CN114777709A - DVC (dynamic voltage waveform) microcrack characterization method based on daughter block separation - Google Patents
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Abstract
The invention discloses a DVC micro-crack characterization method based on daughter block separation, which comprises the following steps: screening a preparation point possibly containing a crack for crack identification calculation according to a displacement field result calculated by the traditional DVC; the assumed crack parameters are used for fitting a crack surface, the sub-block containing the cracks is divided into a main block and a slave block, then the reference sub-block and the target sub-block added with the crack parameters from the block part are subjected to correlation matching, the crack parameters corresponding to the optimal correlation are initial crack parameters of the target sub-block, the calculated sub-block containing the cracks is confirmed through the opposite number relationship of the initial crack parameters on the two sides of the cracks, and crack identification is realized; and then calculating the sub-voxel displacement of the main body block parts on two sides of the crack, and obtaining a high-precision crack width parameter by performing difference. According to the method, DVC is combined with daughter block separation, and three-dimensional high-precision characterization of microcracks is realized.
Description
Technical Field
The invention relates to a DVC (digital volume correlation) microcrack characterization method based on daughter block separation.
Background
The cracking of the material can affect the function of the material, and according to the severity of the cracking, the aesthetic property of the material is slightly affected, and the mechanical property of the material structure is affected and the corrosion resistance is reduced if the cracking is serious. The process of material cracking generally starts from micro cracks which can not be identified by human eyes, and then gradually develops into macro visible large cracks, so that the whole structure is influenced. Therefore, the research on the initiation of the microcracks in the material has very important significance for obtaining the subsequent extension and expansion rule, thereby providing scientific guidance for the prevention and treatment of the cracks. To study the characteristics of the initiation and the propagation of the microcracks, the existence of the microcracks needs to be identified and characterized, and the main means for observing the microcracks can be divided into two types, one type is destructive detection, for example: and (4) microscopic observation, namely, sample preparation is carried out, and then the appearance of the crack is observed under a high-resolution microscope, but the original crack can be damaged in the sample preparation process. Another type of inspection technique is non-destructive inspection: for example, in the acoustic emission crack detection technology, the information of cracks is obtained by collecting and analyzing acoustic signals generated by the cracks, but the requirement on the acoustic signals is high when data is processed, the result is seriously influenced by noise, and the capability of identifying the microcracks is limited; in addition, with the development of the computational imaging technology, the technology of identifying cracks by combining the image processing technology with different algorithms is also widely applied, but the method has higher requirements on gray level representation of the cracks in the image, usually identifies the cracks on the surface of the sample, and is not beneficial to identifying the microcracks with smaller sizes.
In addition to the above methods, Digital Image Correlation (DIC) is increasingly used for material damage detection as a non-contact method for measuring surface deformation of an object. DIC was proposed in 1983 by Sutton et al (Sutton M A, Wolters W J, Peters W H, et al. determination of displacement using an improved digital correlation method [ J ]. Image and vision computing, 1983, 1(3): 133-: and matching the gray level correlation degrees of the two groups of image data before and after deformation, then performing subtraction to obtain the deformed displacement data, and performing point-by-point calculation to obtain the displacement field data of the whole plane. However, when a deformed Image has a Discontinuous region, such as a crack region, Displacement data obtained by calculation of the DIC will fail, and the result of the Displacement field near the crack region is inaccurate, for such a situation, different solutions are provided for domestic and foreign related studies, such as a subregion Splitting method proposed by Poissant et al (J poise, F barthelat.a Novel "Subset Splitting" Procedure for Digital Image Correlation on discrete Fields [ J ], experimental Mechanics, 2010, 50(3):353-364 ], the crack is simulated by using a geometric straight line, the complete subregion is divided into two parts to be calculated respectively, so that a correct Displacement field around the crack is obtained, and a crack line is obtained at the same time, so that direction information of the crack in the subregion can be specifically characterized, but the method does not give information about crack width and cannot specifically describe the crack; tung et al (Tung S H, Shih M H, Sun W P. development of digital image correlation method to analysis crack variations of masonnary wall [ J ]. Sadhana,2008,33(6):767-779.) calculate the deformation field of DIC wall material surface, then judge the position of the crack according to the concentration of high deformation, thus realizing crack identification, but the method can only roughly position the crack, and cannot carry out quantitative analysis on the parameters of the crack; soup and the like (Tangwen, Shohan bin, Zhoushi. discontinuous image digital correlation method applied in crack reconstruction [ J ] mechanics science report, 2019, 051(004): 1101-. The digital volume correlation method (DVC) is used as the extension of DIC on the three-dimensional layer, plays a great role in researching the three-dimensional deformation of materials, and fills up the short plates of DIC on the three-dimensional layer. DVC was proposed in 1999 by Bay et al (Bay B K, Smith T S, Fyherie D P, et al, digital volume correction: Three-dimensional structural mapping X-ray tomographics [ J ]. Experimental Mechanics,1999,39(3): 217-226), which is similar in principle to DIC and is similarly misaligned when calculating to discrete regions. Based on the advantages of DVC three-dimensional detection, methods of characterizing lesions using DVC have also been developed: for example, Morgneyer et al (Morgneyer T F, Helfen L, Mubarak H, et al, 3D digital volume correction of synchronous radiation analysis images of reduced crack initiation: an initial crack initiation [ J ]. Experimental Mechanics,2013,53(4):543-556.) in order to understand the law of ductile crack development, an aluminum alloy material was tested, the deformation field at the crack initiation site was measured using DVC technology, and the strain concentration at the tip of the crack was shown after calculation, indicating the existence of the crack, but the specific morphological characteristics of the crack could not be obtained by the deformation field. Mostafavi et al (Mostafavi M, McDonald S A, Mummery P M, et al. Observation and qualification of the crack-propagation in poly-nuclear graph [ J ] Engineering crack mechanisms, 2013,110:410-420.) when studying the Fracture behavior of crack propagation at the gap of a sample, calculate the displacement field near the gap by DVC, and calculate the information of crack width change by this displacement field, but the displacement field calculated by DVC near the crack may appear misaligned, so the obtained crack width information is not perfect and the crack shape characteristics are not specifically characterized. According to the method, a DVC algorithm is used for calculating a deformation field in a cement drying process, then the existence of a crack is presumed according to the large nonuniformity of strain, the deformation displacement near the crack is calculated by using the improved DVC algorithm, the existence of the crack and the range of the presumed crack width are confirmed through results, but the crack width obtained by the method is not accurate and can only be given a large range, and the position of the crack can be roughly positioned through the high strain concentration near the crack by the method, so that the trend characteristic of the crack cannot be more accurately described. In conclusion, the existing microcrack characterization methods cannot perform complete morphological characteristic characterization on the three-dimensional microcrack.
Disclosure of Invention
The purpose of the invention is as follows: the invention aims to solve the technical problem of providing a DVC microcrack characterization method based on daughter block separation, which can realize three-dimensional lossless high-precision characterization of microcracks in materials.
The technical scheme is as follows: the DVC microcrack characterization method based on daughter block separation comprises the following steps:
(1) performing traditional DVC displacement field calculation on a group of three-dimensional image data, and screening out points with calculation errors (comparing the correlation degree between calculation points, and if the correlation degree of the calculation points is lower than a set threshold value, the points are points with calculation errors) as preparation points for subsequent crack identification calculation according to the obtained displacement field data (the displacement field data represents the deformation size of the group of three-dimensional data, when the deformation is continuous, the displacement field data cannot be subjected to large-area error, and when the deformation is discontinuous, namely, cracks are included, the displacement field data near the cracks can have the phenomenon of calculation errors, but the cracks can not be subjected to error when the width of the cracks is smaller), and entering step (2), wherein if no obvious wrong calculation points exist, all points in calculation areas can be directly taken as the preparation points and then enter step (3);
(2) selecting preparation points according to the step (1) to correct the overall element displacement, firstly judging whether the overall element displacement result of the preparation points (comparing the overall element displacement of the preparation points with the overall element displacement of the surrounding calculation correct points, judging whether the overall element displacement is consistent or not, and if the overall element displacement is consistent or not, judging whether the overall element displacement is correct or not), if the overall element displacement result is correct, directly entering the step (3), if the overall element displacement result is not consistent, referring to the surrounding calculation correct points (the overall element displacement of the calculation error points is possibly wrong, and generally, the overall element displacement does not change greatly within a certain calculation range, so the overall element displacement of the calculation error points can be modified by referring to the overall element displacement of the surrounding calculation correct points), and entering the step (3) after modification;
(3) finding a target daughter block possibly containing cracks in a target image through a preparation point and correct monobloc displacement thereof, then presetting a crack width parameter b (bx, by, bz) and a distance d0 from a central point of the target daughter block to the crack, constructing a crack plane F in the target daughter block according to the preset parameters, dividing the target daughter block into a main body block and a slave body block by the crack plane, keeping the position of partial voxel points of the main body block unchanged, adding the preset crack width parameter b to the voxel point coordinates of the body block part to obtain a new daughter block, matching the obtained new daughter block with a reference daughter block in a correlation degree, and then continuously changing parameters b and d0 until the correlation degree is optimal, wherein the corresponding b is a primary crack width parameter in the target daughter block;
(4) DVC sub-voxel displacement calculation is carried out on the main body block part corresponding to the preliminary crack width parameter b obtained in the step (3) to obtain the sub-voxel displacement of the main body block part, namely the displacement of one side of the crack;
(5) similarly, the preparation point on the other side of the crack is processed by the same method to obtain a primary crack width parameter and a main body block displacement result, and the existence of the crack is confirmed by comparing the primary crack width parameters on the two sides of the crack in an opposite number relationship;
(6) and subtracting the displacement fields on the two sides of the crack obtained by calculation of the main body block to obtain the crack width parameter at the sub-voxel level, namely obtaining the final crack width.
In step (3), the body block and the slave block refer to two parts of the calculation of the sub-body block divided by the assumed fracture plane, wherein the part containing the center point of the sub-body block and containing most of the voxels is called the body block, and the part containing less voxels is called the slave block.
In the step (3), a crack surface F is constructed, and the equation of the crack surface is shown as the following formula:
wherein bx, by and bz are the size and direction of the crack width in three spatial directions, (x0, y0, z0) is the center point of the calculated operator block, and d0 is the distance from the center point to the plane.
In step (3), the target daughter block is divided into a master block and a slave block according to the crack plane, the voxel coordinates of the slave block portion are added with a preset crack width parameter b, and a new daughter block g '(x, y, z) is formed with the voxel points in the master block, and then correlation matching is performed on the new daughter block g' (x, y, z) and the reference daughter block f (x, y, z), and the formula is as follows:
in the formula, f is a reference daughter block, g' is a target daughter block to which a crack parameter is added, and C is a correlation coefficient indicating the degree of similarity between the two daughter blocks.
The three-dimensional image data refers to three-dimensional image data obtained by X-ray tomography, gamma ray tomography, nuclear magnetic resonance tomography, confocal microscope imaging or neutron tomography.
Has the advantages that: compared with the current mainstream crack detection method, the method has the advantages of no damage, high precision, simple principle, convenient data collection, easy calculation and the like, and the identification and characterization of the cracks are not limited to the gray scale of the cracks in the image any more; compared with a method for representing cracks by using a two-dimensional DIC (digital computer), the method can realize the representation of the cracks on a three-dimensional layer, and represents the extension and width information of the three-dimensional cracks by using the form of a fitting space plane; compared with the existing DVC crack detection technology, the method has the advantages that the crack parameters are assumed in the target daughter block containing the cracks, then the crack surface equation fitting is carried out, the direction information of the cracks is obtained, the crack width information is obtained according to the displacement fields recalculated on two sides of the cracks, the appearance of the cracks in one daughter block is more specifically described, and after point-by-point calculation, the direction and the width of the microcracks can be completely represented, so that a basis is provided for the subsequent research on the germination and the expansion of the cracks.
Drawings
FIG. 1 is a schematic diagram of the method of the present invention;
FIG. 2 is a flow chart of the method of the present invention;
FIG. 3 is a slice view and a three-dimensional view of a target image after writing an I-type crack in step 1 of example 1 of the present invention;
FIG. 4 is a schematic view of displacement fields in the u-direction and the v-direction for writing I-type cracks in step 1 of example 1 of the present invention;
FIG. 5 is a comparison graph of the correlation between the conventional DVC and the DVC with the added crack parameters in step 3 of example 1 of the present invention;
FIG. 6 is a statistical plot of the results of the method on the recognition rate along the crack length direction and the recognition rate along the crack width direction;
FIG. 7 is a graph comparing the results of the method identification calculation of crack width with theoretical crack width results;
FIG. 8 is a CT slice of a sample containing an actual crack at step 1 of example 2 according to the present invention;
FIG. 9 is a schematic of the DVC full field displacement of a crack containing sample at step 2 of example 2 of the present invention;
FIG. 10 is a comparison graph of the correlation between the conventional DVC and the DVC with crack parameters added in step 3 of example 2 of the present invention;
FIG. 11 is a graph comparing the results of the method identification calculation of crack width with the theoretical crack width results.
Detailed Description
Example 1
Example 1 is a method of the present invention for identifying and verifying a type I crack field written in a sample, specifically:
in the formula, KIFor the stress intensity factor, E and v are young's modulus and poisson's ratio, respectively, and the plane strain k ═ 3-4 v;
secondly, the purpose of obtaining a crack field formula is to apply the crack displacement field in a reference image for simulating displacement changes of two sides of the crack during real crack cracking;
in the above formula
Since the crack displacement values are usually non-integer, the voxel point in the reference image after being shifted by adding the crack displacement field to the original voxel point position appears at a non-integer coordinate position, and therefore, the processing needs to be performed by interpolation, and then the theoretical crack field is applied to each layer of slices of the reference image (the processing needs to be performed by Mat)The lab's own function interp3 is interpolated, and then the theoretical crack field is applied to each slice of the reference image to generate three-dimensional image data containing cracks), fig. 3 is a theoretical crack field written with an I-type crack, and it can be seen that the theoretical crack field shows a gradual opening trend in the v direction, fig. 4 is a slice diagram and a three-dimensional schematic diagram of the target image data written with an I-type crack, and it can be seen that the crack width gradually narrows from bottom to top, the maximum width is 4voxel, and the minimum width reaches 0.1 voxel;
and 3, after confirming that the whole element displacement result of the preparation point is correct, presetting a crack width parameter b ═ bx, by and bz and a target daughter block central point (x)0,y0,z0) Distance d to crack0And constructing a crack surface F as shown in the following formula:
formula (2) is a crack surface equation, bx, by and bz are the size and direction of the crack width in three spatial directions respectively, (x0, y0 and z0) are the central points of the calculation sub-block, and d0 is the distance from the central point to the plane;
then the target sub-block is divided into a main block and a slave block according to the crack surface, the voxel coordinate of the slave block part is added with a preset crack width parameter b to form a new sub-block g '(x, y, z) with the voxel point in the main block, and then the new sub-block g' (x, y, z) is subjected to correlation matching with a reference sub-block f (x, y, z), and the formula is as follows:
in the formula (3), f is a reference daughter block, g' is a target daughter block to which a crack parameter is added, and C is a correlation coefficient indicating the degree of correlation (degree of similarity) between the two daughter blocks.
The DVC calculation needs to use two groups of image data, namely a reference image and a target image, the reference daughter block is a cubic daughter block with a certain width and centered on a voxel point in the reference image, the target daughter block corresponding to the reference daughter block can be found in the target image through whole voxel calculation, but when the target image is cracked due to deformation, the reference daughter block and the target daughter block cannot be matched, namely, calculation errors occur.
The closer the result of C in the above formula is to 0, the higher the correlation between two daughter blocks, and then the parameters in the crack plane are changed continuously (the daughter block size is selected to be 25 × 25 × 25 volume, the crack parameter b is smaller than 1/5 of the daughter block, i.e. 0-5 volume range, d0 is smaller than 1/3 of the daughter block size, i.e. 0-10 volume range, the step size is 1 volume, and then through exhaustive attempts, until the best correlation is reached), until the best correlation is reached, the crack width parameter at this time is the initial crack width parameter, and the corresponding crack plane is the initial crack plane. Fig. 5 is a comparison between the correlation of the added crack parameter DVC and the correlation of the two calculated daughter blocks of the conventional DVC, and it can be seen that the correlation after the added crack parameter is well matched.
wherein L (u, v, w) is a displacement result on the crack side.
And 5, similarly, after the other side of the crack is processed by the steps 3 and 4, obtaining the displacement result R (u, v, w) of the crack width parameter and the other side of the crack, and confirming that the calculated daughter block contains the crack by comparing the opposite number relationship between the crack width parameters on the two sides.
And 6, subtracting the displacement data L and R of the main body blocks obtained by calculation in the steps 4 and 5 to obtain a sub-voxel level result of the crack width, and finally, calculating point by point (the point by point refers to a point where an error is calculated near the crack, namely calculating the main body blocks, using the main body blocks in the calculating sub-body blocks to participate in calculation to obtain a displacement result, and the corresponding points of the n main body blocks refer to the calculation points) to obtain high-precision width information of the whole crack area, and replacing the calculated crack width b with the crack trend information obtained according to a crack surface equation in the formula (2) to obtain the crack surface equation.
Fig. 6 is a statistical result of the recognition rate of the method in the length direction and the width direction of the crack, respectively, and it can be seen that the recognition rate of the algorithm is substantially maintained at 90% in the length direction of the crack, and the recognition rate of the rest positions is substantially 90% in the width direction except that the recognition rate cannot be achieved when the center point of the sub-volume is just at the crack. FIG. 7 is a comparison of the results of the algorithm identification calculation of crack width with the theoretical width, and it can be seen that along the length of the crack, from 0.1voxel at the crack tip to 4 voxels at the crack widest, the results can be identified and remain substantially consistent with theory.
Example 2
Example 2 identification verification of real cracks in samples was performed.
and 2, performing traditional DVC calculation on the reference image f and the target image g, collecting a point with an error calculated near the crack as a preparation point for crack identification, wherein a deformation field obtained by the traditional DVC calculation is shown in FIG. 9, and can find that discontinuous fluctuation occurs in u and v directions and a calculation point in a crack area has an error, then setting a correlation threshold value to be 0.6, and collecting the point as the preparation point when the correlation of the calculation point is lower than the threshold value.
And 3, identifying cracks by using the preparation points, and calculating to obtain crack surfaces, wherein the specific content is the same as that of the step 3 in the embodiment 1. Fig. 10 is a comparison of correlation between the added crack parameter DVC and the conventional DVC calculated for two daughter blocks, and the correlation after adding the crack parameter is well matched.
And 4, dividing the sub-body block into two parts by using the crack surface, and then performing sub-voxel displacement calculation on the main body block, wherein the specific content is the same as that in the step 4 in the embodiment 1.
And step 5, obtaining the trend and width information of the cracks in the same way as the step 5 of the embodiment 1.
FIG. 11 is crack information obtained from the present invention identifying real cracks, wherein the crack widths in the three directions u, v, and w do not vary much along the length of the crack, and the average sizes are-0.2100 volume, 0.2639 volume, -0.0476 volume, respectively, wherein the minus sign indicates the direction; along the crack depth direction, both the u direction and the v direction show a tendency to become larger, and the width in the w direction fluctuates around 0. In conclusion, the method can obtain the width information of the cracks in three dimensions, and the trend information of the cracks is obtained by combining the width information with the crack surface equation, so that the specific representation of the appearance characteristics of the cracks is realized.
Claims (5)
1. A DVC microcrack characterization method based on daughter block separation, comprising the steps of:
(1) carrying out traditional DVC displacement field calculation on a group of three-dimensional image data, screening out points with calculation errors according to the obtained displacement field data, taking the points as preparation points for subsequent crack identification calculation, and entering the step (2), and if no obvious error calculation points exist, directly entering the step (3) after taking the points in all calculation areas as the preparation points;
(2) selecting a preparation point according to the step (1) to correct the overall element displacement, firstly judging whether the overall element displacement result of the preparation point is correct or not, if so, directly entering the step (3), if not, calculating the overall element displacement of the correct point by referring to the periphery, and entering the step (3) after the correction;
(3) finding a target sub-block possibly containing a crack in a target image through a preparation point and correct integer pixel displacement of the preparation point, then presetting a crack width parameter b as (bx, by, bz) and a distance d0 from a central point of the target sub-block to the crack, constructing a crack surface F in the target sub-block according to the preset parameters, dividing the target sub-block into a main block and a slave block by the crack surface, keeping the position of a voxel point of the main block part unchanged, obtaining a new sub-block by adding the preset crack width parameter b to the coordinate of the voxel point of the main block part, matching the obtained new sub-block with a reference sub-block in a correlation degree, and then continuously changing parameters b and d0 until the correlation degree is optimal, wherein the corresponding b is a primary crack width parameter in the target sub-block;
(4) performing DVC sub-voxel displacement calculation on the main body block part corresponding to the initial crack width parameter b obtained in the step (3) to obtain the sub-voxel displacement of the main body block part, namely obtaining the displacement of one side of the crack;
(5) similarly, the preparation point on the other side of the crack is processed by the same method to obtain a primary crack width parameter and a main body block displacement result, and the existence of the crack is confirmed by comparing the primary crack width parameters on the two sides of the crack in an opposite number relationship;
(6) and subtracting the displacement fields on the two sides of the crack obtained by calculation of the main body block to obtain the crack width parameter at the sub-voxel level, namely obtaining the final crack width.
2. The method for DVC microcrack characterization based on daughter block separation of claim 1, wherein: in step (3), the body block and the slave block refer to two parts of the calculation of the sub-body block divided by the assumed crack plane, wherein the part containing the center point of the sub-body block and containing most of the voxels is called the body block, and the part containing less voxels is called the slave block.
3. The method for DVC microcrack characterization according to claim 1, wherein: in the step (3), a crack surface F is constructed, and the equation of the crack surface is shown as the following formula:
wherein bx, by and bz are the size and direction of the crack width in three spatial directions, (x0, y0, z0) is the center point of the calculated operator block, and d0 is the distance from the center point to the plane.
4. The method for DVC microcrack characterization according to claim 1, wherein: in the step (3), the target daughter block is divided into a master block and a slave block according to the crack surface, the voxel coordinate of the slave block part is added with a preset crack width parameter b to form a new daughter block g '(x, y, z) with the voxel point in the master block, and then the new daughter block g' (x, y, z) is subjected to correlation matching with the reference daughter block f (x, y, z), and the formula is as follows:
in the formula, f is a reference daughter block, g' is a target daughter block to which a crack parameter is added, and C is a correlation coefficient indicating the degree of similarity between the two daughter blocks.
5. The method for DVC microcrack characterization according to claim 1, wherein: the three-dimensional image data refers to three-dimensional image data obtained by X-ray tomography, gamma ray tomography, nuclear magnetic resonance tomography, confocal microscope imaging or neutron tomography.
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