A two-dimensional multi-scale strain measurement method based on digital image correlation

Abstract

The present invention relates to a two-dimensional multi-scale strain measurement method based on digital image correlation, comprising the following steps: (1) preparing multi-scale, high-temperature speckles on the surface of a test piece by combining a particle mixing method with a spraying method; (2) building a two-dimensional in-situ multi-scale strain measurement test bench; (3) synchronously controlling and acquiring real-time speckle images of multiple scales under different loads; (4) calculating out-of-plane displacement based on the in-plane displacement generated by the test piece; and (5) eliminating strain errors introduced by out-of-plane displacement. The present invention can achieve multi-scale strain measurement at high temperatures, prepares multi-scale speckles suitable for temperatures of 800°C, performs microscopic observations simultaneously with macroscopic observations, and corrects false strains caused by out-of-plane displacements, thereby improving the accuracy of the two-dimensional digital image correlation method. The method has high measurement efficiency, convenient operation, high measurement accuracy, and a wide range of applications.

Description

Technical Field

[0001] The present invention relates to the field of aerospace engine technology, and in particular to a two-dimensional multi-scale strain measurement method based on digital image correlation. The method is a high-precision strain measurement method for correcting false strain caused by off-plane displacement in multi-scale strain measurement. Background Art

[0002] A wide variety of materials and structural components are widely used in the aerospace industry. Material failure can significantly reduce the service life of structural components, seriously endangering the normal operation of aerospace equipment. Strain and displacement measurements of structural components and materials at different scales are crucial for studying macroscopic failure patterns and revealing microscopic failure mechanisms, thereby extending the service life of various structural components. Therefore, it is necessary to develop a high-precision and efficient strain measurement method at multiple scales.

[0003] Currently, widely used displacement and strain measurement methods include electrical measurement, optical measurement, and extensometers. Electrical measurement involves attaching strain gauges to the surface being measured and is often applied at the macroscale. Photoelastic measurement offers high accuracy, but its optical system is complex and specimen preparation is tedious. Projection moiré, an experimental method that uses interference moiré as a measurement element to calculate displacement and strain fields, is difficult to apply to microscopic measurements. These measurement methods struggle to balance the advantages of multi-scale measurement with high precision and efficiency.

[0004] With the development of digital cameras and computer technology, digital image correlation method has been widely used in full-field strain measurement due to its advantages such as strong environmental adaptability, simple optical system, and full-field non-contact measurement.

[0005] The existing literature (Sun C, Zhou Y, Li Y, et al. Multiscale segmentation-aided digital image correlation for strain concentration characterization of a turbine blade fir-tree root [J]. Measurement Science and Technology, 2018, 29(4): 045205) uses digital image correlation to measure at different magnifications and observe different ranges of the specimen using two cameras. However, only macroscopic speckles are produced on the specimen surface, which is still at the macroscale, and the false strain caused by off-plane displacement is not eliminated. Chen Zhenning and others developed a multi-scale digital image correlation measurement method (Chen Zhenning, Sun Wei. A multi-scale digital image correlation measurement method [P]. Jiangsu Province: CN112857243A, 2021-05-28.), using oil, ink and paint to print digital speckle fields on the surface of the sample one by one. However, the speckle prepared by this method requires a purple light source to excite different bands, and the imaging system needs to be equipped with corresponding bandpass filters. In addition, the speckle pattern is easy to evaporate and melt under high temperature conditions, making it difficult to apply to high temperature environments. In addition, the ink transfer method is difficult to apply to fine scales.

[0006] In summary, the existing two-dimensional DIC method lacks an efficient and simple multi-scale speckle preparation method for multi-scale measurement. On the other hand, the false strain caused by off-plane displacement is ignored during strain calculation, resulting in large errors in the calculation results. Therefore, traditional strain measurement methods are difficult to achieve multi-scale measurement. Summary of the Invention

[0007] To address the aforementioned technical issues, the present invention provides a two-dimensional, multi-scale strain measurement method based on digital image correlation. This method fully reflects the displacement and strain field of aerospace specimens under load and corrects for spurious strain caused by out-of-plane displacement, thus supporting strain measurement of aerospace structures. This method utilizes a high-temperature, multi-scale speckle pattern generation method that combines particle mixing and spraying, and proposes methods for calculating out-of-plane displacement and correcting for spurious strain.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A two-dimensional multi-scale strain measurement method based on digital image correlation method includes the following steps:

[0010] Step (1) combines a particle mixing method with a spraying method to prepare multi-scale, high-temperature speckles on the surface of the test piece. At the macroscale, high-temperature resistant paint is used for high-pressure spraying, and black and white paint appears as macroscopic speckles under the millimeter-level field of view. At the microscale, high-temperature paint and high-temperature resistant cobalt oxide are selected to prepare a suspension through a particle mixing method, and then sprayed on the surface of the test piece with a high-pressure airbrush. Random black particles on an off-white primer appear as microscopic speckles under the micrometer-level field of view.

[0011] Step (2) building a two-dimensional in-situ multi-scale strain measurement test bench, configuring a blue light source and filters, and building a camera gimbal to ensure that the camera optical path is perpendicular to the specimen;

[0012] Step (3) acquiring real-time speckle images at multiple scales, and collecting reference images and deformation images at multiple scales under different loads;

[0013] Step (4) Calculate the out-of-plane displacement based on the in-plane displacement generated by the specimen: directly use DIC calculation to obtain preliminary full-field displacement strain information, and calculate its out-of-plane displacement through the geometric relationship of optical imaging. Calculating the out-of-plane displacement requires first interpolating sub-pixel points and calculating the displacement of the sub-pixel points;

[0014] Step (5) eliminates the strain error introduced by the off-plane displacement: a false strain correction algorithm is proposed based on the optical imaging model, and the false strain correction calculation is performed using the off-plane displacement calculated in the fourth step to obtain the true strain value; at this point, the two-dimensional multi-scale strain measurement method based on the digital image correlation method is completed.

[0015] Furthermore, in the step (1), in the preparation of the high-temperature resistant multi-scale speckle pattern, black and white epoxy silicone resin paints are respectively mixed with nitro diluent in a ratio of 5:1 to 3:1 to form a low-viscosity liquid, and the mixture is randomly sprayed at an air pressure of 0.4 to 0.6 MPa; high-purity cobalt oxide powder is mixed into the white liquid with a solid-liquid volume ratio of 1:3 to form an off-white suspension, and the suspension is placed in an ultrasonic oscillator and oscillated for at least 5 minutes to remove bubbles in the suspension and to uniformly disperse the particles; after oscillation, the suspension is randomly and uniformly sprayed at an air pressure of about 0.5 MPa using a spray pen, and after drying, a macroscopic speckle pattern of off-white spots on the surface of the black primer and a microscopic speckle pattern of random black particles on the off-white primer are formed.

[0016] Furthermore, in step (4), the calculation method for calculating the out-of-plane displacement ω based on the in-plane displacement is:

[0017]

[0018] d0=dd F

[0019] Where R represents the image width, d is the in-plane displacement calculated by the DIC method, d0 represents the in-plane displacement caused by the out-of-plane displacement, and d F is the actual in-plane displacement due to the load, and θ represents the angle between the specimen surfaces before and after deformation;

[0020] The interpolation sub-pixel displacement algorithm is used to interpolate the grayscale value of a single pixel into the grayscale values ​​of 10 0.1 pixel points in the x and y directions respectively. The displacement value of the sub-pixel point is calculated by the grayscale value, that is, 100 sub-pixel point displacements are calculated within a pixel point at the center of the optical axis, and the average value is taken as the d in the calculation process. F , and use this as a correction to further calculate d0 of each sub-area.

[0021] Furthermore, in step (5), the calculation method of the false strain Δε based on the optical imaging model is:

[0022]

[0023] Where L represents the real-time lens-object distance during image acquisition, ω is the off-plane displacement calculated in step (4), Δε represents the false strain, and Y(Z) represents the y-coordinate of the measured point on the imaging surface.

[0024] Furthermore, in step (5), the step of determining the true strain based on the false strain is as follows: when the true strain is tensile strain, the false strain is corrected and subtracted; and when the true strain is compressive strain, the opposite is true, and the false strain is added during correction.

[0025] The advantages of the present invention compared with the prior art are:

[0026] (1) The particle mixing and high-pressure gas spraying method adopted in the present invention can prepare multi-scale speckles, which can simultaneously show macroscopic speckles of alternating black and white paints and microscopic speckles of alternating gray-white primers and black particles in millimeter and micrometer fields of view, meeting the needs of multi-scale observation.

[0027] (2) The selected silicone resin paint and cobalt oxide particles are both resistant to high temperatures, so the multi-scale speckle pattern prepared by the present invention is also resistant to high temperatures. Therefore, high-temperature tests can be carried out in combination with a two-dimensional in-situ multi-scale strain measurement test bench.

[0028] (3) The present invention adds the calculation of interpolated sub-pixel displacement on the basis of the digital image correlation method, further improving the accuracy of displacement and strain calculation.

[0029] (4) The present invention makes full use of the in-plane displacement information and full-field strain calculation results obtained by two-dimensional digital image correlation analysis, introduces the principle of interpolation sub-pixel displacement algorithm to perform off-plane displacement calculation and false strain correction, improves the accuracy of two-dimensional multi-scale strain measurement, overcomes the shortcomings of third-party auxiliary tools that are difficult to apply at microscopic scales, and improves the practicality of digital image correlation method in multi-scale measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of a two-dimensional multi-scale strain measurement method based on digital image correlation method of the present invention;

[0031] Figure 2 The geometric diagram for calculating out-of-plane displacement from in-plane displacement;

[0032] Figure 3 This is an interpolation diagram of the sub-pixel interpolation principle;

[0033] Figure 4 Schematic diagram of the geometric structure of the false strain correction method based on the optical model, where (a) is the out-of-plane displacement diagram far away from the lens, and (b) is the out-of-plane displacement diagram close to the lens;

[0034] Figure 5 Comparison chart of the direct DIC calculation results and the results after correcting false strain for the two experiments. DETAILED DESCRIPTION

[0035] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0036] The following further illustrates the technical solution of a two-dimensional multi-scale strain measurement method based on digital image correlation method of the present invention with reference to the accompanying drawings.

[0037] The present invention relates to a two-dimensional multi-scale strain measurement method based on digital image correlation method, such as Figure 1 As shown, considering multi-scale observation, high temperature test, and multi-scale strain measurement method resulting from false strain caused by off-plane displacement, the present invention provides a two-dimensional multi-scale strain measurement method based on digital image correlation method, which is specifically implemented as follows:

[0038] In the first step, the test piece is subjected to high-temperature resistant multi-scale speckle preparation by using the method of particle mixing and high-pressure gas spraying. The first is the preparation of macroscopic speckles. The black and white epoxy silicone resin paints are respectively mixed with nitro diluent in a ratio of 5:1 to 3:1 to form a low-viscosity liquid. Then, the black liquid is evenly sprayed onto the surface of the test piece through a siphon spray gun at an air pressure of 0.4 to 0.6 MPa to form a thin and uniform layer of black primer. The uniform black primer can improve the contrast of the test piece surface and prevent reflection, while providing a flat surface for the preparation of microscopic speckles. After the black primer is fully dried, a white paint spray gun is used, and the spray gun nozzle is adjusted to enhance the atomization effect and reduce the paint flow. Then, random dot spraying is performed at an air pressure of about 0.5 MPa to form a macroscopic speckle of white spots of black primer.

[0039] To prepare micro-scale speckle patterns, use the white, low-viscosity solution prepared in the macro-speckle preparation as a base. High-purity cobalt oxide powder is mixed into the solution at a solid-to-liquid ratio of approximately 1:3. This solution is thoroughly stirred to form an off-white suspension. This suspension is then placed in a glass or metal container and placed in an ultrasonic vibrator for at least 5 minutes to remove air bubbles and ensure uniform dispersion of the particles. No further stirring or other operations are permitted after the suspension has been shaken. The shaken suspension is then placed in an airbrush and sprayed evenly onto the test piece at a pressure of approximately 0.5 MPa. Dry at room temperature for 10-20 minutes to create a micro-speckle pattern of random black particles on the off-white primer.

[0040] The second step is to build a two-dimensional in-situ multi-scale strain measurement test bench based on the requirements of high-temperature testing. A camera with interchangeable magnification is used to meet multi-scale observation requirements. The camera imaging optical path is ensured to be perpendicular to the load direction. A blue light source is installed, and a filter is placed in front of the lens to reduce the impact of thermal radiation.

[0041] The third step is to obtain real-time speckle images at multiple scales. The camera's focus knob is adjusted until the speckle image is clearly displayed. After the load is reset, the specimen is clamped and images are collected under objective lenses of different magnifications as reference images. During the loading process, images under objective lenses of different magnifications are collected under the same load as deformation images.

[0042] The fourth step is to calculate the out-of-plane displacement based on the in-plane displacement generated by the specimen. Using the speckle images acquired at multiple scales under different loads using the aforementioned steps 1, 2, and 3, the reference and deformation images are analyzed using digital image correlation to obtain preliminary full-field strain calculations and full-field in-plane displacement information. The out-of-plane displacement is calculated based on the geometric relationship of optical imaging. An in-plane displacement solution algorithm based on sub-pixel interpolation and normalized cross-correlation is introduced to calculate the sub-pixel displacement value at the center of the optical axis.

[0043] The calculation of the off-plane displacement is based on the actual load test, and the DIC method is used to calculate the preliminary in-plane displacement information. The off-plane displacement algorithm is:

[0044]

[0045] d0=dd F

[0046] Where d is the total in-plane displacement calculated by the DIC method, d0 represents the in-plane displacement caused by the out-of-plane displacement, and d F is the actual in-plane displacement due to the load, R represents the width of the image, and θ represents the angle between the specimen surfaces before and after deformation;

[0047] The calculation process takes the average displacement of a single pixel at the center of the optical path as d F :In the specific calculation process, the sub-pixel displacement of 0.1 pixel point at the center of the optical axis is obtained by interpolation sub-pixel displacement algorithm, and then the average value is taken as d in the algorithm. F , we can further calculate d0 of each sub-area.

[0048] The interpolation sub-pixel displacement algorithm refers to dividing 1 pixel by a mathematical interpolation method on the basis of the whole pixel to obtain the grayscale value of 0.5 pixel or smaller pixel, such as Figure 3 As shown in the figure, for a two-dimensional digital image, grayscale interpolation is first performed in the horizontal direction, and vertical interpolation is performed based on the interpolation point. Considering the smoothness of the interpolation, two-point cubic Hermite interpolation is selected to perform horizontal interpolation on a certain pixel point, as shown in the figure. Its interpolation function is:

[0049]

[0050] Where x represents the coordinate of the point being sought, x2 and x3 are the interpolation points on both sides respectively; g(x) is the initial grayscale value, and the boundary condition satisfies the first-order derivative of the grayscale G'(x) = g'(x). The derivative is calculated using the Barron operator with higher accuracy:

[0051]

[0052]

[0053] Where y represents the vertical coordinate of the point to be interpolated, and x0 to x5 are the horizontal coordinates of the three integer pixels on the left and right sides of the interpolation point.

[0054] That is, the grayscale value between two points after interpolation of the integer pixel point is expressed as:

[0055]

[0056] After obtaining the interpolation function, one-tenth of a pixel is selected as the step size to calculate the sub-pixel grayscale value, and finally the grayscale value difference of 1 pixel is converted to the grayscale value of 0.1 pixel.

[0057] Then the coordinates of the target points before and after deformation are searched and matched using the correlation coefficient function. The correlation coefficient function is expressed as follows:

[0058]

[0059]

[0060] Where C represents the zero-mean normalized least square distance correlation function; n represents the n×n fitting window centered at the point to be solved (x, y), and f i 、 Represents the sub-pixel grayscale value in the reference image, g i 、 Represents the sub-pixel grayscale value in the deformed image.

[0061] The calculation formula for the in-plane displacement of sub-pixel points is expressed as:

[0062] u=x′-x,v=y′-y

[0063] Where u and v represent the displacement values ​​in the x and y directions, x and y are the coordinates of the point to be determined in the reference image, and x′ and y′ are the coordinates of the point to be determined in the deformed image.

[0064] The geometric model for calculating off-plane displacement is as follows Figure 2 As shown in the figure, the schematic diagram of single pixel interpolation is as follows: Figure 3 shown.

[0065] The fifth step is to eliminate the strain error introduced by the off-plane displacement. A false strain correction algorithm is proposed based on the optical imaging model. The false strain correction calculation is performed using the off-plane displacement calculated in the fourth step to obtain the true strain value.

[0066] The false strain correction algorithm based on the optical imaging model, the optical imaging model diagram is as follows Figure 4 As shown in , when the object moves away from the lens, it will cause the corresponding point on the image to move closer to the center of the image, that is, it will produce a false compressive strain. Conversely, it will cause the corresponding point on the image to move away from the center of the image, that is, it will produce a false tensile strain. Figure 4 (a), Figure 4 The geometric similarity relationship in (b) can be used to obtain the false strain calculation method:

[0067]

[0068] Where L is the object distance of the camera lens, ω is the off-plane displacement calculated in the fourth step, Δε is the false strain, and Y(Z) is the y-coordinate of the measured point on the imaging surface.

[0069] The specific process of correcting false strain is to first calculate the out-of-plane displacement value through the geometric relationship of optical imaging, then calculate the false strain value through the imaging model, and finally eliminate the false strain from the strain result directly calculated by DIC to obtain the corrected strain value.

[0070] Based on the off-plane displacement value, the false strain can be calculated. If the actual strain is tensile strain, the false compressive strain should be added and the false tensile strain should be subtracted during correction, while the compressive strain is just the opposite. In the process of false strain calculation, the false tensile strain and false compressive strain are directly reflected in the positive and negative signs, that is, when the actual strain is tensile strain:

[0071] ε=ε0-Δε

[0072] When the actual strain is compressive strain:

[0073] ε=ε0+Δε

[0074] Where ε0 is the calculated strain value obtained by DIC calculation, and ε is the final true strain result.

[0075] like Figure 5 As shown in the figure, the DIC direct calculation results are compared with the calculation results after correcting the false strain. The left and right figures are comparison diagrams of the results of two different experiments. The relative error of the calculation results after correcting the false strain is reduced from 15% to about 5%. The accuracy of the strain calculation results of the present invention is significantly improved.

[0076] The above embodiments are provided for the purpose of describing the present invention only and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. Various equivalent substitutions and modifications made without departing from the spirit and principles of the present invention are intended to be within the scope of the present invention.

Claims

1. A two-dimensional multi-scale strain measurement method based on digital image correlation, characterized in that: The steps include: Step (1) combines a particle mixing method with a spraying method to prepare multi-scale, high-temperature speckles on the surface of the test piece. At the macroscale, a high-temperature resistant paint is used for high-pressure spraying, and black and white paint appears as macroscopic speckles under the millimeter-level field of view. At the microscale, a suspension is prepared by a particle mixing method using a high-temperature paint and a high-temperature resistant cobalt oxide, and then sprayed on the surface of the test piece with a high-pressure airbrush. Random black particles on an off-white primer appear as microscopic speckles under the micrometer-level field of view. Step (2) Build a two-dimensional in-situ multi-scale strain measurement test bench, configure a blue light source and filter, and build a camera gimbal to ensure that the camera optical path is perpendicular to the specimen; Step (3) acquiring real-time speckle images at multiple scales, and collecting reference images and deformation images at multiple scales under different loads; Step (4) Calculate the out-of-plane displacement based on the in-plane displacement generated by the specimen: directly use DIC calculation to obtain preliminary full-field displacement strain information, and calculate its out-of-plane displacement through the geometric relationship of optical imaging. Calculating the out-of-plane displacement requires first interpolating sub-pixel points and calculating the displacement of the sub-pixel points; The calculation method for the out-of-plane displacement ω based on the in-plane displacement is: Where R represents the image width, d is the in-plane displacement calculated by the DIC method, d0 represents the in-plane displacement caused by the out-of-plane displacement, and d F is the actual in-plane displacement due to the load, and θ represents the angle between the specimen surfaces before and after deformation; The interpolation sub-pixel displacement algorithm is used to interpolate the grayscale value of a single pixel into the grayscale values ​​of 10 0.1 pixel points in the x and y directions respectively. The displacement value of the sub-pixel point is calculated by the grayscale value, that is, 100 sub-pixel point displacements are calculated within a pixel point at the center of the optical axis, and the average value is taken as the d in the calculation process. F , and use this as a correction to further calculate the d of each sub-area 0； Step (5) Eliminate the strain error introduced by the off-plane displacement: A false strain correction algorithm is proposed based on the optical imaging model, and the false strain correction calculation is performed using the off-plane displacement calculated in the fourth step, that is, the true strain value is obtained; At this point, the two-dimensional multi-scale strain measurement method based on digital image correlation is completed; The calculation method of false strain Δε based on the optical imaging model is: Where L represents the real-time lens-object distance during image acquisition, ω is the off-plane displacement calculated in step (4), Δε represents the false strain, and Y(Z) represents the y-coordinate of the measured point on the imaging surface.

2. The two-dimensional multi-scale strain measurement method based on digital image correlation according to claim 1, characterized in that: In the step (1), in the preparation of the high-temperature resistant multi-scale speckle pattern, black and white epoxy silicone resin paints are respectively mixed with nitro diluent in a ratio of 5:1 to 3:1 to form a low-viscosity liquid, and the mixture is randomly sprayed at an air pressure of 0.4 to 0.6 MPa; high-purity cobalt oxide powder is mixed into the white liquid with a solid-liquid volume ratio of 1:3 to form an off-white suspension, and the suspension is placed in an ultrasonic oscillator and oscillated for at least 5 minutes to remove bubbles in the suspension and to disperse the particles evenly; after oscillation, the suspension is randomly and evenly sprayed at an air pressure of about 0.5 MPa using a spray brush, and after drying, a macroscopic speckle pattern of off-white spots on the surface of the black primer and a microscopic speckle pattern of random black particles on the off-white primer are formed.

3. The two-dimensional multi-scale strain measurement method based on digital image correlation according to claim 1, characterized in that: In the step (5), the step of determining the true strain based on the false strain is as follows: when the true strain is tensile strain, the false strain is corrected and subtracted; and when the true strain is compressive strain, the opposite is true, and the false strain is added during correction.

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