Method and device for full-field deformation measurement of planar elongated region based on 2D-DIC under small field of view

Abstract

The application discloses a 2D-DIC-based full-field deformation measurement method and device for a planar elongated area under a small field of view, and belongs to the technical field of deformation measurement. The method comprises the following steps: collecting panoramic images of a planar elongated area by using a virtual camera array in a field-of-view limited environment; achieving high-precision strain field measurement by means of double-mirror out-of-plane motion compensation, system static error compensation, and adaptive image splicing and fusion algorithms; finding accurate splicing positions by using a sub-set correlation and frequency domain shift theorem-based sub-pixel compensation method, achieving high-precision image splicing, and obtaining high-resolution images; and replacing a linear weight curve of a traditional fade-in and fade-out method fusion function with a sine trigonometric function, so that second-type discontinuous points generated at a boundary position of an overlapping area are eliminated. The application enables 2D-DIC to be effectively applied to high-resolution and high-precision strain field measurement of a planar elongated area under a small field of view.

Description

Technical Field

[0001] This application relates to the field of deformation measurement technology, and in particular to non-contact deformation measurement under small field of view. Background Technology

[0002] Accurately measuring the deformation of solid materials has always been a challenging task. In engineering, displacement and strain are commonly used to describe material deformation. Traditional contact strain measurement methods, such as strain gauges, extensometers, and sensors, obtain precise strain values ​​by contacting the test sample during deformation. However, this seemingly reliable strain measurement method has significant drawbacks: complex operation, susceptibility to environmental factors, and inability to measure range-bound displacement changes. These problems prevent traditional deformation measurement methods from meeting the measurement requirements of engineering in increasingly complex testing environments. Over the past 40 years, with the development of machine vision and related disciplines, non-contact strain measurement methods, which are more suitable for practical applications and offer superior performance, have gradually come into focus. Digital Image Correlation (DIC) is a classic non-contact deformation measurement method. Its advantages, including ease of operation, adaptability, rapid calculation, accurate measurement, and strong anti-interference capabilities, have made it an important member of solid material deformation measurement methods. Today, DIC is widely used as a highly effective optical measurement tool in fields such as material performance testing and biomechanical property research.

[0003] Digital speckle correlation (DIC), also known as digital speckle correlation, uses a CCD camera to acquire images of the test sample surface before and after deformation. A computer analyzes and compares the changes in speckle texture within a subset of the images to calculate the changes in image coordinates, ultimately obtaining sub-pixel-level full-field displacement and strain information. The speckle image of the sample before deformation is called the reference image, and the speckle image of the sample after deformation is called the deformation image. The speckle features identified by the correlation function can be naturally occurring random textures on the object's surface, or artificially sprayed paint or non-ferrous metals. Regardless, a fundamental premise of DIC is that the speckle characteristics deform along with the sample. Traditional two-dimensional digital image correlation (2D-DIC) involves applying a load to a planar sample with speckle characteristics, using a CCD camera to vertically acquire two-dimensional measurement areas of the sample, and then using DIC analysis to obtain the displacement and strain fields within the plane.

[0004] However, any out-of-plane motion will affect the measurement results of 2D-DIC to some extent: according to the ideal pinhole model of lens imaging, any out-of-plane motion of the sample during image acquisition will cause a small image displacement, resulting in a considerable strain error. While stereoscopic digital image correlation (Stereo-DIC) using a multi-camera system can measure the out-of-plane displacement of the sample through depth displacement information, its high cost, system complexity, and cumbersome operation make it unsuitable for measurement needs in field-limited environments. Furthermore, traditional small-field-of-view deformation measurement methods based on image stitching algorithms still suffer from insufficient stitching accuracy due to the large number and high similarity of speckles, ultimately leading to a large error between the DIC calculation results and the actual deformation, making the deformation measurement unreliable.

[0005] In summary, two key issues still limit the further development of full-field deformation measurement methods under small field-of-view conditions: first, how to calculate full-field deformation under limited measurement space; and second, how to eliminate the problem of strain field discontinuity in DIC measurement results caused by stitching seams resulting from image stitching. In addition to these two points, 2D-DIC itself has limitations in its application scenarios; the out-of-plane motion of the specimen caused by the eccentric load of the testing machine fixture can significantly affect the strain field measurement results.

[0006] Therefore, achieving high-precision stitching of speckle images, minimizing DIC measurement errors caused by insufficient algorithm accuracy, mechanical vibration of equipment, etc., and realizing high-precision full-field deformation measurement under small field of view are technical problems that urgently need to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of low accuracy, high cost and complex system of existing deformation measurement, and to provide a method and device for full-field deformation measurement of planar slender regions based on 2D-DIC under small field of view.

[0008] This invention is achieved through the following technical solution: In one aspect, this invention provides a method for measuring the full-field deformation of a slender planar region based on 2D-DIC under a small field of view, the method comprising:

[0009] Step 1: Select a sample with a flat test surface, create a speckle texture on the sample surface that meets the randomness requirements, and then place it on the testing machine. At the same time, install a dual-mirror imaging system and a virtual camera array acquisition system to form a complete experimental device.

[0010] Step 2: Stitch and fuse the speckle images of different regions on the front and back surfaces of the sample acquired by the virtual camera array to obtain a panoramic image;

[0011] Step 3: Perform 2D-DIC calculations on the panoramic images of each surface of the sample to obtain the high-resolution displacement field;

[0012] Step 4: Measure the static displacement error of the sample through a preliminary experiment;

[0013] Step 5: Perform double-mirror out-of-plane motion error compensation and static error compensation on the displacement field obtained in Step 3 to obtain the displacement field and strain field.

[0014] Furthermore, the splicing described in step 2 specifically includes:

[0015] Set the coordinate offset of any position in the overlapping area between two images to be stitched to be the same in the image coordinate system;

[0016] Select a preset number of subset regions at the bottom of the image f to be stitched. The image f to be stitched is the image to be stitched located in the upper half of the overlapping region. Calculate the displacement of the center point of the subset relative to the image g to be stitched in the v direction. The image g to be stitched is the image to be stitched located in the lower half of the overlapping region.

[0017] Assuming the overlapping portion is a complete rectangular region in the digital image, the Y-direction dimension of the overlapping region is expressed as:

[0018]

[0019] in, The value is the average axial displacement of the center points of all subsets, and Height is the height of image f or g; the Y-coordinate of the boundary of the overlapping region in image g is:

[0020]

[0021] Meanwhile, the dimension H along the Y-axis of the image portion located outside the overlapping region in image g is expressed as:

[0022]

[0023] Where pixel represents the integer pixel result after rounding, and s represents the decimal result;

[0024] High-precision image stitching of sub-pixel overlapping regions is achieved using the frequency domain shift theorem, specifically as follows:

[0025] In the Fourier domain, shifting the image g down by s pixels yields g', as shown below:

[0026] g' = DFT -1 [e -j2π(sv) G(u,v)]

[0027] G(u,v)=DFT[g(x,y)]

[0028] The Y-axis dimension of the non-overlapping portion in image g' is an integer pixel;

[0029] The whole pixel region is cropped and stitched to the bottom of image f to obtain the final stitched image F.

[0030] Furthermore, the fusion in step 2 includes:

[0031] Improved fusion function:

[0032]

[0033] Where point P is any point in the overlapping region of the image, and I returns the pixel value of point P in the fused image. f I g Returns the pixel values ​​of point P in images f and g respectively; H represents the size of the overlapping region along the Y direction; h g This represents the distance of point P in graph g from the upper boundary of the image.

[0034] Furthermore, the preliminary experiment in step 4 specifically includes: calculating the full-field displacements u and v between the same deformation state of the specimen while keeping the test specimen and static load unchanged. These two sets of displacement fields are specifically the static errors generated by the hardware equipment, experimental operation and algorithm on the DIC measurement results.

[0035] Furthermore, the motion error compensation in step 5 specifically includes:

[0036] Based on the principle of displacement change during out-of-plane motion in double-mirror imaging, we can obtain:

[0037]

[0038]

[0039] Where A and A1 represent any pair of mutually symmetrical image points on the front and back surfaces of the sample as presented by the dual mirrors; v represents the displacement. This represents the actual displacement at point A caused by sample deformation; This indicates the displacement caused by the camera's optical axis not being perpendicular to the sample.

[0040] The average displacement between points A and A1 can be used to self-compensate for the out-of-plane error of the front and rear surfaces of the sample, as shown in the following formula:

[0041]

[0042] Furthermore, the static error compensation in step 5 specifically includes: subtracting the corresponding static error displacement field obtained in the preliminary experiment from the displacement field u and v measured in the formal experiment.

[0043] Secondly, the present invention provides an apparatus for measuring the full-field deformation of a planar slender region based on 2D-DIC in a small field of view as described above, the apparatus comprising:

[0044] A metal stand for securing the tray handle; a vertical swivel joint for adjusting the tray vertically along the Y-axis; two plane mirrors fixed to mutually perpendicular optical breadboards, placed on an R-axis turntable; tray adjustment knobs for horizontal adjustment along the X-axis; an R-axis turntable, located on the tray, for rotating the double-reflecting mirror assembly on the upper surface in the ZOX plane; a tensile system including a tensile machine, clamps, and specimen; a CCD camera fixed to a camera clamp; a screw-type fine-tuning knob for adjusting the camera position vertically along the Y-axis to form a virtual camera array; and a camera clamp adjustment knob for horizontal adjustment of the camera position along the X-axis.

[0045] Place the sample's measurement plane at a 45° angle to the two mirrors, ensuring the camera's optical axis passes through the sample's thickness plane and is parallel to the measurement plane. Simultaneously, align the sample's side edge with the mating line between the two mirrors to ensure the camera's optical axis is perpendicular to the sample's front and back surfaces as presented in the mirrors, thus obtaining a symmetrical image.

[0046] Thirdly, the present invention provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and when the processor runs the computer program stored in the memory, it executes the steps of the method for measuring the full-field deformation of a planar slender region based on 2D-DIC under a small field of view as described above.

[0047] Fourthly, the present invention provides a computer-readable storage medium storing a plurality of computer instructions, the plurality of computer instructions being used to cause a computer to execute a method for measuring the full-field deformation of a planar slender region based on 2D-DIC in a small field of view as described above.

[0048] Fifthly, the present invention provides an electronic device, comprising:

[0049] At least one processor; and,

[0050] A memory communicatively connected to the at least one processor; wherein,

[0051] The memory stores instructions that can be executed by the at least one processor, which enable the at least one processor to perform a method for measuring full-field deformation of a planar slender region based on 2D-DIC in a small field of view, as described above.

[0052] The beneficial effects of this invention are:

[0053] This invention provides a deformation measurement system and method based on 2D-DIC, enabling 2D-DIC to be effectively applied to high-resolution, high-precision strain field measurement in slender planar regions with small field of view.

[0054] (1) Only one CCD camera is needed to complete the panoramic image acquisition under a small field of view, which reduces the systematic error between multiple devices, saves costs and simplifies experimental operation.

[0055] (2) Static error compensation is performed on the calculated displacement field based on the results of the preliminary experiment, and the self-compensation technology of the double-reflector out-of-plane displacement is extended to the field measurement. Error self-compensation is performed on the displacement field of the front and rear surfaces of the sample, which reduces the strain error caused by out-of-plane motion and further improves the accuracy of strain field measurement.

[0056] (3) The sub-pixel compensation method based on subset correlation and frequency domain shift theorem is used to find the accurate stitching position, realize high-precision image stitching and obtain high-resolution image, and avoid the problem of insufficient stitching accuracy caused by the high feature point matching error rate of traditional methods.

[0057] (4) The linear weight curve of the traditional fade-in / fade-out fusion function is replaced by a sine trigonometric function, which eliminates the second type of discontinuity points generated at the boundary of the overlapping area, making the gray-scale change of the fusion area in the stitched image smoother and further reducing the impact of stitching error on the DIC algorithm.

[0058] This invention is applicable to full-field deformation measurement of slender planar regions based on 2D-DIC in small field of view. Attached Figure Description

[0059] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0060] Figure 1 This is a schematic diagram of the device structure of the present invention, wherein: a is a schematic diagram of the experimental device, and b is a schematic diagram of the dual reflector composition;

[0061] Figure 2 For the placement and imaging of dual reflecting mirrors;

[0062] Figure 3 Flowchart for finding the splicing position;

[0063] Figure 4 This is a schematic diagram illustrating the principle of image stitching.

[0064] Figure 5 This is a schematic diagram of the X-direction offset of the stitched images;

[0065] Figure 6 This is the weight change curve;

[0066] Figure 7 This outlines the overall implementation steps of the method of the present invention.

[0067] Figure 8 The sample size;

[0068] Figure 9 The image to be stitched is a grayscale image;

[0069] Figure 10 This represents the static error displacement field.

[0070] Figure 11 This represents the axial displacement field (unit: pixels).

[0071] Figure 12 This represents the axial strain field (unit: με). Detailed Implementation

[0072] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0073] Implementation Method 1: A method for measuring the full-field deformation of a slender planar region based on 2D-DIC in a small field of view, the method comprising:

[0074] Step 1: Select a sample with a flat test surface, create a speckle texture on the sample surface that meets the randomness requirements, and then place it on the testing machine. At the same time, install a dual-mirror imaging system and a virtual camera array acquisition system to form a complete experimental device.

[0075] Step 2: Stitch and fuse the speckle images of different regions on the front and back surfaces of the sample acquired by the virtual camera array to obtain a panoramic image;

[0076] Step 3: Perform 2D-DIC calculations on the panoramic images of each surface of the sample to obtain the high-resolution displacement field;

[0077] Step 4: Measure the static displacement error of the sample through a preliminary experiment;

[0078] Step 5: Perform double-mirror out-of-plane motion error compensation and static error compensation on the displacement field obtained in Step 3 to obtain the displacement field and strain field.

[0079] The technical solution of this embodiment can be divided into two parts: the first part is to use a virtual camera array to acquire panoramic images of a narrow planar region in an environment with a limited field of view; the second part is to achieve high-precision strain field measurement through out-of-plane motion compensation of dual mirrors, static error compensation of the system, and adapted image stitching and fusion algorithms.

[0080] Implementation Method Two is a further limitation on the method for measuring the full-field deformation of a slender planar region based on 2D-DIC in a small field of view described in Implementation Method One. In this implementation method, the stitching in step 2 is further defined, specifically including:

[0081] The splicing in step 2 specifically includes:

[0082] Set the coordinate offset of any position in the overlapping area between two images to be stitched to be the same in the image coordinate system;

[0083] Select a preset number of subset regions at the bottom of the image f to be stitched. The image f to be stitched is the image to be stitched located in the upper half of the overlapping region. Calculate the displacement of the center point of the subset relative to the image g to be stitched in the v direction. The image g to be stitched is the image to be stitched located in the lower half of the overlapping region.

[0084] Assuming the overlapping portion is a complete rectangular region in the digital image, the Y-direction dimension of the overlapping region is expressed as:

[0085]

[0086] in, Let be the average axial displacement of the center points of all subsets, and Height be the height of image f or g. The Y-coordinate of the boundary of the overlapping region in image g is:

[0087]

[0088] Meanwhile, the dimension H along the Y-axis of the image portion located outside the overlapping region in image g is expressed as:

[0089]

[0090] Where pixel represents the integer pixel result after rounding, and s represents the decimal result.

[0091] High-precision image stitching of sub-pixel overlapping regions is achieved using the frequency domain shift theorem, specifically as follows:

[0092] In the Fourier domain, shifting the image g down by s pixels yields g', as shown below:

[0093] g' = DFT -1 [e -j2π(sv)G(u,v)]

[0094] G(u,v)=DFT[g(x,y)]

[0095] The Y-axis dimension of the non-overlapping portion in image g' is an integer pixel;

[0096] The whole pixel region is cropped and stitched to the bottom of image f to obtain the final stitched image F.

[0097] It should be noted that, ideally, the coordinate offsets of any position in the overlapping region between two images to be stitched should be the same in the image coordinate system. This implementation uses a nonlinear optimization algorithm in DIC to find the precise stitching position.

[0098] In this embodiment, the image stitching algorithm is a high-precision image stitching algorithm based on subset correlation and frequency domain shift theorem. Compared with traditional methods based on scale-invariant features and homography matrix, this method avoids the problem of insufficient stitching accuracy caused by the large number of speckles on the sample surface, high similarity of shape and structure, and high error rate of feature point matching.

[0099] Implementation method three is a further limitation on the method for measuring the full-field deformation of a planar slender region based on 2D-DIC under a small field of view described in implementation method one. In this implementation method, the fusion in step 2 is further defined, specifically including:

[0100] The fusion in step 2 includes:

[0101] Improved fusion function:

[0102]

[0103] Where point P is any point in the overlapping region of the image, and I returns the pixel value of point P in the fused image. f I g Returns the pixel values ​​of point P in images f and g respectively; H represents the size of the overlapping region along the Y direction; h g This represents the distance of point P in graph g from the upper boundary of the image.

[0104] In this embodiment, the image fusion algorithm is a speckle fusion algorithm based on sine trigonometric functions. Compared with traditional fade-in / fade-out methods, this method can further mitigate the impact of image grayscale gradient changes caused by insufficient stitching accuracy and uneven brightness in the stitched image on the DIC algorithm. Traditional mixing function weight curves exhibit second-type discontinuities at the boundaries of the overlapping region. These discontinuities cause abrupt changes in brightness along the Y-direction, resulting in a significant grayscale gradient in the fused panoramic image and affecting the DIC measurement results. This embodiment uses a sine curve to replace the straight lines within the overlapping region to eliminate this discontinuity. The modified mixing function weight curve makes the curve changes at the boundaries of the overlapping region smoother.

[0105] Implementation Method Four: This implementation method further defines the method for measuring the full-field deformation of a slender planar region based on 2D-DIC under a small field of view described in Implementation Method One. In this implementation method, the preliminary experiment in step 4 is further defined, specifically including:

[0106] The preliminary experiment in step 4 specifically includes: calculating the full-field displacements u and v of the specimen under the premise of keeping the test specimen and static load unchanged. These two sets of displacement fields are specifically the static errors generated by hardware equipment, experimental operation and algorithm on DIC measurement results.

[0107] In this embodiment, the pre-experiment in step 4 refers to calculating the full-field displacements u and v of the specimen between the same deformation states using experimental steps 2 and 3 of this application, while keeping the test specimen and static load unchanged. These two sets of displacement fields represent the static errors generated by the hardware equipment, experimental operation, and algorithm on the DIC measurement results. Setting up the pre-experiment in this way allows the static displacement error to be adapted to the quasi-static experiment that follows, making the static error compensation more reliable.

[0108] Implementation method five is a further limitation on the method for measuring the full-field deformation of a slender planar region based on 2D-DIC under a small field of view described in implementation method one. In this implementation method, the motion error compensation in step 5 is further limited, specifically including:

[0109] The motion error compensation in step 5 specifically includes:

[0110] Based on the principle of displacement change during out-of-plane motion in double-mirror imaging, we can obtain:

[0111]

[0112]

[0113] Where A and A1 represent any pair of mutually symmetrical image points on the front and back surfaces of the sample as presented by the dual mirrors; v represents the displacement. This represents the actual displacement at point A caused by sample deformation; This indicates the displacement caused by the camera's optical axis not being perpendicular to the sample.

[0114] The average displacement between points A and A1 can be used to self-compensate for the out-of-plane error of the front and rear surfaces of the sample, as shown in the following formula:

[0115]

[0116] In this embodiment, the dual-mirror out-of-plane motion error compensation extends the dual-mirror imaging technology to field measurement. It performs out-of-plane error self-compensation on the displacement field of the front and rear surfaces of the sample, thereby reducing strain errors caused by out-of-plane motion. The average displacement of points A and A1 is used to self-compensate the out-of-plane error of the front and rear surfaces of the sample, reducing displacement errors caused by out-of-plane motion. This embodiment extends this method to field measurement to obtain a high-precision displacement field.

[0117] Implementation method six is ​​a further limitation on the method for measuring the full-field deformation of a slender planar region based on 2D-DIC under a small field of view described in implementation method one. In this implementation method, the static error compensation in step 5 is further limited, specifically including:

[0118] The static error compensation in step 5 specifically includes: subtracting the corresponding static error displacement field obtained in the preliminary experiment from the displacement field u and v measured in the formal experiment.

[0119] In this embodiment, static error compensation is achieved by subtracting the corresponding static error displacement field obtained in the preliminary experiment from the displacement field u and v measured in the formal experiment, thereby compensating for the static errors caused by experimental equipment, operation, and related algorithms during the experiment.

[0120] Implementation method seven, this implementation method is Example 1 of a method for measuring the full-field deformation of a planar slender region based on 2D-DIC in a small field of view as described above, specifically including:

[0121] This invention provides a deformation measurement system and method based on 2D-DIC, enabling 2D-DIC to be effectively applied to high-resolution, high-precision strain field measurement in slender planar regions with small field of view.

[0122] The technical solution of this invention can be divided into two parts: the first part involves using a virtual camera array to acquire panoramic images of a narrow planar region in an environment with a limited field of view; the second part involves achieving high-precision strain field measurement through out-of-plane motion compensation with dual mirrors, system static error compensation, and an adapted image stitching and fusion algorithm. The implementation of the above-mentioned high-precision full-field deformation measurement method based on 2D-DIC in a small field of view includes the following steps:

[0123] Step 1: Select a suitable sample with a flat test surface, create a speckle texture on the surface that meets the randomness requirements, and place it on the testing machine as required. At the same time, install a dual-mirror imaging system and a virtual camera array acquisition system as required to form a complete experimental device.

[0124] Step 2: Stitch and fuse the speckle images of different regions on the front and back surfaces of the sample acquired by the virtual camera array to obtain a panoramic image;

[0125] Step 3: Perform 2D-DIC calculations on the panoramic images of each surface of the sample to obtain the high-resolution displacement field;

[0126] Step 4: Measure the static displacement error of the sample through a preliminary experiment, and then perform subtraction on the displacement field obtained in subsequent experiments to complete the static error compensation;

[0127] Step 5: Perform dual-mirror out-of-plane motion error compensation and static error compensation on the displacement field results from Step 3 to obtain high-resolution, high-precision displacement and strain fields.

[0128] Furthermore, the image stitching algorithm in step 2 above is a high-precision image stitching algorithm based on subset correlation and frequency domain shift theorem. Compared with traditional methods based on scale-invariant features and homography matrices, this method avoids the problem of insufficient stitching accuracy caused by factors such as a large number of speckles on the sample surface, high similarity in shape and structure, and a high error rate in feature point matching.

[0129] The details are as follows:

[0130] Ideally, the coordinate offsets of any position in the overlapping region between two images to be stitched should be the same in the image coordinate system. The precise stitching position is found using a nonlinear optimization algorithm in DIC: a certain number of subset regions are selected at the bottom of the image f (located in the upper half of the overlapping region), and the displacement of the subset's center point relative to the image g (located in the lower half of the overlapping region) in the v direction is calculated.

[0131] Assuming the overlapping portion is a complete rectangular region in the digital image, then the Y-direction dimension of the overlapping region can be expressed as:

[0132]

[0133] in, Let be the average axial displacement of the center points of all subsets, and Height be the height of image f or g. Then, the Y-coordinate of the boundary of the overlapping region in image g is:

[0134]

[0135] Meanwhile, the dimension H along the Y-axis of the image portion located outside the overlapping region in image g can be expressed as:

[0136]

[0137] Where pixel represents the integer pixel result after rounding, and s represents the decimal result.

[0138] Finally, the shift theorem in the frequency domain is used to achieve high-precision image stitching of sub-pixel overlapping regions: In the Fourier domain, the image g is shifted down by s pixels to obtain g', as shown below:

[0139] g' = DFT -1 [e -j2π(sv) G(u,v)]

[0140] G(u,v)=DFT[g(x,y)]

[0141] At this point, the Y-direction dimension of the non-overlapping part in image g' is an integer pixel; this integer pixel region is cropped and stitched to the bottom of image f to obtain the final stitched image F, thus completing the high-precision image stitching.

[0142] Next, the image fusion algorithm in step 2 above is a speckle fusion algorithm based on the sine trigonometric function. Compared with the traditional fade-in / fade-out method, this method can further mitigate the impact of image grayscale gradient changes caused by insufficient stitching accuracy and uneven brightness of the stitched image on the DIC algorithm. The specific details are as follows:

[0143] Generally, to eliminate stitching seams, a blending function needs to be defined in the overlapping region to superimpose the pixel values ​​of the overlapping region according to certain weights and form a new image. Traditional fade-in / fade-out algorithms use a blending function with linearly increasing weights, as shown in the following equation:

[0144]

[0145] Where point P is any point in the overlapping region of the image, and I returns the pixel value of point P in the fused image. f I g Returns the pixel values ​​of point P in images f and g respectively; H represents the size of the overlapping region along the Y direction; h f h gLet P represent the distance from point P to the lower boundary of the image in graph f, and the distance from point P to the upper boundary of the image in graph g, respectively.

[0146] Clearly, the traditional fusion function weight curve exhibits a second type of discontinuity at the boundary of the overlapping region. These discontinuities cause abrupt changes in brightness along the Y-direction, resulting in a significant gray-level gradient in the fused panoramic image and impacting DIC measurement results. This paper uses a sine curve to replace the straight lines within the overlapping region to eliminate this discontinuity. The modified fusion function weight curve smooths the curve changes at the boundary of the overlapping region. The improved fusion function can be expressed as follows:

[0147]

[0148] Point P is any point in the image located within the overlapping region, and I returns the pixel value of point P in the merged image. f I g Returns the pixel values ​​of point P in images f and g respectively; H represents the size of the overlapping region along the Y direction; h g This represents the distance of point P in graph g from the upper boundary of the image.

[0149] Specifically, the preliminary experiment in step 4 refers to calculating the full-field displacements u and v of the specimen between the same deformation states by using the experimental steps and algorithms in this invention, while keeping the test specimen and static load unchanged. These two sets of displacement fields represent the static errors generated by the hardware equipment, experimental operation and algorithm on the DIC measurement results.

[0150] In step 5, the out-of-plane motion error compensation of the dual-mirror system extends the dual-mirror imaging technology to field measurement, performing out-of-plane error self-compensation on the displacement field of the front and rear surfaces of the sample, thereby reducing strain errors caused by out-of-plane motion. Based on the principle of out-of-plane motion displacement change in dual-mirror imaging, the following formula can be obtained:

[0151]

[0152]

[0153] Where A and A1 represent any pair of mutually symmetrical image points on the front and back surfaces of the sample as presented by the dual mirrors; v represents the displacement. This represents the actual displacement at point A caused by sample deformation; This represents the displacement caused by the camera's optical axis not being perpendicular to the sample. The average displacement between points A and A1 can be used to self-compensate for the out-of-plane error of the sample's front and rear surfaces, reducing displacement errors caused by out-of-plane motion, as shown in the following formula. This invention extends this method to obtain high-precision displacement fields in field measurements.

[0154]

[0155] The static error compensation performed in step 5 is to subtract the corresponding static error displacement field obtained in the preliminary experiment from the displacement field u and v measured in the formal experiment, so as to compensate for the static error caused by experimental equipment, operation and related algorithms during the experiment.

[0156] Implementation method eight is embodiment 2 of the method for measuring the full-field deformation of a planar slender region based on 2D-DIC in a small field of view as described above, specifically including:

[0157] Building upon existing DIC methods, this embodiment presents a high-precision full-field deformation measurement method for slender planar regions with small field of view, combining a virtual camera array and dual-mirror imaging. The method includes a new experimental procedure based on traditional DIC experiments, as well as the algorithm code written in MATLAB for the software portion.

[0158] The implementation method of the experiment is as follows: Steps 1 and 2:

[0159] Step 1: Select dumbbell-shaped 6061 aluminum alloy tensile parts as test specimens. Use speckle stickers to create speckle textures on the surface of the specimens that meet the randomness requirements, and then place them on the testing machine as required. At the same time, install the dual-mirror imaging system and the virtual camera array acquisition system as required to form a complete experimental device.

[0160] Step 2: Use a virtual camera array to acquire speckle images of different areas on the front and back surfaces of the sample, which will be used as program input for the preliminary experimental measurement of static displacement error and the formal experimental measurement of deformation results.

[0161] Step 3: Stitch and fuse the speckle images of different areas on the front and back surfaces of the sample acquired by the virtual camera array to obtain a panoramic image;

[0162] Step 4: Perform 2D-DIC calculations on the panoramic images of each surface of the sample to obtain the high-resolution deformation field;

[0163] Step 5: Perform dual-mirror out-of-plane motion error compensation and static error compensation on the results of Step 3 to obtain high-resolution, high-precision displacement and strain fields.

[0164] Specifically, the image acquisition system in step 1 consists of a CCD camera 7, camera clamps (including 8 and 9), and a camera clamp mounting platform 10. The camera is held in place by a camera clamp with a fine-tuning knob 8, and a ring light source is mounted at the front end. Adjusting the fine-tuning knob moves the front end of the camera clamp to the upper and lower limit positions (approximately 1.5 cm apart), forming a virtual camera array.

[0165] The dual-mirror system consists of two plane mirrors 3 fixed on mutually perpendicular optical breadboards (see details). Figure 1 b) It consists of an R-axis platform 5 (specifically an R-axis rotating platform) and an iron frame 1, using a camera clamp with built-in level and adjustable up and down as a tray; while ensuring that the two plane mirrors are perpendicular to each other, it can achieve up, down, left, right and rotational fine adjustment so that the sample can be imaged symmetrically.

[0166] The tensioning system consists of a universal tensioning machine and clamps.

[0167] The above equipment consists of the following components: Figure 1 As shown in diagram a: Item 1 is an iron stand used to fix the tray handle; Item 2 is a vertical adapter that allows the tray to be adjusted up and down along the Y-axis; Item 3 consists of two plane mirrors fixed to mutually perpendicular optical breadboards and placed on an R-axis turntable; Item 4 is an adjustment knob for the tray, used to adjust the tray horizontally along the X-axis; Item 5 is an R-axis turntable located on the tray, which allows the double-reflecting mirror assembly on the upper surface to be rotated in the ZOX plane; Item 6 is a tensile system, including a tensile machine, fixtures, and sample 11; Item 7 is a CCD camera fixed to a camera fixture; Item 8 is a screw fine-tuning knob that allows the camera position to be adjusted up and down along the Y-axis to form a virtual camera array; Item 9 is an adjustment knob for the camera fixture that allows the camera position to be adjusted horizontally along the X-axis.

[0168] The installation requirements are as follows: Position the sample's measuring plane at a 45° angle to the two reflecting mirrors, ensuring the camera's optical axis passes through the sample's thickness plane and is parallel to the measuring plane. Simultaneously, align the sample's side edges with the contact line between the two reflecting mirrors, ensuring the camera's optical axis is perpendicular to the front and rear surfaces of the sample as presented in the two reflecting mirrors to obtain a symmetrical image. Figure 2 As shown

[0169] The image acquisition process in step 2 is as follows: (1) Install the double reflector device, the tensile system and the image acquisition system, and reasonably adjust the camera position, focal length, aperture and ring light source to ensure that the front and back surfaces of the sample presented by the double reflector are clearly imaged; adjust the position of the double reflector device so that the side edge of the sample coincides with the bonding line between the double reflector, and ensure that the optical axis of the camera is perpendicular to the front and back surfaces of the sample presented in the double reflector to obtain a symmetrical image; (2) The computer controls the tensile machine to apply pre-tightening force to relieve the eccentricity between the upper and lower clamps and keep the load constant; adjust the screw fine adjustment knob to adjust the camera to the highest and lowest positions to form a virtual camera array, and the camera acquires images at each position to obtain two images with overlapping areas to be stitched as reference images; (3) In the preliminary experiment, keep the load constant while in the formal experiment, change the load size, repeat the above steps to acquire two images with overlapping areas to be stitched as deformed images.

[0170] The specific implementation method of image stitching and fusion in step 3 is as follows:

[0171] The precise stitching position is found using the second-order inverse combination Gauss-Newton method in the DIC algorithm: A certain number of subset regions are selected at the bottom of the images to be stitched (f, located in the upper half of the overlapping region). The displacement v of the center point of each subset relative to the image to be stitched (g, located in the lower half of the overlapping region) is calculated. The average displacement is taken as the precise positional information for the subsequent frequency domain shift operation of the stitched images. The flowchart of the above process is as follows: Figure 3 Illustration. The high-precision image stitching process based on subset correlation and the frequency domain shift theorem in this embodiment is as follows: Figure 4 The specific implementation method is as follows:

[0172] If the overlapping portion is a complete rectangular region in the digital image, then the Y-direction dimension of the overlapping region can be expressed as:

[0173]

[0174] in, Let be the average axial displacement of the center points of all subsets, and Height be the height of image f or g. Then, the Y-coordinate of the boundary of the overlapping region in image g is:

[0175]

[0176] Among them, Y int For Y overlap The integer part of Y non-int The non-integer part. Meanwhile, the dimension H along the Y-axis of the image portion located outside the overlapping region in image g can be expressed as:

[0177]

[0178] Where pixel represents the integer pixel result after rounding, and s represents the decimal result.

[0179] Finally, the shift theorem in the frequency domain is used to achieve high-precision image stitching of sub-pixel overlapping regions: In the Fourier domain, the image g is shifted down by s pixels to obtain g', as shown below:

[0180] g' = DFT -1 [e -j2π(sv) G(u,v)]

[0181] G(u,v)=DFT[g(x,y)]

[0182] Where G(u,v) is the Fourier transform of image g(x,y), j is the imaginary unit, and u and v are the independent variables in the Fourier domain. At this time, the Y-direction dimension of the non-overlapping part in image g' is an integer pixel; the integer pixel region is cropped and stitched to the bottom of image f to obtain the final stitched image F, thus completing the high-precision image stitching.

[0183] Objective factors such as eccentric loading of the fixture can cause out-of-plane rotation of the sample, which appears as a sideways and upward movement of the sample in a dual-mirror system. The resulting small tilt angle θ will cause a deviation Δu in the X-direction of the stitched image at the stitching position, such as... Figure 5 As shown. To address this problem, this embodiment uses the displacements of several previously calculated subset windows to perform sub-pixel compensation on image g using the same method: the average value of the u displacement of the subset windows is calculated to replace Δu, and then applied to image g using the frequency domain shift theorem to complete sub-pixel compensation, eliminating the offset in the X direction at the stitching position after image stitching. The g' obtained from the above process can be expressed as follows:

[0184] g' = DFT -1 [G(u,v)e -j2π(Δu×u) ]

[0185] Where Δu is the deviation in the X direction (e.g., Figure 5 As shown), G(u,v) is the Fourier transform of image g, j is the imaginary unit, and u and v are the independent variables in the Fourier domain. After applying the above two shift transforms to image g, the final g' can be expressed as follows, with the symbols having the same meaning as in the above equation:

[0186] g' = DFT -1 [G(u,v)e -j2π(Δu×u+sv) ]

[0187] Finally, a sine trigonometric function is used in the overlapping area to achieve speckle fusion, making the gray-level changes in the fused area of ​​the stitched image smoother and further reducing the impact of stitching errors on the DIC algorithm. The modified fusion function weight curve is shown below. Figure 6 As shown. The improved fusion function of this embodiment can be expressed as follows:

[0188]

[0189] Point P is any point in the image located within the overlapping region, and I returns the pixel value of point P in the merged image. f I g Returns the pixel values ​​of point P in images f and g respectively; H represents the size of the overlapping region along the Y direction; h g This represents the distance of point P in graph g from the upper boundary of the image.

[0190] The static error compensation and dual-mirror out-of-plane motion error compensation in step 5 are performed based on the u and v displacement field results obtained in step 4. For example... Figure 7 As shown, static error compensation is achieved by subtracting the static error displacement field from the displacement fields of the two surfaces of the specimen in each image to be stitched under a single load state. Finally, the u displacement field and v displacement field of the symmetrical region of the front and rear surfaces of the specimen are superimposed and averaged to obtain a high-precision displacement field. Finally, a high-precision strain field is obtained by 2D-DIC calculation.

[0191] An application example of this invention is a uniaxial tensile test on a specimen with a planar slender measurement region, with the specific configuration as follows:

[0192] The specific dimensions of the dumbbell-shaped 6061 aluminum alloy stretching part used in the example are as follows: Figure 8 As shown. The image size captured by the camera is 2748×3840 pixels. The grayscale images of the two speckle patterns to be stitched together, obtained by the virtual camera array, are shown below. Figure 9 As shown. According to Figure 7 The operation procedure resulted in a single-surface image of the cut sample with a size of 2748×737 pixels. The stitched image of the entire field had a size of 3680×737 pixels, improving resolution by approximately 34%. The rectangular ROI region for DIC analysis was located on each measurement surface of the sample and was axially symmetric, with a size of 3000×251 pixels. Tensile testing was conducted under a pre-tightening force of 1200N to measure the static error displacement field. The calculation results of the static error displacement field in the experimental example are shown below. Figure 10 As shown. The tensile force was increased to 2400 N, and the final axial displacement and strain fields relative to the 1200 N tensile state were calculated using the measurement method of this invention. The results are as follows: Figure 11 , 12 As shown.

[0193] The strain field exhibits micro-strain fluctuations of approximately ±600 με, generally showing a uniform distribution, with an average axial strain of 748.0127 με. Simultaneously, conventional 2D-DIC measurements on a single measurement surface of the specimen using the same algorithm parameters yielded an average axial strain of 823.4407 με. A total of six strain gauges were attached to the front and back surfaces of the specimen to measure axial strain values, with results of 748.2 με, 741.2 με, 755.0 με, 747.8 με, and 744.8 με, respectively. με and 760.3με; the relative errors of axial strain obtained based on the strain gauge results are 0.0250%, 0.9191%, 0.9255%, 0.0284%, 0.4314%, and 1.6161%, respectively. Compared with the traditional 2D-DIC measurement method, the relative errors of axial strain are all reduced by 10.0312%, 10.1765%, 8.1395%, 10.0867%, 10.1272%, and 6.6886%, respectively.

[0194] The changes in the displacement field and the overall data show that the calculation results of the proposed 2D-DIC-based high-precision full-field deformation measurement method for slender planar regions are very close to those of strain gauge measurements. It improves measurement resolution while exhibiting smaller strain errors in a small field of view, achieving high-resolution, high-precision quasi-static displacement and strain field measurements with a single camera in a small field of view, thus verifying the feasibility and effectiveness of the proposed measurement method. Furthermore, the virtual camera array in this example only deployed two camera positions; theoretically, more camera positions could be added to further expand the measurement range, demonstrating significant development potential.

Claims

1. A method for measuring the full-field deformation of a slender planar region based on 2D-DIC in a small field of view, characterized in that, The method includes: Step 1: Select a sample with a flat test surface, create a speckle texture on the sample surface that meets the randomness requirements, and then place it on the testing machine. At the same time, install a dual-mirror imaging system and a virtual camera array acquisition system to form a complete experimental device. Step 2: Stitch and fuse the speckle images of different regions on the front and back surfaces of the sample acquired by the virtual camera array to obtain a panoramic image; Step 3: Perform 2D-DIC calculations on the panoramic images of each surface of the sample to obtain the high-resolution displacement field; Step 4: Measure the static displacement error of the sample through a preliminary experiment; Step 5: Perform double-mirror out-of-plane motion error compensation and static error compensation on the displacement field obtained in Step 3 to obtain the displacement field and strain field; The splicing in step 2 specifically includes: Set the coordinate offset of any position in the overlapping area between two images to be stitched to be the same in the image coordinate system; Images to be stitched f Select a preset number of subset regions at the bottom of the image to be stitched. f For the image to be stitched, located in the upper half of the overlapping region, calculate the subset center point relative to the image to be stitched. g exist v The displacement in the direction, the image to be stitched g is the image to be stitched located in the lower half of the overlapping region; Assuming the overlapping portion is a complete rectangular region in the digital image, the overlapping region... Y The directional dimension is expressed as: in, This represents the average axial displacement of the center points of all subsets. Height For image f or g The height; at g The boundary of overlapping regions in the image Y The coordinates are: in, for The integer part, The non-integer part; Meanwhile, located in the image g Image portions outside the overlapping region along Y Shaft dimensions H Represented as: H = Height – Y overlap = = s + pixel in, pixel This represents the integer pixel result after rounding. s Represents the decimal result; High-precision image stitching of sub-pixel overlapping regions is achieved using the frequency domain shift theorem, specifically as follows: In the Fourier domain, the image g translate downwards s Pixels obtained g’ As shown below: image g’ The overlapping parts of China and Africa Y The directional dimension is an integer pixel; The integer pixel region is cropped and stitched to the image. f At the bottom, the final stitched image is obtained. F .

2. The method for measuring full-field deformation of a slender planar region based on 2D-DIC in a small field of view according to claim 1, characterized in that, The fusion in step 2 includes: Improved fusion function: Among them, point P For any point in the image located within the overlapping region, I Return to the fused image P pixel value of a point I f , I g Return points respectively P In the image f , g Pixel values ​​in; H Indicates the overlapping area along Y Dimensions of direction; h g Point P In the figure g The distance from the center to the top boundary of the image.

3. The method for measuring full-field deformation of a slender planar region based on 2D-DIC in a small field of view according to claim 1, characterized in that, The preliminary experiment in step 4 specifically includes: calculating the full-field displacement of the specimen under the same deformation state while keeping the test specimen and static load constant. u , v These two sets of displacement fields specifically represent the static errors generated by hardware equipment, experimental operations, and algorithms on the DIC measurement results.

4. The method for measuring full-field deformation of a slender planar region based on 2D-DIC in a small field of view according to claim 1, characterized in that, The motion error compensation in step 5 specifically includes: Based on the principle of displacement change during out-of-plane motion in double-mirror imaging, we can obtain: in, A and A 1 represents any pair of mutually symmetrical image points on the front and back surfaces of the sample as presented by the two reflecting mirrors; v Indicates the amount of displacement; This represents the actual displacement at point A caused by sample deformation; This indicates the displacement caused by the camera's optical axis not being perpendicular to the sample. Pick A and A The average displacement of a single point can self-compensate for the out-of-plane error of the front and rear surfaces of the sample, as shown in the following formula: 。 5. The method for measuring full-field deformation of a slender planar region based on 2D-DIC in a small field of view according to claim 1, characterized in that, The static error compensation mentioned in step 5 specifically includes: compensating for the displacement field measured in the formal experiment. u , v The result is the result minus the corresponding static error displacement field obtained in the preliminary experiment.

6. An apparatus for measuring the full-field deformation of a planar slender region based on 2D-DIC in a small field of view, as described in any one of claims 1-5, characterized in that, The device includes: A metal stand for securing the tray handle; a vertical swivel joint for adjusting the tray vertically along the Y-axis; two plane mirrors fixed to mutually perpendicular optical breadboards, placed on an R-axis turntable; tray adjustment knobs for horizontal adjustment along the X-axis; an R-axis turntable, located on the tray, for rotating the double-reflecting mirror assembly on the upper surface in the ZOX plane; a tensile system including a tensile machine, clamps, and specimen; a CCD camera fixed to a camera clamp; a screw-type fine-tuning knob for adjusting the camera position vertically along the Y-axis to form a virtual camera array; and a camera clamp adjustment knob for horizontal adjustment of the camera position along the X-axis. Place the sample's measurement plane at a 45° angle to the two mirrors, ensuring the camera's optical axis passes through the sample's thickness plane and is parallel to the measurement plane. Simultaneously, align the sample's side edge with the mating line between the two mirrors to ensure the camera's optical axis is perpendicular to the sample's front and back surfaces as presented in the mirrors, thus obtaining a symmetrical image.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, The steps of the method according to any one of claims 1 to 5 are performed when the processor runs the computer program stored in the memory.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a plurality of computer instructions, which are used to cause a computer to perform the method of any one of claims 1 to 5.

9. An electronic device, characterized in that, include: At least one processor; as well as, A memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor to enable the at least one processor to perform the method of any one of claims 1 to 5.

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