CN111859747B - Layered quantification method based on damage mapping finite element grid - Google Patents
Layered quantification method based on damage mapping finite element grid Download PDFInfo
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Abstract
A layering quantification method based on damage mapping finite element grids comprises the steps of conducting luminance value blocking statistics on images obtained after conducting ultrasonic scanning on a drilling layering area, establishing an initial finite element grid which is completely consistent with a blocking topological structure, conducting material attribute replacement, conducting smoothing processing on the finite element grid, obtaining an optimized finite element model, and exporting the optimized finite element model to an INP text file. According to the invention, the damage information is obtained from the ultrasonic C scanning image, the damage degree can be reflected on each unit, the layering degree of different areas is quantized, and the simulation precision is improved; the whole automation degree is high, the direct generation from the ultrasonic scanning image to the finite element calculation model is realized, and the process of constructing a geometric model is avoided. Finally, the effect of quantifying the layered damage of the drill hole is achieved.
Description
Technical Field
The invention relates to a technology in the field of material mechanics, in particular to a composite material drilling layering quantification method based on a damage mapping finite element grid.
Background
Carbon fiber reinforced Composite (CFRP) is an advanced composite material formed by co-thermosetting carbon fibers and an epoxy resin matrix, and when mechanical connection is performed with other materials, bolts or rivet holes need to be drilled in the CFRP, and delamination generated in the drilling process can affect the residual strength of the structure. In order to quantify the layering damage condition, firstly, the layering appearance needs to be acquired by means of external equipment, and then the obtained image is analyzed to quantify the layering degree. In the process, concepts of the layering factors and the equivalent layering radii are proposed and developed to facilitate subsequent mechanical modeling. However, most of the existing equivalent layering radii can only reflect the overall layering degree in the application process, and cannot reflect the layering area distribution and the layering gradient.
Ultrasonic flaw detection is a method widely used for acquiring layered information of composite materials, and is a technology for extracting echo information perpendicular to a specified section of an acoustic beam to form a two-dimensional image. For the composite material, the sound pressure of the echo is positively correlated with the layering degree, and the sound pressure information is converted to obtain an ultrasonic scanning image capable of reflecting the damage degree by the color value. Most of the existing methods using ultrasound images calculate the area or contour of the damaged area after thresholding, and lose the information of the damage degree.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a hierarchical quantification method based on a damage mapping finite element grid, which obtains damage information from an ultrasonic C scanning image, can reflect the damage degree on each unit, quantifies the hierarchical degree of different areas and improves the simulation precision; the whole automation degree is high, the direct generation from the ultrasonic scanning image to the finite element calculation model is realized, and the process of constructing a geometric model is avoided. Finally, the effect of quantifying the layered damage of the drill hole is achieved.
The invention is realized by the following technical scheme:
the invention relates to a layering quantification method based on damage mapping finite element grids, which is characterized in that an image obtained after ultrasonic scanning is carried out on a drilling layering area is subjected to brightness value blocking statistics, an initial finite element grid completely consistent with a blocked topological structure is established, material property replacement is carried out, then the finite element grid is subjected to smoothing processing, an optimized finite element model is obtained, and an INP text file is exported.
The INP text file can be directly read by ABAQUS software, and a model generated after being read in not only accurately describes the real boundary of an original drilling hole by a smooth structured grid, but also clearly shows the material properties of different layering areas, achieves the effect of quantifying layering damage, and can provide guidance for more precise simulation tests and deep solution of a drilling hole layering mechanism.
The luminance value block statistics means: according to the brightness value of each pixel point in the image, dividing the image blocks into the same number according to the preset number of finite element grid units, counting the average value of the pixel brightness in each image block, setting the maximum value and the minimum value in the average value to respectively correspond to a hole area and a sound area, and setting other image blocks to correspond to areas with different damage degrees caused by layering.
The material attribute replacement means: endowing the image block grids corresponding to the hole areas with the material properties of the intact areas; and assigning the image blocks corresponding to the damaged areas to the degraded material properties.
The smoothing treatment is as follows: and moving the grid nodes around the hole area through a smoothing algorithm to smooth the boundary, namely: and finding out all peripheral output nodes including the boundary points of each hole area, respectively averaging the horizontal coordinates and the vertical coordinates of all the nodes to obtain node centers, moving the boundary points to the middle points of connecting lines between the boundary points and the node centers and updating the boundary points to new boundary points, and obtaining smoother boundaries after traversing all the boundary points.
The invention relates to a system for realizing the method, which comprises the following steps: image processing module, finite element mesh initialization module, mesh smoothing module and the model file generation module that connects gradually in series, wherein: the image processing module extracts RGB information of an original layered image in blocks, the finite element mesh initialization module generates mesh units containing different material attributes according to the RGB information, and the mesh smoothing module performs boundary smoothing on the mesh units to better describe the boundaries of raw materials.
The system is preferably further provided with a model file generation module for generating input and output information of each module in the system.
Technical effects
The invention integrally solves the problem that the prior art can not fully excavate the information of the layered damage image of the composite material drill hole; according to the invention, the damage degree is reflected on each unit according to the block brightness value, namely the material attribute with the declining damage degree value, so that the simulation precision is improved; the quadrilateral mesh smoothing algorithm not only ensures the smooth boundary, but also does not influence the quadrilateral topological connection structure of the whole mesh, thereby facilitating the residual mechanical property of the subsequent calculation model.
Compared with the prior art, the method has the advantages that the damage information obtained by ultrasonic C scanning is fully utilized, the brightness value of the image block is used for representing the damage degree, the information of the damaged area is rarely lost, and the representing mode is convenient for engineering personnel to understand. The quadrilateral mesh is established by a simple topological structure, the scale of the finite element mesh can be conveniently controlled, and the mesh quality is high. The whole process has high automation degree, realizes the direct generation from the ultrasonic scanning image to the finite element calculation model, and avoids the process of constructing the geometric model.
Drawings
FIG. 1 is an ultrasonic C-scan image of a perforated carbon fiber composite;
FIG. 2 is a schematic flow chart of the present invention;
FIG. 3 is a schematic diagram of tile partitioning;
FIG. 4 is a graph of a layering factor as a function of tile brightness;
FIG. 5 is a finite element mesh generated from tiles;
FIG. 6 is a finite element mesh with different cell properties;
FIG. 7 is a schematic diagram of a smoothing algorithm;
fig. 8 is a diagram of the final effect of the embodiment.
Detailed Description
As shown in fig. 1, the present embodiment relates to a method for quantifying drilling layering based on a damage mapping finite element mesh, which includes the following steps:
the first step is as follows: introducing an image which is subjected to ultrasonic C scanning and contains p × q pixel points of the holed carbon fiber composite material shown in the figure 1 into MATLAB software, and reading the RGB value of the image; the RGB values are then stored in a matrix of p × q × 3; then, calculating the average value of the third dimension and converting the average value into an integer; and finally, storing the average value in a p × q two-dimensional array D, wherein each element in D is the brightness of the pixel point at the corresponding position.
The second step is that: the image is discretized into m × n blocks with the required accuracy of the finite element mesh, each block defining the number i, i ═ 1,2, …, m × n, as shown in fig. 3. Calculating the average value of the brightness of all pixel points in each image block, converting the average value into an integer quantity, and storing the integer quantity in an m x n two-dimensional array L, wherein each element in the L is the brightness value of the corresponding ith image block and also reflects the layering degree of the composite material corresponding to the image block.
The third step: clustering elements of the same value (i.e. tiles of the same luminance value) in L: firstly, establishing an N-dimensional vector K containing all different elements in the L, wherein N is the number of the different elements in the L; then, an N-dimensional cell array C is established, and the block numbers i with the same L (i) numerical value are stored in the same cell.
The cells in the cell array C have a one-to-one correspondence relationship with the elements in the vector K.
The fourth step: because the hierarchical damage coefficient F of the area with the maximum brightness value (hole area) is 1, the hierarchical damage coefficient of the area with the minimum brightness value (good area) is 0, the hierarchical damage coefficients of other areas are positively correlated with the brightness value, and the hierarchical damage coefficient correlated with the brightness value is calculated
Wherein: the maximum value in the vector K is maxk, the minimum value is mink, the jth element is K (j), and the function image is shown in fig. 4.
The fifth step: establishing a finite element mesh with the same structure according to the topological structure of the discrete graph blocks, as shown in fig. 5, the unit number and the node number both adopt a numbering mode that abqus can read, specifically: and setting a control threshold T, not outputting all units in the cells C (j) corresponding to the F (K (j)) > T in the subsequent process, namely not outputting the units, outputting the rest other units as output units, and respectively establishing output and non-output unit number sets set-1 and set-0.
And a sixth step: calculating generalized stiffness matrices E (j) ═ E (1-F (k (j))) of all the numbered units in the unit cell c (j), wherein: the generalized stiffness matrix of the material used for the simulation is E. Thus, the elements with different layering degrees in the finite element mesh are identified, different material attributes are given to the elements according to the layering degrees, and the finite element mesh with different material attributes is given, as shown in fig. 6.
The seventh step: the pixelization boundary between the output unit and the non-output unit is smoothed in the following specific flow
Solving intersection of a node set of an output unit and a node set of a non-output unit to obtain a boundary point set NsetB;
secondly, for each boundary point P in the boundary point set NsetB, finding all (output) nodes including the boundary point P, respectively averaging the horizontal coordinates and the vertical coordinates of all the nodes to obtain a node center Pc, moving the point P to a middle point on a connecting line of the point P and the Pc to be used as an updated point P coordinate, and obtaining a smoother boundary after traversing all the boundary points as shown in FIG. 7.
And thirdly, after one traversal is finished, whether the traversal times need to be increased or not is selected according to the actual smooth effect.
Eighth step: the above obtained finite element node coordinates, element node numbers, material properties, etc. are written into a text file with the suffix of. inp, which can be read by the Abaqus software, so that a finite element mesh with different material properties and smooth boundaries is obtained, as shown in fig. 8. And finally, completing the layering quantification of the finite element mesh model based on damage mapping, wherein the model and the quantification result are used for more refined drilling layering evaluation and performance simulation experiments of residual mechanical properties after drilling and the perforated connecting piece.
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (6)
1. A hierarchical quantification method based on damage mapping finite element grids is characterized in that brightness value block statistics is carried out on an image obtained after ultrasonic scanning is carried out on a drilling layered area, an initial finite element grid which is completely consistent with a block topological structure is established, material property replacement is carried out, then smoothing processing is carried out on the finite element grid, an optimized finite element model is obtained, and an INP text file is exported, and the method specifically comprises the following steps:
the first step is as follows: reading the RGB value of an image which is subjected to ultrasonic C scanning and contains p multiplied by q pixel points and is made of the carbon fiber composite material with the holes; the RGB values are then stored in a matrix of p × q × 3; then, calculating the average value of the third dimension and converting the average value into an integer; finally, storing the average value in a p × q two-dimensional array D, wherein each element in D is the brightness of a pixel point at a corresponding position;
the second step is that: discretizing the image into m × n blocks according to the precision of the required finite element grid, wherein each block defines the number i, i is 1,2, the.
The third step: clustering elements of the same value in L, i.e. blocks of the same luminance value: firstly, establishing an N-dimensional vector K containing all different elements in the L, wherein N is the number of the different elements in the L; establishing an N-dimensional cell array C, and storing block numbers i with the same L (i) numerical value in the same cell;
the fourth step: because the hierarchical damage coefficient F of the area with the maximum brightness value, namely the hole area, is 1, the hierarchical damage coefficient of the area with the minimum brightness value is 0, the hierarchical damage coefficients of other areas are positively correlated with the brightness value, and the hierarchical damage coefficient correlated with the brightness value is calculated
Wherein: the maximum value in the vector K is maxk, the minimum value is mink, and the jth element is K (j);
the fifth step: establishing a finite element grid with the same structure according to the topological structure of the discrete graph blocks, wherein the unit number and the node number are both in a numbering mode that abaqus can read, and the method specifically comprises the following steps: setting a control threshold value T, not outputting all units in the cells C (j) corresponding to the cells F (K (j)) > T in the subsequent process, namely not outputting the units, outputting the rest other units as output units, and respectively establishing output and non-output unit number sets set-1 and set-0 according to the output units and the non-output unit number sets;
and a sixth step: calculating generalized stiffness matrices E (j) ═ E (1-F (k (j))) of all the numbered units in the unit cell c (j), wherein: the generalized rigidity matrix of the material used for simulation is E, so that units with different layering degrees in the finite element mesh are identified, different material attributes are given to the units according to the layering degrees, and the finite element mesh with different material attributes is given to the finite element mesh;
the seventh step: the method for smoothing the pixelization boundary between the output unit and the non-output unit specifically comprises the following steps:
solving intersection of a node set of an output unit and a node set of a non-output unit to obtain a boundary point set NsetB;
finding all surrounding output nodes including the boundary point P for each boundary point P in the boundary point set NsetB, respectively averaging the horizontal coordinates and the vertical coordinates of all the nodes to obtain a node center Pc, moving the point P to a middle point on a connecting line of the point P and the Pc to be used as an updated point P coordinate, and obtaining a smoother boundary after traversing all the boundary points;
thirdly, after one traversal is finished, whether the traversal times need to be increased or not is selected according to the actual smooth effect;
eighth step: writing the obtained finite element node coordinates, the unit node numbers and the material properties of different units into a text file with the suffix of inp, which can be read by Abaqus software, so as to obtain the finite element mesh with different material properties and smooth boundaries.
2. The method as claimed in claim 1, wherein said luminance value blocking statistics are selected from the group consisting of: according to the brightness value of each pixel point in the image, dividing the image blocks into the same number according to the preset number of finite element grid units, counting the average value of the pixel brightness in each image block, setting the maximum value and the minimum value in the average value to respectively correspond to a hole area and a sound area, and setting other image blocks to correspond to areas with different damage degrees caused by layering.
3. The method of claim 1, wherein the material property substitution is selected from the group consisting of: endowing the image block grids corresponding to the hole areas with the material properties of the intact areas; and assigning the image blocks corresponding to the damaged areas to the degraded material properties.
4. The method as claimed in claim 1, wherein the smoothing process comprises: and moving the grid nodes around the hole area through a smoothing algorithm to smooth the boundary, namely: and finding out all peripheral output nodes including the boundary points of each hole area, respectively averaging the horizontal coordinates and the vertical coordinates of all the nodes to obtain node centers, moving the boundary points to the middle points of connecting lines between the boundary points and the node centers and updating the boundary points to new boundary points, and obtaining smoother boundaries after traversing all the boundary points.
5. A system for implementing the method for hierarchical quantization based on finite element mesh mapping of damage as claimed in any one of claims 1 to 4, comprising: image processing module, finite element mesh initialization module, mesh smoothing module and the model file generation module that connects gradually in series, wherein: the image processing module extracts RGB information of an original layered image in blocks, the finite element mesh initialization module generates mesh units containing different material attributes according to the RGB information, and the mesh smoothing module performs boundary smoothing on the mesh units to better describe the boundaries of raw materials.
6. The system of claim 5, further comprising a model file generation module for generating INP files for facilitating reading and subsequent modification of input and output information of each module in the system.
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