A high-precision full-field measurement method for transparent specimens subjected to concentrated loads

Abstract

The present invention provides a high-precision full-field measurement method for a transparent specimen subjected to a concentrated load. The experimental device of the measurement method consists of a speckle plate, a transparent specimen to be measured, an external light source and an optical measurement device. The method comprises the following steps: (1) preparing monochromatic fluorescent random speckle patterns (red, green and blue) of different colors on the surface, back and speckle plate of the transparent specimen to be measured; (2) using two 3CCD color cameras to build an optical measurement device, and placing the transparent specimen to be measured and the speckle plate; (3) calibrating the internal and external parameters of the optical measurement device, and calibrating the photomechanical parameters C of the transparent specimen to be measured. σ ; (4) Collect images of the tested piece during the concentrated load process; (5) Calculate the surface deformation of the front and back sides of the tested transparent piece and the image displacement field of the speckle plate in the main camera δ x and δ y Calculate; (6) Calculate the full-field stress distribution of the transparent specimen under test.

Description

Technical Field

[0001] The invention relates to the field of solid mechanics of optical measurement experiments, and in particular to a high-precision full-field measurement method for a transparent specimen subjected to concentrated loads. Background Art

[0002] Digital speckle correlation method is a non-contact optical full-field measurement method. This method tracks the changes in speckle patterns on the surface of an object and calculates the changes in the speckle grayscale value of the area of ​​interest, so that the displacement field and strain field on the surface of the object can be accurately measured. This measurement method has the advantages of non-contact, easy operation, high precision, and full-field measurement. At present, digital image correlation technology is widely used in the engineering field. However, the digital speckle method has certain limitations on the measurement object during measurement. Its principle is to calculate the displacement of the point of interest by measuring the spatial coordinates of the speckle at different times, and then further calculate the strain field. Since the constitutive parameters of the measured material cannot be known, the stress of the measured material cannot be directly or indirectly measured.

[0003] For measuring the stress distribution of transparent specimens, a digital gradient sensitive method that utilizes the transparent characteristics of materials has been derived based on the digital speckle correlation method. A single camera is used to collect images of the object during concentrated loads to obtain the effect of the load on the refractive index of the object. The full-field normal stress and gradient of the transparent specimen are calculated through the relationship between the stress field and the refractive index. If the digital speckle correlation method and the digital gradient sensitive method are integrated, the characteristics of fluorescent speckles can be used to obtain information-rich full-field morphology, displacement, strain, stress and other information of the transparent specimen at the same time, which can achieve high-precision measurement of physical quantities at various points on the front and back of the transparent specimen during concentrated loads. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a high-precision full-field measurement method for a transparent specimen subjected to concentrated load, which is easy to operate and can realize real-time measurement of the displacement field, strain field and stress field on the front and back sides of the transparent material.

[0005] Technical solution: A high-precision full-field measurement method for a transparent specimen subjected to a concentrated load, the method comprising the following steps:

[0006] (1) Monochromatic fluorescent random speckle patterns are prepared on the front and back surfaces of the transparent specimen under test and the surface of the speckle plate, and the colors of the fluorescent speckles on the three planes are different, namely, red, green, and blue;

[0007] (2) Use two 3CCD color cameras to build an optical measurement device, and place the transparent test piece and the speckle plate. One camera is used as the main camera, and its optical axis is perpendicular to the plane of the test piece and the plane of the speckle plate. The other camera is used as the auxiliary camera, and its optical axis forms a certain angle with the optical axis of the main camera.

[0008] (3) Keep the position of the camera in step (2) unchanged, and calibrate the internal and external parameters of the optical measurement device; calibrate the photo-mechanical parameter C of the transparent specimen to be measured σ for calculating the surface deformation and stress distribution of the transparent specimen to be measured;

[0009] (4) Collect images of the specimen to be tested under concentrated load;

[0010] (5) Use the internal and external parameters of the optical measurement device in step (3) and the images obtained in (4), and calculate the surface deformations of the front and back sides of the transparent specimen to be measured at different times by the digital speckle method, and calculate the image displacement fields δ x and δ y ;

[0011] (6) Use the photo-mechanical parameter C of the transparent specimen to be measured calibrated in step (3) σ and the image displacement fields δ x and δ y of the light of the speckle plate passing through the transparent specimen to be measured and refracted in the main camera to realize the measurement of the full-field stress distribution of the transparent specimen to be measured.

[0012] Furthermore, in the method of the present invention, the monochromatic fluorescent speckles prepared on the front, back and surface of the speckle plate of the transparent specimen to be measured in step (1) have different colors, and should be one of red, green and blue. The reason is that a 3CCD color camera can output the values of red, green and blue for each pixel, and a CCD is configured for each color. Since the fluorescent speckles on the front, back and speckle plate of the transparent specimen have different wavelengths, the images of the three speckle planes are respectively processed by different CCD image sensors under the irradiation of a specific light source, avoiding interference between the fluorescent speckles on different planes. And the speckles should be random fluorescent spots with a certain contrast to the background color of the plane. Calculate the deformation field of the front side of the transparent specimen to be measured, the deformation field of the back side and the stress field through the CCD images of the three colors respectively.

[0013] Furthermore, in the method of the present invention, in the optical measurement system in step (2), when placing the transparent specimen to be measured and the speckle plate, the plane of the transparent specimen to be measured and the surface of the speckle plate should be kept parallel. l is the distance from the plane of the transparent specimen to be measured to the plane of the speckle plate, and L is the distance from the plane of the transparent specimen to be measured to the lens of the main camera. It is required that l < L. When the optical axis angle of the two 3CCD color cameras is 35° ± 5°, the measurement accuracies of different-sized measurement areas in their planes are highly consistent.

[0014] Furthermore, in the method of the present invention, the internal parameters described in step (3) are (u0, v0, S x ,S y ), where (u0, v0) is the image coordinate of the intersection point O of the main camera optical axis and the image plane of the main camera, and the equivalent focal length (S x ,S y ) is the ratio of the focal length of the lens f to the horizontal and vertical dimensions of the pixel respectively; the external parameters are the rotation matrix R and the translation vector T, R and T describe the relative position and posture relationship between the camera coordinate system and the world coordinate system; the image coordinates (u, v) of any point on the transparent specimen under test in a certain camera image and the coordinates (X, Y, Z) in the world coordinate system have the following mapping relationship:

[0015]

[0016] Among them, Z c It is the projection length of the distance from the spatial point to the optical center of the camera in the direction of the optical axis.

[0017] Photomechanical parameters C of the tested transparent specimen σ It is an inherent parameter of the material, but it is easily affected by the preparation process, temperature control and other conditions. Different batches of materials with the same composition often have different photomechanical parameters C σ There are also differences, so they need to be calibrated before the experiment.

[0018] Furthermore, in the method of the present invention, the image displacement field δ in step (5) x and δ y It refers to the image displacement field in the x and y directions presented by the speckle plate in the main camera. This displacement field is the displacement field presented on the image by the refraction of light through the transparent specimen under test, and is not the real displacement field of the speckle plate.

[0019] Furthermore, in the method of the present invention, the stress field of the transparent specimen to be tested described in step (6) and the image displacement field obtained in step (5) have the following relationship:

[0020]

[0021] Wherein, B is the thickness of the specimen; the point of action of the concentrated load on the transparent specimen is taken as the origin, the direction x is perpendicular to the boundary of the transparent specimen and points to the inside of the specimen, and the direction along the boundary of the transparent specimen is taken as the y direction to establish a coordinate system, and the y direction is the x direction rotated 90° in the counterclockwise direction; x and y are the coordinates of any point on the transparent specimen in the coordinate system, σ x and σ y is the stress distribution of the tested transparent specimen in the x and y directions, F refers to the concentrated load on the tested transparent specimen, τ xyis the shear stress distribution of the tested transparent specimen.

[0022] Beneficial results: Compared with the prior art, the technical solution of the present invention has the following advantages:

[0023] The traditional digital speckle method has certain limitations on the measurement object during measurement. Since the constitutive parameters of the measured material cannot be known, the stress of the measured material cannot be directly or indirectly measured. Compared with the traditional digital speckle method, the present invention utilizes the characteristics of fluorescent speckle and transparent materials, integrates the digital speckle correlation method and the digital gradient sensitivity method, and realizes high-precision measurement of physical quantities at various points on the front and back of the transparent specimen during concentrated load, such as three-dimensional morphology, displacement field, strain field, stress field, etc. The method of the present invention has the characteristics of simple operation, non-contact measurement, and the ability to simultaneously measure the three-dimensional morphology, overall deformation and stress field of the front and back of the transparent specimen. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a flow chart of the method of the present invention;

[0025] Figure 2 This is a diagram of the experimental equipment of the method of the present invention;

[0026] Figure 3 This is the schematic diagram of the digital gradient sensitivity method. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to specific implementation examples in conjunction with the accompanying drawings.

[0028] (1) Monochrome fluorescent random speckle patterns are prepared on the three planes of the front, back and speckle plate surface of the transparent specimen under test, and the colors of the fluorescent speckles on the three planes are different, being one of red, green and blue. The reason is that the 3CCD color camera can output the values ​​of the three colors of red, green and blue at each pixel, and a CCD is configured for each color. Since the wavelengths of the fluorescent speckles on the front, back and speckle plate of the transparent specimen are different, the three speckle plane images are processed by different CCD image sensors under the illumination of a specific light source to avoid mutual interference between the fluorescent speckles on different planes. And the speckles should be random fluorescent spots with a certain contrast with the background color of the plane. The deformation field on the front, back and stress field of the transparent specimen under test are calculated respectively through the three-color CCD images;

[0029] (2) Use two 3CCD color cameras to build a Figure 2The optical measurement device shown has one camera as the main camera, whose optical axis is perpendicular to the plane of the test piece and the plane of the speckle plate, and another camera as the auxiliary camera, whose optical axis forms an angle of 35° ± 5° with the optical axis of the main camera. When the included angle between the optical axes of the two 3CCD color cameras is 35°, the measurement accuracies of different-sized measurement areas within their planes are highly consistent. l is the distance from the plane of the transparent test piece to the plane of the speckle plate, and L is the distance from the plane of the transparent test piece to the lens of the main camera, with the requirement that l < L. The transparent test piece under test is subjected to a static load F. Due to the deflection of light, point P corresponding to point O when no load is applied becomes point Q when a concentrated load is applied, and δ x and δ y respectively represent the displacements of a certain point on the image in the x-direction and y-direction caused by the deflection of light after the test piece is stressed.

[0030] (3) Before loading, calibrate the internal and external parameters of the optical measurement device, and calibrate the photo-mechanical parameter C σ of the transparent test piece under test, which is used to calculate the surface deformation and stress distribution of the transparent test piece under test. The internal parameters are (u0, v0, S x , S y ), where (u0, v0) are the image coordinates of the intersection point O of the optical axis of the main camera and the image plane of the main camera, and the equivalent focal lengths (S x , S y ) are the ratios of the lens focal length f to the horizontal and vertical dimensions of the pixel respectively; the external parameters are the rotation matrix R and the translation vector T, and R and T describe the relative position and attitude relationship between the camera coordinate system and the world coordinate system; for any point on the transparent test piece under test, there is the following mapping relationship between its image coordinates (u, v) in a certain camera image and its coordinates (X, Y, Z) in the world coordinate system:

[0031]

[0032] where Z c is the projection length of the distance from this spatial point to the optical center of the camera in the direction of the optical axis.

[0033] The photo-mechanical parameter C σ of the transparent test piece under test is an inherent parameter of the material, but this parameter is easily affected by conditions such as preparation process and temperature control. For materials of the same composition but different batches, their photo-mechanical parameters C σ also vary. Therefore, it is necessary to calibrate it before the experiment;

[0034] (4) Collect the images of the test piece under a concentrated load;

[0035] (5) Using the internal and external parameters of the optical measurement device in step (3), the front and back sides of the transparent specimen under test are three-dimensionally reconstructed at different times, and the surface deformation is calculated using the digital speckle correlation method, as well as the image displacement field δ of the speckle plate in the main camera after the light is refracted by the transparent specimen under test. x and δ y . Image displacement field δ x and δ y It refers to the image displacement field in the x and y directions presented by the speckle plate in the main camera. This displacement field is the displacement field presented on the image by the refraction of light through the transparent specimen under test, and is not the real displacement field of the speckle plate.

[0036] (6) Use the photomechanical parameters C of the transparent specimen calibrated in step (3) σ and the image displacement field δ of the speckle plate in the main camera after the light is refracted by the transparent specimen in step (5) x and δ y , the full-field stress distribution of the transparent specimen can be measured. The stress field of the transparent specimen and the image displacement field obtained in step (5) have the following relationship:

[0037]

[0038] Wherein, B is the thickness of the specimen; the point of action of the concentrated load on the transparent specimen is taken as the origin, the direction perpendicular to the boundary of the transparent specimen and pointing to the inside of the specimen is the x direction, the direction along the boundary of the transparent specimen is the y direction, and the y direction is the x direction rotated 90° in the counterclockwise direction; x and y are the coordinates of any point on the transparent specimen in this coordinate system, σ x and σ y is the stress distribution of the tested transparent specimen in the x and y directions, F refers to the concentrated load on the tested transparent specimen, τ xy is the shear stress distribution of the tested transparent specimen.

Claims

1. A high-precision full-field measurement method for a transparent specimen subjected to a concentrated load, characterized in that: The method comprises the following steps: (1) Monochromatic fluorescent random speckle patterns are prepared on the front and back surfaces of the transparent specimen under test and the surface of the speckle plate, and the colors of the fluorescent speckles on the three planes are different, namely, red, green, and blue; (2) Use two 3CCD color cameras to build an optical measurement device, and place the transparent test piece and the speckle plate. One camera is used as the main camera, and its optical axis is perpendicular to the plane of the test piece and the plane of the speckle plate. The other camera is used as the auxiliary camera, and its optical axis forms a certain angle with the optical axis of the main camera. (3) Keeping the camera position in step (2) unchanged, calibrate the internal and external parameters of the optical measurement device; calibrate the optical-mechanical parameters C of the transparent specimen to be measured. σ Calibration is performed to calculate the surface deformation and stress distribution of the transparent specimen under test; (4) Collect images of the test piece when it is subjected to concentrated load; (5) Using the internal and external parameters of the optical measurement device in step (3) and the image obtained in step (4), the surface deformation of the front and back surfaces of the transparent specimen under test at different times is calculated by the digital speckle method, and the image displacement field δ of the light of the speckle plate in the main camera after being refracted by the transparent specimen under test is calculated. x and δ y ; (6) Use the photomechanical parameters C of the transparent specimen calibrated in step (3) σ and the image displacement field δ of the light of the speckle plate in step (5) after being refracted by the transparent specimen under test in the main camera x and δ y , to achieve the measurement of the full-field stress distribution of the transparent specimen under test; The stress field of the transparent specimen under test described in step (6) and the image displacement field obtained in step (5) are related as follows: Wherein, B is the thickness of the specimen; the point of action of the concentrated load on the transparent specimen is taken as the origin, the direction x is perpendicular to the boundary of the transparent specimen and points to the inside of the specimen, and the direction along the boundary of the transparent specimen is taken as the y direction to establish a coordinate system, and the y direction is the x direction rotated 90° in the counterclockwise direction; x and y are the coordinates of any point on the transparent specimen in the coordinate system, σ x and σ y is the stress distribution of the tested transparent specimen in the x and y directions, F refers to the concentrated load on the tested transparent specimen, τ xy is the shear stress distribution of the tested transparent specimen, and l is the distance from the plane of the tested transparent specimen to the plane of the speckle plate.

2. A high-precision full-field measurement method for a transparent specimen subjected to concentrated load according to claim 1, characterized in that: In step (2), the angle between the optical axis of the secondary camera and the optical axis of the main camera is 35°±5°.

3. A high-precision full-field measurement method for a transparent specimen subjected to concentrated load according to claim 1, characterized in that: In step (2), when the transparent specimen to be tested and the speckle plate are placed, the plane of the transparent specimen to be tested and the surface of the speckle plate should be kept parallel, L is the distance from the plane of the transparent specimen to the main camera lens, and l<L.

4. A high-precision full-field measurement method for a transparent specimen subjected to concentrated load according to claim 1, characterized in that: The internal parameters described in step (3) are (u0, v0, S x , S y ), where (u0, v0) is the image coordinate of the intersection point O of the main camera optical axis and the image plane of the main camera, and the equivalent focal length (S x , S y ) is the ratio of the focal length of the lens f to the horizontal and vertical dimensions of the pixel respectively; the external parameters are the rotation matrix R and the translation vector T, R and T describe the relative position and posture relationship between the camera coordinate system and the world coordinate system; the image coordinates (u, v) of any point on the tested transparent specimen in a certain camera image and the coordinates (X, Y, Z) in the world coordinate system have the following mapping relationship: Among them, Z c It is the projection length of the distance from the spatial point to the optical center of the camera in the direction of the optical axis.

5. A high-precision full-field measurement method for a transparent specimen subjected to concentrated load according to claim 1, characterized in that: The image displacement field δ described in step (5) x and δ y It refers to the image displacement field in the x and y directions presented by the speckle plate in the main camera. This displacement field is the displacement field presented on the image by the refraction of light through the transparent specimen under test, and is not the real displacement field of the speckle plate.

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