Abstract:
A core of sandwich structural material comprising an assembly of several sheets comprising a first sheet and a second sheet that are provided on one of the faces thereof with of periodically distributed polygonal-based pyramid frustums, the superposition of these sheets in the thickness direction forming a three-dimensional network of hollow tubular channels which extend from one face of one sheet to one face of another sheet along a channel direction, and that are each defined by a peripheral wall, this peripheral wall being provided both by the first sheet and by the second sheet.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a core of structural material, a sheet as a core component, a sandwich structural material comprising such a core, as well as the method for producing such a core. 
         [0002]    Sandwich structural materials are generally composed of two outer skins secured onto opposite sides of a core. Said core can be created from a wide variety of component materials and is made to have a high structural compressive strength and flexural strength while maintaining a minimal weight. These structural materials have many applications, for example in the aerospace and automotive industries. 
         [0003]    Among these materials, the best known are those with a honeycomb core. These cores are made of sheets that are shaped and then fastened together at specific points to form a honeycomb of cells with hexagonal profiles, possibly deformed, which extend perpendicularly to said outer skins. Many materials can be used for constructing this core, for example cardboard, aluminum, plastic, composites, or metals. 
         [0004]    As the sheets forming the core of the structural material are arranged orthogonally to the surface of the sandwich structure and their structure can be extended, the mechanical properties of honeycomb cores are highly anisotropic. They exhibit, for example, a lower flexural strength than compressive strength. Various structures have been developed to attempt to overcome this disadvantage. 
       TECHNICAL BACKGROUND 
       [0005]    Document FR-A-2 922476 describes an example of such a structure, formed of a stack of sheets equipped with a plurality of parallel ribs on their faces, said ribs being complementary between sheets. These sheets are stacked to form a non-extendable structure providing improved flexural and compressive rigidity. 
       OBJECTS OF THE INVENTION 
       [0006]    The present invention aims to provide a material and a core structure having improved strength characteristics while weighing less and offering an economically viable alternative to known solutions. 
         [0007]    To this end, the core of sandwich structural material according to the invention is noteworthy in that:
       it comprises at least a first sheet and a second sheet that are provided with a plurality of periodically distributed polygonal-based truncated pyramids (frustums),   superimposing said first and second sheets forms a three-dimensional network of hollow tubular channels that extend from one face of one sheet to one face of another sheet along a channel direction, and that are each defined by a peripheral wall, said peripheral wall being provided both by the first sheet and by the second sheet.       
 
         [0010]    “Three-dimensional network of channels” is understood to mean that the network comprises at least three channels for which the channel directions are non-coplanar. 
         [0011]    Each of said channels has a high compressive rigidity along its channel direction. As the network of tubular channels is three-dimensional, the mechanical strength of the structure is therefore significant in all spatial directions, which gives the material a significant flexural as well as compressive rigidity. The core of structural material according to the invention also has very good properties as an absorber of energy, for example impacts. Lastly, the channels or the spaces between channels allow the passage of cables and simplify the installation of sensors or gauges. 
         [0012]    Advantageously, said first and second sheets are identical. The second sheet is turned over relative to the first sheet, on an axis lying within the plane of said sheet, in order to create said network of hollow channels when said sheets are placed atop one another. In this way, the cost and complexity of producing the cores can be significantly reduced. 
         [0013]    Preferably, said truncated pyramids each comprise a base, sides, a top, and edges connecting the base to the top, said edges delimiting recessed areas between peripheral adhesion surfaces. The recessed areas of a first sheet are supplemented by the facing surfaces of a second sheet, defining the peripheral wall of the hollow tubular channels. These two sheets are secured together at the peripheral adhesion surfaces by an adhesion process, which for example may be welding, thermal bonding, or joining. 
         [0014]    In order to obtain the best mechanical properties, the network of tubular channels will be three-dimensional, meaning it will comprise channels extending in non-parallel channel directions. 
         [0015]    According to one embodiment of the invention, the first and second sheets may be superimposed in a staggered manner, the second sheet then being placed so it straddles multiple first sheets arranged side by side. This will ensure a good mechanical connection between said first sheets. 
         [0016]    To extend the structural material in the thickness direction, the core of the present invention is advantageously such that:
       it further comprises third and fourth sheets having the same characteristics as the first and second sheets and which when superimposed in the thickness direction form a three-dimensional network of hollow tubular channels,   said first and fourth sheets are superimposed and secured to each other by the tops of each of said truncated pyramids of said first and fourth sheets.       
 
         [0019]    To ensure continuity of the hollow channels when several such sheet assemblies are superimposed, the tops of said truncated pyramids of said first and fourth sheets advantageously present a flat surface. 
         [0020]    According to another embodiment of the present invention, the tops of the truncated pyramids of said first and fourth sheets have angled portions to ensure the alignment of the channel directions of said hollow channels when multiple sheet assemblies are superimposed. This alignment will give the strongest mechanical properties to the structure. 
         [0021]    The invention also relates to a sheet having a plurality of periodically distributed polygonal-based truncated pyramids. Such pyramids have recessed areas between peripheral adhesion surfaces, such recessed areas of two superimposed sheets then defining hollow tubular channels. 
         [0022]    A core according to the present invention can easily be provided with at least one outer skin attached to at least one of its opposite faces to form a sandwich structural material. 
         [0023]    Finally, the invention also relates to a method for producing a core of sandwich structural material comprising an assembly of multiple sheets including at least a first sheet and a second sheet that are provided on at least one of the faces thereof with a plurality of periodically distributed polygonal-based truncated pyramids, said pyramids having recessed areas between the peripheral adhesion surfaces, the recessed areas of said superimposed sheets defining hollow tubular channels, said method characterized by its comprising at least the following steps:
       a step of forming said truncated pyramids in said sheets, and   a step of superimposing said sheets and attaching them to one another.       
 
         [0026]    According to a first embodiment of this production method, the step of forming the truncated pyramids in said sheets includes a step of folding said sheets. 
         [0027]    According to a second embodiment of this production method, the step of forming the truncated pyramids in said sheets includes a step of plastic deformation of the sheets. 
         [0028]    According to a variant of these embodiments, a plurality of openings are created beforehand in the sheets to reduce the amount of structural material in the core. 
         [0029]    It is also possible to shape and assemble a third sheet and a fourth sheet in a manner identical to said first and second sheets, then to stack and attach said first and fourth sheets to one another by the tops of their truncated pyramids. 
         [0030]    In general, some embodiments of the invention provide the following advantages. The production method is easily applied on an industrial scale because the steps are simple to implement using existing technical means. The sheets can all be identical and the formation of the network of hollow tubes and then of the structural material is done simply by turning the sheets over before they are superimposed and assembled. The core of sandwich structural material can be of arbitrary thickness and dimensions. As the mechanical strength of the material, whether flexural or compressive, depends only on the hollow tubular network, the core of the sandwich structural material can be significantly lightened by creating openings in the sheets that constitute the structure. Finally, as the tubular network is three-dimensional, the mechanical strength of the material is much higher, at equal volume, than that of honeycomb-type materials. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Other features and advantages of the invention will become apparent from the following description. 
           [0032]    In the drawings: 
           [0033]      FIG. 1  is perspective view of a sheet of the assembly constituting the core of sandwich structural material, 
           [0034]      FIG. 2   a  shows a unit cell of said sheet, 
           [0035]      FIG. 2   b  and  FIG. 2   c  show how two unit cells of two sheets are superimposed to create the network of hollow channels, 
           [0036]      FIGS. 2   d  and  2   e  correspond to cross-sections along IID and IIE and illustrate the position of the hollow tubular channels created by superimposing the first and second sheets, 
           [0037]      FIG. 3  shows an embodiment in which openings are created in the sheets, leaving only the hollow channels and peripheral adhesion surfaces, 
           [0038]      FIG. 4  shows the assembly of two sandwiches, each constructed by superimposing a first and a second sheet, 
           [0039]      FIGS. 5   a ,  5   b  and  5   c  detail the structures located at the tops of the truncated pyramids, which allow stacking the sandwiches of  FIG. 4 , 
           [0040]      FIG. 6  shows a staggered assembly of multiple sheets to form a structure extending over a large area. 
           [0041]      FIGS. 7   a ,  7   b  and  7   c  show other embodiments according to the present invention, in which the bases of the truncated pyramids, and the network formed by these pyramids, are different, 
           [0042]      FIGS. 8   a ,  8   b  and  8   c , as well as  FIGS. 9   a  and  9   b , show two methods for producing a sheet of the invention by folding a previously embossed sheet, 
           [0043]      FIG. 10  shows a manufacturing process of the present invention according to one embodiment. 
       
    
    
       [0044]    In the various figures, the same references designate identical or similar elements. 
       DETAILED DESCRIPTION 
       [0045]      FIG. 1  shows a sheet  1  from an assembly constituting the core of sandwich structural material according to an embodiment of the present invention. This sheet  1  extends in extension directions X and Y and has an upper face  2  and a lower face  3  located on either side of a central plane  99 , said faces therefore facing away from each other along the thickness direction Z. 
         [0046]    On the lower face  3 , the sheet has a periodic network of truncated pyramids  4  each shaped to have a top  11 , edges  10 , and a triangular base  5  on the central plane  99 . On the upper face  2 , the sheet has a periodic network of truncated pyramids  6  each consisting of a base  7 , edges  12 , and a top  13 . The bases  7  are located on the central plane  99 . They are generally triangular with recessed sides  8  at the tops. These recessed sides  8  extend along the edges  12  to form recessed areas  9  running from the base  7  to the top  13  of the truncated pyramids  6 . 
         [0047]    The network formed by said bases  5  and said bases  7  on the central plane  99  is a Kagome lattice, meaning that the bases  5  are each surrounded by three bases  7  which are each connected to their respective base  5  by a pyramid top. The tops of said bases  5  slightly penetrate the tops of said bases  7 , which creates said recessed sides  8  at said tops of said bases  7 . Flat hexagons  35  complete the central plane, each of their sides corresponding to a separate truncated pyramid. As shown in  FIG. 1 , the quadrilateral faces of the truncated pyramids and the hexagons  35  can be cut out to lighten the structure. 
         [0048]    Other networks and other shapes for the polygonal bases  5  and  7  of said truncated pyramid  4  and  6  can be considered, as indicated below. The central plane  99  may be omitted in some embodiments. 
         [0049]    The sheet  1  may be made for example of a material such as a plastic, metal, alloy, composite, or resin. The thickness of the component material of the sheet  1  can range from 5 micrometers to 2 millimeters. 
         [0050]      FIGS. 2   a  to  2   e  illustrate the assembly of two sheets in a variant of the embodiment of  FIG. 1 , in order to form a network of hollow tubular channels. 
         [0051]    In  FIG. 2   a , a unit cell  77  has been isolated from a variant embodiment of the sheet  1 . This unit cell consists of a truncated pyramid  4  and a truncated pyramid  6 . The recessed areas  9  form halves of the hollow tubular channels and are surrounded by peripheral adhesion surfaces  14 . 
         [0052]      FIG. 2   b  shows how two sheets  16  and  17 , identical to the sheet  1  of  FIG. 1 , must be positioned relative to each other before being stacked in order to create said network of hollow channels. Sheet  16  is flipped over relative to sheet  17 , on an axis of rotation located within the plane of said sheet  16 , which is the plane (X, Y). This operation places the truncated pyramids  6  of sheet  17  so they are facing the truncated pyramids  4  of sheet  16 , and places the peripheral adhesion surfaces  14  so they are facing the parallel peripheral adhesion surfaces  15 . 
         [0053]    Sheets  16  and  17  are then brought into contact with one another and fastened together at surfaces  14  and  15  in order to obtain the sandwich  19  of  FIG. 2   c . Superimposing the truncated pyramids  6  of sheet  17  and the truncated pyramids  4  of sheet  16  completes the recessed areas  9  of truncated pyramid  6  with the walls of the truncated pyramid  4 , obtaining hollow tubular channels  18  having walls formed by both sheet  16  and sheet  17 . The sandwich  19  of  FIG. 2   c  thus comprises truncated pyramids  20  on the upper face and truncated pyramids  21  on the lower face, said pyramids  20  and  21  being symmetrical to each other relative to plane (X, Y), formed by the assembly of the two sheets  16  and  17 , and each comprising hollow tubular channels  18 . 
         [0054]    Sectional view  2   d  of the sandwich  19 , along the plane (X, Z), provides a view of a hollow tubular channel  18   a  sliced off at a plane containing its channel direction, said hollow channel therefore being visible along its largest dimension, as well as two hollow tubular channels  18   b  whose channel directions run in the Y direction and are therefore sliced transversely to said channel direction. The walls of channels  18   a  and  18   b  are formed in part by sheet  16  and in part by sheet  17 . Channel  18   a  extends from the top  22  of pyramid  20  to the top  23  of pyramid  21 . Its channel direction thus includes non-zero components in the X and Z directions. The directions of the channels  18   b  include non-zero components in the Y direction. Other hollow tubular channels  18  of the sandwich  19  possess channel directions extending in other spatial directions. 
         [0055]    Sectional view IIE of the sandwich  19 , along plane (X, Y) through the truncated pyramid  20 , provides a view of three hollow tubular channels  18  located at the tops of base  7  of said truncated pyramid  20 . Sectional view IIE also shows how the peripheral surfaces  14  and  15  are superimposed in order to attach sheets  16  and  17  together. 
         [0056]      FIG. 3  shows a sandwich  19  according to another embodiment of the invention. The network of hollow channels is three-dimensional and the channels extend in all spatial directions, which gives advantageous mechanical properties to the structure. In the embodiment shown in  FIG. 3 , openings have been made in the sheets  16  and  17 , leaving only the walls of the hollow tubular channels and the peripheral adhesion surfaces, in order to lighten the structure. Thus, this embodiment does not provide a central plane; the base of a pyramid is represented with dotted lines. In addition, adjacent pyramid tops are interconnected, for example in threes, by a radial structure  36 ,  37 . 
         [0057]    According to one embodiment of the present invention shown in  FIG. 4 , two sandwiches  24  and  25 , each consisting of two superimposed sheets  16  and  17  as detailed in  FIG. 2   c  for sandwich  19 , can be stacked to extend the core  27  in thickness. Sandwiches  24  and  25  are interconnected by assembling  26  the tops  23  of the truncated pyramids  21  of sandwich  24  and the tops  22  of the truncated pyramids  20  of sandwich  25 . 
         [0058]    According to other embodiments of the present invention, other sandwiched sheets  19  may be superimposed onto said core  27 , thus allowing the formation of a core of arbitrary thickness. 
         [0059]      FIGS. 5   a ,  5   b  and  5   c  show two embodiments of the assembly  26  of said tops  22  and  23  of sandwiches  24  and  25 .  FIG. 5   a  is a top view of the core  27  of  FIG. 4 , showing the plane for sectional views  5 B and  5 C. 
         [0060]    In a first embodiment of tops  22  and  23 , shown in  FIG. 5   b , said tops are formed by flat surfaces  27 . The assembly  26  of said flat surfaces  27  places the ends of the hollow tubular channels  18  of sandwiches  24  and  25  so they are facing one another. 
         [0061]    In a second embodiment, shown in  FIG. 5   c , the tops  22  and  23  have angled portions  28  which allow aligning the channel directions of the channels  18  of sandwich  24  and sandwich  25 . The plane of sectional view  5 C of said recessed portions  28  is orthogonal to the channel direction of the channels  18 . This embodiment provides better transfer of the forces exerted on the tubular channels  18  from one sandwich to another. 
         [0062]    Other embodiments of said assembly  26  can be used by a person skilled in the art by varying the geometry of the tops  22  and  23 . 
         [0063]    Finally, as shown in  FIG. 6 , a sandwich  19  of two sheets can be formed in a staggered manner to further extend a core of sandwich structural material. In one embodiment, to achieve this, sheet  34  is superimposed and secured such that it straddles four sheets  30 ,  31 ,  32  and  33 , forming a strong structure. Such an assembly significantly improves the mechanical properties when assembling multiple sheets in the extension plane (X, Y), in comparison to a simple bonding of the ends of said sheets or a simple extendable stacking. 
         [0064]    In other embodiments of the invention, the recessed sides  8  can be placed in other areas of the base  7 , for example on the sides of said base, and in all embodiments can be replaced by or associated with protruding sides, extending via the edges  12  as protruding areas, on the truncated pyramids  4  or  7 . 
         [0065]      FIG. 7   a  shows another embodiment of the sheet  1  from the assembly constituting the core of sandwich structural material. In this embodiment, the truncated pyramids have a hexagonal base and hollowed triangular faces  38  alternating with quadrilateral faces  39 . The recessed areas  8  are formed in the quadrilateral faces  39  of the truncated pyramids. The area surrounding the recessed area forms the peripheral adhesion surface  14 . The central plane  99  is absent in this embodiment. 
         [0066]      FIG. 7   b  shows another embodiment of said sheet  1  in which the truncated pyramids are assembled in a triangular lattice, the triangular bases of said pyramids being joined at their sides. 
         [0067]      FIG. 7   c  shows yet another embodiment of the sheet of  FIG. 1 , in which the truncated pyramids have a square base, with recessed areas at the edges of said squares. 
         [0068]    A method for producing a sheet according to the invention is shown in  FIGS. 8   a ,  8   b  and  8   c . Starting with a sheet  41  in which openings have been created, two folding steps shown in  FIGS. 8   a  and  8   b  result in the sheet  1  shown in  FIG. 8   c.    
         [0069]    Another method for producing a sheet of the invention is shown in  FIGS. 9   a  and  9   b , starting with a sheet  41  having previously created openings as shown in  FIG. 9   a . A single folding step results in the sheet  1  of  FIG. 9   b.    
         [0070]    Finally, a method for producing a core of sandwich structural material according to an embodiment of the invention is shown in  FIG. 10 , wherein an optional first step  100  consists of creating holes in a sheet to reduce the material and where necessary to allow easier folding of said sheet. In step  200 , the truncated pyramids are formed in said sheet by folding the sheet, plastic deformation, embossing, injection molding the sheet, and/or any other method or combination of the above methods for creating such a sheet providing truncated pyramids. A step  300  then consists of superimposing and attaching pairs of sheets providing truncated pyramids in order to obtain the network of hollow tubular channels. Finally, during step  400 , the sandwiches obtained in step  300  are stacked and attached to one another to form a core of arbitrary dimensions. 
         [0071]    A sandwich structural material can be obtained by superimposing, on at least one of the opposite faces of a core obtained by one of the embodiments of the production method described above, an outer skin or two outer skins. Said skins are for example formed of a composite material of fiber reinforced plastic.