K2, is the actual distance between scale lines K1 and K2, It is the actual distance between scale lines K1 and K3.

[0017] In step (b), the first target point and the second target point are marked as C and D respectively, the scales at the upper and lower boundaries on the left side of the digital image are read out and marked as K1 and K2, and the scale at the lower boundary in the right image is marked as K3. The calculation formula of the pixel distance s between the first target point and the second target point is as follows:

[0018]

[0019] In the formula, l 12 is the pixel distance between scale lines K1 and K2, is the actual distance between scale lines K1 and K2, is the actual distance l between scale lines K1 and K3 1C is the pixel distance from scale line K1 to point C, l 3D is the pixel distance from scale line K3 to point D.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0021] (1) Non-destructive measurement: Compared with contact measurement technologies such as electronic extensometers and strain gauges in the industrial field, the present invention adopts optical measurement technology, which does not require direct contact with the sample to be tested, has no additional mass, does not damage the sample, and does not limit the deformation of the sample;

[0022] (2) Small impact of out-of-plane displacement: The arrangement of two rhombic prisms can converge two distant target points into a common lens through light path reflection, and the target point is selected at the horizontal midline of the image. The out-of-plane displacement will not cause false displacement and false strain of the target point in the vertical direction.

[0023] (3) Improved measurement accuracy: The strain measurement accuracy of the optical extensometer is limited by the size of the selected gauge length, and the gauge length cannot exceed the resolution of the digital camera, which limits the strain measurement accuracy. The arrangement of two rhombic prisms can magnify the distance between the two target points several times, thereby greatly improving the measurement accuracy of uniform strain.

[0024] (4) Easy adjustment: When the existing four-mirror device realizes field of view separation, it is necessary to finely adjust the deflection angle of each mirror one by one, and the adjustment process is relatively cumbersome. However, the present invention only uses two rhombus prisms, which greatly simplifies the steps of realizing field of view separation. It is also easy to integrate into an overall measuring device, which is directly connected to the lens and does not need to be adjusted every time measurement is performed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Fig. 1 (a)-(b) is a schematic diagram of the optical extensometer measurement of the present invention; Fig. 1 (a) is an overall diagram of the optical extensometer, and Fig. 1 (b) is an arrangement diagram of two rhombus prisms;

[0026] Figure 2 Schematic diagram of imaging of a single rhombus prism of the present invention;

[0027] Figure 3 It is a schematic diagram of calibration of the distance between two target points (i.e., gauge length) of the present invention;

[0028] Figures 4(a)-(b) are schematic diagrams of the principle of eliminating off-plane displacement according to the present invention, Figure 4(a) is a pinhole model, and Figure 4(b) is an equal-value radial false displacement generated on a dotted circle. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below in conjunction with embodiments.

[0030] As shown in FIG. 1( a ), the optical extensometer based on lens imaging and dual prism reflection of the present invention comprises an rhombus prism 6 and an rhombus prism 7 corresponding to two target points 2 respectively, a housing 8 and a fixed cover plate 9 for fixing the two prisms, a digital camera 3, an ordinary lens 4, a camera tripod 5 and a data processing device.

[0031] First, select two target areas of the optical extensometer on the test sample 1, one above and one below, each including two target points 2; arrange the first and second rhombus prisms 6 and 7 as shown in FIG1(b), and fix them in the concave housing 8, one above and one below, with the two 45-degree inclined surfaces of each prism forming a 45-degree angle with the surface of the test sample 1 (the schematic diagram of the rhombus prism imaging is shown in FIG1(b)). Figure 2 As shown), the lower reflection surface of the rhombus prism 6 and the upper reflection surface of the rhombus prism 7 are located at the same height and are staggered, corresponding to the middle position of the ordinary lens 4. After the front cover plate 9 is covered, only the light from the upper reflection surface of the rhombus prism 6 and the lower reflection surface of the rhombus prism 7 are allowed to pass through. In this way, after being reflected by the two reflection surfaces of the upper rhombus prism 6, the light emitted by the first target point is translated downward by a distance and incident on the ordinary lens. After being reflected by the two reflection surfaces of the lower rhombus prism 7, the light emitted by the second target point is translated upward by a distance and incident on the ordinary lens. These two groups of light are imaged on the left and right half image planes of the camera, respectively.

[0032] An ordinary lens 4 and a digital camera 3 are installed on a camera tripod 5. The tripod is adjusted so that the optical axis of the ordinary lens 4 is perpendicular to the surface of the sample to be tested. A concave shell 8 with a prism installed is connected to the front of the ordinary lens 4. The ordinary lens 4 is used to image two target points on the surface of the sample to be tested 1. The characteristic that the ordinary lens 4 does not generate vertical false displacement when a point on the horizontal midline is subjected to off-plane displacement is utilized to eliminate the measurement error caused by the off-plane displacement of the sample to be tested 1. A digital camera 3 is used to simultaneously image two target points 2 on the surface of the sample to be tested 1 and form a digital image. The two target points 2 are respectively located on both sides of the image chip of the digital camera 3 to realize the simultaneous acquisition of images of two target areas. The tripod 5 that can carry the digital camera ensures that the digital camera 3 can be stably carried, so that the position of the digital camera 3 can be adjusted within a certain range.

[0033] The data processing device includes a calibration module, a correlation operation module and a post-processing module.

[0034] The calibration module in the data processing device is used to calibrate the pixel distance between the first target point and the second target point after the field of view is separated. The detailed calibration principle is as follows: Figure 3 As shown. To calibrate the pixel distance s between the first and second target points, a ruler with a scale is placed in front of the sample and a digital image is taken as the calibration image. The scales near the upper and lower boundaries on the left side of the image are read out and marked as K1 and K2. Similarly, the scale at the lower boundary in the right image is marked as K3. Since the lens magnification in the entire image is a constant, the image pixel distance between K1 and K3 can be calculated according to the proportional relationship of the following formula:

[0035]

[0036] In the formula, l 12 is the pixel distance between scale lines K1 and K2, which can be directly obtained from the image on the left. is the actual distance between scale lines K1 and K2, is the actual distance between the scale lines K1 and K3, and these two distances can be read from the scale lines.

[0037] Since the first target point and the second target point in the present invention are located on the horizontal center line of the left and right images, marked as C and D. Therefore, the pixel distance l from the scale line K1 to point C is read from the calibration image. 1C The pixel distance l from the scale line K3 to point D 3D Then, the pixel distance s between the first target point and the second target point can be obtained according to the following formula:

[0038]

[0039] In the formula, the parameter l 12 , Has the same meaning as above.

[0040] The correlation operation module in the data processing device is used to perform correlation operation on the digital image collected by the digital camera to obtain the vertical displacement information of two target points located on the horizontal midline of the image.

[0041] The post-processing module in the data processing device performs certain smoothing filtering on the displacement information obtained by the relevant operation module to reduce the displacement error, and then calculates the uniform strain information of the surface of the test sample 1 in combination with the distance information of the two target points.

[0042] The optical extensometer and strain testing method based on lens imaging and double rhombus prism field of view separation of the present invention can eliminate the false displacement and false strain caused by the off-plane displacement of the test sample. The principle of generating false displacement and strain is shown in Figure 4. Ordinary camera imaging follows the pinhole model shown in Figure 4 (a). If the target point is set to a point A on the surface of the test sample far away from the optical axis, an image will be formed at point a on the image plane. When a small off-plane displacement close to the lens occurs on the surface of the test sample, causing point A to move to point B, an image will be formed at point b at a certain distance from point a on the image plane according to the pinhole model. Under the condition that the test sample is not deformed, the image displacement on the image plane of the same point is generated only by the off-plane displacement of the test sample, that is, a false displacement. The corresponding strain data can be calculated based on the displacement data, which is the false strain. The closer the target point A is to the optical axis, the smaller the false displacement is. The false displacement of the point on the optical axis is exactly zero, that is, the point is not affected by the out-of-plane displacement. However, there is only one point with this characteristic in the entire image, which cannot be used as the two target points of the optical extensometer. Therefore, the equal radial false displacement generated on the dotted circle in Figure 4(b) is analyzed and it is found that under the action of the out-of-plane displacement, the point located on the horizontal center line of the image has only a horizontal false displacement, but no vertical false displacement. Therefore, the two target points C and D are selected on the horizontal center line in the left and right image fields of view, which will not generate vertical false displacement and false strain. Therefore, the vertical strain measurement accuracy of the optical extensometer can be improved.

[0043] The embodiments of the present invention disclose a high-precision optical extensometer and strain testing method based on ordinary lens imaging and double rhombus prisms. The light translation characteristics of the double rhombus prisms are used to bring two target points 2 on the surface of the test sample closer together and record them in a digital image, thereby greatly increasing the distance between the two target points in the optical extensometer, which is beneficial to improving the strain measurement resolution and test accuracy.

[0044] An embodiment of the present invention discloses a uniform strain testing method based on the above optical extensometer, comprising the following steps:

[0045] Step 1: Create randomly distributed speckles on the surface of the test sample as a carrier of deformation information; then install the sample on the chuck of the testing machine;

[0046] Step 2, install two rhombic prisms in a concave housing, one above and one below, with the upper reflection surface of the upper rhombic prism and the lower reflection surface of the lower rhombic prism facing the direction of the tested sample, and the lower reflection surface of the upper rhombic prism and the upper reflection surface of the lower rhombic prism form an X shape and are located at the same height; install a front cover plate at the front of the concave housing, allowing only the upper and lower reflection surfaces to be incident with light;

[0047] Step 3, connect the ordinary lens to the camera and fix them together on the tripod, adjust the tripod so that the optical axis of the lens is perpendicular to the surface of the test sample; connect the concave shell with the prism installed to the front of the ordinary lens through threads, and adjust the long side of the concave shell to be in a vertical state;

[0048] Step 4: Adjust the camera aperture and focal length to obtain a clear image; place a ruler with a scale on the surface of the sample, and photograph the upper and lower target areas of the ruler to obtain a digital image; calculate the pixel distance generated by the field of view separation based on the scale line readings of the left and right areas of the image through the proportional relationship;

[0049] Step 5, using a camera to capture a speckle image of the surface of the test sample during the loading process; taking the first target point and the second target point of the optical extensometer on the horizontal midline of the left and right regions of the first digital image, respectively, and combining the field of view separation pixel distance obtained previously, calculating the initial spacing s between the first and second target points;

[0050] Step 6: Use the digital image correlation algorithm to obtain the vertical displacements y1 and y2 of the first and second target points during loading. Combined with the distance s between the two target points obtained by the previous calibration module, the uniform strain of the measured surface is calculated to be (y2-y1) / s.

[0051] In step 2, after the double rhombus prism and the front cover are installed to the concave shell, there is no need to repeat the installation in subsequent strain measurements. It is only necessary to connect the concave shell to the front end of the lens through threads, which greatly saves the preparation time for the experiment.

[0052] The present invention uses a common lens and a double rhombus prism to separate the field of view to image two specific target points on the surface of the test sample. Compared with the traditional single camera, the distance between the two target points, that is, the distance between the two prisms, is multiplied, and the vertical displacement of the target point is not affected by the off-plane displacement, thereby greatly improving the accuracy and resolution of the optical extensometer for strain measurement.

[0053] The object distance of the camera is 500 mm, and the target points in the image are 1600 pixels apart. If the tested sample has an out-of-plane displacement of 0.1 mm during the test, the use of ordinary lens imaging will produce a strain error of 0.1 / 500=200με; even if no out-of-plane displacement occurs, the strain accuracy of the optical extensometer is only (0.01+0.01) / 1600=12.5με; and if the dual prism field of view separation device in this embodiment is used and the gauge length s is increased to 8000 pixels, the strain accuracy of the optical extensometer can be improved to (0.01+0.01) / 8000=2.5με.

Claims

1. A uniform strain test method of an optical extensometer based on lens imaging and double rhombus prism reflection, comprising the following steps: Step 1: Create randomly distributed speckles on the surface of the test sample as a carrier of deformation information; then install the sample on the chuck of the testing machine; Step 2, install two rhombic prisms in a concave housing, one above and one below, with the upper reflection surface of the upper rhombic prism and the lower reflection surface of the lower rhombic prism facing the direction of the tested sample, and the lower reflection surface of the upper rhombic prism and the upper reflection surface of the lower rhombic prism form an X shape and are located at the same height; install a front cover plate at the front of the concave housing, allowing only the upper and lower reflection surfaces to be incident with light; Step 3, connect the ordinary lens to the camera and fix them together on the tripod, adjust the tripod so that the optical axis of the lens is perpendicular to the surface of the test sample; connect the concave shell with the prism installed to the front of the ordinary lens through threads, and adjust the long side of the concave shell to be in a vertical state; Step 4: Adjust the camera aperture and focal length to obtain a clear image; place a ruler with a scale on the surface of the sample, and photograph the upper and lower target areas of the ruler to obtain a digital image; calculate the pixel distance generated by the field of view separation based on the scale line readings of the left and right areas of the image through the proportional relationship; Step 5, using a camera to capture a speckle image of the surface of the test sample during the loading process; taking the first target point and the second target point of the optical extensometer on the horizontal midline of the left and right regions of the first digital image, respectively, and combining the field of view separation pixel distance obtained previously, calculating the initial spacing s between the first and second target points; Step 6: Use the digital image correlation algorithm to obtain the vertical displacements y1 and y2 of the first and second target points during loading. Combined with the distance s between the two target points obtained by the previous calibration module, the uniform strain of the measured surface is calculated to be (y2-y1) / s.

2. The uniform strain testing method of the optical extensometer based on lens imaging and double rhombus prism reflection according to claim 1 is characterized in that: In step 5, the scales at the upper and lower boundaries on the left side of the digital image are read out and marked as K1 and K2, the scale at the lower boundary in the right image is marked as K3, and the image pixel distance l between K1 and K3 is calculated. 13 : In the formula, l 12 is the pixel distance between scale lines K1 and K2, is the actual distance between scale lines K1 and K2, It is the actual distance between scale lines K1 and K3.

3. The uniform strain testing method of the optical extensometer based on lens imaging and double rhombus prism reflection according to claim 1 is characterized in that: In step 5, the first target point and the second target point are marked as C and D respectively, the scales at the upper and lower boundaries on the left side of the digital image are read out and marked as K1 and K2, and the scale at the lower boundary in the right image is marked as K3. The calculation formula of the pixel distance s between the first target point and the second target point is as follows: In the formula, l 12 is the pixel distance between scale lines K1 and K2, is the actual distance between scale lines K1 and K2, is the actual distance l between scale lines K1 and K3 1C is the pixel distance from scale line K1 to point C, l 3D is the pixel distance from scale line K3 to point D.

4. The optical extensometer based on lens imaging and dual prism reflection according to any one of claims 1 to 3, characterized in that: The device comprises a digital camera (3), an imaging lens (4), a first rhombus prism (6), a second rhombus prism (7) and a data processing device; the two rhombus prisms are arranged between the imaging lens (4) and the tested sample (1), a reflection surface of each rhombus prism faces the lens direction of the digital camera (3), and the digital camera (3) is connected to the data processing device.

5. The optical extensometer based on lens imaging and dual prism reflection according to claim 4, characterized in that: The two rhombic prisms are of the same size.

6. The optical extensometer based on lens imaging and dual prism reflection according to claim 4, characterized in that: The two 45-degree inclined surfaces of each rhombic prism are coated surfaces.

7. The optical extensometer based on lens imaging and dual prism reflection according to claim 4, characterized in that: One 45-degree inclined surface of the two rhombic prisms is located at the same height.

8. The optical extensometer based on lens imaging and dual prism reflection according to claim 4, characterized in that: The imaging lens (4) is a fixed-focus lens or a zoom lens.

9. The optical extensometer based on lens imaging and dual prism reflection according to claim 4, characterized in that: The data processing device comprises a calibration module, a calculation module and a post-processing module. The calibration module is used to calibrate the pixel distance between the first and second target points in the separation field of view. The calculation module obtains displacement information of the first and second target points along the measurement direction according to the received digital image. The post-processing module uses the displacement information obtained by the calculation module and combines it with the pixel distance between the first and second target points to obtain uniform strain information on the surface of the tested sample (1).

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