Planar structures

Abstract

To provide a technique capable of adjusting a planner structural body being deformable into a plane form and a three-dimensional form into three-dimensional shape aiming at a whole planner structural body and capable of easily holding the three-dimensional shape.SOLUTION: A planner structural body comprises: a plurality of surface parts 12; and a folding line 14 that is provided at boundaries of the plurality of surface parts 12 and at least partially curved. Each of the surface parts 12 is anisotropic in bending rigidity by linearly extending from the folding line 14 and having a plurality of hollow parts in closed cross-sectional shape. The plurality of hollow parts forms linear elements 26 when the planner structural body is folded along a curve. In the whole variable range area of the folding angle on the folding line 14, the planner structural body can be folded along a curve without crossing all of the surface parts 12 while maintaining the linear elements 26, and maintaining the linear elements 26 formed by the hollow parts adjacent to each other in the positional relation without torsion, in all of the surface parts 12.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a planar structure that can be transformed into a planar form and a three-dimensional form. [Background technology]

[0002] Patent Document 1 discloses a blank as a planar structure that can be transformed into a planar shape and a three-dimensional shape. The blank is generally flat when in a planar shape and container-shaped when in a three-dimensional shape. The blank mainly comprises a rectangular bottom, a pair of long side pieces that are continuous with the long sides of the bottom via fold lines, and a pair of short side pieces that are continuous with the short sides of the bottom via fold lines. When transforming the blank from a planar shape to a three-dimensional shape, a transformation process is performed in which the pair of short side pieces are individually folded relative to the bottom, and then the pair of long side pieces are individually folded relative to the bottom. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 54-078283 Summary of the Invention [Problem to be solved by the invention]

[0004] The technology disclosed in Patent Document 1 does not allow for adjustment of the overall shape of a planar structure in a three-dimensional form. The inventors of the present application have recognized that there is room for improvement in the technology disclosed in Patent Document 1 in terms of adjusting the three-dimensional shape of the entire planar structure and then maintaining the targeted three-dimensional shape.

[0005] One of the objectives of the present disclosure is to provide a technology that allows a planar structure that can be transformed into a planar form and a three-dimensional form to be adjusted as a whole to a desired three-dimensional shape and to easily maintain that three-dimensional shape. [Means for solving the problem]

[0006] The planar structure of the present disclosure comprises a plurality of surface portions and fold lines provided at the boundaries of the plurality of surface portions and curved in at least a portion thereof, each of the plurality of surface portions having a plurality of hollow portions extending linearly from the fold line and forming a closed cross-sectional shape, thereby giving the structure anisotropy in bending rigidity, and the plurality of hollow portions forming straight elements when the planar structure is curved, and the planar structure can perform the curved folding without crossing all of the surface portions, while maintaining the straight elements in all of the surface portions throughout the entire range of variable folding angles at the fold lines and maintaining the straight elements formed by adjacent hollow portions in a twist-free positional relationship. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 2 is a perspective view of a planar structure of the embodiment. [Figure 2] FIG. 2 is a view of the planar structure of FIG. 1 as seen from one side in the thickness direction. [Figure 3] 3 is a cross-sectional view taken along the line AA in FIG. 2. [Figure 4] FIG. 2 is a perspective view of a planar structure in a three-dimensional form according to an embodiment. [Figure 5] 10A and 10B are diagrams for explaining how the planar structure of the embodiment undergoes bending deformation. [Figure 6] 6A to 6C are schematic diagrams showing the states before and after curved surface folding in the BB cross section of FIG. 5. [Figure 7] 6A to 6C are schematic diagrams showing the states before and after curved surface folding in the CC cross section of FIG. 5. [Figure 8] 2A to 2C are schematic diagrams showing the states before and after curved surface folding in the DD cross section of FIG. 1. [Figure 9] FIG. 10 is a perspective view of a planar structure in a first modified form. [Figure 10] FIG. 10 is a view of a planar structure of a first modified embodiment as viewed from one side in the thickness direction. [Figure 11] FIG. 10 is a perspective view of a planar structure in a three-dimensional form of a first modified embodiment. [Figure 12]10A and 10B are diagrams illustrating how the planar structure of the first modified embodiment undergoes bending deformation. [Figure 13] FIG. 4 is a cross-sectional view of a planar structure of a second modified embodiment, seen from the same viewpoint as in FIG. 3. [Figure 14A] FIG. 10 is a partial cross-sectional view of a planar structure of a third modified embodiment. [Figure 14B] FIG. 10 is a partial cross-sectional view of a planar structure of a fourth modified embodiment. [Figure 15A] FIG. 10 is a partial cross-sectional view of a planar structure of a fifth modified embodiment. [Figure 15B] FIG. 10 is a partial cross-sectional view of a planar structure of a sixth modified embodiment. [Figure 15C] FIG. 11 is a partial cross-sectional view of a planar structure of a seventh modified embodiment. [Figure 16] FIG. 11 is a view of a planar structure of an eighth modified embodiment, seen from the same viewpoint as in FIG. [Figure 17A] FIG. 13 is a partial cross-sectional view of a planar structure of a ninth modified embodiment. [Figure 17B] FIG. 19 is a partial cross-sectional view of a planar structure of a tenth modified embodiment. [Figure 18] FIG. 4 is a diagram showing a state in which the planar structure of FIG. 3 is bent and deformed. [Figure 19] FIG. 1(a) is a front view showing a jig used to measure the curvature retention rate, (b) is an FF cross-sectional view of (a), and (c) is a top view thereof. [Figure 20] 10 is a graph showing the relationship between curvature retention rate and bending rigidity ratio. [Figure 21] (a) shows the cross-sectional shape of the model used in the eigenvalue analysis, and (b) shows the cross-sectional shape of another model. [Figure 22] (a) is a graph showing the relationship between the thickness of the face material and the eigenvalue ratio, (b) is a graph showing the relationship between the pitch and the eigenvalue ratio, and (c) is a graph showing the relationship between the number of layers and the eigenvalue ratio. DETAILED DESCRIPTION OF THE INVENTION

[0008] First, the background that led to the idea of ​​the planar structure of the present disclosure will be explained. Refer to Figures 1 and 4. The planar structure 10 of the present disclosure employs the following two features to adjust the three-dimensional shape of the entire planar structure 10 and then facilitate maintaining the targeted three-dimensional shape.

[0009] As a first feature, the planar structure 10 has fold lines 14 that are curved at least in part and are provided at the boundaries between the multiple planar portions 12. This allows the planar structure 10 to perform curved folding, in which bending deformation of the planar portions 12 and folding deformation at the fold lines 14 are performed simultaneously, as will be described later.

[0010] When performing this curved folding, consider the case where each surface portion 12 of the planar structure 10 has isotropic bending rigidity (described later). In this case, if the portion of the planar structure 10 that is bent and deformed is considered to be a ruled surface, it becomes difficult to determine the position of the linear elements (described later) that form the ruled surface. For this reason, each time the planar structure 10 is curved and bent, it becomes difficult to determine the shape of the bent and deformed surface portion 12 into a fixed three-dimensional shape.

[0011] As a countermeasure to this, as a second feature, the planar structure 10 has anisotropy in bending rigidity in each of the plurality of planar portions 12. To achieve this, each of the plurality of planar portions 12 has a plurality of hollow portions 22 (not shown) extending linearly from the folding line 14. As a result, when the planar structure 10 is folded in a curved shape, the hollow portions 22 can hold the linear elements 26 in fixed positions.

[0012] These features combine to allow the entire planar structure 10 to operate as a mechanism with one degree of freedom, as will be described later. As a result, the entire planar structure 10 can be easily adjusted to a desired three-dimensional shape, and can be easily maintained in that desired three-dimensional shape, as will be described later.

[0013] The planar structure 10 of the embodiment will be described in detail below. Hereinafter, identical components will be assigned the same reference numerals, and duplicate explanations will be omitted. For ease of explanation, components will be omitted, enlarged, or reduced in size in each drawing as appropriate. The drawings should be viewed in accordance with the orientation of the reference numerals.

[0014] (First embodiment) See Figures 1 and 2. A planar structure 10 comprises a plurality of surface portions 12 and fold lines 14 provided at the boundaries between the plurality of surface portions 12. In this embodiment, the plurality of surface portions 12 comprise two surface portions 12A and 12B.

[0015] The fold line 14 is formed by folding adjacent surface portions 12. The fold line 14 includes a curved region 16 that is provided in at least a portion of the fold line 14 and has a curved shape. The curved region 16 of this embodiment forms a wave shape that rises and falls in a linear direction X (described later) of one of the hollow portions 22 in the surface portion 12 that is continuous with the fold line 14. The curved region 16 of this embodiment includes a convex portion 18 that has a curved shape that is convex toward one side of the linear direction X. The curved region 16 of this embodiment is provided along the entire fold line 14. Alternatively, the fold line 14 may include a straight region that is provided in the remaining portion of the fold line 14 other than the curved region 16 and has a straight line shape.

[0016] Refer to Figure 3. The surface portion 12 comprises at least one surface material 20 and a hollow-forming member 24 that is integrated with the at least one surface material 20 and forms multiple hollow portions 22. The surface portion 12 of this embodiment comprises two surface materials 20 as the at least one surface material 20, and one hollow-forming member 24. The multiple surface materials 20 of this embodiment are provided at intervals in the thickness direction Z of the surface portion 12. The hollow-forming member 24 of this embodiment is provided between surface materials 20 adjacent to each other in the thickness direction Z.

[0017] The materials of the face material 20 and the hollow-forming material 24 are not particularly limited. The face material 20 and the hollow-forming material 24 may be made of the same material, or may be made of different materials. The face material 20 and the hollow-forming material 24 are integrated by sewing, gluing, or the like. The hollow-forming material 24 may form the hollow portion 22 in cooperation with the face material 20, as in this embodiment, or the hollow portion 22 may be formed by the hollow-forming material 24 alone (see Figures 15A to 15C). The hollow-forming material 24 of this embodiment is made of a corrugated plate material in which peaks and valleys are arranged alternately.

[0018] 2 and 3. Each of the surface portions 12 has a plurality of hollow portions 22 extending linearly from the folding line 14. In FIG. 2, the hollow portions 22 are indicated by the center line La of the hollow portions 22 (see FIG. 3). Here, the direction along the straight line formed by the hollow portions 22 is referred to as the linear direction X. The straight line formed by the hollow portions 22 is set along the center line La of the hollow portions 22. In addition, in a cross section of the hollow portions 22 perpendicular to the linear direction X, the direction perpendicular to the thickness direction Z and the linear direction X is referred to as the width direction Y.

[0019] The hollow portion 22 has a closed cross-sectional shape in a cross section perpendicular to the linear direction X. The closed cross-sectional shape here means that the cross-sectional shape is continuous over the entire circumference around the center line La of the hollow portion 22 in a cross section perpendicular to the linear direction X. The hollow portion 22 does not have an open cross-sectional shape that is open to the outside in a cross section perpendicular to the linear direction X. The open cross-sectional shape here is assumed to be, for example, either a concave shape or a slit shape that is open to the outside in a cross section perpendicular to the linear direction X.

[0020] Each of the multiple surface portions 12 can have anisotropy in bending rigidity by including multiple linearly extending hollow portions 22. Here, "having anisotropy in bending rigidity" means that the bending rigidity against a warping force that tries to warp the center line La of the hollow portion 22 in the thickness direction Z is different from the bending rigidity against a curved bending force that tries to bend the surface portion 12 into a curved shape around the center line La of the hollow portion 22. The surface portion 12 of this embodiment can have anisotropy in bending rigidity so that the bending rigidity against the warping force is large and the bending rigidity against the curved bending force is small.

[0021] 1 and 4. As described above, the planar structure 10 can be curved-folded by providing the curved fold lines 14. Here, curved folding refers to deformation that integrally performs bending deformation that bends the surface portion 12 into a curved shape at a location that continues from the curved region 16 of the fold line 14 in the linear direction X, and folding deformation along the fold line 14 that changes the folding angle at the fold line 14.

[0022] In addition, as described above, the planar structure 10 has anisotropic bending rigidity by providing each of the plurality of surface portions 12 with a plurality of hollow portions 22 extending linearly from the fold line 14. This makes it easier for an external force F1 to be input to one surface portion 12 (e.g., surface portion 12A) to change the amount of bending deformation, and for this external force F2 to change the folding angle of the fold line 14 that is continuous with that surface portion 12. At the same time, the external force F1 is easier to propagate as an internal force F3 that changes the amount of bending deformation in another surface portion 12 (here, surface portion 12B) that is continuous with the fold line 14. In other words, the external force F1 input to one surface portion 12 is easier to propagate to other locations on the planar structure 10 as internal forces F2 and F3 that change the amount of curved folding of the entire planar structure 10 (the amount of bending deformation of the surface portion 12 and the folding angle of the fold line 14).

[0023] As a result, the entire planar structure 10 capable of performing curved surface folding can be operated as a mechanism with one degree of freedom. A planar structure 10 operating in this manner will have a deformation behavior in which the amount of bending deformation of the multiple planar portions 12 is determined once the folding angle θa of the folding line 14 is determined. When there are multiple folding lines 14, once the folding angle θa of any one folding line 14 is determined, the folding angles θa of the remaining folding lines 14 will also be determined.

[0024] The planar structure 10 described above can be transformed into a planar form Fa (see FIG. 1) and a three-dimensional form Fb (see FIG. 4). When the planar structure 10 is in the planar form Fa, the planar structure 10 as a whole is planar. The planar structure 10 can be transformed from the planar form Fa to the three-dimensional form Fb by bending it along a curve. The planar structure 10 can be unfolded into the planar form Fa by reducing the amount of bending (amount of bending deformation and folding angle) when it is in the three-dimensional form Fb.

[0025] Consider the folding angle θa of the folding line 14. The folding angle θa is at its minimum, zero, when the planar structure 10 is in the planar configuration Fa. The folding angle θa reaches its maximum folding angle θa2 (see FIG. 4) when one surface portion 12 connected to the folding line 14 contacts the other surface portion 12 so as to sink into it. The maximum folding angle θa2 is the angle immediately before the adjacent surface portions 12 that are contacting each other begin to deform so that they are no longer able to maintain a twist-free positional relationship (described below) with respect to the linear elements formed by the adjacent hollow portions 22. When the maximum folding angle θa2 is exceeded, deformation begins at either the contact points of the surface material 20 and the hollow-forming material 24 of the adjacent surface portions 12, or at the bending outer side of the folding line 14 relative to these contact points. The folding angle θa of the folding line 14 of the planar structure 10 can be changed by curved folding within a range from zero to the maximum folding angle θa2.

[0026] When the planar structure 10 is in the three-dimensional form Fb, the surface portion 12 has a simple curved shape at the portion continuing from the curved region 16 of the fold line 14 in the linear direction X. When the planar structure 10 is in the three-dimensional form Fb, if the fold line 14 has a linear region, the surface portion 12 has a planar shape at the portion continuing from the linear region. Here, a simple curved surface refers to a curved surface that is a developable surface, and is either a cylindrical surface, a conical surface, or a tangent curved surface. Here, a developable surface is a type of ruled surface, and refers to a surface that can be developed into a plane without distortion. A ruled surface refers to a curved surface obtained by the trajectory of a continuously moving generatrix (straight line). The generatrix representing this ruled surface is a straight line element.

[0027] See FIGS. 4 to 7. A single curved surface is also a curved surface in which, when k1 and k2 are the pair of principal curvatures (maximum and minimum values ​​of curvature) at any point P of the curved surface portion formed by the surface portion 12, the Gaussian curvature (k1 × k2) is zero. When in the three-dimensional form Fb, each of the multiple surface portions 12 undergoes bending deformation so that one principal curvature defining the single curved surface remains zero while the other principal curvature changes. In this case, the principal direction corresponding to "one principal curvature (the principal curvature that becomes zero)" is the linear direction X of the hollow portion 22, and the principal direction corresponding to "the other principal curvature (the principal curvature that changes)" is the direction perpendicular to the linear direction X at point P. The cross-sectional shape of the three-dimensional form Fb in FIGS. 6 and 7 is a cross-section cut along a normal plane along which the "other principal curvature" is taken. When in the three-dimensional form Fb, each of the multiple surface portions 12 undergoes bending deformation so that the curvature of this cross-section changes. The amount of bending deformation of the surface portion 12 when the planar structure 10 is curved is expressed by this "other principal curvature (changing principal curvature)." The "principal direction" here refers to the tangential direction at point P on the normal plane that gives the principal curvature passing through point P.

[0028] In Figure 5, (+) indicates that when in the three-dimensional form Fb, the surface portion 12 is convex toward the front side (upper side of the paper in Figure 6), and (-) indicates that the surface portion 12 is convex toward the rear side (lower side of the paper in Figure 7). The surface portion 12 forms a single curved surface corresponding to the shape of the fold line 14 at a location that continues from the curved region 16 of the fold line 14 in the linear direction X. When the planar structure 10 in the planar form Fa is viewed from one side in the thickness direction Z (perspective in Figure 5), the direction in which the convex portion 18 of the curved region 16 is convex is called the convex direction Da. In this case, when viewed from one side in the thickness direction Z (perspective in Figure 5), a fold deformation that is convex toward the front side is called a mountain fold, and a fold deformation that is convex toward the rear side is called a valley fold.

[0029] Consider a case where a mountain fold is made at fold line 14. In this case, surface portion 12A located in the convex direction Da relative to convex portion 18 of fold line 14 is provided with a convex curved surface portion 28 that is bent and deformed into a curved surface that is convex toward the front side when viewed from one side in thickness direction Z (the viewpoint of FIG. 5 ). Convex curved surface portion 28 is provided at a location that continues from convex portion 18 in the linear direction X. At the same time, surface portion 12B located on the opposite side of convex direction Da relative to fold line 14 is provided with a concave curved surface portion 30 that is bent and deformed into a curved surface that is convex toward the back side when viewed from one side in thickness direction Z. Concave curved surface portion 30 is provided at a location that continues from convex portion 18 in the linear direction X. When a valley fold is made at fold line 14, the direction of the convex bending deformation at surface portion 12 is opposite to that when a mountain fold is made at fold line 14. That is, a concave curved surface portion 30 is provided on the surface portion 12 in the convex direction Da relative to the convex portion 18 at the fold line 14, and a convex curved surface portion 28 is provided on the surface portion 12 on the opposite side of the convex direction Da relative to the convex portion 18.

[0030] The surface portion 12 has a single curved surface shape corresponding to the arrangement pattern of the multiple hollow portions 22. In this specification, the term "shape" does not only mean a shape that strictly matches the shape indicated by the immediately preceding term, but also includes a shape that approximately matches that shape. For example, when the multiple hollow portions 22 are arranged in parallel, as in this embodiment, the surface portion 12 has a cylindrical shape. Here, a cylindrical shape refers to a surface in which all the linear elements forming the ruled surface are parallel. When the multiple hollow portions 22 are arranged radially (see surface portion 12A in Figure 16), the surface portion 12 has a conical shape. Here, a conical shape refers to a surface in which all the linear elements forming the ruled surface pass through a fixed point. In this case, the linear elements formed by the multiple hollow portions 22 also have a radial shape.

[0031] The planar structure 10 can be curved so as to satisfy the following two conditions. The first condition is that the planar structure 10 is curved while maintaining the linear elements 26 formed by the hollow portions 22 in all of the planar portions 12 throughout the entire range of the folding angle θa at the folding lines 14. FIG. 4 schematically illustrates only some of the linear elements 26 formed by the hollow portions 22. The variable range of the folding angle θa here refers to the range from zero to the maximum folding angle θa2. This means that the planar structure 10 can be curved without bending the linear elements 26 themselves during the curved folding process. Here, one of the linear elements 26 formed by the hollow portions 22 is formed along the center line La of the hollow portion 22 at a position that overlaps with the hollow portion 22 when the planar structure 10 is viewed from the thickness direction Z.

[0032] The second condition is that the planar structure 10 performs curved folding while maintaining a twist-free positional relationship between the linear elements 26 formed by adjacent hollow portions 22 in all of the surface portions 12 throughout the entire range of variation in the folding angle θa at the folding line 14. There are three types of positional relationships between multiple straight lines: parallel, intersecting, and twisting. The twist-free positional relationship here refers to either parallel or intersecting among these three types of positional relationships. The second condition means that curved folding can be performed without causing twisting deformation of the linear elements 26 formed by adjacent hollow portions 22. This condition is satisfied for all hollow portions 22 provided in one surface portion 12. The intersecting positional relationship here refers to a positional relationship in which, when multiple linear elements 26 are arranged radially as described above, the lines extending from the multiple linear elements 26 intersect at a fixed point.

[0033] In addition, the planar structure 10 can be curved without any of the surface portions 12 intersecting over the entire range of variation in the folding angle θa at the folding line 14. Intersection here refers to the edge 12a of one surface portion 12 that is not connected to another surface portion 12 via the folding line 14 abutting on another surface portion 12. This means that the surface portion 12 can move without abutting on other surface portions 12 at locations other than the folding line 14 over the entire range of variation in the folding angle θa. This allows curved folding to be performed over the entire range of variation in the folding angle θa at the folding line 14 so as to satisfy the first and second conditions described above.

[0034] To satisfy the first and second conditions, for example, the bending rigidity of the face material 20 and the hollow-forming material 24 may be adjusted. The smaller the bending rigidity of the face material 20 relative to the bending rigidity of the hollow-forming material 24, the easier it is to satisfy the above-mentioned conditions. To satisfy this condition, the bending rigidity of the face material 20 may be smaller than the bending rigidity of the hollow-forming material 24. Here, the bending rigidity of the hollow-forming material 24 refers to the bending rigidity when the hollow-forming material 24 is unfolded in a planar shape. To adjust the bending rigidity, it is sufficient to adjust the thickness of the material mentioned, as well as the physical properties of the material. Furthermore, to satisfy the above-mentioned conditions, the cross-sectional shape of the face portion 12 may be adjusted. For example, as described below, if the face material 20 of the face portion 12 is configured to include only a single neutral face material 20A, the above-mentioned conditions are more easily satisfied. Alternatively, the face material 20 may be pre-bent into the desired shape to create a bending tendency.

[0035] See FIG. 8. In order to satisfy the first and second conditions described above, the planar structure 10 may have a notch 34 provided at a location of the fold line 14, which partially divides adjacent surface portions 12 in the thickness direction Z. The notch 34 leaves one component out of the multiple components constituting the surface portion 12 and divides the other components. These multiple components refer to the surface material 20 and the hollow-forming material 24. Adjacent surface portions 12 are continuous at the location of the fold line 14 except for the portion along the notch 34. The notch 34 of this embodiment leaves only one surface material 20 and divides the remaining components (the remaining surface material 20 and the hollow-forming material 24). The notch 34 of this embodiment leaves only a surface material 20 that is on the outer side of the multiple surface materials 20 in the thickness direction Z and divides the remaining components. Alternatively, the notch 34 may leave only a surface material 20 that is on the inner side of the multiple surface materials 20 and divide the other components. This makes it possible to avoid a situation in which the entire surface portion 12 is crushed and deformed in the thickness direction Z due to folding deformation at the fold line 14 at a location close to the fold line 14, and makes it easier to perform curved folding so as to satisfy the first and second conditions described above. In addition, the thickness of the surface portion 12 may be thinned to an extent that the influence of crushing deformation of the surface portion 12 due to folding deformation at the fold line 14 can be ignored while satisfying the first and second conditions described above.

[0036] The effects of the planar structure 10 described above will now be described.

[0037] (A) The planar structure 10 has the first and second features described above, and can therefore operate as a mechanism with one degree of freedom. Therefore, by applying an external force to a part of the planar structure 10, the three-dimensional shape of the entire planar structure 10 can be easily adjusted.

[0038] (B) In addition, when the planar structure 10 is in the three-dimensional form Fb, restricting the movement of a part of it also makes it possible to restrict the movement of the entire planar structure 10. Therefore, simply by fixing a part of the planar structure 10, the entire planar structure 10 can be fixed in a desired three-dimensional shape. Consequently, the entire planar structure 10 can be easily maintained in a desired three-dimensional shape.

[0039] For example, the entire planar structure 10 can be adjusted to a desired three-dimensional shape by applying an external force to a portion of the planar structure 10 so that the folding angle θa is set at the folding line 14. In addition, the planar structure 10 can be maintained in the desired three-dimensional shape by simply fixing a portion of the planar structure 10 while keeping it in the desired three-dimensional shape.

[0040] The target three-dimensional shape may be a three-dimensional shape somewhere in the range where the folding angle θa can be changed, or a three-dimensional shape where the folding angle θa is at the maximum folding angle θa2. Whichever three-dimensional shape is used, satisfying the first and second conditions described above can suppress the occurrence of distortion that would cause the single-curved surface portion 12 to become twisted. Furthermore, when the planar structure 10 is stationary at any position within the range where the folding angle θa can be changed, the occurrence of an elastic repulsive force due to distortion of the surface portion 12 can be suppressed. This makes it easier to hold the entire planar structure 10 stationary at any position.

[0041] According to this embodiment, the above-mentioned effects can be obtained without the need for an additional mechanism such as a frame that restricts the movement of the planar structure 10 .

[0042] According to this embodiment, the planar structure 10 can operate as a mechanism with one degree of freedom, and therefore there are no restrictions on the locations where an external force should be applied when adjusting the overall shape of the planar structure 10.

[0043] The surface portion 12 of the planar structure 10 has anisotropy in bending rigidity. Therefore, the shape of the surface portion 12 is less likely to become disordered than when the bending rigidity is isotropic. As a result, the shape can be easily maintained whether the surface portion 12 is in the planar form Fa or the three-dimensional form Fb. Here, "having isotropy in bending rigidity" means that the bending rigidity against bending forces around two mutually perpendicular directional axes parallel to the surface portion 12 in the planar form is equivalent.

[0044] Each of the planar structures 10 has anisotropic bending rigidity. Therefore, the planar structures 10 have high rigidity against warping forces and high strength against buckling deformation in the linear direction X. This makes it easy to make the planar structures 10 in the three-dimensional form Fb self-supporting without warping deformation or buckling deformation in the linear direction X due to warping forces in the planar structures 12.

[0045] The above effects can be obtained even for relatively large structures. The structure here refers to building materials the size of a door or enclosure that a person can enter. The planar structure 10 can be used for furniture such as chairs and beds, as well as doors, movable partitions, movable eaves, etc.

[0046] In obtaining the above-described planar structure 10, the core material or hollow material of existing corrugated cardboard may be used as the hollow forming material 24. This simplifies the manufacturing process.

[0047] A functional material may be disposed within the hollow portion 22 of the planar structure 10 to impart a predetermined function to the planar structure 10. The functional material may be, for example, a heat insulating material for imparting heat insulation, a sound insulating material for imparting sound insulation, a sound insulating material for imparting sound insulation, or a light blocking material for imparting light blocking properties. This allows a variety of functions to be imparted to the planar structure 10.

[0048] Next, various modifications of the planar structure 10 will be described.

[0049] (First Modified Form) See Figures 9 to 11. In this modified form, three surface portions 12A to 12C are provided as the multiple surface portions 12. The three surface portions 12A to 12C include a first surface portion 12A, a second surface portion 12B, and a third surface portion 12C. The three surface portions 12A to 12C are arranged in the order of first surface portion 12A, second surface portion 12B, and third surface portion 12C in the linear direction X of any one of the hollow portions 22 in one surface portion 12A in the planar form Fa.

[0050] The curved region 16 of this modified embodiment includes a first convex portion 18A that is curved and convex toward one side of the linear direction X (upper side in FIG. 10), and a second convex portion 18B that is curved and convex toward the other side of the linear direction X (lower side in FIG. 10).

[0051] 11 and 12. When the planar structure 10 is in the three-dimensional form Fb, the plurality of planar portions 12A to 12C are provided with convex curved surface portions 28 and concave curved surface portions 30 at locations continuous with the respective convex portions 18A, 18B.

[0052] When the planar structure 10 of this modified form is in the three-dimensional form Fb, the first surface portion 12 and the third surface portion 12 function as legs, allowing the planar structure 10 to be placed on an object such as a floor.

[0053] (Second modified embodiment) See Figure 13. The face portion 12 of this modified embodiment has three face members 20 as at least one face member 20 and two hollow-forming members 24. As such, the number of face members 20 is not particularly limited. The number of face members 20 may be one (see Figures 17A and 17B described below), two (see Figure 3), three (see Figure 13), or four or more.

[0054] 14A and 14B. The hollow-forming member 24 may have a plate shape in a cross section perpendicular to the linear direction X, connecting adjacent panel members 20 in the thickness direction Z. A plurality of such plate-shaped hollow-forming members 24 are provided at intervals in the width direction Y in the cross section perpendicular to the linear direction X.

[0055] The angle θb between the face material 20 and the hollow-forming member 24 is not particularly limited. The angle θb may be inclined with respect to the face material 20, as shown in FIG. 14A. The angle θb may be an angle equivalent to 90°, as shown in FIG. 14B. Here, "equivalent" includes cases where the conditions are the same as those mentioned, as well as cases where the conditions are almost the same. In this case, the face material 20 and the hollow-forming member 24 adjacent in the thickness direction Z are arranged in a truss shape.

[0056] 14A and 14B, the hollow portions 22 are provided between adjacent hollow-forming members 24. The cross-sectional shape of the hollow portions 22 is, for example, trapezoidal (see FIG. 14A) or rectangular (see FIG. 14B).

[0057] (Fifth to Seventh Modified Embodiments) See FIGS. 15A to 15C. In the embodiments of FIGS. 3 and 13, the hollow portion 22 is formed by the face material 20 and the hollow-forming material 24. Alternatively, the hollow-forming material 24 itself may have a hollow cross-sectional shape, so that the hollow portion 22 is formed only by the hollow-forming material 24. In the modified embodiments of FIGS. 15A to 15C, the hollow portion 22 is formed inside the hollow-forming material 24, and is also formed outside the hollow-forming material 24 by the face material 20 and the hollow-forming material 24. The cross-sectional shape of the hollow-forming material 24 is, for example, circular (see FIGS. 15A and 15C) or polygonal (see FIG. 15B). Adjacent hollow-forming materials 24 in the width direction Y may be spaced apart in the width direction Y (see FIG. 15A) or may be in contact with each other (see FIG. 15C).

[0058] (Eighth Modified Form) Refer to Fig. 16. In the embodiment of Fig. 2, an example has been described in which the multiple hollow portions 22 are arranged parallel to one another on one surface portion 12. The multiple hollow portions 22 do not have to be arranged parallel to one another as long as they do not intersect with one another on one surface portion 12. As an example of this, as shown in Fig. 16, the multiple hollow portions 22 may be arranged radially on one surface portion 12 (here, surface portion 12A). In this case, as described above, when the planar structure 10 is in the three-dimensional form Fb, the surface portion 12 has a conical surface shape (not shown).

[0059] Here, one of the adjacent surface portions 12 is referred to as the one surface portion 12X, and the other surface portion 12 is referred to as the other surface portion 12Y. The ends of the hollow portions 22 in the one surface portion 12X and the ends of the hollow portions 22 in the other surface portion 12Y are arranged so as to abut against each other. In the embodiment of FIG. 2, the hollow portions 22 in the one surface portion 12X and the hollow portions 22 in the other surface portion 12Y that satisfy this condition are arranged so as to be aligned on the same straight line. Alternatively, as shown in FIG. 16, the extension directions of the straight line (center line La) formed by the hollow portions 22 in the one surface portion 12X and the straight line (center line La) formed by the hollow portions 22 in the other surface portion 12Y may be arranged so as to change at the fold line 14. Here, an example is shown in which the straight lines formed by the multiple hollow portions 22 in the one surface portion 12X are parallel to each other, and the straight lines formed by the multiple hollow portions 22 in the other surface portion 12Y are intersecting. Alternatively, the straight lines formed by the hollow portions 22 on both adjacent surface portions 12X and 12Y may intersect. Alternatively, the ends of the hollow portions 22 on one surface portion 12X and the ends of the hollow portions 22 on the other surface portion 12Y may be arranged to be offset in the direction along the fold line 14.

[0060] (Ninth and tenth variants) See Figures 17A and 17B. The face portion 12 in these variants has only a single neutral face material 20A as at least one face material 20. This means that the face portion 12 does not have a face material 20 other than the neutral face material 20A. The neutral face material 20A is located on a bending neutral plane 32 where expansion and contraction in the width direction Y are unlikely to occur when bending deformation occurs in the face portion 12. This bending neutral plane 32 is not located on the outermost surface of the face portion 12 in the thickness direction Z, but rather at a position midway in the thickness direction Z. The neutral face material 20A may be composed of either a single or multiple face members.

[0061] The hollow-forming members 24 are individually provided on both sides of the neutral surface member 20A on one side of the surface portion 12 in the thickness direction Z. This means that the hollow-forming members 24 are provided on both outer sides of the surface portion 12 in the thickness direction Z, and no surface member 20 is provided. The hollow-forming members 24 in this embodiment are made of corrugated plate material. In the modified form of Figure 17A, the hollow-forming members 24 on both sides in the thickness direction Z are provided so as to be symmetrical with respect to the neutral surface member 20A. In the modified form of Figure 17B, the hollow-forming members 24 on both sides in the thickness direction Z are provided so as to be aligned in phase with each other in the width direction Y.

[0062] The above advantages will be explained with reference to Fig. 18. Consider a case where the surface portion 12 is provided with a surface material 20 that is exposed to the outside in the thickness direction Z. In this case, when bending deformation occurs in the surface portion 12, the surface material 20 undergoes large shrinkage deformation in the width direction Y, which increases the bending rigidity against the curved surface bending force at the shrinking point Pa of the surface material 20.

[0063] In contrast, consider a case where the surface portion 12 is provided with only the neutral surface material 20A. In this case, when bending deformation occurs in the surface portion 12, the neutral surface material 20A is located on the bending neutral plane 32, so it is possible to avoid a situation where the neutral surface material 20A is significantly contracted and deformed. Consequently, compared to a case where other surface materials 20 are provided, bending deformation of the surface portion 12 can be easily performed, and a highly rigid material can be used for the surface material 20.

[0064] In addition, the number of surfaces 12 and folding lines 14 in the planar structure 10 is not particularly limited.

[0065] Next, conditions for facilitating curved folding in the planar structure 10 will be described. Generally, the bending rigidity R [GPa] of an object against bending in a specific direction can be expressed as the product (E×I) of the bending modulus E and the second moment of area I. If the cross-sectional shape of the object is a rectangle with a thickness h and a width b, the second moment of area I is expressed as b×h 3 Using these, the bending rigidity R of an object is E×b×h 3 It can be expressed as a number proportional to

[0066] The relationship between the bending rigidity R1, bending modulus E1, thickness h1, and width b1 of the face material 20 can be expressed by the following formula (1). The relationship between the bending rigidity R2, bending modulus E2, thickness h2, and width b2 of the hollow forming material 24 when the hollow forming material 24 is unfolded into a plane can be expressed by the following formula (2). In formulas (1) and (2), k is a coefficient. R1=k×E1×b1×h1 3 ···(1) R2=k×E2×b2×h2 3 ···(2)

[0067] Assuming that these widths b1 and b2 are common, the bending rigidity ratio R2 / R1 of the surface portion 12, which is the ratio of the bending rigidity R2 of the hollow forming material 24 to the bending rigidity R1 of the surface material 20, can be expressed by the following formula (A) from formulas (1) and (2). R2 / R1=(E2 / E1)×(h2 / h1) 3 (A)

[0068] This bending rigidity ratio R2 / R1 represents the relative difficulty in bending of the face material 20 and the hollow-forming material 24, expressed by the thicknesses h1, h2 and bending moduli E1, E2 of the face material 20 and the hollow-forming material 24. The larger this bending rigidity ratio R2 / R1, the more difficult it is for the hollow-forming material 24 to bend and deform relative to the face material 20. Therefore, the larger the bending rigidity ratio R2 / R1, the greater the bending rigidity against a warping bending force and the smaller the bending rigidity against a curved surface bending force.

[0069] The inventors of the present application have newly discovered that the larger the bending rigidity ratio R2 / R1, the better the bending compliance. Here, bending compliance refers to the ease with which bending deformation follows a bending deformation in one portion of the surface portion 12 without torsional deformation in other portions of the surface portion 12. If this bending compliance deteriorates, when an external force F1 is applied to one portion of the surface portion 12, torsional deformation occurs in other portions of the surface portion 12. As a result, it becomes difficult to propagate the external force F1 applied to one surface portion 12 to other surfaces 12 as an internal force F3 for changing the amount of bending deformation, making it difficult for the entire planar structure 10 to operate as a one-degree-of-freedom mechanism. On the other hand, the better the bending compliance, the easier it becomes to perform curved folding while maintaining the linear elements 26 in a twist-free positional relationship. Ultimately, it becomes easier to propagate the external force F1 applied to one surface portion 12 to other surfaces 12, making it easier for the entire planar structure 10 to operate as a one-degree-of-freedom mechanism.

[0070] The bending rigidity ratio R2 / R1 of the surface portion 12 in this embodiment is preferably 100 or more. By satisfying this condition, good bending compliance can be obtained in relation to the bending rigidity ratio R2 / R1. To obtain even better bending compliance, it is more preferable that the bending rigidity ratio R2 / R1 be 800 or more. This is the finding obtained through experimental and analytical studies described below. There is no particular upper limit to the bending rigidity ratio R2 / R1. This condition may be satisfied for all or some of the surface portions 12 in the planar structure 10.

[0071] To achieve the desired bending stiffness ratio R2 / R1, either the relative thicknesses h1, h2 of the face material 20 and the hollow-forming material 24 or the relative bending moduli E1, E2 of the face material 20 and the hollow-forming material 24 may be adjusted. For example, the bending stiffness ratio R2 / R1 can be increased by increasing the ratio of the thickness h2 of the hollow-forming material 24 to the thickness h1 of the face material 20 (= h2 / h1). This can be achieved, for example, by making the face material 20 thinner than the hollow-forming material 24. Alternatively, the bending stiffness ratio R2 / R1 can be increased by increasing the ratio of the bending modulus E2 of the hollow-forming material 24 to the bending modulus E1 of the face material 20 (= E2 / E1). This can be achieved, for example, by making the material of the face material 20 softer than the material of the hollow-forming material 24.

[0072] Next, a test conducted to confirm the aforementioned effect on bending conformability will be described. In this test, a sample was created that resembled the surface portion 12 of the planar structure 10 in FIG. 3. The sample had an X-direction dimension of 500 mm and a Y-direction dimension of 300 mm. The sample had a cross-sectional shape that resembled the shape shown in FIG. 21(a), and its dimensional conditions were as shown in Table 1 below. h3 in Table 1 is the height of the hollow-forming material 24. As a dimensional condition not listed in Table 1, the pitch p of the hollow-forming material 24 was set to 3 mm. Here, the pitch p of the hollow-forming material 24 refers to the distance between the centers in the thickness direction Z of adjacent hollow-forming materials 24 in the width direction Y. A polyester resin was used for the surface material 20 of the samples. Soft polyvinyl chloride was used for the hollow-forming material 24 in Sample No. 3, and epoxy resin was used for the other samples.

[0073] [Table 1]

[0074] To evaluate the bending followability of the sample, the curvature retention rate, which will be described below, was measured. A plate-shaped jig 40 shown in FIG. 19 was used to measure the curvature retention rate. The jig 40 had an arc-shaped through-hole 42 formed therethrough. By inserting a portion of a sample 44 through the through-hole 42 of the jig 40, the outer surface of the portion on the inner side of the bend was held at a curvature radius of the arc formed by the through-hole 42 (hereinafter referred to as the set curvature radius). The set curvature radius was set to 286.5 mm. The sample 44 was pulled out 300 mm from the through-hole 42 of the jig 40, and the tip of the sample 44 (range Sa in the figure) was used as the measurement point, and the curvature radius of the measurement point (referred to as the measured curvature radius) was measured. The radius of curvature of the measurement point is determined by finding an arc passing through three points: the center position P1 in the Y direction on the outer surface of the inside of the bend at the tip of the sample 44, and a position P2 20 mm inward from both ends in the Y direction. This radius of curvature is measured using image measurement software such as ImageMeasure.

[0075] The curvature retention rate is expressed as the ratio of the measured curvature to the set curvature (= (1 / measured curvature radius) / (1 / set curvature radius)). This curvature retention rate indicates the extent to which the radius of curvature at the external force input point (set curvature radius) is maintained at a position away from the external force input point when an external force for bending deformation is input to a part of the surface portion 12 and maintained in that state. The higher this curvature retention rate, the easier it is to bend and deform the entire surface portion 12 while maintaining a positional relationship without twisting as a whole, resulting in good bending followability.

[0076] In this test, samples with a curvature retention rate of 90% or more were deemed to have passed. This passing criterion for curvature retention rate merely indicates a guideline for determining the bending stiffness ratio, and is not a condition that must be met in relation to the bending stiffness ratio.

[0077] FIG. 20 shows the relationship between the bending stiffness ratio and the curvature retention rate for Samples No. 2 to 8. As shown in Table 1 and FIG. 20, it can be seen that when the bending stiffness ratio is 100 or more, a good curvature retention rate (for example, a curvature retention rate of 0.90 or more) can be obtained. In particular, it can be seen that the trend in the curvature retention rate changes abruptly around the point where the bending stiffness ratio R2 / R1 becomes 100. When the bending stiffness ratio is less than 100, the curvature retention rate deteriorates abruptly. On the other hand, when the bending stiffness ratio is 100 or more, the curvature retention rate remains good and stable. As can be seen, by setting the bending stiffness ratio R2 / R1 to 100 or more, good bending followability can be obtained in relation to the bending stiffness ratio R2 / R1. It can also be seen that when the bending stiffness ratio is 800 or more, an even better curvature retention rate (for example, a curvature retention rate of 0.97 or more) can be obtained.

[0078] Furthermore, for example, by comparing Samples Nos. 1, 2, 4, 5, and 6, it can be seen that the relationship with curvature retention is influenced by the thickness ratio h2 / 1 of the face material 20 and the hollow-forming material 24, which determines the bending rigidity ratio R2 / R1. Additionally, by comparing Samples Nos. 3 and 6, it can be seen that the relationship with curvature retention is influenced by the bending modulus ratio E2 / E1 of the face material 20 and the hollow-forming material 24, which determines the bending rigidity ratio R2 / R1. Additionally, by comparing Samples Nos. 4, and 7 to 9, it can be seen that the height h3 of the hollow-forming material 24 does not have a significant effect on obtaining good bending followability.

[0079] Next, we will explain the eigenvalue analysis that was performed to confirm the effect on the bending followability mentioned above. For this eigenvalue analysis, Altair HyperWorks 2017 was used. For this eigenvalue analysis, a model was created that imitated the surface portion 12 of the planar structure 10. Two models were created, as shown in Figures 21(a) and (b).

[0080] In this eigenvalue analysis, two different eigenmodes were identified under conditions in which various parameters related to the surface portion 12 of the planar structure 10 were changed, and the eigenvalues ​​(natural frequencies) of those eigenmodes were obtained. The two different eigenmodes here are a bending deformation mode in which the entire surface portion 12 is bent in a curved shape around the center line La of the hollow portion 22, and a torsional deformation mode in which the entire surface portion 12 is twisted so that the linear elements formed by the multiple hollow portions 22 are in twisted positions.

[0081] The following relational expression (3) holds between the eigenvalue F corresponding to an eigenmode and the modal mass m and modal stiffness K corresponding to the eigenmode. From this relational expression (3), the following relational expression (4) holds. As can be seen from this relational expression (4), the smaller the eigenvalue F, the smaller the modal stiffness K of the eigenmode corresponding to that eigenvalue F, making that eigenmode more likely to occur. F ∝ (K / m) 1 / 2 ···(3) F×(m) 1 / 2 ∝(K) 1 / 2 ···(4)

[0082] Based on this, the eigenvalues ​​obtained by the above-mentioned eigenvalue analysis were used to calculate the eigenvalue ratio (=F2 / F1), which is the ratio of the eigenvalue F2 of the torsional deformation mode to the eigenvalue F1 of the bending deformation mode. The larger this eigenvalue ratio F2 / F1, the more easily the bending deformation mode occurs while suppressing the occurrence of the torsional deformation mode in the surface portion 12. In other words, the larger the eigenvalue ratio F2 / F1, the more easily the entire surface portion 12 is bent while maintaining a positional relationship without twisting as a whole, meaning that good bending followability can be obtained.

[0083] This eigenvalue analysis was performed under conditions in which the following parameters (1) to (5) were changed to confirm the effect on the eigenvalue ratio F2 / F1. For (1) to (3), the eigenvalue ratio F2 / F1 was obtained under conditions in which the parameters for the model in Figure 21(a) were changed. For (4), the eigenvalue ratio F2 / F1 was obtained for the models in Figures 21(a) and 21(b). For (5), the eigenvalue ratio F2 / F1 was obtained for the model in Figure 21(a) when both face plates 20 were present and when one face plate 20 was present. For (1) to (5), the standard conditions were as follows: the thickness of the face plate 20 was 0.05 mm, the thickness of the hollow-forming material 24 was 0.5 mm, the height of the hollow-forming material 24 was 6 mm, the pitch p of the hollow-forming material 24 was 3 mm, and the number of layers was 1. For (1) to (3), only the parameters to be changed were changed from the standard conditions. (1) Thickness h1 of the surface material 20 (2) Pitch p of hollow forming material 24 (3) The number of layers of the surface material 20 and the hollow forming material 24 combined together (4) Shape of the hollow forming material 24 (5) Presence or absence of one of the face plates 20

[0084] As shown in Figure 22(a), it can be seen that the thickness h1 of the face plate 20 used in the bending stiffness ratio R2 / R1, i.e., the thickness ratio h2 / h1 of the face plate 20 and the hollow-forming member 24, has a significant effect on the eigenvalue ratio F2 / F1. Specifically, it can be seen that the smaller the thickness h1, the larger the thickness ratio h2 / h1, which in turn increases the eigenvalue ratio F2 / F1 and improves bending compliance. Figure 22(a) shows the location where the bending stiffness ratio R2 / R1 becomes 100 under the analytical conditions used here. The analytical results show a common trend with the test results, in that the larger the thickness ratio h2 / h1, the larger the bending stiffness ratio R2 / R1, which in turn increases the curvature retention rate (good bending compliance).

[0085] 22(b) and (c), it was found that the parameters (2) and (3) do not have as great an effect on the eigenvalue ratio F2 / F1 as parameter (1). In addition, although not shown, it was found that parameters (4) and (5) have almost no effect on the eigenvalue ratio F2 / F1 compared to parameters (1), (2), and (3). This supports the idea that the pitch of the hollow-forming material 24, the number of layers, the shape of the hollow-forming material 24, and the presence or absence of one of the face materials 20 do not have a significant effect on bending followability.

[0086] The above-described embodiments and variations are merely examples. The abstract technical concepts should not be interpreted as being limited to the content of the embodiments and variations. Many design modifications, such as changes, additions, and deletions of components, are possible in the content of the embodiments and variations. In the above-described embodiments, the term "embodiment" is used to emphasize the content in which such design modifications are possible. However, design modifications are also permitted even in content without such notation. Hatching on cross sections in the drawings does not limit the material of the hatched object. The structures referred to in the embodiments and variations naturally include structures that are shifted by an amount of error that can be considered identical when manufacturing errors, etc. are taken into account.

[0087] Any combination of the above elements is also valid. For example, any description of an embodiment and another embodiment may be combined with a variant. [Explanation of symbols]

[0088] 10...planar structure, 12...surface portion, 14...fold line, 20...surface material, 20A...neutral surface material, 22...hollow portion, 24...hollow forming material, 26...straight element.

Claims

1. A plurality of surfaces; A fold line is provided at the boundary of the plurality of surface portions and is curved at least in part, Each of the plurality of surface portions has a plurality of hollow portions extending linearly from the folding line and forming a closed cross-sectional shape, thereby having anisotropy in bending rigidity; the plurality of hollow portions form linear elements when the planar structure is curved; The surface portion includes at least one surface material and a hollow forming material that is integrated with the at least one surface material and forms the hollow portion, By satisfying the following conditions, the present planar structure can perform the curved folding while maintaining the straight elements in all of the surface portions throughout the entire range of change in the folding angle at the folding line, and the curved folding can be performed without crossing any of the surface portions while maintaining the straight elements formed by adjacent hollow portions in an untwisted positional relationship in all of the surface portions throughout the entire range of change in the folding angle at the folding line. The condition is that when the face material and the hollow forming material are referred to as components that constitute the face portion, a notch is provided at a location where the fold line is present, and only one face material is left, and the other components that constitute the adjacent face portion are separated, A planar structure in which, when the ratio of the bending rigidity R2 of the hollow forming material to the bending rigidity R1 of the surface material, expressed by the following formula (A), is taken as the bending rigidity ratio R2 / R1 of the surface portion, the bending rigidity ratio R2 / R1 of the surface portion is 100 or more. R2 / R1=(E2 / E1)×(h2 / h1) 3...(A) (In formula (A), E1 represents the flexural modulus of the face material [GPa], h1 represents the thickness of the face material [mm], E2 represents the flexural modulus of the hollow-forming material [GPa], and h2 represents the thickness of the hollow-forming material [mm].)

2. The surface portion includes only a single neutral surface material as the at least one surface material, The planar structure according to claim 1 , wherein the hollow-forming members are provided individually on both sides of the neutral surface member in the thickness direction of the surface portion.

Images