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CN109696356B - Geosynthetic material tensile sample global strain field measuring device and method - Google Patents

Geosynthetic material tensile sample global strain field measuring device and method Download PDF

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CN109696356B
CN109696356B CN201910060750.0A CN201910060750A CN109696356B CN 109696356 B CN109696356 B CN 109696356B CN 201910060750 A CN201910060750 A CN 201910060750A CN 109696356 B CN109696356 B CN 109696356B
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吴海民
张振
束一鸣
程醒
田振宇
冯路明
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Hohai University HHU
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/08Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01BMEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00Measuring arrangements characterised by the use of optical techniques
    • G01B11/16Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00Investigating strength properties of solid materials by application of mechanical stress
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01N2203/0017Tensile
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00Investigating strength properties of solid materials by application of mechanical stress
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Abstract

The invention discloses a device and a method for measuring a global strain field of a geosynthetic material tensile sample. The invention obviously improves the measuring efficiency, has strong adaptability and simple and efficient measuring method.

Description

Geosynthetic material tensile sample global strain field measuring device and method
Technical Field
The invention relates to a strain field measuring device and method, in particular to a device and method for measuring a global strain field of a geosynthetic material tensile sample.
Background
In the existing geosynthetic material test regulations or standards, the characterization and calculation of the test tensile deformation both adopt approximate nominal strain or elongation, namely, the change value of the distance between clamps at two ends of a sample is compared with the original length of the sample to obtain a nominal strain value. When the tensile deformation of the geosynthetic material sample is small, the difference between the nominal strain and the real strain is not large, but when the tensile deformation is large, the strains in the middle part of the geosynthetic material sample and the two ends close to the clamp are not uniformly distributed, so that the nominal strain and the real strain obtained by approximate calculation have large difference. Therefore, if the tensile mechanical properties of the geosynthetic material are to be accurately studied, it is necessary to more accurately measure the true strain and distribution at any point on the sample, i.e., the global strain field of the entire sample, during the tensile test. At present, for the unidirectional tensile test of the geosynthetic material, the linear strain of a sample measurement area can be accurately obtained through an extensometer, but no measurement method can well solve the measurement problem of the sample global true strain field in the bidirectional tensile test of the geosynthetic material.
The measurement method can be classified into a contact measurement method and a non-contact measurement method according to whether the test instrument is in contact with the test material. The contact type measuring method tool is a quite common measuring method at present due to simplicity, easiness in control and low price, and the methods for contacting a sample can be roughly divided into three types, namely adhesion, barb and manual supporting. However, for geosynthetics having the characteristics of soft texture, large deformation, thin thickness, easy damage, extremely uneven strain distribution and the like, adhesive strain measurement tools such as a resistance strain gauge and the like often have a larger modulus compared with the geosynthetics, and easily influence the test result of the mechanical property test of the geosynthetics; hook-and-stab type strain measuring tools, such as extensometers and the like, are easy to damage the geosynthetic materials, so that the test is influenced; strain measurement tools that require manual bracing, such as rulers, are obviously impractical when continuous tensile testing and multi-point measurement are required. Therefore, for the strain field measurement in the biaxial tension test of the geosynthetic material, the contact strain measurement method cannot meet the requirement, and a non-contact strain field measurement device or method is needed.
Disclosure of Invention
The purpose of the invention is as follows: the invention aims to provide a device for measuring the global strain field of a tensile sample of a geosynthetic material, which can measure strain without contacting with a measuring object, and has the advantages of simplicity, high efficiency and strong adaptability.
The invention also aims to provide a global strain field measurement method for the geosynthetic material tensile sample.
The technical scheme is as follows: the invention provides a device for measuring a global strain field of a geosynthetic material tensile sample, which comprises a high-definition camera, a sample to be measured, a computer and a data acquisition and analysis system, wherein the high-definition camera is connected with the computer and the data acquisition and analysis system through data lines, a mark grid is arranged on the surface of the sample to be measured, and a lens of the high-definition camera is right opposite to the mark grid on the surface of the sample to be measured. The high-definition camera is a high-pixel shooting device for acquiring high-definition digital images. The marked grid refers to an orthogonal grid-shaped graph or a dot matrix drawn in an observation area on the surface of the geosynthetic material sample to be detected according to the load direction. The computer and the data acquisition and analysis system are used for storing, processing and analyzing digital image data and calculating a strain field.
The surface to be measured of the sample to be measured is opposite to the lens of the high-definition camera, the field range of the digital image is adjusted by adjusting the focal length and the position of the high-definition camera, and meanwhile, the mark grids in the digital image are clear and bright enough by focusing and adjusting the aperture.
Furthermore, the edge of the sample to be tested is clamped by a clamp, and the clamp draws the sample to be tested to deform under the action of test load.
Further, the high definition camera is located the cloud platform on fixed bolster upper portion, the fixed bolster is used for supporting the high definition camera, the cloud platform is used for adjusting high definition camera shooting angle.
Due to the large deformation of the geosynthetic material, the field of view should be large enough to ensure that the complete grid of marks is captured throughout the course of the test by adjusting the high definition camera and the fixed support.
The method for measuring the global strain field of the geosynthetic material tensile sample comprises the following steps of:
(1) drawing a marked grid in an observation area of a sample to be tested, and loading the sample;
(2) fixing the high-definition camera by using a fixing support, and adjusting a holder on the fixing support to enable a lens of the high-definition camera to be opposite to a mark grid on the surface of the sample to be detected;
(3) adjusting the light and shadow conditions of a shooting site, and adjusting the focal length and the aperture of the high-definition camera and the position of the fixed support, so that the marking grids in the digital image are clear and bright, the light and shadow effect is good, and the field of view is sufficient;
(4) starting a high-definition camera for image acquisition, and starting a test at the same time;
(5) after the test is finished, stopping image acquisition, and then transmitting the digital image acquired by the high-definition camera to a computer and a data acquisition and analysis system;
(6) and reading the pixel coordinates of each grid point at each acquisition moment through a computer and a data acquisition and analysis system, and calculating to obtain a displacement field and a strain field in the observation area of the sample to be measured through the pixel coordinates.
Further, the calculation of the strain field in the step (6) comprises the following steps:
(1) reading the pixel coordinates of each grid point at each acquisition time through a computer and a data acquisition and analysis system, and recording the pixel coordinates of the grid point j at the ith acquisition time as
Figure BDA0001953280910000021
(2) Calculating a calibration proportion k;
(3) calculating the coordinates of each grid point in the world coordinate system corresponding to the pixel coordinate system at each acquisition time, and recording the actual coordinates of the grid point j at the ith acquisition time as
Figure BDA0001953280910000022
Wherein:
Figure BDA0001953280910000023
(4) calculating the displacement of each grid point relative to the initial time at each acquisition time, and recording the displacement of the grid point j relative to the initial time at the ith acquisition time as
Figure BDA0001953280910000024
Wherein:
Figure BDA0001953280910000025
(5) assuming that the grid cells in the marked grid are sequentially grid points a, b, c and d from the top right vertex clockwise, obtaining a displacement field in an observation area at each acquisition time by an interpolation method, and recording the displacement function of the ith acquisition time in the first grid cell as ui(x, y) and vi(x,y);
(6) And calculating the strain tensor by using the Green strain tensor, assuming that a certain grid is sequentially grid points a, b, c and d from a clockwise vertex of an upper right vertex and a central point of the grid is an o point, calculating a strain log in an observation area at each acquisition time by the following formula, and recording the strain of the central point o of the grid at the ith acquisition time as
Figure BDA0001953280910000031
Figure BDA0001953280910000032
Wherein,
Figure BDA0001953280910000033
the strain value of the grid central point o at the ith acquisition time can be obtained by derivation of a displacement function in the grid and substituting the displacement function into the coordinate of the grid central point o, and the strain values of all the grid central points are calculated according to the method, so that a strain field in an observation area can be obtained by interpolation.
Further, the method for calculating the calibration ratio k in the step (2) comprises the following steps: the method is obtained by comparing the digital image acquired at the initial undeformed moment with the actual condition:
Figure BDA0001953280910000034
wherein L is0Is the pixel distance between two points on the surface of the geosynthetic material sample to be measured at the initial moment; l0Is the actual distance between the corresponding two points at the initial moment.
Has the advantages that: the invention belongs to a non-contact measurement method, which can not damage a sample, can not influence the normal operation of a test, and can not influence the accuracy of a test result; the invention records the movement of the grid points on the surface of the sample and the deformation of the grid units in the form of digital images, can realize the continuous measurement of the large-area and long-time universe displacement field and strain field, and can greatly improve the testing work efficiency; the displacement strain is calculated by processing and analyzing the digital image, and if the equipment is high-end and advanced, the measurement precision is enough to meet the requirement; the measuring method has high automation degree, and can reduce the workload of experimenters.
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FIG. 1 is a schematic structural view of the present invention;
FIG. 2 is a schematic structural diagram of the sample to be tested in FIG. 1;
fig. 3 is a schematic diagram of the labels in the grid cell displacement field calculation and the grid center point strain calculation according to the present invention.
Detailed Description
As shown in fig. 1-3, the geosynthetic material tensile sample global strain field measurement device of this embodiment includes a high definition camera 1, a sample 4 to be measured, a computer and a data acquisition and analysis system 3, where the high definition camera 1 is connected to the computer and the data acquisition and analysis system 3 through a data line 6, a mark grid 5 is disposed on the surface of the sample 4 to be measured, and a lens 10 of the high definition camera 1 faces the mark grid 5 on the surface of the sample 4 to be measured. The edge of the sample 4 to be tested is clamped by the clamp 9, and the clamp 9 draws the sample 4 to be tested to deform under the action of the test load 8. High definition camera 1 is located the cloud platform 7 on fixed bolster 2 upper portions, and fixed bolster 2 is used for supporting high definition camera 1, and cloud platform 7 is used for adjusting 1 shooting angles of high definition camera.
The high-definition camera 1 is an industrial camera which can adjust the shooting brightness and the shooting focal length and transmits the acquired digital images to the computer and the data acquisition and analysis system 3 through a data line 6. The fixed support 2 adopts a tripod, and the tripod is provided with a tripod head 7 which can be used for fixing an industrial camera and adjusting the all-directional angle of the industrial camera, so that the industrial camera can be conveniently shot. The shooting mode, shooting interval and the like are set by the computer and the data acquisition and analysis system 3. The sample 4 to be tested is made of a PVC geomembrane, the color is gray, the marking grids 5 are 10 multiplied by 10 square grid lines drawn in the center of the PVC geomembrane to be tested according to the load direction by adopting an oil pen and a ruler with relatively bright colors, and the distance between every two grid lines is 0.5 cm.
The method for measuring the global strain field of the geosynthetic material tensile sample comprises the following steps of:
(1) and (3) drawing a marked grid 5 in the observation area, namely the midpoint, of the bidirectional stretching cross sample of the PVC geomembrane to be tested by using a red oily pen, wherein the central point of the marked grid is superposed with the central point of the sample to be tested 4, and the transverse and longitudinal directions of the marked grid are consistent with the direction of a test load 8 to be applied. Then clamping the sample 4 to be tested by using a clamp 9 according to the test requirement, and preparing for starting the test;
(2) the industrial camera is fixed through a tripod head 7, the shooting angle of the industrial camera is adjusted through the tripod head 7 to be aligned to the mark grid 5 on the surface of the sample 4 to be measured, and a lens 10 of the industrial camera is required to be aligned to the mark grid 5 on the surface of the sample 4 to be measured. Then, opening the industrial camera, and adjusting the light and shadow conditions of the test site and the brightness, distance and definition of the image until the digital image has enough vision field, good definition and uniform brightness, wherein in order to enable the digital images acquired in the test to have the same pixel coordinate system, the high-definition camera 1 must be fixed in the test process, and the shooting environment must be kept stable;
(3) the shooting mode of the industrial camera is set to be a continuous shooting mode through the computer and the data acquisition and analysis system 3, and the shooting interval is 1 min. The shutter 11 was then clicked and the test was started. The industrial camera and the digital image acquisition are not required to be changed or interfered any more in the test process;
(4) after the test is finished, stopping the acquisition of the digital image, and transmitting the digital image acquired by the industrial camera to the computer and data acquisition and analysis system 3 by using the data line 6;
(5) reading the pixel coordinate information of each grid point 12 at each acquisition time by MATLAB software on a computer and a data acquisition and analysis system 3, and recording the pixel coordinate of a grid point j at the ith acquisition time as
Figure BDA0001953280910000041
(6) Calculating the ratio of the distance between the specific two points on the surface of the sample to the distance between the corresponding two points in the digital image acquired at the initial moment by the following formula to obtain a calibration ratio k:
Figure BDA0001953280910000042
in the formula: l is0Is the pixel distance between two points on the surface of the geosynthetic material sample to be measured at the initial moment; l0Is the actual distance between the corresponding two points at the initial moment;
(7) calculating the actual coordinate information of each grid point 12 at each acquisition time by the following formula, and recording the actual coordinate of the grid point j at the ith acquisition time as
Figure BDA0001953280910000051
Figure BDA0001953280910000052
(8) Calculating the displacement information of each grid point 12 at each acquisition time by the following formula, and recording the displacement of the grid point j at the ith acquisition time relative to the initial time as
Figure BDA0001953280910000053
Figure BDA0001953280910000054
(9) The displacement field in the observation zone at each acquisition instant is calculated by the following formula. Assuming that the clockwise vertexes of a certain grid from the upper right vertex are grid points a, b, c and d in turn, and obtaining a displacement function of the ith acquisition time in the grid through a linear interpolation method:
Figure BDA0001953280910000055
Figure BDA0001953280910000056
in the formula:
Figure BDA0001953280910000057
X-direction displacement of the grid points a, b, c and d at the moment respectively;
Figure BDA0001953280910000058
the y-direction displacement of the grid points a, b, c and d at the moment respectively; l is the side length of a unit grid in the marking grid and is 0.5 cm; x is the number ofc、ycRespectively the initial coordinates of grid point c. Further obtain the displacement field (u) of the ith acquisition time of the whole observation areai,vi);
(10) Calculating the strain value of the midpoint 13 in each grid at each acquisition time according to the green strain tensor, and assuming the sequence numbers of the four vertexes of the marked grid, wherein the strain of the midpoint in the grid at the ith acquisition time is as follows:
Figure BDA0001953280910000059
wherein:
Figure BDA00019532809100000510
the strain of the middle point of the grid at the ith acquisition moment can be obtained by derivation of a displacement function in the grid and then substitution into the coordinate of the middle point of the grid;
(11) calculating the strain field (epsilon) of the observation zonex,εy,εxy). According to the known strain of the midpoint 13 of each grid, the interpolation method of step (9) can be used to obtain the strain field of the whole observation area.

Claims (2)

1. A geosynthetic material tensile sample global strain field measurement method is characterized in that: the geosynthetic material tensile sample global strain field measuring device is used for measuring and comprises a high-definition camera (1), a sample to be measured (4), a computer and a data acquisition and analysis system (3), wherein the high-definition camera (1) is connected with the computer and the data acquisition and analysis system (3) through a data line (6), a mark grid (5) is arranged on the surface of the sample to be measured (4), and a lens (10) of the high-definition camera (1) is right opposite to the mark grid (5) on the surface of the sample to be measured (4); the edge of the sample (4) to be tested is clamped by a clamp (9), and the clamp (9) is acted by a test load (8) to pull the sample (4) to be tested to deform; the high-definition camera (1) is positioned on a tripod head (7) at the upper part of the fixed support (2), the fixed support (2) is used for supporting the high-definition camera (1), and the tripod head (7) is used for adjusting the shooting angle of the high-definition camera (1);
the measuring method comprises the following steps:
(1) drawing a mark grid (5) in an observation area of a sample (4) to be tested, and loading the sample;
(2) fixing the high-definition camera (1) by using the fixing support (2), and adjusting a holder (7) on the fixing support (2) to ensure that a lens (10) of the high-definition camera (1) is opposite to a mark grid (5) on the surface of the sample (4) to be detected;
(3) adjusting the light and shadow conditions of a shooting site, and adjusting the focal length and the aperture of the high-definition camera (1) and the position of the fixed support (2) to ensure that the marking grid (5) in the digital image is clear and bright, the light and shadow effect is good and the field of view is sufficient;
(4) starting a high-definition camera (1) to acquire images, and starting a test;
(5) after the test is finished, stopping image acquisition, and then transmitting the digital image acquired by the high-definition camera (1) to a computer and a data acquisition and analysis system (3);
(6) reading the pixel coordinates of each grid point at each acquisition moment through a computer and a data acquisition and analysis system (3), and calculating to obtain a displacement field and a strain field in an observation area of a sample (4) to be measured through the pixel coordinates;
the calculation of the strain field in the step (6) comprises the following steps:
(1) reading the pixel coordinates of each grid point at each acquisition time through a computer and a data acquisition and analysis system (3), and recording the pixel coordinates of the grid point j at the ith acquisition time as
Figure FDA0002804275870000011
(2) Calculating a calibration proportion k;
(3) calculating each acquisitionThe coordinates of each grid point (12) at the moment in an absolute coordinate system corresponding to the pixel coordinate system are recorded as the actual coordinates of the grid point j at the i-th acquisition time
Figure FDA0002804275870000012
Wherein:
Figure FDA0002804275870000013
(4) and calculating the displacement of each grid point (12) at each acquisition time relative to the initial time, and recording the displacement of the grid point j at the ith acquisition time relative to the initial time as
Figure FDA0002804275870000014
Wherein:
Figure FDA0002804275870000015
(5) assuming that the grid cells in the marked grid (5) are grid points a, b, c and d sequentially from the top right vertex clockwise, obtaining a displacement field in an observation area at each acquisition time by an interpolation method, and recording the displacement function of the ith acquisition time in a certain grid cell as ui(x, y) and vi(x,y);
(6) And the strain tensor needs to be calculated by using a Green strain tensor, assuming that a certain grid is sequentially grid points a, b, c and d from a clockwise vertex of an upper right vertex and a grid central point (13) is a point o, calculating a strain log in an observation area at each acquisition time by the following formula, and recording the strain of the grid central point o at the ith acquisition time as
Figure FDA0002804275870000021
Figure FDA0002804275870000022
Wherein,
Figure FDA0002804275870000023
the strain value of the grid central point o at the ith acquisition time can be obtained by derivation of a displacement function in the grid and substituting the displacement function into the coordinate of the grid central point o, and the strain values of all the grid central points (13) are calculated according to the method, namely, a strain field in an observation area can be obtained by interpolation.
2. The geosynthetic tensile specimen global strain field measurement method of claim 1, wherein: the method for calculating the calibration proportion k in the step (2) comprises the following steps: the method is obtained by comparing the digital image acquired at the initial undeformed moment with the actual condition:
Figure FDA0002804275870000024
wherein L isOIs the pixel distance between two points on the surface of the geosynthetic material sample to be measured at the initial moment; lOIs the actual distance between the corresponding two points at the initial moment.
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