A device and method for measuring the principal stress on the surface of a specimen

Abstract

The present invention discloses a device and method for measuring the principal stress on the surface of a specimen, comprising: a photoelastic patch adhered to the component to be measured; and a measuring assembly disposed on one side of the component to be measured adhered with the photoelastic patch, including a light source, a color filter, a polarizer, a quarter-wave plate, an analyzer and a CCD detachably fixed on a platform. By moving the positions of some devices in the measuring assembly, the isochromatic fringe order and the strain value on the surface of the specimen are measured, and the principal stress on the surface of the specimen is obtained. The present invention can measure the full-field principal stress value on the surface of the specimen and can more intuitively display the stress distribution on the surface of the specimen in the form of photoelastic fringes.

Description

Technical Field

[0001] The present invention relates to an optical stress measurement system, and particularly to a device and method for measuring the principal stress on the surface of a specimen. Background Art

[0002] In engineering practice, there are many dangerous points on stressed components that are in a complex stress state. At this time, the failure of the material is related to the three principal stresses. And the principal stresses can be combined in countless ways to study the ultimate stress value when it reaches the dangerous state. Based on long-term practice and a large number of test results, people have put forward various hypotheses about the cause of material failure, and these hypotheses are usually called strength theories.

[0003] The photoelastic coating method is an experimental stress analysis method that pastes a photoelastic coating with relatively high strain optical sensitivity on the surface of the component to be measured, and obtains the strain distribution on the surface of the component to be measured by measuring the order of isochromatic interference fringes generated by the coating with the deformation of the surface of the component to be measured. It can directly measure the full-field distribution of strain on the surface of an engineering component and accurately measure the stress concentration phenomenon of the component. Therefore, the commonly used photoelastic experimental technology can be applied to on-site measurement in industry. The photoelastic coating method is an analysis method for full-field observation, so it can discover some high-stress areas that are difficult to predict, such as stress concentration areas caused by assembly, reinforcement, and welding; it can not only measure static elastic stress, but also measure dynamic stress, elastoplastic stress, residual stress, and thermal stress; it can not only test metal material structures, but also test structures or parts made of materials such as concrete, wood, composite materials, rocks, and rubbers. In addition, in the study of fracture mechanics, this method can also be used to measure the elastoplastic strain field at the crack tip and the crack propagation process.

[0004] Paste a thin sheet with photoelastic effect on the specimen to be analyzed. When the specimen is loaded, its surface strain is transmitted through shear, and the strain field of the specimen can be reproduced in the coating. To perform quantitative analysis of stress, principal stress separation must be carried out. The stress separation methods mainly include the strip method, the auxiliary strain gauge method, the cutting method, and the stress separation method of one-time normal incidence and one-time oblique incidence. The disadvantages of the above first three methods are that full-field data cannot be measured. The stress separation method of one-time normal incidence and one-time oblique incidence on the coating can measure full-field data, but the test and calculation processes are very complex.

[0005] In view of the above problems, this patent proposes to perform principal stress testing based on the photoelastic coating method and the digital image correlation method, and design a principal stress testing device. The digital image correlation method determines the surface displacement and strain of an object based on the statistical correlation of the light intensity of randomly distributed speckles on the object surface before and after deformation. The measurement process involves using a CCD camera to collect the speckle pattern on the object surface, and these images are converted to represent the gray values of each pixel point. The digital image correlation method uses the gray values of two images before and after surface deformation to perform a correlation operation, thereby achieving the purpose of solving the surface displacement and strain of the deformed body. In the bonding process, an appropriate amount of toner is added to the adhesive as random spots. Using the digital image correlation method, the surface strain of the component can be measured, and the principal stress on the surface of the component can be calculated based on the strain. Combining with the principal stress difference measured by the photoelastic coating method, the principal stress can be separated to achieve the testing of the principal stress of the component to be measured. Summary of the Invention

[0006] The object of the present invention is to provide a device and method for testing the principal stress on the surface of a specimen, which can measure the full-field principal stress value on the surface of the specimen and can more intuitively display the stress distribution on the surface of the specimen in the form of photoelastic fringes.

[0007] To achieve the above object, the specific technical solutions of a device and method for testing the principal stress on the surface of a specimen of the present invention are as follows:

[0008] A device for testing the principal stress on the surface of a specimen includes: a photoelastic coating, which is pasted on the component to be measured; and a measurement component, which is arranged on one side of the component to be measured with the photoelastic coating bonded thereto, and includes a light source, a color filter, a polarizer, a quarter-wave plate, an analyzer, and a CCD that are detachably arranged on a platform. By moving the positions of some devices in the measurement component, the order of isochromatic fringes and the strain value on the surface of the specimen are measured, and the principal stress on the surface of the specimen is obtained.

[0009] A method for testing the principal stress on the surface of a specimen includes the following steps:

[0010] Step 1, place some devices in the measurement component on the same axis, and measure the order of isochromatic fringes and the strain value on the surface of the specimen;

[0011] Step 2, based on the measured order of isochromatic fringes and the strain value on the surface of the specimen, perform the principal stress test on the surface of the specimen, and perform formula derivation to measure the full-field principal stress value on the surface of the specimen.

[0012] The advantages of a device and method for testing the principal stress on the surface of a specimen of the present invention are as follows:

[0013] 1) It can measure the full-field principal stress value on the surface of the specimen;

[0014] 2) It can more intuitively display the stress distribution on the surface of the specimen in the form of photoelastic fringes. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1a FIG. 1 is a schematic diagram of the CCD captured image in the present invention;

[0016] Figure 1b FIG. 2 is a schematic diagram of the CCD captured image in the present invention;

[0017] Figure 2a FIG. 3 is a top view of the measuring device in the present invention;

[0018] Figure 2b FIG. 4 is a front view of the measuring device in the present invention.

[0019] In the figures: 1, light source; 2, color filter; 3, polarizer; 4, quarter-wave plate; 5, analyzer; 6, CCD; 7, platform; 8, base with screw; 9, nut. DETAILED DESCRIPTION OF THE INVENTION

[0020] In order to better understand the purpose, structure and function of the present invention, the following further describes in detail a device and method for measuring the principal stress on the surface of a specimen in the present invention with reference to the accompanying drawings.

[0021] As Figures 1a to 2b shown, it shows a device and method for measuring the principal stress on the surface of a specimen in the present invention. The principal stress is measured by the photoelastic coating method and the digital image correlation method. The measuring device includes a photoelastic coating, a specimen to be measured and a measuring component. Among them, the photoelastic coating in the present invention is pasted on the specimen to be measured. A patch material is made by using an epoxy resin with a lower viscosity and a room temperature curing agent, and adding an appropriate amount of diluent. The photoelastic coating is pasted on the surface of the specimen to be measured through an adhesive. The adhesive is an epoxy adhesive cured at room temperature. When encountering the surface of a part with poor reflectivity, a small amount of aluminum powder can be added to the adhesive to increase the intensity of the reflected light. In addition, an appropriate amount of toner is added to the adhesive as random speckles for digital image correlation method testing.

[0022] Furthermore, the measuring component is arranged on one side of the specimen to be measured with the photoelastic coating pasted thereon, and includes a light source 1, a color filter 2, a polarizer 3, a quarter-wave plate 4, an analyzer 5 and a CCD 6 (CCD is a charge-coupled device) detachably arranged on the platform 7. By moving the positions of some devices in the measuring component, the order of isochromatic fringes and the strain value on the surface of the specimen are measured, and the principal stress on the surface of the specimen is obtained.

[0023] Furthermore, a plurality of parallel slides are provided on the platform 7. The light source 1 and the CCD 6 are located on the first slide 71 of the platform 7, the color filter 2 is located on the second slide 72 of the platform 7, the polarizer 3 and the analyzer 5 are located on the third slide 73 of the platform 7, and the quarter-wave plate 4 is located on the fourth slide 74 of the platform 7.

[0024] Furthermore, the bottom of the measuring assembly is respectively fixed on the slide of the platform 7 through a detachable assembly. The detachable assembly includes a base 8 with a screw and a nut 9, and can be disassembled when some devices are not needed. In addition, the light source 1, the color filter 2, the polarizer 3, the quarter-wave plate 4, the analyzer 5 and the CCD 6 slide relative to the platform 7 through the detachable assembly, so that when some devices are not used, they can be moved to an area that does not affect the test, making the operation more convenient.

[0025] The present invention also discloses a method for testing the principal stress on the surface of a specimen, which performs principal stress testing through the photoelastic coating method and the digital image correlation method.

[0026] Step 1, place some devices in the measuring assembly on the same axis, and measure the isochromatic fringe order through the photoelastic coating method and the strain value on the surface of the specimen through the digital image correlation method.

[0027] (1) When the light source 1, the polarizer 3 and the quarter-wave plate 4 are placed on the same axis, and the other quarter-wave plate 4, the analyzer 5 and the CCD 6 are placed on the same axis, at this time, the color filter 2 is not needed and is moved to an area that does not affect the test. At this time, the light source is white light, and a colored isochromatic fringe pattern of the specimen is obtained, which can clearly show the stress distribution.

[0028] (2) When the light source 1, the color filter 2, the polarizer 3 and the quarter-wave plate 4 are placed on the same axis, and the quarter-wave plate 4, the analyzer 5 and the CCD 6 are placed on the same axis, at this time, since the color filter 2 is used, the light source 1 becomes monochromatic light, and the image captured by the CCD is as Figure 1a shown, and an isochromatic fringe pattern with light and dark intervals of the specimen is obtained, which can determine the isochromatic fringe order.

[0029] (3) When the light source 1 and the CCD 6 are placed on the same slide, at this time, the color filter 2, the polarizer 3, the quarter-wave plate 4 and the analyzer 5 are not needed and are moved to an area that does not affect the test. The image captured by the CCD is as Figure 1b shown, and a speckle pattern on the surface of the specimen is obtained. According to the speckle patterns before and after deformation, the strain value on the surface of the specimen is measured.

[0030] The present invention more intuitively shows the stress distribution on the surface of the specimen in the form of photoelastic fringes.

[0031] Step 2: Test and derive the principal stresses on the surface of the specimen through the photoelastic coating method and digital image correlation method.

[0032] Based on the measured isochromatic fringe order and the strain values on the surface of the specimen above, the principal stress test on the surface of the specimen can be carried out, and the formula derivation is as follows:

[0033] Examine the photoelastic effect when polarized light is normally incident on the photoelastic coating, reflected by the interface after passing through the photoelastic coating, and then passes through the photoelastic coating again and exits. According to the plane photoelastic principle, the principal stress difference at any point on the photoelastic coating is:

[0034]

[0035] Among them, in formula (1): and represent the principal stress and principal strain of the photoelastic coating; and represent the principal stress and strain of the corresponding point on the surface of the measured component; n is the isochromatic fringe order; is the stress fringe value of the photoelastic coating; h is the thickness of the photoelastic coating.

[0036] Analyze the stress-strain relationship through Hooke's law in general form:

[0037]

[0038]

[0039] Among them, in formula (3): and are the elastic moduli of the photoelastic coating and the measured component material respectively; and are the Poisson's ratios of the photoelastic coating and the measured component material respectively.

[0040] Substitute formula (1) into formula (3), noting that the strain values of the corresponding points on the photoelastic coating and the measured component are equal, and we get:

[0041]

[0042] Among them, in formula (4): mm / fringe, defined as the strain fringe value of the photoelastic coating material.

[0043] From this, the difference in principal stresses on the surface of the measured component is derived as:

[0044]

[0045] The calculation formula for the principal stress on the surface of the measured component is:

[0046]

[0047] Adding the principal stresses gives:

[0048]

[0049] Analyze the stress-strain relationship on the surface of the component under test through Hooke's law in generalized form:

[0050]

[0051]

[0052] Substituting Equation (9) into Equation (7) can obtain the sum of the principal stresses:

[0053]

[0054] Combining Equation (5) and Equation (10) can calculate the principal stresses and achieve the separation of the principal stresses:

[0055]

[0056] Among them, in Equation (11):

[0057] and are measured by using the digital image correlation method through the speckle patterns before and after deformation collected;

[0058] The order n of the isochromatic fringe is calculated from the isochromatic fringe pattern collected.

[0059] A device and method for testing the principal stresses on the surface of a specimen according to the present invention can measure the full-field principal stress values on the surface of the specimen; and can more intuitively display the stress distribution on the surface of the specimen in the form of photoelastic fringes.

[0060] The above further describes the present invention with the aid of specific embodiments. However, it should be understood that this specific description should not be construed as a limitation on the essence and scope of the present invention. Various modifications made by those of ordinary skill in the art to the above embodiments after reading this specification all fall within the scope protected by the present invention.

Claims

1. A main stress testing device for the surface of a test piece, characterized in that Including: A photoelastic patch, which is pasted on the component to be measured; And A measuring component, which is arranged on one side of the component to be measured with the photoelastic patch adhered thereto, and includes a light source (1), a color filter (2), a polarizer (3), a quarter-wave plate (4), an analyzer (5) and a CCD (6) that are detachably arranged on a platform (7). By moving the positions of some devices in the measuring component, the isochromatic fringe order and the strain value on the surface of the specimen are measured, and the principal stress on the surface of the specimen is obtained; A plurality of parallel slideways are arranged on the platform (7), where: The light source (1) and the CCD (6) are located on the first slideway (71) on the platform (7); The color filter (2) is located on the second slideway (72) on the platform (7); The polarizer (3) and the analyzer (5) are located on the third slideway (73) on the platform (7); The quarter-wave plate (4) is located on the fourth slideway (74) on the platform (7); The bottom of the measuring component is respectively fixed on the slideways of the platform (7) through a detachable component, and the detachable component includes a base with a screw (8) and a nut (9); The light source (1), the polarizer (3) and the quarter-wave plate (4) are placed on the same axis, and the other quarter-wave plate (4), the analyzer (5) and the CCD (6) are placed on the same axis to obtain a color isochromatic fringe pattern of the specimen, showing the stress distribution; The light source (1), the color filter (2), the polarizer (3) and the quarter-wave plate (4) are placed on the same axis, and the other quarter-wave plate (4), the analyzer (5) and the CCD (6) are placed on the same axis. The light source (1) is monochromatic light to obtain a light and dark isochromatic fringe pattern of the specimen, and the isochromatic fringe order is determined; The light source (1) and the CCD (6) are placed on the same slideway to obtain a speckle pattern on the surface of the specimen. According to the speckle patterns before and after deformation, the strain value on the surface of the specimen is measured.

2. A method for testing the principal stress on the surface of a specimen, which uses the device for testing the principal stress on the surface of a specimen described in claim 1, is characterized in that Including the following steps: Step 1, place some devices in the measuring component on the same axis to measure the isochromatic fringe order and the strain value on the surface of the specimen; Step 2, according to the measured isochromatic fringe order and the strain value on the surface of the specimen, perform the principal stress test on the surface of the specimen, and conduct formula derivation to test the full-field principal stress value on the surface of the specimen.

3. The method for testing the principal stress on the surface of a test piece according to claim 2, characterized in that, In Step 2: Examine the photoelastic effect when polarized light is normally incident on the photoelastic patch, reflected by the interface after passing through the photoelastic patch, and then emitted through the photoelastic patch again. The principal stress difference at any point on the photoelastic patch is: Wherein, in formula (1): Indicates the principal stress and principal strain of the photoelastic patch; n is the isochromatic fringe order; is the stress fringe value of the photoelastic patch; h is the thickness of the photoelastic patch.

4. The method for testing the principal stress on the surface of the test piece according to claim 3, wherein Analyze the stress-strain relationship through Hooke's law of elasticity: Wherein, in formula (3): Indicates the principal strain of the photoelastic patch; and represent the principal stress and strain of the corresponding points on the surface of the component under test; and are the elastic moduli of the photoelastic patch and the material of the component under test, respectively; and are the Poisson's ratios of the photoelastic patch and the material of the component under test, respectively.

5. The method for testing the principal stress on the surface of the test piece according to claim 4, characterized in that, Substitute formula (1) into formula (3). The strain values of the corresponding points on the photoelastic patch and the component to be measured are equal, and we get: Wherein, in formula (4): mm / strip, which is the strain fringe value of the photoelastic patch material; From this, the difference in the principal stress on the surface of the component to be measured is deduced as: 。 6. The method for testing the principal stress on the surface of the test piece according to claim 5, characterized in that, The calculation formula for the principal stress on the surface of the component to be measured is: Adding the principal stresses together, we get: Analyze the stress-strain relationship on the surface of the component to be measured through Hooke's law of elasticity: 。 7. The method for testing the principal stress on the surface of the test piece according to claim 6, wherein Substitute formula (9) into formula (7) to obtain the sum of the principal stresses: The principal stress can be calculated by combining Equation (5) and Equation (10) to achieve the separation of principal stresses: Among them, in Equation (11): and Measured by using the digital image correlation method with the speckle patterns before and after deformation collected; The arithmetic fringe order n is calculated from the collected arithmetic fringe pattern.

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