WO2024189849A1

WO2024189849A1 - Auxetic structure and cushioning material

Info

Publication number: WO2024189849A1
Application number: PCT/JP2023/010139
Authority: WO
Inventors: 龍野木村; 陽松下; 光雄葛川
Original assignee: 株式会社フコク
Priority date: 2023-03-15
Filing date: 2023-03-15
Publication date: 2024-09-19

Abstract

An auxetic structure (10) comprising an elastic body (11) having a plurality of gap parts (12), the gap parts each having a shape inscribed to three sides of a virtually set virtual triangle, and the outer peripheries of the gap parts each forming an integrated shape by connecting first recessed parts (A), linear parts (B), protruding parts (C), and second recessed parts (D). The first recessed parts (A) are each formed by an arc of an inscribed circle inscribed to each of two sides located on either side of a vertex of the virtual triangle. The linear parts (B) each form part of the side of the virtual triangle, and connect one end of a first recessed part (A) and one end of a second recessed part (D). The protruding parts (C) are each formed by an arc having a center on a vertical bisector of each side of the virtual triangle and having a radius greater than the radius of an inscribed circle of a first recessed part (A). The second recessed parts (D) each connect one end of a protruding part (C) and one end of a linear part (B), and are formed by an arc of which the center is present inside the virtual triangle.

Description

Auxetic structures and cushioning materials

The present invention relates to an auxetic structure that exhibits macroscopic behavior that has a negative Poisson's ratio, and to a cushioning material that uses the auxetic structure.

General materials have a positive Poisson's ratio, and when the material is compressed in a certain direction, it deforms so that it expands in a direction perpendicular to that direction. However, when multiple voids or openings are distributed in the material to form a structure also called a void structure or cellular solid, the voids may work together to deform under macroscopic stress and strain loads, causing the structure to behave as if it has a negative Poisson's ratio. In other words, when the structure is compressed in a certain direction, it also shrinks in a direction perpendicular to that direction. Structures that behave with a negative Poisson's ratio macroscopically are called auxetic structures, and are used, for example, to relieve stress caused by thermal expansion. Auxetic structures also shrink in a direction perpendicular to a certain direction when subjected to compressive stress from that direction, so they can be used favorably as cushioning materials to relieve mechanical shocks.

Patent Document 1 discloses an auxetic structure in which multiple elongated openings are arranged in a lattice pattern with their longitudinal orientations shifted by 90°. Patent Document 1 gives examples of the shapes of the elongated openings, such as an elongated oval, a concave hole shape, and an H-shape stretched out horizontally.

International Publication No. 2014/197059

In the auxetic structure disclosed in Patent Document 1, the ratio of the length of the long axis to the length of the short axis at the opening, i.e., the aspect ratio, is large, for example, from 5 to 40, so that the volume of the opening is large even when subjected to compressive stress, and there is a problem that the shock absorption capacity is low when used as a cushioning material, etc. Because the aspect ratio is large, the auxetic structure disclosed in Patent Document 1 also has the problem that local strain increases when deformed, reducing durability.

The object of the present invention is to provide an auxetic structure that combines shock absorbing capacity with long-term durability, and a cushioning material that uses such an auxetic structure.

The auxetic structure of one embodiment of the present invention is an auxetic structure made of an elastic body having voids,
the void portion has a shape inscribed in three sides of an imaginary triangle, and the outer periphery of the void portion forms an integrated shape by connecting each of the first recess (A), the straight line portion (B), the convex portion (C) and the second recess (D);
the first recess (A) has a center on each axis of symmetry of the imaginary triangle, is formed by an arc of an inscribed circle inscribed in each of two sides that sandwich a vertex of the imaginary triangle, and both ends of the arc are on the two sides of the imaginary triangle, and has a concave shape that curves outwardly of the imaginary triangle when viewed from the center of gravity of the imaginary triangle,
The straight line portion (B) is a line segment forming a part of each side of the virtual triangle, and connects an end of the first recess (A) and one end of the second recess (D),
the convex portion (C) has a center on the perpendicular bisector of each side of the virtual triangle, is formed by an arc having a radius equal to or greater than the radius of the inscribed circle of the first concave portion (A), and has a convex shape toward the center of gravity of the virtual triangle; the second concave portion (D) is an arc tangent to an end of the convex portion (C) and the other end of the straight portion (B), and further, the center of the arc is inside the virtual triangle, and has a concave shape that curves outward from the virtual triangle when viewed from the center of the arc;
The plurality of voids arranged in the elastic body are arranged such that the centers of gravity of the imaginary triangles in which the voids are inscribed are located at the lattice points of a triangular lattice.

The above-mentioned auxetic structure has a void portion having a shape inscribed in a virtual triangle virtually set in an elastic body, and the void portion has an outer periphery formed by connecting a first concave portion (A) provided at each vertex of the virtual triangle, inscribed in two sides of the virtual triangle, and curved outward from the virtual triangle, a straight portion (B) on a side of the virtual triangle and one end of which is connected to an end of the first concave portion (A), a convex portion (C) made of an arc curving toward the center of gravity of the virtual triangle, and a second concave portion (D) made of an arc connecting the end of the convex portion (C) and the other end of the straight portion (B) to form an integral shape. Thus, the auxetic structure according to the present invention having a void portion having a shape inscribed in a virtual triangle by connecting the first concave portion (A), the straight portion (B), the convex portion (C), and the second concave portion (D) is not limited to a void portion having a shape inscribed in a virtual triangle. For example, an auxetic structure having a void portion having a shape inscribed in a convex rectangle virtually set in an elastic body is also included in the scope of the present invention.

Thus, according to another aspect of the invention, the auxetic structure comprises:
An auxetic structure made of an elastic body having voids,
The gap portion includes a first gap portion and a second gap portion,
the first gap has a shape inscribed in four sides of a virtual rhombus having a first diagonal and a second diagonal perpendicular to each other, and the outer periphery of the first gap forms an integrated shape by connecting each of the first recess (A), the straight line portion (B), the protrusion (C), and the second recess (D);
the first recess (A) has a center on a first diagonal of the virtual rhombus, is formed by an arc of an inscribed circle inscribed in each of two sides that sandwich a vertex that is both ends of the first diagonal of the virtual rhombus, and both ends of the arc of the inscribed circle are on the two sides of the virtual rhombus, and has a concave shape that curves outward when viewed from the intersection of the first diagonal and the second diagonal,
The straight line portion (B) is a line segment forming a part of two sides sandwiching a vertex that is both ends of the first diagonal of the virtual rhombus, and connects an end of the first recess (A) and an end of the second recess (D),
the convex portion (C) has a center on the second diagonal of the virtual rhombus, is formed by an arc having a radius larger than the radius of the inscribed circle of the first concave portion (A), and has a convex shape curved toward an intersection of the first diagonal and the second diagonal of the virtual rhombus,
the second recess (D) is an arc tangent to an end of the protrusion (C) and the other end of the straight line portion (B), the center of the arc is inside the imaginary rhombus, and the second recess (D) has a concave shape that is curved outward from the imaginary rhombus when viewed from the center of the arc,
the second gap portion has a shape obtained by rotating the first gap portion by 90° around an intersection point of the first diagonal line and the second diagonal line of the virtual rhombus,
The first gap and the second gap disposed in the elastic body are arranged in a lattice pattern with the first gap and the second gap serving as one unit.

According to yet another aspect of the present invention, the cushioning material for a battery module is characterized by being made of the above-mentioned auxetic structure.

The invention makes it possible to obtain an auxetic structure that combines shock absorbing capacity with long-term durability, and a cushioning material that uses such an auxetic structure.

FIG. 2A is a plan view illustrating the auxetic structure of the first embodiment, and FIG. 2B is an enlarged view of a void portion formed in the auxetic structure of the first embodiment. FIG. 2 is a diagram illustrating the shape of a void in the auxetic structure shown in FIG. 1 . 1A to 1C are diagrams illustrating deformation when compressive stress is applied to the auxetic structure of the first embodiment. 1A is a plan view illustrating an auxetic structure according to a second embodiment, and FIG. 1B is an enlarged view of a void portion formed in the auxetic structure according to the second embodiment. 5 is a diagram illustrating the shape of a first void portion of the auxetic structure shown in FIG. 4. 11A to 11C are diagrams illustrating deformation when compressive stress is applied to the auxetic structure of the second embodiment. FIG. 2 is a schematic front view of the battery module. FIG. 2A is a plan view illustrating an auxetic structure of a comparative example, and FIG. 2B is an enlarged view of a void portion formed in the auxetic structure of the comparative example. FIG. 1 shows strain distribution in the auxetic structure of Example 1. FIG. 13 shows strain distribution in the auxetic structure of Example 2. FIG. 13 shows strain distribution in an auxetic structure of a comparative example. FIG. 2 is a stress-strain diagram of an auxetic structure.

Next, a form for carrying out the present invention will be described with reference to the drawings. The auxetic structure according to the present invention is constructed by arranging a plurality of voids in a substrate, i.e., an elastic body, made of an elastic material. When a force is applied in a certain direction and the auxetic structure is compressed, it has the property of contracting in a direction different from the direction in which the force is applied, and has a negative Poisson's ratio macroscopically.

[First embodiment]
FIG. 1(a) shows an auxetic structure according to a first embodiment of the present invention, and FIG. 1(b) is an enlarged view of an arbitrary void portion shown in FIG. 1(a). The illustrated auxetic structure 10 is configured by arranging a plurality of void portions 12 in a substrate 11 made of an elastic material. The details of the shape of the void portion 12 are shown in FIG. 2. The void portion 12 may be exposed as an opening on the surface of the substrate 11, or may be formed inside the substrate 11 without being exposed on the surface. When an XY orthogonal coordinate system is defined as shown in FIG. 1, the auxetic structure 10 has the property of shrinking in the X direction when a compressive stress is applied from the outside along the Y direction and compressed.

The shape of the void will be described in detail below with reference to Figures 1(b) and 2. The void 12 has a shape inscribed in three sides of an equilateral triangle, i.e., a virtual triangle 15, which is virtually set in the base material 11, which is an elastic body. The three vertices of the virtual triangle 15 are T1, T2, and T3. The perpendicular lines drawn from the vertices T1, T2, and T3 to the opposing sides are lines L1, L2, and L3, respectively. Here, since the virtual triangle 15 is an equilateral triangle, the lines L1 to L3 pass through the center of gravity G of the virtual triangle 15 and are the symmetry axes of the virtual triangle 15, and the perpendicular lines L1, L2, and L3 are also the perpendicular bisectors of the corresponding sides.

The outline, i.e., the periphery, of the gap 12 in the XY plane is represented by a closed curve, but is divided by 18 points, P1 to P18. As a result, the periphery of the gap 12 is composed of 18 sections connected together to form a single shape. The 18 sections are classified into a first recess A, a straight section B, a convex section C, and a second recess D. In the following explanation, for example, a side, line segment, or arc connecting point U and point V will be represented as U-V.

As shown in FIG. 1(b), the first recess A is a section that is arranged at each vertex of the virtual triangle 15, is defined by an arc of an inscribed circle inscribed in both of the two sides that sandwich the vertex, and curves away from the center of gravity G of the virtual triangle 15. As shown in FIG. 2, for the vertex T1, the arc P18-P1 defined by the circle C1 inscribed in both sides T1-T2 and T3-T1 is the first recess A. Similarly, for the vertex T2, the arc P6-P7 defined by the circle C2 inscribed in both sides T1-T2 and T2-T3 is the first recess A, and for the vertex T3, the arc P12-P13 defined by the circle C3 inscribed in both sides T2-T3 and T3-T1 is the first recess A. In FIG. 2, the centers of the circles C1-C3 are indicated by points O1-O3, respectively. The centers O1 to O3 of the circles C1 to C3 are on the straight lines L1 to L3, which are the axes of symmetry of the imaginary triangle 15. The radii of the circles C1 to C3 are all Ra.

In addition, straight line portion B is a section of a line segment that is arranged at each end of first recess A and has one end connected to the end of first recess A, and this line segment is on a side of imaginary triangle 15. In other words, straight line portion B is a section of a line segment that is part of a side of imaginary triangle 15. For side T1-T2, line segments P1-P2 and P5-P6 are straight line portion B, and both of these line segments are on side T1-T2. Similarly, for side T2-T3, line segments P7-P8 and P11-P12 are straight line portion B, and for side T3-T1, line segments P13-P14 and P17-P18 are straight line portion B.

Furthermore, the convex portion C forms a constriction in the void portion 12 by protruding from the center of each side of the imaginary triangle 15 toward the center of the void portion 12. More specifically, the convex portion C is a section that is arranged on each side of the imaginary triangle 15, is defined by an arc of a circle having a center on the perpendicular bisector of the side and a radius equal to or greater than the radius of the inscribed circle that defines the arc of the first recess A, is located inside the imaginary triangle, and is curved to approach the center of gravity G of the imaginary triangle 15. For side T1-T2, the arc P3-P4 has a center O4 on the straight line L3 that is the perpendicular bisector, and is part of the circumference of the circle C4 with a radius of Rb, and is the convex portion C. Here, Rb ≧ Ra. Similarly, for side T2-T3, the arc P9-P10 has a center O5 on the straight line L1 and is part of the circumference of the circle C5 with a radius of Rb, and is the convex portion C. Similarly, for side T3-T1, arc P15-P16, which is part of the circumference of circle C6 with center O6 on line L2 and radius Rb, is convex portion C.

In addition, the second recess D is a section defined by arcs that are arranged at both ends of the convex portion C for each side of the imaginary triangle 15 and smoothly connect the ends of the convex portion C to the other end of the straight portion B. Smooth connection here means that no bending occurs at the connection position, that is, the formed arcs are in contact with each other at their respective ends. With respect to side T1-T2, arcs P2-P3 and P4-P5 correspond to the second recess D. Arc P2-P3 is an arc of circle C7 whose center is point O7. This circle C7 is tangent to side T1-T2 at point P2, and has a common tangent with circle C4 at point P3. Therefore, arc P2-P3 smoothly connects to line segment P1-P2, which is straight portion B, and also smoothly connects to arc P3-P4, which is convex portion C. Similarly, the arc P4-P5 is an arc of the circle C8 whose center is the point O8, and smoothly connects to the arc P3-P4 and the line segment P5-P6. On the side T2-T3, the arc P8-P9 of the circle C9 whose center is the point O9 and the arc P10-P11 of the circle C10 whose center is the point O10 correspond to the second recess D. The arc P8-P9 smoothly connects to both the line segment P7-P8 and the arc P9-P10, and the arc P10-P11 smoothly connects to both the arc P9-P10 and the line segment P11-P12. On the side T3-T1, the arc P14-P15 of the circle C11 whose center is the point O11 and the arc P16-P17 of the circle C12 whose center is the point O12 correspond to the second recess D. The arc P14-P15 smoothly connects to both the line segment P13-P14 and the arc P15-P16, and the arc P16-P17 smoothly connects to both the arc P15-P16 and the line segment P17-P18. The radii of the circles C7 to C12 are all Rc.

Next, the arrangement of the voids 12 in the base material 11, which is an elastic body, will be described with reference to Figures 1(a) and 2. As shown in Figure 2, a regular triangular lattice is set in the base material 11, and each void 12 is arranged so that the perpendicular lines, i.e., the straight lines L1 to L3, drawn from each vertex of the imaginary triangle 15 that defines the voids 12 to the opposing sides are parallel to the lattice lines of the regular triangular lattice. Also, as shown in Figure 1(a), the distance K between the centers of gravity G of adjacent voids 12 must be greater than 2/3 of the height H of the imaginary triangle 15, as shown in Figure 2. When the distance K is 2/3 or less of the height H, the adjacent voids 12 overlap each other, and the condition that the voids 12 are inscribed in the imaginary triangle 15 is not satisfied. On the other hand, when the distance K exceeds the height H, the distance between the adjacent voids 12 becomes too large, and when a compressive stress is applied to the auxetic structure 10, the deformation obtained by the cooperation of the voids 12 may not be performed smoothly. Therefore, it is preferable that K≦H.

Figure 3 shows the change in shape of the auxetic structure 10 of the first embodiment when compressive stress is applied in the Y direction. In Figure 3, (a) shows the shape of the auxetic structure 10 when no compressive stress is applied, and (b) shows the shape when compressive stress is applied. As shown in the figure, when compressive stress is applied along the Y direction of the auxetic structure 10, the voids 12 are crushed, and the auxetic structure 10 also shrinks in the X direction as a result. When the compressive stress is removed, the shape of the auxetic structure 10 returns to its original state.

In this way, in the auxetic structure 10 of the first embodiment, by providing the straight portion B on the outer periphery of the void portion 12, when a compressive stress is applied from the outside, the structure 10 is deformed without local stress strain occurring in the auxetic structure 10. This indicates that the shock absorption capacity of the auxetic structure 10 is high because the local strain is small. It is also possible to directly connect the straight portion B and the convex portion C without providing the second concave portion D, but in that case, the strain at the connection point between the straight portion B and the convex portion C may become large, and the durability of the auxetic structure may deteriorate. Therefore, in this embodiment, the second concave portion D is interposed between the straight portion B and the convex portion C to reduce the local strain and increase the durability of the auxetic structure 10. Note that in the first embodiment of the present invention, the fact that the local strain is small when a compressive stress is applied to the auxetic structure 10 will be described in detail later using Figures 9 to 11. Details of the shock absorption capacity will also be described later.

As described above, the virtual triangle 15 has been described as being an equilateral triangle. However, according to the mathematical definition, the interior angles of each vertex of an equilateral triangle are all strictly 60°. However, in the present invention, the virtual triangle 15 may have a shape that deviates slightly from a strictly equilateral triangle, so long as the structure obtained by disposing the voids 12 in the substrate 11 exhibits macroscopic behavior of a negative Poisson's ratio. For example, if the interior angles at each vertex of the virtual triangle 15 are in the range of 50° to 70°, the virtual triangle 15 can be treated as being an equilateral triangle to form the auxetic structure 10 of the first embodiment.

Second Embodiment
FIG. 4(a) shows an auxetic structure according to a second embodiment of the present invention, and FIG. 4(b) is an enlarged view of an arbitrary void portion described in FIG. 4(a). The illustrated auxetic structure 20 is a structure in which a plurality of first void portions 22 and a plurality of second void portions 23 are arranged in a substrate 21 made of an elastic material, as in the first embodiment. Details of the shape of the first void portion 22 are shown in FIG. 5. The second void portion 23 is a simple rotation of the first void portion 22 in the substrate 21, which is an elastic body, so a detailed description of the shape of the second void portion 23 is omitted. Note that the first void portion 22 and the second void portion 23 may be exposed as openings on the surface of the substrate 21, or may be formed inside the substrate 21. When an XY orthogonal coordinate system is defined as shown in FIG. 4, the auxetic structure 20 of the second embodiment has the property of shrinking in the X direction as well when a compressive stress is applied from the outside along the Y direction and compressed.

The shapes of the

voids

22 and 23 will be described in detail below with reference to Fig. 4(b) and Fig. 5. As shown in Fig. 5, the first void 22 has a shape inscribed in the four sides of a rhombus, i.e., a virtual rhombus 25, which is virtually set in the substrate 21. The four vertices of the virtual rhombus 25 are S1, S2, S3, and S4. Of the two diagonals of the virtual rhombus 25, the diagonal connecting the vertices S1 and S3 is the first diagonal L11, and the diagonal connecting the vertices S2 and S4 is the second diagonal L12. Since it is a rhombus, the first diagonal L11 and the second diagonal L12 are perpendicular to each other, and the intersection point is M. The intersection point M is also the center of gravity of the virtual rhombus 15. The second void 23 has a shape obtained by rotating the first void 22 by 90° around the intersection point M in the XY plane.

The outline, i.e., the periphery, of the first void portion 22 in the XY plane is expressed by a closed curve, but is divided by 12 points Q1 to Q12. As a result, the periphery of the first void portion 22 is configured as an integral shape with 12 sections connected together. As shown in FIG. 4(b), the 12 sections are classified into a first recess A, a straight section B, a convex section C, and a second recess D, as in the first embodiment.

The first recess A is disposed at each of the vertices S1 and S3 located at both ends of the first diagonal line L11, and is a section that is an arc of an inscribed circle inscribed in both of the two sides that sandwich the vertex, and both ends are defined by arcs on the two sides, and curves away from the intersection point M. With respect to the vertex S1, the arc Q12-Q1 defined by the circle C21 inscribed in both sides S1-S2 and S1-S4 is the first recess A. Similarly, with respect to the vertex S3, the arc Q6-Q7 defined by the circle C22 inscribed in both sides S2-S3 and S3-S4 is the first recess A. In FIG. 5, the centers of the circles C21 and C22 are indicated by points O21 and O22, respectively. The radii of the circles C21 and C22 are both Ra.

Straight line portion B is a section of a line segment that is arranged at each end of first recess A and has one end connected to the end of first recess A, and this line segment is on a side of imaginary rhombus 25. In other words, straight line portion B is a section of a line segment that is part of a side of imaginary rhombus 25. For side S1-S2, line segment Q1-Q2 is straight line portion B, and this line segment is on side S1-S2. Similarly, for side S2-S3, line segment Q5-Q6 is straight line portion B, for side S3-S4, line segment Q7-Q8 is straight line portion B, and for side S4-S1, line segment Q11-Q12 is straight line portion B.

The convex portion C forms a convex portion in the first gap portion 22 by protruding toward the center of the first gap portion 22 along the second diagonal line L12. More specifically, the convex portion C is a section that is located on both sides of the first diagonal line L11, is defined by an arc of a circle having a center on the second diagonal line L12 or an extension thereof and having a radius equal to or greater than the radius of the inscribed circle that defines the arc of the first recess portion A, is located inside the imaginary rhombus 25, and curves (=convex portion) to approach the intersection point M. In the region to the left of the first diagonal line L11 in FIG. 5, the convex portion C is the arc Q3-Q4 that is part of the circumference of a circle C23 that has a center O23 on the second diagonal line L12 at a position to the left of the intersection point M and has a radius Rb. Here, Rb ≧ Ra. Similarly, in the region to the right of the first diagonal L11, the arc Q9-Q10, which is part of the circumference of a circle C24 with a radius Rb and a center O24 on the second diagonal L12 to the right of the intersection point M, is the convex portion C.

The second recess D is a section defined by arcs arranged on each side of the imaginary rhombus 25 and tangent to the end of the convex portion C and the other end of the straight portion B. For side S1-S2, arc Q2-Q3 corresponds to the second recess D. Arc Q2-Q3 is an arc of circle C25 whose center is point O25. This circle C25 is tangent to side S1-S2 at point Q2 and has a common tangent with circle C23 at point Q3. Therefore, arc Q2-Q3 smoothly connects to line segment Q1-Q2, which is straight portion B, and also smoothly connects to arc Q3-Q4, which is convex portion C. For side S2-S3, arc Q4-Q5 of circle C26 whose center is point O26 corresponds to the second recess D. Arc Q4-Q5 smoothly connects to both arc Q3-Q4 and line segment Q5-Q6. On side S3-S4, arc Q8-Q9 of circle C27, whose center is point O27, corresponds to second recess D. Arc Q8-Q9 smoothly connects to both line segment Q7-Q8 and arc Q9-Q10. On side S4-S1, arc Q10-Q11 of circle C28, whose center is point O28, corresponds to second recess D. Arc Q10-Q11 smoothly connects to both arc Q9-Q10 and line segment Q11-Q12. The radii of circles C25 to C28 are all Rc.

Next, the arrangement of the first voids 22 and the second voids 23 in the substrate 21 will be described. As described above, the second voids 23 have a shape obtained by rotating the first voids 22 by 90° around the intersection point M of the first diagonal line L11 and the second diagonal line L12 in the first voids 22, that is, a shape obtained by turning the first voids 22 on its side. Since the intersection point M is the center of rotation, the intersection point M of the second voids 23 can also be determined as the intersection point of a pair of diagonals in the virtual rhombus. In the auxetic structure 20, a plurality of first voids 22 and a plurality of second voids 23 are arranged in a lattice pattern so that the first voids 22 and the second voids 23 are alternately arranged in two mutually orthogonal directions, specifically, in the X direction and the Y direction, in the substrate 21, which is an elastic body.

As shown in FIG. 4(b), when the dimensions of the first gap 22 are expressed by the length h and the maximum width w, the length h is the length of the first gap 22 measured along the first diagonal L11, specifically, the distance from the intersection of the arc Q12-Q1, which is one of the first recesses A, and the first diagonal L11 to the intersection L12 of the arc Q6-Q7, which is the other first recess A, and the first diagonal. The maximum width w is the maximum value of the dimension of the first gap 22 in the direction perpendicular to the first diagonal L11 (i.e., the direction in which the second diagonal L12 extends). Since the second recess D is the one that bulges the most in the direction in which the second diagonal L12 extends in the first gap 22, the maximum width w is the length of the longest line segment connecting the second recesses D that are located in line symmetrical positions with respect to the first diagonal L11.

In this way, by defining the length h and the maximum width w, the ratio h/w can be determined. This ratio h/w is a parameter equivalent to the aspect ratio mentioned in Patent Document 1, except that the ratio h/w can be less than 1. In this embodiment, the ratio h/w is preferably 1 or more and 1.5 or less. If the ratio h/w is less than 1, when compressive stress is applied to the auxetic structure 20, the deformation obtained by the cooperation of the

voids

22 and 23 does not occur smoothly. On the other hand, if the ratio h/w exceeds 1.5, when compressive stress is applied to the auxetic structure 20, the difference in the amount of deformation between the

voids

22 and 23 becomes large, and one void 23 is easily crushed while the other void 22 is not easily crushed, resulting in a problem that the auxetic structure 20 cannot deform uniformly and the shock absorption capacity is reduced. The aspect ratio of the auxetic structure described in Patent Document 1 is 5 to 20, but in comparison, the aspect ratio of the auxetic structure 20 of this embodiment is sufficiently small at 1 to 1.5. Therefore, in the auxetic structure 20 of this embodiment, when a compressive stress is applied to the structure 20, the

voids

22 and 23 can each deform uniformly, so that the structure 20 also deforms uniformly, improving the shock absorption capacity.

Furthermore, it is preferable that the central angle θ of the arcs (arc Q12-Q1 and arc Q6-Q7) constituting the first recess A is 70° or more and 100° or less. If the central angle θ is less than 70°, when compressive stress is applied to the auxetic structure 20, the deformation obtained by the cooperation of the

voids

22, 23 may not occur smoothly. On the other hand, if the central angle θ exceeds 100°, the auxetic structure 20 cannot deform uniformly, and the shock absorption capacity decreases.

In addition, if the distance between the intersections M of adjacent first and

second voids

22 and 23 in the auxetic structure 20 is L, then the distance L must be greater than the maximum width w. If the distance L is less than or equal to the maximum width w, the adjacent first and

second voids

22 and 23 will overlap and merge with each other, and the condition that the first void 22 is inscribed in the imaginary rhombus 25 will no longer be satisfied. On the other hand, if the distance L exceeds the length h, the distance between the adjacent first and

second voids

22 and 23 will become too large, and the first and

second voids

22 and 23 will not be able to deform in cooperation with each other. Therefore, it is preferable that L≦h.

FIG. 6 shows the change in shape when compressive stress is applied in the Y direction in the auxetic structure 20 of the second embodiment. In FIG. 6, (a) shows the shape of the auxetic structure 20 when no compressive stress is applied, and (b) shows the shape when compressive stress is applied. As shown in the figure, when compressive stress is applied in the Y direction, the first void portion 22 and the second void portion 23 contract, and the auxetic structure 20 contracts in the X direction accordingly. When the compressive stress is removed, the auxetic structure 20 returns to its original shape. Note that when compressive stress is applied to the auxetic structure 20 in the X direction, the structure 20 contracts in the Y direction. This can be easily understood because the structure 20 has a negative Poisson's ratio.

As described above, in the auxetic structure 20 of the second embodiment, similarly to the auxetic structure 10 of the first embodiment, by providing straight portion B on the outer periphery of the first void portion 22 and the second void portion 23, when compressive stress is applied from the outside, the auxetic structure 20 deforms without local stress strain. This indicates that the shock absorption capacity of the auxetic structure 20 is high because the local strain is small. Here, it is also possible to directly connect the straight portion B and the convex portion C without providing the second concave portion D, but in this case, as in the case described in the first embodiment, strain may increase at the connection point between the straight portion B and the convex portion C, resulting in a deterioration in durability.

According to the mathematical definition, a rhombus is a quadrilateral with four sides of equal length, but in the present invention, the lengths of the four sides of the virtual rhombus 25 may vary somewhat as long as the structure obtained by disposing the voids 12 in the substrate 11 exhibits macroscopic behavior with a negative Poisson's ratio. For example, if a quadrilateral has a reference length and the length of each side is within a range of ±10% of the reference length, the quadrilateral can be used as the virtual rhombus 25 to form the auxetic structure 20 of the second embodiment.

[Materials constituting the elastic body]
Next, an elastic body that can be used in the auxetic structure according to the present invention will be described. In the auxetic structure according to the present invention, including the

auxetic structures

10 and 20 of the above-mentioned embodiments, any material having elasticity, such as metal, ceramics, or resin, can be used as the elastic body. Among them, it is preferable to use a resin material or an elastomer material exhibiting high elasticity as the elastic body.

Resin materials that can be used as the elastic body include general-purpose plastics such as polyethylene (PE), polypropylene (PP), ABS resin (ABS), polyethylene terephthalate (PET), and methacrylic resin (PMMA), as well as general-purpose engineering plastics such as polyamide (PA), polycarbonate (PC), polyacetal (POM), polybutylene terephthalate (PBT), polyphenylene ether (PPE), and epoxy resin (EP). In addition, super engineering plastics such as fluororesin, polyimide (PI), polyethersulfone (PES), and polyetherimide (PEI) may also be used.

Elastomers that can be used as the elastic body include thermosetting elastomers such as silicone rubber (Q), urethane rubber (U), natural rubber (NR), ethylene propylene diene rubber (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), chloroprene rubber (CR), and butyl rubber (isobutylene-isoprene rubber) (IIR), as well as thermoplastic elastomers such as polystyrene thermoplastic elastomer (TPS), olefin thermoplastic elastomer (TPO) (also called alkene thermoplastic elastomer), polyvinyl chloride thermoplastic elastomer (TPVC), polyurethane thermoplastic elastomer (TPU), polyester thermoplastic elastomer (TPC), and polyamide thermoplastic elastomer (TPAE).

[Method of manufacturing auxetic structure]
The auxetic structure according to the present invention is made of an elastic body having a plurality of voids, and can be manufactured by dispersing and forming a plurality of voids in a base material that is an elastic body. Specifically, it can be manufactured by laser cutting or water jet cutting, or by molding using a mold or a 3D (three-dimensional) printer, similar to the molding of general resin materials and elastomer materials. The manufacturing method of the auxetic structure is not limited to the method described here, and any manufacturing method may be used as long as it is a method that can form voids in a dispersed manner.

[Application examples of auxetic structures]
Hereinafter, as an application example of the auxetic structure according to the present invention, an example in which the auxetic structure is applied to a cushioning material for a battery module will be described.

FIG. 7 is a schematic front view showing a battery module 40. The battery module 40 is an integrated device in which a plurality of battery cells 45 are held by a restraining member 41 so as to obtain a desired output voltage and capacity. The battery cells 45 are unit cells of a rechargeable secondary battery, and preferably unit cells of a lithium-ion battery. In the battery module 40, the battery cells 45 and the cushioning material 50 are alternately stacked along the X direction shown in the figure, and the restraining member 41 restrains each battery cell 45 from both ends of the stacking direction (X direction shown in the figure). As the restraining member 41, an L-shaped member or a case-shaped member is used as shown in the figure. The material of the restraining member 41 is a metal such as aluminum, but a hard resin such as polyamide-imide resin (PAI) or polyether-ether-ketone resin (PEEK) may also be used. .

In the illustrated example, four battery cells 45 are provided, and the internal structure of the second battery cell 45 from the left is shown. The battery cell 45 is, for example, a metal case 45a with an electrode body 45b sealed inside. The electrode body 45b is a spirally wound battery cell 45 with a positive electrode and a negative electrode sandwiched between separators (not shown). When the battery cell 45 is charged, the negative electrode in the electrode body 45b absorbs ions and expands, and at the same time, the electrode body 45b generates heat and expands thermally. For this reason, the battery cell 45 expands mainly in the stacking direction, that is, in the X direction in FIG. 7, when the battery cell 45 is charged, and the amount of expansion of the battery cell 45 is maximized when the battery cell 45 is fully charged. When the battery cell 45 is discharged, the battery cell 45 returns to its initial state. However, when the battery cell 45 is repeatedly charged and discharged, the distance between the electrodes of the electrode body 45b increases, and the electrical characteristics of the battery cell 45 deteriorate.

Therefore, the cushioning material 50 arranged between the battery cells 45 or between the battery cells 45 and the restraining member 41 is required to be able to absorb the deformation caused by the expansion and contraction (return to the initial state) of the battery cells 45. Furthermore, when the cushioning material 50 is compressed due to the expansion of the battery cells 45, if the reaction force of the cushioning material 50 becomes excessively large, the battery cells 45 may be damaged. The auxetic structure based on the present invention has a small change in stress (or reaction force) relative to the amount of deformation (or strain), so it can be suitably used as the cushioning material 50 for the battery module 40. In particular, when the number of battery cells 45 constituting the battery module 40 is large, the change in volume caused by the expansion and contraction of the battery cells 45 becomes large, so it is particularly preferable to use the cushioning material 50 made of the auxetic structure based on the present invention in order to reliably absorb the volume change of the battery cells 45. Note that the auxetic structure 20 described in the second embodiment of the present invention is used for the cushioning material 50 shown in FIG. 7.

The present invention will be described in detail based on examples and comparative examples. In the examples and comparative examples, the strain distribution was evaluated by simulation, and the relationship between the compression ratio and stress was measured using an actually fabricated auxetic structure, and the impact absorption capacity was evaluated. A 3D printer (manufactured by Formlabs, product name: Form 3L) was used to fabricate the auxetic structure, and an ultraviolet (UV) curable silicone-based elastomer (manufactured by Formlabs, product name: Elastic 50A) was used as the elastic material. The specifications of the fabricated auxetic structure are described below.

In Example 1, the auxetic structure 10 of the first embodiment was used, and in Example 2, the auxetic structure 20 of the second embodiment was used. Furthermore, as a comparative example, an auxetic structure 60 having the shape shown in FIG. 8 was used. The auxetic structure 60 of the comparative example is, like the auxetic structure 20 of the second embodiment, an elastic base body 61 in which first voids 62 and second voids 63 are alternately arranged in a lattice pattern. However, the auxetic structure 60 of the comparative example differs from the auxetic structure 20 of the second embodiment in the shape of the first voids 62. Specifically, the first void of the comparative example does not have a straight line portion B and a second recessed portion D, and has a shape in which a first recessed portion A consisting of an arc with a radius Ra and a convex portion C consisting of an arc with a radius Rb are directly connected, and is an auxetic structure having voids as shown in Patent Document 1.

Table 1 shows the dimensions of the auxetic structures of Example 1, Example 2 and Comparative Example, the distance between voids, and parameters indicating the shape of each void. Here, the height of the auxetic structure is the overall length of the auxetic structure along the Y direction in Figures 1, 4 and 8, and similarly the width is the overall length of the auxetic structure along the X direction. Furthermore, the thickness of the auxetic structure is the thickness in a direction perpendicular to both the X and Y directions. Furthermore, the distance between voids is the distance K between adjacent voids 12 shown in Figure 1 in Example 1, and is the distance L between adjacent

first voids

22, 62 and

second voids

23, 63 shown in Figures 4 and 8 in Example 2 and Comparative Example. The parameters that indicate the shape of each void are the height H of the virtual triangle 15, the height h and maximum width w of the

first voids

22 and 62, the radius Ra of the circle corresponding to the arc that defines the first recess A, the radius Rb of the circle corresponding to the arc that defines the protrusion B, and the radius Rc of the circle corresponding to the arc that defines the second recess D.

The evaluation results will be explained below.
[Strain distribution evaluation]
The simulation software used was Abaqus/CAE 2020 from Dassault Systems, Inc., and the distribution of strain in the auxetic structure when compressed by 20% was analyzed. The analysis results of the strain distribution in Example 1 are shown in FIG. 9, the analysis results in Example 2 in FIG. 10, and the analysis results in the comparative example in FIG. 11. In FIG. 9 to FIG. 11, the greater the strain, the lower the brightness, and the smaller the strain, the higher the brightness.

From Figures 9 to 11, it can be seen that in the auxetic structure of the comparative example, the strain is particularly large near the first recess A and near the convex C of each void, and the strain is also large throughout the structure. In contrast, in the auxetic structures of Examples 1 and 2 based on the present invention, the strain is small overall, and the strain is particularly small near the first recess A and near the convex C compared to the comparative example. Furthermore, when comparing Example 1 and Example 2, it can be seen that Example 1 has smaller variation in strain, and that the strain is distributed evenly throughout the auxetic structure.

[Evaluation of impact absorption capacity]
The shock absorbing capacity of the auxetic structure was measured under the following test conditions, and the relationship between the compression rate (%) and the stress (kPa) was used to confirm whether the structure had a high shock absorbing capacity, i.e., whether low stress was achieved even with a high compression rate. Note that the stress was measured using the surface pressure during compression.

(Test conditions)
Compression tester: Universal material testing machine (Instron, model number: 5585H)
Compression conditions: 0.5 to 30 ± 5%
Test speed: 1 mm/min
・Compression measurements: 3 times

Figure 12 shows stress-strain diagrams (FS diagrams) for each of Example 1, Example 2, and Comparative Example. As shown in Figure 12, in the auxetic structure of the comparative example, stress increases gradually in the region where the compression ratio is 20% or less, and a rapid increase in stress is observed in the region where the compression ratio is 20% or more. In contrast, in the auxetic structure of Example 1, the stress increases gradually up to a compression ratio of approximately 30%. Furthermore, in the auxetic structure of Example 2, in the region where the compression ratio is 5% or more and 20% or less, there is almost no increase in stress relative to the compression ratio. Furthermore, even in the region where the compression ratio is 20% or more, the increase in stress relative to the compression ratio is gradual.

If the average stress in the region where the compression ratio is 5% or more and 10% or less is s1, the average stress in the region where the compression ratio is 25% or more and 30% or less is s2, and s2/s1 is the shock absorption capacity index, the auxetic structure has a shock absorption capacity index s2/s1 of 1≦s2/s1≦3 (1)
It is preferable to satisfy the above. If the shock absorption capacity index s2/s1 is within the range defined by formula (1), the auxetic structure can be evaluated as having good shock absorption capacity. If the shock absorption capacity index is smaller than 1, the structure will break when an impact is input. On the other hand, if the shock absorption capacity index is larger than 3, the input impact will be transmitted without being absorbed. In other words, the shock absorption capacity will not be exerted. Table 2 shows the average stresses s1, s2 and the shock absorption capacity index s2/s1 actually measured in the auxetic structures of Example 1, Example 2, and Comparative Example.

As shown in Table 2, compared to the auxetic structure of the comparative example, the auxetic structures of Examples 1 and 2 have shock absorption capacity indices s2/s1 within the range defined by formula (1), and therefore exhibit good shock absorption capacity.

From the results of each of the examples and comparative examples described above, it can be seen that the auxetic structure based on the present invention can increase the shock absorption capacity and alleviate local strain compared to auxetic structures of the prior art. For this reason, it goes without saying that the auxetic structure of the present invention can be applied to fields where auxetic structures have not been applicable until now due to concerns about long-term durability, such as cushioning materials for battery modules, insulating materials for battery modules, vibration-proofing materials for electric vehicles, shock-absorbing flooring materials, etc.

10, 20, 60

Auxetic structure

11, 21, 61

Substrate

12, 22, 23, 62, 63 Vacant portion 15 Virtual triangle 16 Virtual rhombus 40 Battery module 45 Battery cell 50 Cushioning material A First recess B Straight portion C Convex portion D Second recess

Claims

An auxetic structure made of an elastic body having voids,
the void portion has a shape inscribed in three sides of an imaginary triangle, and the outer periphery of the void portion forms an integrated shape by connecting each of the first recess (A), the straight line portion (B), the convex portion (C) and the second recess (D);
the first recess (A) has a center on each axis of symmetry of the imaginary triangle, is formed by an arc of an inscribed circle inscribed in each of two sides that sandwich a vertex of the imaginary triangle, and both ends of the arc are on the two sides of the imaginary triangle, and has a concave shape that curves outwardly of the imaginary triangle when viewed from the center of gravity of the imaginary triangle,
The straight line portion (B) is a line segment forming a part of each side of the virtual triangle, and connects an end of the first recess (A) and one end of the second recess (D),
the convex portion (C) has a center on the perpendicular bisector of each side of the imaginary triangle, is formed by an arc having a radius equal to or greater than the radius of the inscribed circle of the first concave portion (A), and has a convex shape toward the center of gravity of the imaginary triangle,
the second recess (D) is an arc tangent to an end of the protrusion (C) and the other end of the straight line portion (B), the center of the arc is inside the imaginary triangle, and the second recess (D) has a concave shape that is curved outwardly of the imaginary triangle as viewed from the center of the arc,
An auxetic structure in which the multiple voids arranged in the elastic body are arranged such that the centers of gravity of the virtual triangles in which the voids are inscribed are positioned at the lattice points of a triangular lattice.
The auxetic structure of claim 1, wherein the distance between the plurality of voids arranged at the lattice points of the triangular lattice is equal to the distance between the centers of gravity of the virtual triangles inscribed by the voids, and the distance is greater than 2/3 of the height of the virtual triangles and less than or equal to the height of the virtual triangles.
An auxetic structure made of an elastic body having voids,
The gap portion includes a first gap portion and a second gap portion,
the first gap has a shape inscribed in four sides of a virtual rhombus having a first diagonal and a second diagonal perpendicular to each other, and the outer periphery of the first gap forms an integrated shape by connecting each of the first recess (A), the straight line portion (B), the protrusion (C), and the second recess (D);
the first recess (A) has a center on a first diagonal of the virtual rhombus, is formed by an arc of an inscribed circle inscribed in each of two sides that sandwich a vertex that is both ends of the first diagonal of the virtual rhombus, and both ends of the arc of the inscribed circle are on the two sides of the virtual rhombus, and has a concave shape that curves outward when viewed from the intersection of the first diagonal and the second diagonal,
The straight line portion (B) is a line segment forming a part of two sides sandwiching a vertex that is both ends of the first diagonal of the virtual rhombus, and connects an end of the first recess (A) and an end of the second recess (D),
the convex portion (C) has a center on the second diagonal of the virtual rhombus, is formed by an arc having a radius larger than the radius of the inscribed circle of the first concave portion (A), and has a convex shape curved toward an intersection of the first diagonal and the second diagonal of the virtual rhombus,
the second recess (D) is an arc tangent to an end of the protrusion (C) and the other end of the straight line portion (B), the center of the arc is inside the imaginary rhombus, and the second recess (D) has a concave shape that is curved outwardly of the imaginary rhombus when viewed from the center of the arc,
the second gap portion has a shape obtained by rotating the first gap portion by 90° around an intersection point of the first diagonal line and the second diagonal line of the virtual rhombus,
An auxetic structure, wherein the first void portion and the second void portion arranged in the elastic body are arranged in a lattice pattern with the first void portion and the second void portion as one unit.
When the length of the first gap portion is h and the maximum width of the first gap portion is w,
1.0≦h/w≦1.5
The auxetic structure according to claim 3, which satisfies:
A distance L between the intersections of the first diagonal and the second diagonal of the virtual rhombus in the first gap portion and the second gap portion adjacent to each other is
w<L≦h
The auxetic structure according to claim 4, which satisfies:
When compressive stress is applied to the auxetic structure, the average stress in the region where the compression rate is 5% or more and 10% or less is defined as s1, and the average stress in the region where the compression rate is 25% or more and 30% or less is defined as s2.
1≦s2/s1≦3
The auxetic structure according to any one of claims 1 to 5, which satisfies the above.
A cushioning material for a battery module, comprising the auxetic structure according to any one of claims 1 to 5.