Patent Publication Number: US-2010115883-A1

Title: Load-bearing space lattice structure, lightweight construction element and process for the preparation thereof

Description:
The invention relates to a load-bearing space lattice structure, lightweight construction elements comprising such structure, and a process for the preparation of such lightweight construction elements. 
     Sandwich lightweight construction elements comprising a core layer having a honeycomb structure are known. Such elements are characterized by a high compressive strength and shear strength in longitudinal direction. However, their resistance against shock load and torsion is low since the honeycomb structure is connected (e.g., by adhesive bonding) with the cover layers only at the edges and such connection is easily detached. EP-527109-A1 proposes to improve this condition by connecting honeycomb edges with the cover layers through adhesive beads. 
     DE-10252207-B discloses a molded part as a core of a sandwich with humps formed perpendicular to the center plane of the core, the side faces of the humps being flat and bondable to cover plates. This improves the bonding of the core to the cover layers. 
     U.S. Pat. No. 5,615,528 describes a load bearing structure made of a continuous material that is in the form of rhombic dodecahedra truncated at four vertices. Such polyhedrons cannot be arranged in a space-filling way, and the remaining voids are to dissipate the tensions occurring upon loading to minimize tensile stress. 
     It is the object of the invention to provide a load-bearing space lattice structure and a lightweight construction element that can equally withstand tensile, compressive, flexural, shear and torsional forces while requiring as low an amount of material as possible. Another object of the invention relates to a process for the preparation of such a lightweight construction element. 
     These objects are achieved by a space lattice structure according to claim  1 , a lightweight construction element according to claim  8  and a process according to claim  23 . 
     Within the meaning of the present invention, “truncated octahedron” means a solid that can be considered as formed by cutting all six vertices from a regular octahedron. The severing of the vertices takes place in such a way that one third each of the edges coming into the vertex is removed together with each vertex. This results in a so-called Archimedean polyhedron bounded by six squares and eight regular hexagons. All the 36 edges are of equal length (one third of the edge length of the starting octahedron). The 24 vertices are also equal in that they are adjacent to one square and two hexagons. A particular property is the fact that a plurality of equal truncated octahedrons can be arranged to fill a region of space without voids. For example, if equal truncated octahedrons are laid out on a flat substrate as a layer in such a way that each contacts the substrate with one square, and adjacent solids contact each other congruently with the squares on the sides, a square lattice is obtained having depressions in the middle of the cells, which can receive exactly one further similar layer, but shifted by half the cell diagonal and elevated by half the height of the solids (measured through opposing squares). Space can be filled without voids (tessellated) by adding further layers in the same way. 
     In this space-filling arrangement, faces and edges of truncated octahedrons contacting each other in a geometrical sense coincide and are merely two-dimensional or one-dimensional, respectively. However, for realizing the invention in practice, three-dimensional elements are required. Therefore, the terms “faces” and “edges” herein are to be understood to mean plate-shaped or rod-shaped items that may belong to several contacting truncated octahedrons of the space lattice structure, but may also result from the bonding of faces and edges of contacting truncated octahedrons. By bonding the contacting faces together or by using surface elements common to the adjacent truncated octahedrons, the load-bearing space lattice structure according to the invention is formed. In the following, “bonding” is intended to mean both the attaching of congruently contacting surface elements to each other and the use of surface elements common to adjacent truncated octahedrons. 
     If the space lattice structure is to be curved or bent rather than flat or be adapted to some irregularly shaped surface of another component, it is within the scope of the invention to appropriately shorten or extend individual edges without letting them, disappear altogether. While in general the truncated octahedrons of the structure are equally sized and congruent, they deviate from congruence in this special case. 
     If faces of the truncated octahedron are present in the structure, their sides also form corresponding edges belonging to the structure as a matter of course. The invention also includes embodiments in which the surface elements are thinner in the middle region, i.e., remote from the edges, as compared to the edge region, and embodiments in which only rod-like edge elements are left. 
     In addition to complete space filling, the truncated octahedron among the regular polyhedrons also has a ratio of surface area to volume that is very close to the optimum ratio found in spheres. This contributes to meeting the demand for a minimum expense of material while space filling is optimal. 
     The space lattice structure according to the invention allows for a generally isotropic distribution of the forces acting thereupon without concentrating the mechanical stresses to particular potential breaking points. Therefore, it is equally compression-resistant, flexurally rigid, shockproof, resistant to shock and contact loads and shear-resistant in both longitudinal and transversal directions. 
     The truncated octahedrons may be arranged in one layer, wherein the square faces of adjacent solids respectively coincide as described above. Such a layer has depressions on the top and bottom sides each of which corresponds to half a truncated octahedron. Such a layer may already be connected with other components through the square lateral surfaces facing up and down. 
     In another embodiment, the space lattice structure includes two or more layers of truncated octahedrons arranged on top of one another in a space-filling manner. Such a structure can keep other components at a distance, for example. 
     If the edges of the truncated octahedrons are of equal length, a layer produced in this way will be flat. However, within the scope of the invention, it is also possible to produce curved or warped layers if the lengths of individual edges of the truncated octahedrons are slightly adapted without eliminating the space-filling property. By analogy, the structure can also be matched to uneven surfaces of other components by appropriately extending or shortening individual edges. 
     A preferred embodiment according to the invention relates to a sandwich-type lightweight construction element having two outer cover layers and one core layer, wherein the core layer comprises a space lattice structure according to the invention. 
     In a preferred embodiment, the core layer is formed of one layer of truncated octahedrons bonded to each of the cover layers through opposing square faces. For space filling, the individual solids are arranged in such a way that each truncated octahedron contacts and is bonded with one square face of an adjacent solid through each of the four square faces not bonded to the cover layers. 
     Another preferred embodiment relates to a sandwich-type lightweight construction element whose core is formed of two layers of truncated octahedrons stacked in a space-filling manner. The solids of the second layer come to lie in the depressions of the first layer, so that the entire core is higher by half as compared to one consisting of a single layer. The bonding to the cover layers is again achieved through the square faces of the truncated octahedrons exposed at the top and bottom. 
     Further preferred embodiments are obtained if further layers of truncated octahedrons are stacked to form the core in a space-filling manner. For example, these may be three, four or five layers in total. 
     A lightweight construction element in which the space lattice structure of the core is reduced to a space-filling arrangement of truncated octahedrons cut in half is also possible. “Cut in half” herein means a division through a plane parallel with and equidistant to opposing square faces, dividing four of the square faces diagonally. The thus obtained halves are equal and can be made to coincide by a rotation by 180°. Such an arrangement is possible if the adjacent halves of the truncated octahedron are oriented in opposite directions, i.e., if bottom and top halves alternate. In this case too, the bonding with the cover layers is effected through the undivided square faces. 
     In another embodiment of the lightweight construction element according to the invention, the sandwich core is composed of at least three layers of truncated octahedrons of which at least one of the middle layers has a height of the truncated octahedrons differing from that of the outermost layers. Particularly preferred are space lattice structures having a truncated octahedron in the middle region that is twice the height of the surface layers. When the layers are joined together, every fourth square of the surface layer would come to lie on a large square of the middle layer. Very thin intermediate layers can improve the diaphragm action between the outer and middle layers. 
     In order to further reduce the basis weight of the lightweight construction element in a preferred embodiment, at least part of the surface elements of the structure are thinned out in their middle region. This does not substantially affect the stability of the structure since the space lattice structure causes force absorption and dissipation into the space. The force is deliberately decomposed in the space through the framework and thus allows for an isotropic force absorption. Since an accumulation of matter is produced at the edges, the flow of forces is transmitted in a defined way over the edges of the solids and newly decomposed and decreased at each node. However, it may be advantageous to exclude the square surface elements serving for bonding with the cover layers from thinning out in order that the stability of the bonding with the cover layer is not challenged. 
     In another preferred embodiment, the space lattice structure of the sandwich core is realized by a framework. The rods lie on the edges of the truncated octahedrons and are interconnected at the vertices. One may also imagine that such a framework is derived from a structure with surface elements if the above mentioned thinning out of the middle regions is continued until openings are formed in the surface elements. The bonding with the cover layers is then effected through the rods that bound the corresponding bonding square. It is also possible to design these bonding squares as surface elements in the framework structure. 
     As materials for the cover and intermediate layers of the lightweight construction element according to the invention, metals, plastic materials or fibrous materials, such as cardboard, may preferably be used. The plastic material may be a thermoset or thermoplastic polymer or may be fiber-reinforced. In order to achieve as isotropic mechanical properties as possible, reinforcement by woven or non-woven fabrics may also be used. 
     Suitable materials for the surface elements of the space lattice structure include metals, such as aluminum, plastic materials, such as thermoset and thermoplastic polymers, paper or cardboard, optionally impregnated with strength enhancers. For the framework structures, metals or plastic materials reinforced by fibers (e.g., glass fibers) are preferably employed. Thermoplastic polymers are preferred. 
     The mutual bonding of the surface elements in the space lattice structure preferably results from the fact that larger plate-shaped semifabricated products are reformed for the production and the integrity of the surface elements is thus maintained. On the other hand, individual surface elements may also be connected with one another, such as by welding, soldering or adhesive bonding. 
     The rod elements of the framework structure may be connected with one another at the vertices of the truncated octahedrons, for example, by welding, soldering or adhesive bonding, or else by bosshead elements. 
     The bonding of the space lattice structure to the cover and intermediate layers through the connecting squares is realized depending on the materials employed. For example, adhesive bonding, riveting, soldering or welding are suitable methods. With reinforced plastic materials or fibrous materials, a material connection through the plastic material or the impregnating agent which are cured after the joining by cooling, drying or chemical reaction is also possible. 
     It is advantageous to provide fixing aids on the connecting square faces and/or the cover or intermediate layers that improve the joining and stability of the space lattice structure and the cover or intermediate layer. For example, such fixing aids may be roughened spots for adhesive bonding, preformed holes for riveting or screwing, especially latches attached to rod elements for riveting, screwing or adhesive bonding. 
     The lightweight construction elements according to the invention are characterized by excellent strength properties, especially compressive strength, flexural rigidity, resistance to shock and contact loads, shear resistance in longitudinal and transversal directions, while having low basis weights. The space lattice structures between the cover layers also have a vibration-damping effect, even when the cover layers are in resonance. Therefore, they act against the transmission of vibrations from one cover layer to the other, so that the lightweight construction elements according to the invention are excellently suitable as sound-protection elements despite their low basis weight. 
     In the space lattice structures and lightweight construction elements according to the invention, forces acting thereon are dissipated, so that load peaks cannot occur. A high section modulus is realized thereby. 
     The space lattice structures and lightweight construction elements according to the invention can be employed in many fields of technology, including automobile construction, e.g., for engine hoods, trunk lids, rear shelves, trunk floors and side trims, especially for doors, in aviation technology, e.g., for a tubular exterior wall or for wing profiles, and in construction engineering, e.g., for sound-insulating walls. 
     The invention also involves a process for the preparation of the lightweight construction elements. In principle, it is possible to prepare the truncated octahedron solids individually as hollow bodies and then to arrange them in the space lattice structure according to the invention. A simpler process results from the property of truncated octahedrons that its halves (as defined above) are identical and can be made to coincide by a rotation by 180°. Further, the truncated octahedrons are arranged in a space-filling way in the space lattice structure so that they contact each other and are interconnected. Therefore, an open mold of adjacent halves of truncated octahedrons can be prepared, which is much simpler than the production of individual hollow bodies. The arrangement of the complete truncated octahedrons is then obtained by rotating part of such mold by 180° or by shifting it by half a cell diagonal, placing it on and connecting it with the rest through the square faces. 
     Thus, the process for the preparation of the lightweight construction elements according to the invention comprises the following steps:
         providing sheet materials for the cover and intermediate layers;   preparing molded parts as sandwich cores represented by the faces of truncated octahedrons cut in half and arranged in a space-filling manner;   interconnecting two sandwich cores by connecting square faces to form a structure of whole truncated octahedrons;   connecting the structure obtained with a cover layer on one side and a cover or intermediate layer on the other side through square faces.       

     For example, the sandwich core molded parts can be formed from a sheet-like thermoplast (such as polyamide, optionally fiber-reinforced) by hot forming into plates representing an arrangement of the surface elements of interconnected half truncated octahedrons. As mentioned above, an arrangement of lower halves interconnected through the squares diagonally cut in half equally bounds an arrangement of upper halves on their bottom side. Thus, it is sufficient to place a second sandwich core molded part on a first one in such a way that the complete squares coincide (shifted by half a cell diagonal) and to interconnect them, e.g., by adhesive bonding, to obtain the structure of complete truncated octahedrons. This structure may now be connected with cover layers and optionally intermediate layers through the still exposed square faces. 
     Of course, thicker cores can be prepared by placing one or more further sandwich core molded parts onto the structure consisting of two layers and connecting them through the square faces. An intermediate layer may also be inserted anywhere in the order. After further sandwich core molded parts have been applied and connected, the lightweight construction element is then concluded with a cover layer. 
     The process according to the invention enables a simple and economically efficient preparation of the subject matters of the invention. It does not require any new technology and can be performed with the known methods, such as calendaring, injection molding, embossing, injection embossing or compression molding. 
     Using this process, the space lattice structures and lightweight construction elements according to the invention can be adapted to the respective intended use in a very flexible way. Possible applications are, for example, in automobile construction, in aerospace technology, in construction engineering, especially in lightweight construction, and in sound protection. 
    
    
     
       The invention will now be illustrated further by means of Examples and the accompanying drawings, wherein: 
         FIG. 1  shows a truncated octahedron in relation to the generating octahedron in perspective view; 
         FIG. 2  shows a space-filling arrangement of truncated octahedrons in perspective view; 
         FIG. 3  shows four truncated octahedrons from  FIG. 2  in top plan view; 
         FIG. 4  shows a space lattice structure constituted by two layers of half truncated octahedrons forming complete truncated octahedrons in side elevational view; 
         FIG. 5  shows the structure of  FIG. 4  in top plan view; 
         FIG. 6  shows the structure of  FIG. 4  in perspective view; 
         FIG. 7  shows the structure of  FIG. 4  as a core in a lightweight construction element. 
         FIG. 8   a  shows a cross-sectional view of a molded part. 
         FIG. 8   b  shows a cross-sectional view of the same molded part with thinned-out surface elements. 
         FIG. 9  shows a lightweight construction element with inner layers having different heights of the truncated octahedrons. 
     
    
    
       FIG. 1  shows some essential properties of the truncated octahedron  1  and its relationship with the generating octahedron  2 . As can be seen, the truncated octahedron is obtained by cutting all the six vertices  3  of octahedron  2 . For a regular truncated octahedron to be produced, each octahedral edge  4  must be cut in three equal parts a. Since the faces of the regular hexagons  5  in part form common edges with squares  6 , we have the result that all 36 edges of the truncated octahedron are of equal length. 
       FIG. 2  shows an arrangement of 16 equal truncated octahedrons  1  juxtaposed in such a way that their square faces  6  respectively facing towards the neighbor  1  coincide. The arrangement is a square lattice. The capacity of space filling can be seen from the fact that the depressions formed between the solids exactly have the shape of half a truncated octahedron. If further truncated octahedrons  1 ′ were inserted into these depressions, a layer identical with the first layer would be obtained that would only be shifted by half a cell diagonal and elevated by half the height of the truncated octahedron. A third layer would correspond to the first with elevation by one truncated octahedron height. Thus, a space-filling arrangement of a body-centered cubic lattice is obtained. 
       FIG. 3  shows a detail from  FIG. 2  in top plan view. The four truncated octahedrons  1  bound a void  7  between them formed by the hexagons  5  and the open square  6 ′, which is evidently complementary to each upper half  1 ″ of the octahedrons. Due to the symmetry of the solids, the same picture would be obtained in the bottom layer. This means that a truncated octahedron can be inserted into the void  7  from both above and below. Like the lateral square faces  6  of the truncated octahedrons  1  coincide in this arrangement, the top and bottom squares of these fictitiously inserted truncated octahedrons would also coincide in the open square  6 ′. 
       FIG. 4  shows how two partial layers composed of half truncated octahedrons  1 ″ again form a layer with complete truncated octahedrons. The lower partial layer  11  consists of the hexagonal faces  15  and the square connecting faces  16  (facing outward) and  17  (facing inward). The sideward facing squares  13  are cut in half. The corresponding faces are found in the upper partial layer  12 . Now, the upper partial layer  12  resides on the lower partial layer  11  in such a way that the respectively inner square faces  17  coincide and can be interconnected (e.g., by adhesive bonding). This produces an arrangement of complete truncated octahedrons  18 . 
       FIGS. 5 and 6  show the same arrangement as  FIG. 4 , but in top plan view and in perspective view for a better understanding. The Figures show that a partial layer can be easily prepared by correspondingly reforming a plate-shaped semifabricated product. 
       FIG. 7  shows a lightweight construction element with the structure of  FIG. 4  as a sandwich core. The lower cover layer  19  and the upper cover layer  20  are adhesively bonded to the core through the outer square faces  16 . The distance between the cover layers is equal to the height h of the truncated octahedron, measured as the distance between opposing lateral square faces. If the lightweight construction element is prepared with only one partial layer, the connection to the cover layers is achieved through the outer squares  16  and the “inner” squares  17 , and the distance between the cover layers is h/2. It is also possible to join three or more partial layers into a sandwich core. At any rate, the interconnection of the partial layers and the connection to the cover layers is achieved through squares  16 ,  17 . The distance between the cover layers is nh/2 for n partial layers. 
       FIG. 8   a  shows a detail of a cross-sectional view of a molded part as the partial layer of a sandwich core. Both the hexagonal surface element  25  and the square surface element  26  have a material thickness of 0.5 mm, for example. The entire cross-section of this profile then has an area of 8.7 mm 2 .  FIG. 8   b  shows the effect of the middle regions of the surface elements being thinned out. In accordance with a circular arc (or a spherical cap), the cross-section has been thinned out from both sides until a material thickness of only 0.1 mm was present in the middle of the faces. At the same time, the inner and outer vertices of the truncated octahedron were formed with radii in order to deliberately produce an accumulation of material on the edges, which thus represent a space lattice and framework. The cross-sectional area of the profile was reduced to 4.2 mm 2  thereby, which corresponds to a saving of material of more than 50% without substantially affecting strength. However, the thinned-out region  27  on the connecting outer square face  26  in this form could render the adhesive bonding to other components (partial or cover layers) more difficult. However, it is altogether possible to perform the thinning out of this surface element  26  only from the inner side  28 . 
       FIG. 9  shows a lightweight construction element with inner layers having different heights of the truncated octahedrons. The middle layer has the height 2h, where h is the height of the outer layers. Cover layers  19 ,  20  are provided for outward stabilization. The connection between the middle layer and the outer layers is established by intermediate layers  29 . In this case, where the connecting square of every second truncated octahedron of the outer layer meets one of the inner layer, the intermediate layers may also be omitted. 
     Load-Bearing Space Lattice Structure, Lightweight Construction Element and Process for the Preparation Thereof 
     LIST OF REFERENCE SYMBOLS  
       1 ,  1 ′ truncated octahedron 
       1 ″ half truncated octahedron 
       2  octahedron 
       3  vertex of truncated octahedron 
       4  edge of truncated octahedron 
       5  hexagonal face of truncated octahedron 
       6  square face of truncated octahedron 
       6 ′ open square 
       7  void 
       11  lower partial layer 
       12  upper partial layer 
       13  lateral square faces 
       15  hexagonal faces 
       16  outer square faces 
       17  inner square faces 
       18  assembled complete truncated octahedron 
       19  lower cover layer 
       20  upper cover layer 
       25  hexagonal surface element 
       26  square surface element 
       27  thinned-out region 
       28  inner side 
       29  intermediate layer 
     a edge length of truncated octahedron 
     h height of truncated octahedron