Method for Measuring Poisson's Ratio and Young's Modulus of Irregular Shaped Materials

Abstract

The present invention relates to a method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material, comprising the following steps: S1. Paste a standard area square and a specimen of the cross-section of the irregular-shaped body on the lower surface of the upper transparent plate, and install a camera module above the upper transparent plate to collect in real time the cross-sectional images of the specimen during the compression process; S2. Operate a testing machine equipped with an upper punch and a lower punch to perform uniaxial compression on the specimen and use the camera module to record the whole process; S3. Obtain the force-displacement curve output by the testing machine and find the elastic range. The starting point of the elastic range is set as point a, and the ending point is set as point b; S4. According to the abscissas of points a and b and the downward speed of the universal testing machine, inversely calculate the corresponding moments of points a and b, and intercept the corresponding images in the whole-process video recording; S5. Obtain the cross-sectional areas and the fitted diameters corresponding to points a and b of the specimen through image processing techniques; S5. Calculate the Poisson's ratio and Young's modulus of the specimen.

Description

Technical Field

[0001] The present invention relates to a method for measuring Poisson's ratio and Young's modulus, and in particular to a method for measuring Poisson's ratio and Young's modulus of irregular-shaped materials. Background Art

[0002] Poisson's ratio is defined as the negative value of the ratio of the transverse strain to the corresponding axial strain, which is caused by the axial stress below the proportional limit of the material. Young's modulus is the ratio of the tensile or compressive stress below the proportional limit to the corresponding strain. Commonly used methods for measuring Poisson's ratio and Young's modulus can be divided into mechanical, acoustic and optical methods. The principle of the mechanical method is to apply a uniaxial tensile or compressive axial force within the linear elastic range of the sample, and measure its corresponding lateral deformation by an extensometer or strain gauge. The acoustic method mainly includes Brillouin scattering, surface acoustic wave, acoustic microscopy and other methods. Its main principle is to calculate the elastic parameters of the material according to the acoustic elasticity theory. The optical method mainly includes optical interference method, optical thermoplastic holography method, digital speckle in-plane correlation method, etc., which are mainly based on the pure bending theory of plates or beams in elastic mechanics.

[0003] Among the three types of measurement methods for Poisson's ratio and Young's modulus, the mechanical method is the most widely used and has a relatively low application cost. The acoustic method is mainly used to measure the Poisson's ratio of metal materials. For non-metallic materials, their acoustic resistance and internal damping are usually relatively large, making them difficult to apply. The optical method generally requires the sample to be optically flat, and has high requirements for test cameras and vibration platforms. Although there have been a lot of studies at home and abroad, there is no unified standard, and the experimental error is difficult to estimate. Relatively speaking, the mechanical measurement method of applying uniaxial tension or compression to the sample and measuring its cross-sectional area and lateral strain is relatively mature and stable, and has a high degree of recognition.

[0004] The traditional mechanical measurement methods for Poisson's ratio and Young's modulus are mainly extensometer method and electrical measurement method for measuring the transverse deformation and cross-sectional area of ​​materials under compression or tension. These two methods have good measurement effects on rigid materials with regular shapes such as metals. GB / T 22315-2008 contains detailed application standards for the extensometer method and electrical measurement method to measure the Poisson's ratio and Young's modulus of regular metal specimens. However, for viscoelastic irregular materials, the extensometer and electrical measurement method can only measure the transverse strain at a certain position of the specimen, and cannot express the overall transverse strain of the entire specimen, and cannot measure the cross-sectional area of ​​irregular shapes. In practice, for some specific material parts, their shapes are not regular. If only regular-shaped material parts are used for measurement, the reference significance of the measurement results will be discounted due to the inconsistency with the actual situation.

[0005] Therefore, how to provide a measurement method that can measure the Poisson's ratio and Young's modulus of irregularly shaped materials is a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0006] The present invention mainly improves the measurement of the lateral deformation and cross-sectional area of materials during the compression process in the traditional mechanical measurement method. It collects the cross-sectional images of the specimen during compression in real time, and calculates the cross-sectional area and lateral strain through image recognition technology to measure the Poisson's ratio and Young's modulus of the material, so as to solve the measurement of the Poisson's ratio and Young's modulus of materials with irregular shapes.

[0007] The present invention adopts the following technical solutions:

[0008] A method for measuring the Poisson's ratio and Young's modulus of materials with irregular shapes, characterized by including the following steps

[0009] S1. Use the upper transparent plate as the bottom of the upper pressure head, and use the lower transparent plate as the top of the lower pressure head; paste the standard area square 4 and the specimen 5 of the cross-section of the irregular shape on the lower surface of the upper transparent plate, and install a camera module above the upper transparent plate to collect the cross-sectional images of the specimen during the compression process in real time;

[0010] S2. Operate the universal testing machine equipped with the upper pressure head and the lower pressure head to perform uniaxial compression on the specimen and use the camera module to record the whole process. The picture should include the cross-section of the specimen 5 and the standard area square 4;

[0011] S3. Obtain the force-displacement curve output by the universal testing machine, and find the elastic range. The starting point of the elastic range is set as point a, and the ending point is set as point b;

[0012] S4. According to the abscissas of points a and b in the force-displacement curve and the downward speed of the universal testing machine, inversely calculate the corresponding times of points a and b, and intercept the corresponding images in the whole-process video; the abscissa represents displacement, and the ordinate represents time; it should be noted that the abscissa and the ordinate can be interchanged;

[0013] S5. Obtain the cross-sectional area and the fitted diameter corresponding to the specimen at points a and b through image processing technology;

[0014] S6. Calculate the Poisson's ratio ν and Young's modulus E of the specimen through the following formula:

[0015]

[0016]

[0017] In the formula, ν is the Poisson's ratio, E is the Young's modulus, ε' is the lateral strain, ε is the longitudinal strain, σ is the axial stress, F is the axial pressure, A is the cross-sectional area of the specimen, Δl is the change in the length of the specimen, l is the original length of the specimen, d a is the fitted diameter of the specimen at point a, d bis the fitting diameter of the specimen at point b, l a is the length of the specimen at point a, l b is the length of the specimen at point b, F a is the pressure on the specimen at point a, F b is the pressure on the specimen at point b, A a is the cross-sectional area of the specimen at point a.

[0018] Preferably, the upper transparent plate and the lower transparent plate are made of glass, and the specimen is made of viscoelastic material to avoid damage to the indenter under pressure.

[0019] Preferably, in step S1, a claw-type retaining ring is further included. The claw-type retaining ring and the upper transparent plate and the lower transparent plate together form the indenter for uniaxial compression, and the imaging module 1 is installed in the space between the upper transparent plate and the claw-type retaining ring.

[0020] Preferably, the specimen 5 and the standard area square 4 are adhesively attached to the bottom surface of the upper transparent plate with double-sided adhesive.

[0021] Preferably, the upper and lower surfaces of the specimen are perpendicular to the axis.

[0022] Preferably, a semi-transparent white background paper 6 is provided below the lower transparent plate.

[0023] Further, an upper LED lamp 7 is also provided in the space between the upper transparent plate and the claw-type retaining ring, and a lower LED lamp 11 is also provided in the space between the lower transparent plate and the claw-type retaining ring.

[0024] Further, the claw-type retaining ring includes an upper three-claw retaining ring 2 and a lower three-claw retaining ring 10.

[0025] Preferably, in step S5, the OpenCV2 command set is called in python to perform image processing on the collected images. The steps of image processing are explained in the appendix Figure 5 The image processing effect is explained in the appendix Figure 6 The pixel of the cross-sectional area of the specimen output by the program is P1, and the pixel of the standard area square is P2. Since the true area S2 of the standard area square is known, the cross-sectional area S1 of the specimen is calculated according to the following formula:

[0026]

[0027] Since the cross-sectional shape of the specimen is irregular, the fitting circle diameter with equal area is used as the diameter of the specimen. The specimen diameter d is calculated according to the following formula:

[0028]

[0029] Further, in step S6, the distance between the two transparent plates before compression is measured with a vernier caliper, and l is obtained by combining the force-deformation diagram output by the universal testing machine. a l b F a F b ., A is obtained through image processing. a A b d a d b ..

[0030] The beneficial effects of the present invention are as follows:

[0031] 1) It can collect the cross-sectional images of the specimen under pressure in real time, and calculate the cross-sectional area and transverse strain through image recognition technology to measure the Poisson's ratio and Young's modulus of the material, thus solving the measurement of the Poisson's ratio and Young's modulus of materials with irregular shapes.

[0032] 2) Since the indenter is made of glass material and viscoelastic materials will not damage the indenter, it is particularly suitable for irregular object materials of viscous materials.

[0033] 3) The scheme is ingeniously designed and easy to implement, and has broad prospects for popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a flowchart of the test method according to an embodiment of the present invention;

[0035] Figure 2 is a schematic diagram of a preferred test bench according to the present invention;

[0036] Figure 3 is a schematic diagram of the process of determining the elastic range of the specimen under pressure according to an embodiment of the present invention;

[0037] Figure 4 is a schematic diagram of the measured parameters of the specimen under pressure according to an embodiment of the present invention;

[0038] Figure 5 is a flowchart of the cross-sectional image processing process of the specimen according to an embodiment of the present invention;

[0039] Figure 6 is a schematic diagram of the cross-sectional image processing effect of the specimen according to an embodiment of the present invention.

[0040] In the figure, 1 - camera module, 2 - upper three - jaw retaining ring, 3 - upper glass plate, 4 - standard - area square, 5 - specimen, 6 - semi - transparent white background paper, 7 - upper LED lamp, 8 - fixture, 9 - lower glass plate, 10 - lower three - jaw retaining ring, 11 - lower LED lamp, 12 - universal testing machine, 13 - cross - section of viscoelastic irregular - shaped specimen, 14 - equal - area fitting circle, 15 - outer - contour extraction. Detailed implementation manners

[0041] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0042] See Figures 1-6 , specifically according to the following steps:

[0043] (1) Prepare a columnar viscoelastic specimen with both upper and lower surfaces perpendicular to the axis, and place it on the axis connection line of the new upper and lower indenter of the universal testing machine. And paste a standard - area square on the lower surface of the upper glass plate for subsequent calculation of the cross - sectional area of the specimen. Figure 2 On the axis connection line of the new upper and lower indenter of the universal testing machine. And paste a standard - area square on the lower surface of the upper glass plate for subsequent calculation of the cross - sectional area of the specimen.

[0044] (2) Adjust the image acquisition system, including the focusing of the camera module and the brightness of the upper and lower LED light modules, etc., to ensure that clear - boundary and clean - background images of the specimen and the area standard square can be collected.

[0045] (3) Set the downward speed of the universal testing machine, start uniaxial compression, and record the entire process of the cross - section of the specimen compression through the camera module. The picture should include the cross - section of the specimen and the standard - area square.

[0046] (4) Since the calculation of Poisson's ratio and Young's modulus needs to be within the elastic range of the specimen. Therefore, after the compression is completed, find the elastic range of the material through the force - displacement curve output by the universal testing machine. The specific finding method is as follows: within the elastic range, the pressure on the specimen increases linearly with the increase of displacement, and the force - displacement curve is a straight line. When the elastic range of the material ends, the force - displacement curve shows obvious fluctuations, and the pressure on the specimen does not increase linearly with the increase of displacement. Let the starting point of the elastic range be point a, and the ending point of the elastic range be point b. According to the corresponding displacement and the downward speed of the universal testing machine, the corresponding times of points a and b can be calculated inversely.

[0047] (5) Intercept the corresponding images in the video according to the times corresponding to points a and b, which are the cross - sectional images of the specimen at points a and b.

[0048] (6) Call the OpenCV2 command set in Python to perform image processing on the collected images. The steps of image processing are explained in Appendix Figure 5 The image - processing effect is shown in Appendix Figure 6Explanation. The number of pixels of the cross-sectional area of the specimen output by the program is P1, and the number of pixels of the standard area square is P2. Since the true area S2 of the standard area square is known, the cross-sectional area S1 of the specimen can be calculated according to the following formula:

[0049]

[0050] (7) Since the cross-sectional shape of the specimen is irregular, the diameter of the fitting circle with equal area is used as the diameter of the specimen. The specimen diameter d can be calculated according to the following formula:

[0051]

[0052] (8) The distance between the two glass plates before compression is measured with a vernier caliper. Combining the force-deformation diagram output by the universal testing machine, l a 、l b 、F a 、F b can be obtained. Through image processing, A a 、A b 、d a 、d b can be obtained. The Poisson's ratio and Young's modulus of the viscoelastic irregular specimen can be calculated according to the following formula:

[0053]

[0054]

[0055] In the formula, ν is Poisson's ratio, E is Young's modulus, ε’ is the lateral strain, ε is the axial strain, σ is the axial stress, F is the axial pressure, A is the cross-sectional area of the specimen, Δl is the change in the length of the object, l is the original length of the object, d a is the fitting diameter of the object at point a, d b is the fitting diameter of the object at point b, l a is the length of the object at point a, l b is the length of the object at point b, F a is the pressure on the object at point a, F b is the pressure on the object at point b, A a is the cross-sectional area of the object at point a.

[0056] The known formulas for Poisson's ratio and Young's modulus in the prior art are as follows:

[0057]

[0058]

[0059] Through the above embodiments, it can be seen that the present invention can collect the cross-sectional images of the specimen under pressure in real time, and calculate the cross-sectional area and transverse strain through image recognition technology to measure the Poisson's ratio and Young's modulus of the material, so as to solve the measurement of the Poisson's ratio and Young's modulus of materials with irregular shapes; at the same time, considering that the indenter is made of glass material and the viscoelastic material will not damage the indenter, it is particularly suitable for irregular object materials of viscous materials.

[0060] The above are the preferred embodiments of the present invention. Those of ordinary skill in the art can also make various transformations or improvements on this basis. Without departing from the general concept of the present invention, these transformations or improvements should fall within the scope of protection required by the present invention.

Claims

1. A method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material, characterized in that, It includes the following steps: S1. Use the upper transparent plate as the bottom of the upper platen and the lower transparent plate as the top of the lower platen; paste a standard area square (4) and a specimen (5) with an irregular cross-section on the lower surface of the upper transparent plate, and install a camera module above the upper transparent plate to collect the cross-sectional images of the specimen during the compression process in real time. S2. Operate the testing machine equipped with the upper platen and the lower platen to perform uniaxial compression of the specimen and use the camera module to record the whole process. The video frame should include the cross-section of the specimen (5) and the standard area square (4). S3. Obtain the force-displacement curve output by the testing machine and find the elastic range. The starting point of the elastic range is set as point a, and the ending point is set as point b. S4. According to the abscissas of points a and b and the downward speed of the testing machine, calculate the corresponding moments of points a and b, and intercept the corresponding images from the whole-process video; the abscissa represents displacement. S5. Obtain the cross-sectional areas and fitted diameters of the specimen at points a and b through image processing technology. S6. Calculate the Poisson's ratio (ν) and Young's modulus (E) of the specimen through the following formula: Where ν is the Poisson's ratio, E is the Young's modulus, ε’ is the transverse strain, ε is the longitudinal strain, σ is the axial stress, F is the axial pressure, A is the cross-sectional area of the specimen, Δl is the change in the length of the specimen, l is the original length of the specimen, d a is the fitted diameter of the specimen at point a, d b is the fitted diameter of the specimen at point b, l a is the length of the specimen at point a, l b is the length of the specimen at point b, F a is the pressure on the specimen at point a, F b is the pressure on the specimen at point b, A a is the cross-sectional area of the specimen at point a.

2. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material as described in claim 1, characterized in that: The upper transparent plate and the lower transparent plate are made of glass, and the specimen is made of viscoelastic material to avoid damage to the platen under pressure.

3. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material as claimed in claim 1, wherein: In step S1, it also includes a claw-type retaining ring. The claw-type retaining ring and the upper transparent plate and the lower transparent plate jointly form the platen for uniaxial compression, and the camera module (1) is installed in the space between the upper transparent plate and the claw-type retaining ring.

4. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material as described in claim 1, characterized in that: The specimen (5) and the standard area square (4) are adhered to the lower surface of the upper transparent plate with double-sided tape.

5. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material as described in claim 1, characterized in that: The upper and lower surfaces of the specimen are perpendicular to the axis.

6. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material as described in claim 1, characterized in that: A semi-transparent white background paper (6) is provided below the lower transparent plate.

7. The method for measuring the Poisson's ratio and Young's modulus of the irregular-shaped material according to claim 3, characterized in that: An upper LED lamp (7) is also provided in the space between the upper transparent plate and the claw-type retaining ring, and a lower LED lamp (11) is also provided in the space between the lower transparent plate and the claw-type retaining ring.

8. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material according to claim 3, characterized in that: The claw-type retaining ring includes an upper three-claw retaining ring (2) and a lower three-claw retaining ring (10).

9. The method for measuring the Poisson's ratio and Young's modulus of an irregular-shaped material according to claim 1, wherein: In step S5, call the OpenCV2 command set in Python to perform image processing on the collected images. The number of pixels of the cross-sectional area of the specimen output by the program is P1, and the number of pixels of the standard area square is P2; since the true area S2 of the standard area square is known, the cross-sectional area S1 of the specimen is calculated according to the following formula: Since the cross-sectional shape of the specimen is irregular, the diameter of the fitted circle with an equal area is used as the diameter of the specimen; the diameter d of the specimen is calculated according to the following formula:

10. The method for measuring the Poisson's ratio and Young's modulus of the irregular-shaped material according to claim 9, wherein: In step S6, the distance between the two transparent plates before compression is measured with a vernier caliper, and l is obtained by combining the force-deformation diagram output by the testing machine. a and l b and F a and F b . A is obtained through image processing. a and A b and d a and d b .

Images