A multifunctional negative poisson's ratio cell and honeycomb structure based on mode conversion

Abstract

The application belongs to the technical field of mechanical metamaterials, and particularly relates to a multifunctional negative Poisson's ratio cell and honeycomb structure based on mode conversion. The multifunctional negative Poisson's ratio cell and honeycomb structure based on mode conversion comprises a regular hexagonal honeycomb unit, the regular hexagonal honeycomb unit is sequentially connected by first straight beams, and a second straight beam is connected to the outer end of the connecting point of adjacent first straight beams; each second straight beam is composed of a straight beam segment and a variable cross-section beam, and the topological configuration of the hexagonal honeycomb structure is changed by adjusting the thickness ratio of the first straight beam and the second straight beam, the axial length of the first straight beam and the angle θ of the variable cross-section beam. The multifunctional negative Poisson's ratio honeycomb structure based on mode conversion enables the honeycomb structure to realize the negative Poisson's ratio effect through mode conversion and has more functions, and is accompanied by large-scale rotation of nodes and cells, and the design of the structure also improves the stiffness of the structure, and the application is more convenient.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of mechanical metamaterials, and particularly relates to a multifunctional negative Poisson's ratio cell and honeycomb structure based on mode conversion. BACKGROUND

[0002] Mechanical metamaterials are an important branch of metamaterials, which refer to a group of metamaterials with unique mechanical properties. These unusual properties come from the micro-geometric shape of the structure, not from their material composition. Therefore, mechanical metamaterials are usually constructed by periodically tessellated volume elements, and various unusual mechanical properties, including negative Poisson's ratio and mode conversion, are achieved by designing the cell.

[0003] Negative Poisson's ratio refers to the phenomenon that the structure expands / contracts in the transverse direction when it is axially stretched / compressed, so it is also called auxetic material. Due to the excellent indentation resistance, fracture resistance, shear resistance, curved surface isotropy, variable permeability and energy absorption characteristics of negative Poisson's ratio materials, they are widely used in many fields such as industry, national defense and medicine. Mode conversion refers to the process that when the material is compressed beyond a threshold value, the cell undergoes coordinated buckling, and the lattice undergoes a high degree of reorganization, so that the material form with a certain periodic pattern is converted into a material form with a larger scale. This process is often accompanied by a change in stiffness and may cause the Poisson's ratio to change from positive to negative. By utilizing the mode conversion characteristics, material properties such as adjustable negative Poisson's ratio, chiral pattern, phonon and photon switch, and reprogrammable color display can be customized. Mode conversion is commonly found in elastic periodic porous structures and is expected to be extended to more structural forms. However, the formation of mode conversion requires the size of the ligament and the size of the node of the cell to be coordinated, so the traditional regular honeycomb structure cannot exhibit mode conversion phenomenon. Therefore, it is necessary to change the topological configuration of the honeycomb structure, and currently there is little research on hexagonal honeycomb structures.

[0004] Hexagonal honeycomb structure is one of the many achievements of human learning from nature. Due to its excellent properties such as light weight, high strength, high energy absorption, simple shape, and easy design, it is widely used in aerospace, transportation, machinery, packaging, medicine and many other fields. In order to obtain negative Poisson's ratio effect in hexagonal honeycomb structure, an inner concave hexagonal honeycomb is proposed and widely used. However, due to the large internal pores of the inner concave hexagonal honeycomb, its bearing capacity is limited and it is prone to deformation under load. In addition, the function is relatively single. Therefore, it is desirable to obtain a multifunctional negative Poisson's ratio honeycomb structure with enhanced stiffness. However, in order to obtain excellent mechanical properties of hexagonal honeycomb, various modification ideas have been proposed, such as inner concave design, hierarchical design and gradient design, which are not based on mode conversion.

[0005] A patent application with publication number CN 114542635 A proposes an inner torsion honeycomb structure material and its preparation method. It is a modification of the topology of the traditional hexagonal honeycomb, changing the mechanical properties of the honeycomb structure. The specific method is to keep the cross section of the outer cell wall of the unit unchanged as a regular hexagon, while the inner cell wall is composed of torsion curved surfaces connected in turn. When the unit cell array forms a structure, the adjacent inner torsion hexagonal honeycomb structure unit shares the outer cell wall. By adjusting the process parameters such as torsion angle, unit thickness, side length and height, the honeycomb structure has higher specific energy absorption and specific stiffness.

[0006] A patent application with publication number CN 115479096 A proposes an assembled anti-collision structure with positive or negative Poisson's ratio effect and its design method, which realizes the switching of the Poisson's ratio of the hexagonal honeycomb between positive and negative values. The specific method is to change the position and length of the horizontal plate, an assembled component, to realize the conversion of the hexagonal honeycomb structure and the concave honeycomb structure. When assembled into a hexagonal honeycomb, the Poisson's ratio is positive, while when assembled into a concave honeycomb structure, the Poisson's ratio is negative. Since the above design obtains positive or negative Poisson's ratio by artificially assembling structural components to form a hexagonal honeycomb structure or a concave honeycomb structure, the value of the Poisson's ratio is determined after the structure is assembled. To change the sign of the Poisson's ratio, the structure needs to be reassembled, which is inconvenient to apply.

[0007] A patent application with publication number CN 113958637 A proposes an inner concave negative Poisson's ratio metamaterial unit cell and honeycomb structure, which realizes the negative Poisson's ratio effect through a concave angle honeycomb. Its mechanism is that when the structure is subjected to tension or compression, the inclined bars will rotate in the plane. In order to improve the situation that the concave angle honeycomb structure deforms quickly and has low bearing capacity, symmetric circular arc support cell walls are arranged on the left and right sides of the structure to improve the stiffness of the structure. To ensure a wide range of Poisson's ratio changes, the length of the inclined bars cannot be too small, which limits the bearing capacity of the structure and reduces its resistance to deformation. Moreover, the inner concave hexagonal honeycomb structure has a single function, and no rotation of the nodes and unit cells occurs except for the negative Poisson's ratio effect. SUMMARY

[0008] To solve the above problems, a multifunctional negative Poisson's ratio unit cell and honeycomb structure based on mode conversion are provided, which enables the honeycomb structure to realize the negative Poisson's ratio effect through mode conversion and has more functions, accompanied by a large-scale rotation of the nodes and unit cells. At the same time, the parameter design of the structure also significantly improves its stiffness, making it more convenient to apply and solving the problems existing in the prior art.

[0009] The present application provides one of the following technical solutions:

[0010] A multifunctional negative Poisson's ratio cell based on mode conversion, comprising a regular hexagonal honeycomb unit, the regular hexagonal honeycomb unit is sequentially connected by first straight beams, every two adjacent first straight beams are connected by a connecting point, and a second straight beam is connected to the outer end of each connecting point; each second straight beam is composed of a straight beam segment and a variable cross-section beam; wherein the length of the first straight beam is l , the thickness is t 2 , the length of the straight beam segment is l 1 , the thickness is t 2 , the variable cross-section beam is arranged to be widened first and then narrowed in the direction away from the first straight beam along the straight beam segment, the included angle of the widened side of the variable cross-section beam with the axis of the second straight beam is θ, and the length of the variable cross-section beam is l - l 1 , the distance between the end point of the widened side and the end point of the narrowed side of the variable cross-section beam in the axial direction of the second straight beam is l 3 , and the cross-sectional width of the variable cross-section beam is determined by the following function: .

[0011] Further, the length, thickness of the first straight beam and the second straight beam and the included angle θ of the variable cross-section beam satisfy the following conditions: ; ; .

[0012] Further, t 2 = 3 t 1 ; l = 2 l 1 , and the included angle θ of the variable cross-section beam is 20°.

[0013] The present application provides the following technical solutions two:

[0014] A multifunctional negative Poisson's ratio honeycomb structure based on mode conversion, composed of a plurality of negative Poisson's ratio cells arranged in a topological array.

[0015] Further, the topological configuration of the negative Poisson's ratio cell is spliced by the variable cross-section beams of adjacent cells.

[0016] The multifunctional negative Poisson's ratio honeycomb structure based on mode conversion changes the topological configuration of the hexagonal honeycomb structure by adjusting the thickness ratio of the first straight beam and the second straight beam, the axial length of the first straight beam and the included angle θ of the variable cross-section beam.

[0017] The beneficial effects of the present application are:

[0018] 1. The mechanism by which this invention achieves negative Poisson's ratio differs from existing technologies. By adjusting the thickness of the straight beam and the angle of the variable cross-section beam, the topology of the traditional hexagonal honeycomb is altered, causing the hexagonal honeycomb to generate a negative Poisson's ratio effect due to mode transformation. Furthermore, an adjustable Poisson's ratio can be achieved; that is, the structure initially exhibits a positive Poisson's ratio, but when the pressure exceeds a critical value, the Poisson's ratio transforms into a negative value. Modifying the structural topology also enhances the structure's stiffness.

[0019] 2. The plateau phase resulting from the mode transition in this invention enables the hexagonal honeycomb structure to have a continuous and stable energy absorption effect. The local buckling phenomenon during mode transition can be used for elastic energy dissipation and can be reused, reducing usage and maintenance costs.

[0020] 3. In addition to achieving the negative Poisson's ratio effect, the structure of this invention also generates a large range of cell and node rotation after buckling, with a large range of cell / node rotation. Its rotational characteristics can be used to develop bending / torsion actuators, motion joints of soft robots, etc., further enhancing its application value.

[0021] 4. The structure of this invention is simple, and it is a simple modification of the traditional hexagonal honeycomb. As a multifunctional negative Poisson's ratio honeycomb structure material, it is easy to manufacture and has low cost. It can be realized by traditional methods such as casting.

[0022] In summary, the multifunctional negative Poisson's ratio honeycomb structure of this invention, through modification of its topology, enables the honeycomb structure to achieve the negative Poisson's ratio effect through mode conversion, and also provides other functions such as good energy absorption effect and improved specific stiffness. Compared with the modification of the topology of traditional honeycomb structures in the prior art, it is mainly achieved by adjusting the thickness ratio of the two straight beams of the regular hexagonal honeycomb unit, the axial length, and the angle of the variable cross-section beam, which is simple and flexible in design. Attached Figure Description

[0023] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0024] Figure 1 This is a schematic diagram of the structure of the multifunctional negative Poisson's ratio cell of the present invention;

[0025] Figure 2 This is a schematic diagram of the multifunctional negative Poisson's ratio honeycomb structure of the present invention;

[0026] Figure 3 This invention demonstrates the negative Poisson's ratio effect of the multifunctional negative Poisson's ratio honeycomb structure under compression.

[0027] Figure 4 This is a schematic diagram of the overall buckling of a traditional concave hexagonal honeycomb under uniaxial compression.

[0028] Figure 5 The rotation angles of cells and nodes in the multifunctional negative Poisson's ratio honeycomb structure of this invention under uniaxial compression;

[0029] Figure 6 A comparison of stress-strain results for the three structures is presented.

[0030] Among them, 1 is the second straight beam, 2 is the first straight beam, and 3 is the variable cross-section beam;

[0031] Figure 1 In diagram A, the structure of a multifunctional negative Poisson's ratio cell is shown in the figure. In diagram B, the reference diagram for the design of the geometric parameters of the cell structure of A is shown in the figure. Figure 5 middle and These are the rotation angles of the cell and the node, respectively; Figure 6 (a) shows the stress-strain curves of the concave hexagonal honeycomb (structure 1), the regular hexagonal honeycomb (structure 2), and the honeycomb structure of this application (structure 3); (b) is a schematic diagram of the concave hexagonal honeycomb structure, where all members have a thickness of 0.8 mm, the length of the inclined members is 8 mm, the angle with the horizontal line is 60°, and the length of the horizontal members is 16 mm; (c) is a schematic diagram of the regular hexagonal honeycomb structure, where the thickness and length of the members are 0.8 mm and 8 mm, respectively; (d) is a schematic diagram of the multifunctional negative Poisson's ratio structure of this invention; Structure 1. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments, but the scope of protection of this application is not limited to these embodiments.

[0033] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0034] See Figure 1 , Figure 2 The structures of the multifunctional negative Poisson's ratio cell and the honeycomb structure of the present invention are shown respectively. The multifunctional negative Poisson's ratio cell structure includes regular hexagonal honeycomb units, which are formed by sequentially connecting first straight beams 2. Second straight beams 1 are connected to the outer ends of adjacent first straight beam connection points. Each second straight beam consists of a straight beam segment and a variable cross-section beam 3. The length of the first straight beam is... l Thickness is t 2 The straight beam section is l1 , the thickness of the first straight beam is t 2 , the variable cross-section beam is arranged to be widened first and then narrowed away from the first straight beam along the straight beam section, the included angle between the widened side of the variable cross-section beam and the axis of the second straight beam is θ, and the length of the variable cross-section beam is l - l 1 , the distance between the end point of the widened side and the end point of the narrowed side of the variable cross-section beam in the axial direction of the second straight beam is l 3 , the cross-section width of the variable cross-section beam is determined by the following function: .

[0035] In order to induce the mode conversion of the structure, that is, the local buckling occurs, and then the negative Poisson's ratio effect of the structure is realized, the topological configuration of the structure is designed. The length, thickness of the first straight beam, the second straight beam and the included angle θ of the variable cross-section beam satisfy the following conditions: ; ; . Through the above related parameter design change, the structure stiffness is also improved while the negative Poisson's ratio appears. In addition to the negative Poisson's ratio effect, this design makes the negative Poisson's ratio honeycomb structure produce large-scale rotation of the cell and the node, see Figure 5 .

[0036] In the above structure design, the thickness of the second straight beam 1 is , the length of the second straight beam 1 is , the thickness of the first straight beam 2 is , the length of the first straight beam 2 is , the included angle of the variable cross-section beam is , and the length of the variable cross-section beam is , which are the main design points of the present application. Among them, the length , , l 3 are the axial lengths.

[0037] On the basis of the regular hexagonal honeycomb geometry, the geometric constraint relationship of the above-mentioned design of the geometric configuration makes the first straight beam 2 and the variable cross-section beam 3 hardly deform under uniaxial compression, and the deformation is concentrated in the second straight beam 1. At the same time, the size of the first straight beam 2 and the variable cross-section beam 3 is greater than that of the second straight beam 1, which enhances the constraint of the two ends of the second straight beam 1, so that the second straight beam 1 is more likely to buckle under compression than to bend and deform. When the structure is compressed beyond the critical value, all the inclined second straight beams 1 occur cooperative buckling, and the horizontal second straight beams 1 also occur bending deformation in the same direction, and the deformation coupling of all the bars makes the structure appear mode conversion phenomenon, which makes the Poisson's ratio of the structure become negative. At the same time, the design of the above-mentioned geometric parameters increases the relative density of the honeycomb structure and improves the stiffness of the structure.

[0038] As one specific embodiment, when , , At that time, the designed multifunctional negative Poisson's ratio honeycomb structure ( Figure 6 The specific stiffness of (d) is respectively that of a traditional regular hexagonal honeycomb ( Figure 6 (c) and traditional concave hexagonal honeycomb ( Figure 6 3.87 times and 4.56 times that of (b). (By) Figure 6 As can be seen from the stress-strain curves of the three structures shown in Figure a, the stiffness of the multifunctional negative Poisson's ratio honeycomb structure of the present invention is significantly improved compared with the other two structures.

[0039] Furthermore, traditional concave hexagonal honeycomb structures are prone to overall buckling under uniaxial compression, such as... Figure 4 As shown, this results in a significant decrease in load-bearing capacity, while the honeycomb structure of this invention (see...) Figure 3 This can effectively avoid the problem.

[0040] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A multifunctional negative Poisson's ratio cell based on mode conversion, characterized in that, The hexagonal honeycomb unit is sequentially connected by first straight beams, each two adjacent first straight beams are connected by a connecting point, and a second straight beam is connected to the outer end of each connecting point; each second straight beam is composed of a straight beam segment and a variable cross-section beam; wherein the length of the first straight beam is l , the thickness is t 2 , the length of the straight beam segment is l 1 , the thickness is t 2 , the variable cross-section beam is arranged to be widened first and then narrowed in the direction away from the first straight beam along the straight beam segment, the included angle between the widened side of the variable cross-section beam and the axis of the second straight beam is θ, and the length of the variable cross-section beam is l - l 1 , the distance between the end point of the widened side and the end point of the narrowed side of the variable cross-section beam in the axial direction of the second straight beam is l 3 , the cross-sectional width of the variable cross-section beam is determined by the following function: ; the length, thickness and included angle θ of the first straight beam and the second straight beam satisfy the following conditions: ; The negative Poisson's ratio cell with the above structure is arranged in a topological array to form a negative Poisson's ratio honeycomb structure, when the honeycomb structure is pressed beyond a critical value, all the inclined second straight beams are cooperatively buckled, and the horizontal second straight beams are also bent in the same direction, the deformation coupling causes a mode conversion phenomenon, and the Poisson's ratio of the honeycomb structure becomes negative.

2. The pattern conversion based multifunctional negative Poisson's ratio cell of claim 1, wherein, t 2 = 3 t 1 ; l = 2 l 1 ; the included angle θ on the variable cross-section beam is 20°.

3. A multifunctional negative Poisson's ratio honeycomb structure based on mode conversion, characterized by, The negative Poisson's ratio honeycomb structure is composed of a plurality of negative Poisson's ratio cells arranged in a topological array, and the topological configuration of the negative Poisson's ratio cell is formed by splicing variable cross-section beams of adjacent cells.

4. The multifunctional negative Poisson's ratio honeycomb structure based on mode conversion according to claim 3, characterized by, The topological configuration of the hexagonal honeycomb structure is changed by adjusting the thickness ratio of the first straight beam and the second straight beam, the axial length of the first straight beam and the included angle θ on the variable cross-section beam.

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