TECHNICAL FIELD

The present invention relates to a sandwich construction element, and more particularly to a sandwich construction element with an open core. BACKGROUND OF THE INVENTION

A problem during design of for example a ship, encountered today is that if for example a hull of a ship is manufactured with modern alloys and design, the hull becomes lightweight and strong. But the deck and the various vertical and horizontal walls used in the construction of a ship becomes heavy weight if standard design is used. Therefore, a need for lightweight and sturdy panels of various dimensions and areas exists.

A promising design for such panels is disclosed in US 8,650,756 B2 which provides a solution that involves either extrusion of panels or additive/subtractive manufacturing. If extrusion is used the available materials for construction is rather limited and a lightweight alloy, such as an alumina alloy would be the most likely candidate. However, if the hull of the ship is manufactured in stainless-steel there is a problem related to joining alumina alloys and stainless steel. There exists an alternative to manufacture panels using additive/subtractive manufacturing, but this alternative is rather expensive if large panels are desired.

Furthermore, the truss design used in US 8,650,756 B2 is rather weak and leaves room for improvement. One of its advantages is that the core of the sandwich material may be formed by bending a metal net to a pyramidal lattice. It is well known in solid mechanics that this truss structure is far from optimal from a structural strength point of view, for example if one of the base corners of the pyramidal lattice is subjected to momentum. It is an object of the present invention to provide a solution that is compatible with modern ship design that involves lightweight hulls.

It is another object of the present invention to provide a more robust and sturdy sandwich construction that simultaneously provides very low density. It is another object of the present invention to provide a sandwich construction that provides excellent resistance against corrosive agents or environments.

SUMMARY OF THE INVENTION

According to the present invention, the above mentioned objects and other advantages are obtained by providing a sandwich construction element according to the independent claim.

In one aspect, the present invention provides a sandwich construction element. The sandwich construction element comprises a first element with a face, extending in a longitudinal direction with a thickness and a height being smaller than the longitudinal length. The sandwich construction element further comprises a second element with a face, extending in the same longitudinal direction as the first element with a thickness and a height being smaller than the longitudinal length, wherein the second element is facing the flat face of the first element. The sandwich construction element further comprises an open core structure arranged between, and operatively connected to the first element and the second element. The sandwich construction element is characterized in, that the open core structure comprises a plurality of close packed tetrahedron structures.

The terms "flat", "close packed", and "unit cell" as used herein are to be interpreted in a broad sense. The term "flat" should be interpreted as essentially flat. The term "close packed" should be interpreted as fully packed, unit cell should be interpreted as an element used to close pack a volume i.e. to completely fill the volume with unit cells.

In one embodiment of the sandwich construction element, the open core structure comprises at least two flat elements arranged between the first element and the second element, wherein the at least two flat elements are arranged between the faces of the first element and the second element, and facing each other with a first distance between the two elements.

In one embodiment of the sandwich construction element, the open core structure comprises a plurality of unit cells arranged between two of the at least two flat elements, wherein each unit cell comprises two flat parallelogram arranged in an overlapping intersecting relation along a diagonal of the two flat parallelogram, with an angle between the two flat parallelogram.

In one embodiment, wherein each of the parallelogram comprises a slit along the diagonal of the parallelogram from a corner to at least the center of the parallelogram, wherein the slit is configured to receive a corresponding parallelogram with a slit such that the two parallelograms are joined along the diagonal of each parallelogram. In this way, a strong and flexible joint between the first parallelogram and the second parallelogram may be obtained. In one embodiment, each of the parallelograms of the unit cell comprises a recesses arranged at a distance from the diagonal. This way allows an open connection between the pluralities of unit cells. Furthermore, this allows the unit cell to be

dimensioned for a specific load without unnecessary mass.

In one embodiment, the diagonal is the shortest diagonal of the two flat

parallelograms. This way, a preferred unit cell is obtained.

In one embodiment, the face of the first element, and the face of the second element are flat. This way, a symmetrical construction element is obtained.

In one embodiment, the plurality of unit cells are formed by two flat arrays, wherein each flat array comprises a repeating triangular wave pattern formed by a first parallelogram having a first edge operatively connected to an edge of an intermediate parallelogram. In this way, an efficient way of manufacturing the unit cells are obtained.

In one embodiment, the sandwich construction comprises sheet metal, and in a preferred embodiment the sheet metal is made of stainless steel. In this way, the sandwich structure may resist corrosive agents and environments simultaneously as the sandwich construction is lightweight and provides excellent structural strength.

A more complete understanding of the invention, as well as further features and advantageous thereof, will be obtained by reference to the following detailed description 5 and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an exploded schematic perspective view of an embodiment of a sandwich structure according to the present invention.

Fig. 2 is a schematic perspective view of an embodiment sandwich construction i o according to the present invention.

Fig. 3 is a schematic perspective view of a unit cell according to the present invention.

Fig. 4 is a schematic plan view of parallelograms according to the present invention.

1 5 Fig. 5 is a planar view of an embodiment of a flat array according to the present invention.

Fig. 6 is a schematic perspective view of an embodiment of a flat array according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

20 The following description is for illustration and exemplification of the invention only and is not intended to limit the invention to the specific embodiments described.

All references cited herein, including patents and patent applications are incorporated by reference in their entirety.

Unless defined otherwise, technical and scientific terms have the same meaning as 25 commonly understood by one of ordinary skill in the art to which this invention belongs. The meaning of the terms "flat", "close packed" and "unit cell" as used herein are as follows.

The term "flat" should be interpreted as essentially flat.

The term "close packed" means that a volume is completely filled with a basic 5 construction in a symmetric and repetitive way.

The "unit cell" is the basic geometry used for close packing of a volume.

Fig. 1 shows an exploded perspective view of a sandwich construction element, generally designated 100, according to a first embodiment of the present invention. The sandwich construction element 100 comprises a first element 101 with a flat face and a i o second element 102 with a flat face being parallel and facing the flat face of the first element 101. The sandwich construction element further comprises an open core structure 103 arranged between the first element 101 and the second element 102. The open core structure 103 is operatively connected to the first element 101 and the second element 102.

1 5 The open core structure 103 comprises a plurality of close packed tetrahedrons structures 201.

Furthermore, the open core structure comprises at least two flat elements 104, 105 arranged between the first element 101 and the second element 101. The at least two flat elements 104, 105 are arranged perpendicular to the flat faces of the first element

101 and the second element 102, and facing each other with a first distance between the two flat elements 104, 105.

The two flat elements 104, 105 and the first element 101 and the second element

102 define a box shaped volume. This volume is filled with unit cells 106 in a repetitive pattern as shown exploded in Fig. 1.

In Fig. 2a the sandwich construction element 100 is shown with the unit cells 106 packed together. In this Fig. 2a it is shown that by packing the unit cells together the box shaped volume is close packed filled with tetrahedron structures 201. In this figure the first element 101 and the second element 102 are not shown since they would obstruct the perspective view of the tetrahedron structures 201. In Fig 2b an additional flat element 202 is added to the at least two flat elements 104, 105 at a distance equal to the first distance, and the volume defined by the flat elements 105, 202 is close packed with tetrahedron structures. By adding more flat elements and unit cells panels of arbitrary sizes can be formed in a repetitive pattern.

In Fig. 3 the unit cell 106 is shown in a perspective view with an added imaginary dotted box 300 as a help structure. The dotted box 300 has a width of b, a depth of d and a height of h in arbitrary units. In a first corner 307 is a coordinate system defined with an X axis along the base line of the width direction of the box 300, a Y axis extends along the baseline of the depth direction of the box 300, and a Z axis extends along the baseline of the height direction of the box 300. In order to define the unit cell 106 six points are needed. These six points are as follows p1 =[0,0,0], p2=[b/2,0,h], p3=[0,d,h], p4=[b/2,d,0], p5=[b,d,h], and p6=[b,0,0], where [x coordinate, y coordinate, z coordinate].

As can be seen from figure 3, the six points in the unit cell, defined above, define two tetrahedrons sharing a diagonal spine. The tetrahedrons may have an XZ plane that is perpendicular to the XY plane and are thus asymmetric tetrahedrons.

Fig. 3 shows that a first parallelogram 301 is formed by point's p2, p3, p4 and p6. A second parallelogram 302 is formed by p1 , p2, p5, and p4. These two flat

parallelograms 301 , 302 are arranged in an overlapping intersecting relation along a diagonal d between point's p2 and p4, with an angle 303 between the two flat

parallelograms 301 , 302. From this figure a triangle is identified in the XZ-plane between point's p1 , p2 and p6 and from a solid mechanics point of view this triangle is most preferably an equilateral triangle.

In a unit cell according to the present disclosure, a volume is formed between the tetrahedrons, which is asymmetric and have two of the planes perpendicular. This has the advantage that a pattern with unit cells that alternate orientation can create a rectangular parallelepiped. I.e. a volume with parallel sides which is good or

advantageous, e.g. for generating sheets, plates, columns and beam profiles. This is in contrast to a unit cell based on symmetric tetrahedrons. A symmetric tetrahedron is defined as having four equilateral triangle faces, where every in-plane angle is 60° and every edge is of equal length. Symmetric tetrahedrons cannot easily be assembled in a pattern that gives flat, parallel sides. Now with reference made to Fig. 4a-c different embodiments of the parallelograms 301 , 302 will be discussed.

In Fig. 4a a first embodiment of a parallelogram 400a is disclosed. The

parallelogram 400a comprises a slit 401 along the diagonal d of the parallelogram from a corner to at least the center 403 of the parallelogram.

The slit 401 is configured to receive a corresponding parallelogram 400a with a slit 401 such that the two parallelograms are joined along the diagonal of each

parallelogram, such that the end portions of the slit of the two parallelograms overlaps. The parallelogram is also asymmetric as two of the sides are longer than the other two, thereby forming a tilted rectangle. When two such parallelograms are joined, e.g. by bringing one into the other via the slits, they create a cross that shape or form the diagonal back of two tetrahedrons.

In Fig. 4b, a second embodiment of a parallelogram 400b is disclosed. This parallelogram 400b comprises recesses 404b arranged at distance from the diagonal d. In this particular embodiment the recesses are triangles but other shapes are of course possible such as holes 404b and 404c as disclosed in Fig. 4c.

The purpose of the recesses may be to provide a more lightweight structure, or for allowing fluid communication between the unit cells. In one embodiment the at least two flat elements comprises recesses for the same purposes.

The parallelogram may also comprise tabs along the sides of the parallelogram facing the at least two flat elements, wherein the flat elements comprises corresponding recesses. In this way, the unit cells may be operatively connected to the at least two flat elements. In one embodiment the unit cells are operatively connected to the first and the second element, with for example a welded joint or an adhesive. Fig. 5 discloses a flat array 701 of parallelograms 702, 705 with intermediate parallelograms 706. This flat array 701 forms a repeating triangular wave pattern by a first parallelogram 702 having a first edge 703 operatively connected to an edge of an intermediate parallelogram 706. In one embodiment, the flat array may comprise folding lines along the common edges 703,704 between adjacent first parallelogram and intermediate parallelogram, wherein the intermediate parallelogram comprises a folding line along its short diagonal.

By bending the parallelograms of a first flat array and a second flat array, along the 5 edges 703 in a first angle and the intermediate parallelograms along the short diagonals in a second angle, a bended array that can be used to obtain an array of unit cells is formed.

In Fig. 6a is a bended first array 601 arranged opposite a bended second array 602, with corresponding recesses aligned to each other. By joining theses arrays 601 , i o 602 an array of unit cells are formed. This has the effect that a large number of unit cells may be efficiently manufactured.

In one embodiment, the sandwich construction element comprises sheet metal. And in a preferred embodiment the sheet metal is stainless steel. This way the sandwich construction element may be efficiently integrated with a modern hull of a ship.

1 5 In one embodiment of the sandwich construction, the unit cells of the open cell core is operatively connected to the at least two flat elements by means of tabs extending from the unit cells into corresponding grooves in the at least two flat elements.

In one embodiment of the sandwich construction element the unit cells of the open cell core is operatively connected to the first element and the second element by means 20 of an adhesive.

In one embodiment of the sandwich construction element the unit cells of the open cell core is operatively connected to the first element and the second element by means of welding.

The present invention is not limited to the above-described preferred embodiments. 25 Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.