Composite electrically anisotropic variable resistance porous structures for damage monitoring and methods of making

Abstract

This invention relates to a composite material with anisotropic variable resistivity porous structure for damage monitoring and its preparation method. Based on the material resistance calculation formula, to enable the porous structure to possess anisotropic variable resistivity characteristics, the resistance is controlled by changing the contact area between the unit cells. On a composite material plate, unit cell surface holes are arranged in an n*m array along the X and Y directions; these unit cell surface holes appear on the cross-section of the composite material, forming a porous structure with length, width, and depth. Using this structure, researchers can prepare composite materials that approximate variable resistivity characteristics. The improved conductive network can enhance the sensitivity and range of damage monitoring. This structure is relatively easy to prepare and improves the application capability of resistance methods.

Description

Technical Field

[0001] This invention belongs to the field of structural health monitoring, and relates to a composite material electrically anisotropic variable resistance porous structure for damage monitoring and its preparation method. Background Technology

[0002] Carbon fiber composites possess high specific strength, high specific stiffness, and excellent corrosion resistance, properties that perfectly align with the lightweight, reliable, and long-life requirements of aerospace, automotive, and other fields. However, due to their inherent characteristics and complex nonlinear coupling factors within their structure, carbon fiber composites are prone to invisible damage such as cracks and delamination under vibration and impact in practical applications. This damage significantly reduces the material's strength, safety, and reliability, which the aerospace industry, prioritizing safety, cannot afford to ignore. To ensure the reliability and safety of carbon fiber composites, researchers often adopt conservative designs, ensuring that the ultimate stress of the structure is far greater than the allowable stress. However, this increases the weight of the composite structure, preventing the full utilization of its performance. Structural health monitoring technology is an emerging technology that collects and analyzes structural response signals to determine the structural health status. Its main function is to monitor for structural damage, locate it, perform in-situ, online monitoring, provide early warning of damage, and predict the remaining service life of the structure. Therefore, the use of structural health monitoring technology helps improve structural safety and reliability and has become a key research focus and frontier in aircraft structural design.

[0003] Carbon fiber composites consist of carbon fibers and a matrix. While the carbon fibers themselves are conductive, the matrix is ​​generally non-conductive. However, in reality, because the carbon fibers in composites are not neatly arranged but rather have some curvature and corrugation, conductive carbon fibers in high-carbon fiber content come into contact with each other, forming a conductive network. When damage occurs, carbon fibers break and separate, causing irreversible damage to the conductive network, and the structural resistance changes accordingly. Therefore, by using an electrode array to monitor the internal conductive network of the composite material and analyzing the monitoring data to infer the changes in the conductive network before and after damage, the occurrence, magnitude, and location of the damage can be determined. Through extensive simulations and experiments on carbon fiber composite damage based on the resistance method, the relationship between damage and changes in material surface resistance is established, thereby achieving damage early warning and diagnosis of composite materials in complex environments.

[0004] Achieving high damage monitoring sensitivity in current research often requires deploying a large number of electrodes, making it difficult to apply in practice. Conversely, maintaining a certain level of damage monitoring sensitivity while using only a few electrodes necessitates embedding them within the structure, which reduces structural strength. To address this, some studies have proposed using variable resistivity electrically anisotropic composite materials to create new conductive networks, thereby improving damage monitoring range and sensitivity while maintaining a small number of measuring electrodes. However, the fabrication of linear gradient variable resistivity composite materials is currently difficult, and electrically anisotropic variable resistivity composite materials are limited in practical application. This hinders the development of resistance-based structural health monitoring methods and limits their application prospects. Summary of the Invention

[0005] Technical problems to be solved

[0006] To avoid the shortcomings of existing technologies, this invention proposes a composite material electrical anisotropic variable resistivity porous structure and its preparation method for damage monitoring. In response to the deficiencies of the existing research mentioned above, it provides a more practical porous structure that can approximately possess the characteristics of variable resistivity materials.

[0007] Based on the material resistance calculation formula, to enable porous structures to possess anisotropic variable resistance characteristics, it is proposed to control the resistance by changing the contact area between structural unit cells. Furthermore, for unit cells at different locations, a variable resistance effect is achieved through area gradients. Moreover, for each structural unit cell, controlling the area at the contact surfaces of different unit cells allows the material to exhibit electrical anisotropy.

[0008] Technical solution

[0009] A composite material electrical anisotropic variable resistance porous structure for damage monitoring is characterized by: n*m unit cell surface pores arranged along the X and Y directions on a composite material plate to form an array of n*m unit cell surface pores; the unit cell surface pores are presented on the cross section of the composite material and are shaped as a pore structure with a length l, a width h, and a depth t, the long side l being parallel to the X direction, the wide side h being parallel to the thickness direction of the composite material plate, and the depth t being parallel to the Y direction.

[0010] Since the outermost unit cell of the composite material plate does not contact other unit cells on its outward-facing surface, no surface holes are designed for the unit cell.

[0011] The size and shape of the pores on the surface of the unit cell are related to the electrical conductivity of the composite material itself and its electrical isotropy.

[0012] If the resistivity gradient of the composite material in one direction is positive (i.e., the resistivity increases with the increase of the coordinate value), the hole gradually decreases; if the resistivity gradient is negative (i.e., the resistivity decreases with the increase of the coordinate value), the hole gradually increases.

[0013] When the composite material plate is an electrically anisotropic composite material plate with length L, width W, and thickness N, the unit cell is set to have length M, width K, and thickness N; the number of holes on the surface of the unit cell is p = L / M arranged along the X direction and q = W / K arranged along the Y direction, where L, W, and N are the length L, width W, and thickness N of the composite material plate; forming a p*q array arrangement.

[0014] The area of ​​the Y-direction aperture on the surface of the unit cell is M and N are the length and thickness of the unit cell, σ = a is the electrical conductivity of the material itself, σ y =c is the conductivity in the Y direction.

[0015] The parameters of the Y-axis hole are: Where l is the length of the hole and h is the width of the hole.

[0016] The area of ​​the X-direction aperture on the surface of the unit cell is Where b is the minimum electrical conductivity of the structure in the X direction.

[0017] The parameters of the X-axis hole are: Where l is the length of the hole and h is the width of the hole.

[0018] A method for preparing a composite material electrically anisotropic variable resistivity porous structure for damage monitoring is characterized by: using composite material as the printing material and preparing it using 3D printing technology according to the designed size and arrangement of the porous structure.

[0019] Beneficial effects

[0020] This invention proposes a composite material anisotropic variable resistivity porous structure for damage monitoring and its preparation method. Based on the material resistance calculation formula, to enable the porous structure to possess anisotropic variable resistivity characteristics, the resistance is controlled by changing the contact area between the unit cells. On a composite material plate, unit cell surface holes are arranged in an n*m array along the X and Y directions. These unit cell surface holes appear on the cross-section of the composite material, forming a pore-like structure with a length l, a width h, and a depth t. The long side l is parallel to the X direction, the wide side h is parallel to the thickness direction of the composite material plate, and the depth t is parallel to the Y direction.

[0021] The advantages of this invention compared to the prior art are:

[0022] 1) Improve the effectiveness of damage and health monitoring: Through this structure, researchers can prepare composite materials that can approximate variable resistance characteristics. The improved conductive network can enhance the sensitivity and range of damage monitoring.

[0023] 2) Promoting the application of resistance methods: The traditional application of resistance methods is to monitor non-variable resistance conductive composite materials, while research has shown that variable resistance materials are more effective for damage monitoring. However, it is currently difficult to prepare variable resistance composite materials. Therefore, this invention proposes a structure that makes the material approximately have variable resistance characteristics. This structure is easier to prepare and can improve the application capability of resistance methods.

[0024] 3) Inspired variable resistance structure design: This structure achieves variable resistance characteristics by changing the contact area between porous unit cells by creating holes in the unit cell. Therefore, other researchers can refer to this method to design variable resistance structures by controlling the conductivity, contact area, etc. of porous unit cells. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a porous single-cell structure.

[0026] Figure 2 This is a schematic diagram of the pore parameters of a porous structure, where l, h, and t represent the length, width, and depth of the pore, respectively.

[0027] Figure 3 This is a schematic diagram of the unit cell arrangement. The first two digits of the number in the diagram represent the row number, and the last two digits represent the column number.

[0028] Figure 4 This is a schematic diagram of a porous structure. The cuboid structures inside the flat plate in the diagram are the pores. Detailed Implementation

[0029] The present invention will now be further described in conjunction with the embodiments and accompanying drawings:

[0030] The resistance of a material is related to its conductivity, length, and cross-sectional area. In practical applications, it is difficult to control the conductivity of a unit cell in different directions individually, and although changing the unit cell length can change the unit cell resistance, the overall resistance of the structure does not change. Therefore, it is considered to achieve a variable resistance effect by changing the contact area between different unit cells. There are many methods to change the contact area; this invention considers changing the contact area by creating holes on the surface of the unit cell. Since the conductivity of the composite material itself is electrically isotropic, the size of the contact area can be controlled by changing the size of the holes on the unit cell surface.

[0031] Next, the unit cell shape is designed. For structures with variable resistance in the X direction, the conductivity increases linearly in the X direction, remains unchanged in the Y direction, and has a much lower conductivity in the Z direction than in the XY direction. When designing the unit cell, because the conductivity in the Z direction is three orders of magnitude lower than in the in-plane direction, it is necessary to control the lower surface area of ​​the unit cell to be extremely small, making stacking in the thickness direction difficult. Therefore, only the contact area in the XY direction is considered. Each unit cell has holes drilled in two directions, called X-axis holes and Y-axis holes, and the surface of each hole contacts the complete surface of other unit cells. Each hole is controlled by three parameters, representing the length, width, and depth of the hole.

[0032] After the unit cell shape design is completed, the unit cell arrangement begins. For the composite plate model, n and m unit cells are arranged in the X and Y directions respectively, forming an n*m array. For a variable resistance in one direction, if the resistivity gradient is positive (i.e., the resistivity increases with the coordinate value), the aperture is gradually reduced; if the resistivity gradient is negative (i.e., the resistivity decreases with the coordinate value), the aperture is gradually increased. The outermost unit cell of the composite plate does not have an aperture because its outward-facing surface does not contact other unit cells.

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. This description is exemplary and does not limit the invention in any way. Any simple modifications to the present invention without any technical innovation shall still fall within the protection scope of the technical solution of the present invention.

[0034] As is well known, at a constant temperature, the resistance of a material can be calculated using the following formula.

[0035]

[0036] Therefore, to enable the structure to exhibit variable resistance characteristics, meaning that the material's conductivity changes with location, it is necessary to control the material's conductivity or surface area. For porous materials, since the conductivity of different unit cells is difficult to control, this invention considers controlling the resistance by controlling the contact area between unit cells. Simultaneously, changing the contact area between unit cells achieves a variable resistance effect. For different orientations of the unit cells, adjusting the contact area achieves an electrical anisotropy effect.

[0037] Unit shape like Figure 1 As shown, each unit cell has holes drilled in two directions, called X-axis holes and Y-axis holes, and the surface of each hole contacts the intact surface of other unit cells. Each hole is controlled by three parameters: l, h, and t, representing the length, width, and depth of the hole, respectively. Figure 2 As shown.

[0038] For an electrically anisotropic composite plate with length L, width W, and thickness N, let the unit cell have length M, width K, and thickness N. Therefore, p = L / M unit cells are arranged along the X direction, and q = W / K unit cells are arranged along the Y direction to form a p*q array arrangement, as shown below. Figure 3 As shown, implementation for example Figure 4 As shown, a 12*7 array arrangement is formed.

[0039] For this composite material plate, if the material's inherent conductivity is σ = a, and we want it to approximately have the following variable resistance characteristics: gradually decreasing in the X direction while maintaining a conductivity of σ... x = αx + b, where α is the potential gradient, b is the minimum conductivity of the structure in the X direction, and x is the coordinate value in the X direction. The conductivity in the Y direction is σ. y =c, where σ>σ x ,σ>σ y First, determine the dimensions of the Y-axis hole; its area is:

[0040]

[0041] The parameters for the Y-axis hole are:

[0042]

[0043] Where l is the length of the pore and h is the width of the pore. The value of t can be determined based on the precision of the equipment used in the actual fabrication process, ensuring that the surface of the pore in the unit cell is separated from the surface of other unit cells.

[0044] For X-axis holes, the approach is similar to that for Y-axis holes; the hole area is:

[0045]

[0046] The parameters of the X-axis hole are:

[0047]

[0048] Where l is the length of the hole and h is the width of the hole. The value of t is the same as that of the Y-direction hole, and it is sufficient to ensure that the surface of the unit cell hole is separated from the surface of other unit cells, depending on the precision of the equipment used in actual fabrication.

[0049] After obtaining the relationship between the unit cell pore parameters and coordinate positions, the parameters for each unit cell pore are then calculated. Since the parameters obtained from the above formula are continuous, while the number of unit cells is finite, the parameters need to be discretized. The results are shown in the table below:

[0050]

[0051] Where i = 1, 2, ..., p, j = 1, 2, ..., q; the example is: i = p = 12, j = q = 7.

[0052] Thus, the parameters of each unit cell pore were obtained, and according to... Figure 3 and Figure 4 Arranging the unit cells in a certain way yields an electrically anisotropic variable resistance composite plate.

[0053] Using composite material slurry as the printing material, the size and arrangement of the designed porous structure are as follows: Figure 4 The structure is prepared using 3D printing technology to obtain an electrically anisotropic variable resistance composite material plate.

[0054] For the variable resistance of this composite material in one direction, if its variable resistance gradient is positive, that is, the resistivity increases with the increase of the coordinate value, the hole gradually decreases; if its variable resistance gradient is negative, that is, the resistivity decreases with the increase of the coordinate value, the hole gradually increases.

[0055] In application, 10*5mm electrode plates are arranged on the upper surface (+z direction) of the porous composite material structure at intervals of 30mm in the X direction and 20mm in the Y direction to monitor potential changes. Electrode plates are also attached to the two sides of the plate (+x and -x directions), one connected to a constant current source and the other grounded. A multi-channel data acquisition instrument is then used to monitor potential changes in real time. When damage occurs in the through-hole, the electrode plate closest to the damage will show a higher degree of potential change than the other electrodes, thus allowing the determination of the damage occurrence and location.

Claims

1. A composite electrically anisotropic varistor porous structure for damage monitoring, characterized by: On a composite material plate, n*m unit cell surface holes are arranged along the X and Y directions to form an array of n*m unit cell surface holes; the unit cell surface holes are presented on the cross section of the composite material and are shaped as a hole structure with a length of l, a width of h and a depth of t, the long side l is parallel to the X direction, the wide side h is parallel to the thickness direction of the composite material plate, and the depth t is parallel to the Y direction. When the composite material plate is an electrically anisotropic composite material plate with length L, width W, and thickness N, the unit cell is defined as length M, width K, and thickness N; the number of holes on the surface of the unit cell is p = L / M arranged along the X direction and q = W / K arranged along the Y direction, where L, W, and N are the length L, width W, and thickness N of the composite material plate; forming a p*q array arrangement. The area of ​​the Y-direction aperture on the surface of the unit cell is M and N are the length and thickness of a single cell. The electrical conductivity of the material itself, The conductivity is in the Y direction; The parameters of the Y-direction hole are: wherein, l is the length of the hole, and h is the width of the hole. The area of the X-direction hole of the single-cell surface hole is wherein: b is the minimum conductivity of the structure in the X direction; The parameters of the X-direction hole are: wherein, l is the length of the hole, and h is the width of the hole.

2. The composite electrically anisotropic variable resistance porous structure for damage monitoring according to claim 1, characterized in that: Since the outermost unit cell of the composite material plate does not contact other unit cells on its outward-facing surface, no surface holes are designed for the unit cell.

3. The composite electrically anisotropic variable resistance porous structure for damage monitoring of claim 1, wherein: The size and shape of the pores on the surface of the unit cell are related to the electrical conductivity of the composite material itself and its electrical isotropy.

4. The composite electrically anisotropic variable resistance porous structure for damage monitoring of claim 1, wherein: If the resistivity gradient of the composite material in one direction is positive (i.e., the resistivity increases with the increase of the coordinate value), the hole gradually decreases; if the resistivity gradient is negative (i.e., the resistivity decreases with the increase of the coordinate value), the hole gradually increases.

5. A method for preparing a composite material electrically anisotropic variable resistivity porous structure for damage monitoring as described in any one of claims 1 to 4, characterized in that: Using composite materials as the printing material, 3D printing technology is used to prepare porous structures according to their size and arrangement.

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