Technical Field

[0001] The invention relates to plate-shaped structures, such as panels and aerofoils. Furthermore, the invention relates to methods for producing such a structure, and to a composite construction which includes such a structure.

Background Art

[0002] Buildings and infrastructure typically include panels, boards, slabs or other plate-shaped elements that are designed to sustain high mechanical loads. Such plate-shaped elements have good structural integrity, and preferably a low weight. A bridge deck, for instance, has to be sufficiently strong to carry the combined weight of the bridge and the traffic it supports, and should have a robust deck surface that stays intact when subjected to changing traffic loads, to impact from falling objects, as well as to weather influences. [0003] Patent publication WO2010/008293 A2 describes a composite panel formed of a plurality of core elements that are placed in a parallel arrangement, and which are interconnected via strips made of fibre material that extend in-between the core elements and continue above and below the core elements in an overlapping manner to form outer skins. The material strips are fixed by a cured resin that is applied via resin transfer moulding (RTM) techniques. This known panel construction method provides a robust connection between the core elements and outer skins, and reduces the probability of outer skins detaching from core elements in response to local loads.

[0004] Patent publication WO2016/085336A1 describes a pultrusion and autoclave technique for forming a semi-finished panel structure from overlapping sheets. These sheets are folded around shaping elements and cured in a continuous manner. No core elements are present in the resulting semi-finished structure, and the spaces enclosed between the overlapping sheets remain void.

[0005] It would be desirable to provide a plate-shaped structure with improved robustness. Summary of Invention

[0006] In a first aspect of the invention, there is provided a plate-shaped structure composed of a plurality of sheets arranged in a sequence and attached to each other. The plate-shaped structure defines first and second surfaces that extend predominantly in first and second directions and are located on opposite sides of the structure with respect to a third direction. The structure further defines a side surface facing towards the first direction. Each respective sheet defines a first lateral strip, a second lateral strip, and a medial strip. The first lateral strip extends predominantly in the first and second directions at or near the first surface, whereas the second lateral strip extends predominantly in the first and second directions at or near the second surface. The medial strip extends along the third direction through the structure, and forms an interconnection between the first and second lateral strips. The sheets are arranged in a mutually overlapping sequence, such that first lateral strips of adjacent sheets mutually overlap, second lateral strips of adjacent sheets mutually overlap, and medial strips of adjacent sheets are mutually separated (interspaced) along the second direction by non-zero spacing distances. At least two sheets further define respective flaps. Each respective flap forms an extension of one of the medial strip, the first lateral strip, and the second lateral strip of the corresponding sheet. These flaps extend predominantly in the second and third directions and overlap each other along the side surface of the structure.

[0007] The term "sheet" is used herein to refer to a piece of material that is very thin in comparison to its length and width dimensions e.g. by at least two orders of magnitude. Each such sheet is pre-formed into a self-supporting three-dimensional surface shape with several distinct flat regions (e.g. by casting or moulding), or can be formed from a planar sheet blank into a developable surface with such distinct flat regions (e.g. via cutting, folding, bending, and/or rolling). The sheets may be composed of various materials, which may be initially rigid but plastically deformable to allow the sheet to be folded along folding lines when subjected to sufficient forces, but leaving other regions of the sheet dimensionally stable. For example, initially rigid sheet materials may consist essentially of aluminium sheet material, steel sheet material, thermoplastic sheet material, or the like. Alternatively, the sheet material may be flexible initially (prior to forming the structure), to allow bending or folding into a three-dimensional surface configuration. Initially flexible sheet material may for example consist essentially of a fabric (e.g. a web, mesh, or mat) of fibre material, which may for instance be composed of continuous fibers (e.g. as mono-filament fibers, or yarns/twines of fibers) that are interlocked/interlaced and extending with their direction of elongation predominantly along sheet surface. The manufacturing of such a fibre mat structure should involve a hardening stage in which the sheets are settled in essentially fixed orientations within the resulting structure. Such a hardening stage may for instance include impregnation of the fibre sheets by a liquid resin, followed by curing of the resin to form a rigid matrix in which the fibres are embedded. Impregnation of the sheet arrangement may for example occur via vacuum assisted resin infusion moulding (RIM) or resin transfer moulding (RTM) techniques. Alternatively, the individual sheets may be impregnated and shaped in advance ("prepreg sheets"), followed by stacking and bonding of the sheets to form a desired profile. In yet another alternative, the sheets may be impregnated directly prior to the arrangement of the sheets into the desired profile e.g. via an immersion bath. In yet alternative implementations, composite constructions with both initially rigid sheets and initially flexible sheets may be used, for example glass reinforced aluminium laminate structures.

[0008] The terms "first lateral strip", "second lateral strip", and "medial strip" refer herein to distinct strip-shaped portions of a respective sheet that are identifiable once the sheet has assumed its three-dimensional surface shape. These particular strips are not isolated bands of sheet material, but are interconnected along edges to form a continuous sheet that extends as a surface shape in three dimensions. The medial strip continues along one of its edges via a first bending/folding line into the first lateral strip, and continues on an opposite edge via a second bending/folding line into the second lateral strip. [0009] Similarly, the term "flap" refers to another part of the sheet once it has assumed its three-dimensional surface shape. The flaps cover (at least part of) the side surface of the plateshaped structure, this side surface being associated with the face end (crosscut) surfaces of the core elements or voids enclosed inside the structure. The mutual overlap between the flaps reinforces this side surface, thereby increasing the structure's resistance to tensile/torsional loads acting along this side surface. The overlapping flaps may further allow the face end sides of the core elements or voids to be sealed by the same sheets that also form the structure, so that dust or moisture are prevented from entering the structure.

[0010] Selective ones or all of the strips (i.e. the medial / first lateral / second lateral strips) may define respective flaps. The flaps may be conceptually grouped into one, two, or three overlapping sequences, depending on whether these flaps are present on one, two, or all three of the strips. All of the sheets or only selected ones of the sheets in the structure may define such flaps. For instance, all of the sheets may define only a medial flap, or only first and second lateral flaps. Instead, the presence of flaps may alternate along consecutive sheets, for instance in a periodically alternating pattern such as "sheet with one flap - next sheet with no flap - next sheet with one flap - etc." or such as "sheet with first lateral flap - next sheet with second lateral flap - next sheet with medial flap - etc.". Overlapping flaps are preferably mechanically attached to each other. Each flap preferably overlaps and is attached to at least two flaps of adjacent sheets (either directly, or indirectly via another interposed flap).

[0011] The term "overlap" and the phrase "A overlaps (with) B" are used herein to indicate that part or all of object A extends over and covers at least a part or all of object B. Furthermore, the expression "A overlaps (with) B in/along direction Q" is used herein to indicate that A extends in the above-mentioned manner over part or all of B along the Q-direction. As a result, object A (partly) covers object B if viewed along at least one direction perpendicular to Q. The overlap of objects A and B may but does not necessarily imply that A and B are in direct physical contact. The overlap defines a reciprocal spatial relation, in that "A overlaps B" also implies that "B overlaps A".

[0012] According to embodiments, the flaps include medial flaps. A respective medial flap forms an extension of the medial strip of the corresponding sheet. The medial flaps may be folded towards the second direction to overlap each other and form an imbricated sequence along the side surface of the structure.

[0013] The term "imbricated" refers herein to a collection of overlapping objects that is stacked in a repetitive sequence, in which the trailing end of one object is overlapping the leading end of the next object, similar to roof tiles. The resulting medial flaps may for example form an obliquely layered sequence of overlapping flaps, in which the flaps are slightly tilted relative to the side surface.

[0014] In further embodiments, a respective medial flap extends in the second direction over a length Lmf that is at least three times the spacing distance DU between the medial strips, i.e. Lmf > 3 DU, so that the medial flap overlaps at least two adjacent medial flaps of adjacent sheets. For instance, all medial flaps of the structure may extend over lengths of at least three times the medial strip interspacing distance. The flap overlap may for example include four to six layers, to obtain a good balance between strength and manufacturing complexity of the structure.

[0015] In further embodiments, the overlapping medial flaps of adjacent sheets form an obliquely imbricated sequence, in which each medial flap is tilted about a rotation axis in the vertical direction and outwards with respect to the side surface, at a tilt angle ym in a range 0° < ym < 5° towards the first direction. The tilt angle ym may for instance be in a range of 0° < ym < 2°. This obliquely layered arrangement with small tilt angle improves the resistance to tensile strength loads acting along the side surface of the structure.

[0016] In embodiments, the flaps may include first lateral flaps and/or second lateral flaps, alternative or in addition to the medial flaps. A respective first lateral flap forms an extension of the first lateral strip of the corresponding sheet. These first lateral flaps are folded towards the third direction to overlap each other and form a first imbricated sequence along the side surface of the structure. A respective second lateral flap forms an extension of the second lateral strip of the corresponding sheet. These second lateral flaps are folded towards the third direction to overlap each other and form a second imbricated sequence along the side surface of the structure.

[0017] In further embodiments wherein both first and second lateral flaps are present, the first lateral flaps may adjoin the second lateral flaps in a non-overlapping manner, such that their two common edges extend in the second direction along a seamline across the side surface.

[0018] In embodiments that include second lateral flaps, the second lateral flaps of adjacent sheets may overlap such as to form a second obliquely imbricated sequence. Each second lateral flap may be tilted about a rotation axis in the vertical direction and outwards with respect to the side surface towards the first direction, at a second tilt angle y2 in a range 0° < y2 < 5°.

[0019] In a further embodiment that includes both medial flaps and second lateral flaps, the second lateral flap of a respective sheet may directly overlap the medial flap of the same sheet, such that the second flaps and the medial flaps overlap each other in an alternating manner. [0020] In yet a further embodiment, the second lateral flap may overlap only a second portion of the medial flap without covering a first portion of the medial flap. In this case, the first portion may be tilted about the rotation axis in the vertical direction and outwards with respect to the side surface, at the tilt angle ym, whereas the second portion and second lateral flap are jointly tilted outwards with respect to the side surface, at the second tilt angle y2 that exceeds the tilt angle ym. The second tilt angle y2 may for instance be in a range ym < y2 < 5°.

[0021] In embodiments that include first lateral flaps, the overlapping first lateral flaps of adjacent sheets may overlap such as to form a first obliquely imbricated sequence. Each first lateral flap may be tilted about a rotation axis in the vertical direction and outwards with respect to the side surface, at a first tilt angle y1 in a range -5° < y1 < 0°. The first tilt angle may be in a range -2° < y1 < 0°.

[0022] In embodiments that include both medial flaps and first lateral flaps, each first lateral flap may be displaced outwards along the first direction over at least a distance relative to the medial flap of the corresponding sheet. In this case, the entire first imbricated sequence of first lateral flaps may cover the entire imbricated sequence of medial flaps along the side surface. This yields a smoother arrangement of overlapping flaps on the side surface, and an improved tensile load capacity.

[0023] In embodiments, the sheets are arranged to form a sequence of Z-shaped sheets, when viewed in cross-sectional planes along the second and third directions. In such a Z-shaped sheet arrangement, the first lateral strips mutually overlap and extend in a negative second direction along the first surface, the second lateral strips mutually overlap and extend in a positive second direction along the second surface, and the medial strips extend through the structure and each interconnecting respective first and second lateral strips of the corresponding sheet.

[0024] Examples of known Z-shaped sheet arrangements can be found in patent document W02010/008293A2. In the present embodiments, however, the sheets additionally include mutually overlapping flaps along the side surface, which may help to improve the strength of the structure. It should be understood that the various Z-shaped arrangements may be formed based on either a right-handed or a left-handed coordinate system.

[0025] In alternative embodiments, the sheets are arranged to form a sequence of U-shaped (P-shaped) sheets, when viewed in cross-sectional YZ-planes. In such U-shaped arrangements, the first lateral strips mutually overlap and extend towards a positive second direction along the first surface of the structure, the second lateral strips mutually overlap and extend towards the positive second direction along the second surface of the structure, and the medial strips extend through the structure and interconnect respective first and second lateral strips.

[0026] Examples of known U-shaped sheet arrangements can also be found in document W02010/008293A2. In the present embodiments, however, the sheets additionally include mutually overlapping flaps along the side surface. Again, it should be understood that the U- shaped arrangements may be formed based on either a right-handed or a left-handed coordinate system.

[0027] When both medial flaps and first lateral flaps are present, the first lateral flap may overlap the medial flap of the same sheet (U-shaped arrangement) or overlap the medial flap of the preceding sheet (Z-shaped arrangement). When both medial flaps and second lateral flaps are present, the second lateral flap may overlap the medial flap of the same sheet (in both U- and Z-shaped arrangements).

[0028] In embodiments, a height of the medial flap equals a height of the medial strip. Alternatively or in addition, a width of the first lateral flap may equal a width of the first lateral strip. Alternatively or in addition, a width of the second lateral flap may equal a width of the second lateral strip. Alternatively or in addition, a length of the medial flap may equal a width of the first lateral flap or of the second lateral flap. Each of the above-mentioned flap dimensions contributes in maximizing the coverage of the side surface by the flaps as well as the available flap area for interconnecting the flaps, in order to obtain a smoother and stronger side surface.

[0029] In embodiments, the combined lengths of the first lateral flap and the second lateral flap along the third direction are approximately equal to the height of the medial strip along the third direction. In particular, the length of the first lateral flap may be equal to a length of the second lateral flap. Preferably, this length is about half the height of the medial strip.

[0030] In various embodiments, the spaces within the structure that are enclosed by the medial strips from the second directions, by the first and second lateral strips from the third directions, and by the flaps from the first direction, may remain void in order to reduce overall weight of the structure. Alternatively, the spaces may accommodate core elements to improve the mechanical integrity of the structure or to facilitate its fabrication.

[0031] In embodiments including core elements, the elements are arranged in a sequence along the second direction and mutually parallel along the first direction, whereas the medial strip of each sheet is sandwiched between two adjacent core elements. The first lateral strip of each sheet may extend across first core sides that are located near the first surface and face towards the positive third direction, whereas the second lateral strip of each sheet may extend across second core sides that are located near the second surface and face towards the negative third direction. Preferably, each first and second lateral strip extends across at least three consecutive core elements.

[0032] The core elements may for instance be shaped as geometric prisms having polygonal cross-sectional shapes. The cross-sectional shapes may for instance be triangular, quadrilateral, pentagonal, or hexagonal shapes, and are preferably triangular or rectangular.

[0033] The arrangements of overlapping flaps in accordance with the first aspect (and its various embodiments) may be similarly applied on an opposite side surface of the structure, viewed along the first direction. This overlapping flap arrangement on the opposite side surface may be an identical mirror-image of the overlapping flap arrangement of the side surface relative to a central YZ-plane, but may also differ from that flap arrangement.

[0034] According to different embodiments, the plate-shaped structure may be substantially flat along the third direction, or may have a concave shape with (possibly smooth) curvature(s) in the third direction as a function along the first and/or second directions. The term "plate-shaped structure" refers herein to a self-supporting structural element having main surfaces that extend in two transverse directions and that predominantly face towards mutually opposite third directions, and having a thickness defined along the third direction and between the main surfaces that is considerably smaller than its transverse dimensions. The two main surfaces may be flat, like in a rectangular cuboid panel or board. However, plate-shaped structures with profiled main surfaces are also contemplated, for instance bridge decks or aero-/hydrofoils with curved main surfaces. Alternatively or in addition, the outer peripheral shape of the structure in the transverse directions may differ from a rectangle. Other quadrilateral peripheral shapes (e.g. a parallelogram, trapezium, or rhombus) or more complex peripheral shapes (e.g. gradually curved) are also possible.

[0035] In a second aspect of the invention, and in accordance with the advantages and effects described above, there is provided a reinforced construction including a structure according to the first aspect. The construction may for instance be a bridge, a bridge deck, a bulkhead, a lock gate, or a support platform. The construction may be a turbine blade, wing, airscrew, or aerofoil. The construction may further be a rudder blade, ship propeller, or hydrofoil.

[0036] A third aspect of the invention relates to a method of manufacturing the plate-shaped structures according to the first aspect and its various embodiments. [0037] A fourth aspect of the invention relates to a sheet element formed as a self-supporting three-dimensional surface element adapted to be combined with a plurality of similar sheet elements to form a structure according to the first aspect. The sheet defines a first lateral strip, a second lateral strip, and a medial strip. The first and second lateral strips extend predominantly in the first and second directions but at a mutual distance along the third direction. The medial strip extends predominantly along the third direction and interconnects the first and second lateral strips. The sheet further defines at least one flap, which forms an extension of one of: the medial strip, the first lateral strip, and the second lateral strip, and which extends predominantly in the second and third directions. The preformed sheet is adapted to be combined with a plurality of similar shaped sheets into a mutually overlapping sequence to form the structure, such that first lateral strips of adjacent sheets mutually overlap to form a first structure surface, that second lateral strips of adjacent sheets mutually overlap to form a second structure surface that faces away from the first surface, that medial strips of adjacent sheets are interspaced along the second direction by non-zero spacing distances, and that the flaps of subsequent sheets overlap each other along a side surface of the structure facing towards the first direction. The self- supporting sheet element may include any of the features (or combinations thereof) described with reference to embodiments of the first aspect. The first, second, and medial strips of the sheet element may for instance be arranged in a Z-shape.

Brief Description of Drawings

[0038] Embodiments will now be described, by way of example, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts. In the drawings, like numerals designate like elements. Multiple instances of an element may each include separate letters appended to the element number. For example, two specific instances of a particular element “46” may be labelled with appended letters “46a” and “46b”. The element label may be used with an index i or j (e.g. “46i”) to refer to an unspecified instance of the element, while the element label may be used without an appended letter (e.g. “46”) to generally refer to every instance of the element. [0039] Figure 1a schematically shows a perspective view of a structure according to an embodiment;

[0040] Figure 1 b schematically shows a perspective cross-sectional view of parts of the structure from figure 1a;

[0041 ] Figure 2 schematically shows a side view of the structure from figures 1 a-1 b; [0042] Figure 3 schematically shows a perspective view with dimensional indications of parts of the structure from figures 1a-2;

[0043] Figure 4 schematically shows a perspective view of a sheet and a partial core element in a structure according to another embodiment;

[0044] Figure 5 schematically shows a top view of the structure from figure 4; [0045] Figure 6 schematically shows a frontal cross-sectional view of the structure from figures

4-5, and

[0046] Figure 7 schematically shows a perspective cross-sectional view of parts of a structure according to yet another embodiment.

[0047] The figures are meant for illustrative purposes only, and do not serve as restriction of the scope or the protection as laid down by the claims.

Description of Embodiments

[0048] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the figures. In the figures, Cartesian coordinates will be used to describe spatial relations for exemplary embodiments of the panel.

[0049] Figure 1 a schematically shows a perspective view of a first embodiment of a structure 10, which in this example is formed as a composite panel. The panel 10 has a rectangular cuboid shape with dimensions in the longitudinal and transverse directions X, Y that are considerably larger than the thickness of the panel 10 in the vertical direction Z. This panel 10 may for instance be part of a bridge deck, a lock gate, a blast panel, platform, or a bulkhead. The panel 10 defines first and second surfaces 12, 14, which are located on opposite sides of the panel 10 and are facing towards positive and negative Z-directions, respectively. The panel 10 further defines a side surface 16 that faces predominantly towards the X-direction, and a further side surface 18 that faces predominantly towards the Y-direction. Similar side surfaces present on opposite sides of the panel 10 are not indicated in figure 1a. [0050] Figure 1 b schematically shows a perspective cross-sectional view of part of the panel 10 of figure 1a. The panel 10 is partially cut away to illustrate its structure at side surface 16. The panel 10 includes a series of sheets 30a, 30b, 30c, 30d, which are folded along multiple folding lines 32, 34, 48, 50, 52, and which are arranged in an overlapping sequence that has a periodicity in the Y-direction. The sheets 30 mutually overlap, and have partially exposed portions 36, 38,

42, 44, 46 that jointly give shape to various surfaces 12, 14, 16 of the panel 10.

[0051] Individual ones of the respective sheets 30i (i = a, b, c, d, etc.) are formed into developable three-dimensional surface structures. In particular, each sheet 30i is formed as a piece-wise planar 3D surface, which is composed of a first lateral strip 36i, a second lateral strip 38i, a medial strip 40i, and several flaps 42i, 44i, 46i. These strips and flaps 36i-46i are interconnected and form an integral part of this same sheet 30i, but assume different relative orientations by being folded along respective folding lines 32i, 34i, 48i, 50i, 52i. In this example, the three strips 36, 38, and 40 of each sheet 30 are folded to form an essentially right-angled Z- shape, when viewed along the X-direction and in cross-sectional planes along the Y- and Z- directions (figure 1b).

[0052] The first lateral strip 36i of each sheet 30i extends predominantly in the X- and Y- directions at or near the first surface 12. The second lateral strip 38i also extends predominantly in the X- and Y-directions at or near the second surface 14. The medial strip 40i extends with a major component along the Z-direction through the panel 10, and interconnects the first and second lateral strips 36i, 38i of the sheet 30i. In this example, the medial strips 40i of adjacent sheets 30i extend parallel to each other and largely parallel to the XZ-plane.

[0053] The first lateral strip 36i and medial strip 40i of each sheet 30i are connected along first folding line 32i, which extends in the X-direction. The second lateral strip 38i and the medial strip 40i are connected along second folding line 34i, which also extends in the X-direction. Each first strip 36 is folded away from its corresponding medial strip 40 along its folding line 32, to extend in negative Y-direction. Each second strip 38 is folded away from its medial strip 40 along folding line 34, to extend in positive Y-direction. The corresponding medial strip 40 extends through the thickness of the panel 10, and interconnects the first and second strips 36, 38.

[0054] The sheets 30 are arranged in a mutually abutting manner, and form a sequence of Z- shapes (figure 1 b). Each subsequent sheet 30j (j = b, c, d,..) is placed with its first and second strips 36j, 38j on top of corresponding first/second strips 36i, 38i of its directly preceding sheet 30i, in such a way that the first lateral strips 36 of adjacent sheets 30 mutually overlap, the second lateral strips 38 of adjacent sheets 30 mutually overlap, and the medial strips 40 of adjacent sheets 30 are mutually separated by non-zero spacing distances DU along the Y- direction. The sheets 30 are attached to each other, for instance by cured resin, by diffusion bonding, or by welding, in accordance with compatible methods for the selected sheet material (e.g. fibre mat, thermoplastic, steel).

[0055] The first strips 36 overlap and are fixed to each other, forming a periodic imbricated arrangement of strips 36 that defines the macroscopic outer contour of the first surface 12. Similarly, the second strips 38 overlap and are fixed to each other, forming a further periodic imbricated arrangement of strips 38 that defines the macroscopic outer contour of the second surface 14.

[0056] In this example, cuboid spaces are formed inside the structure 10, which each space being bounded by a first lateral strip 36, a second lateral strip 38, and pair of adjacent medial strips 40. These spaces are filled with cuboid core elements 20 made of polyurethane (PU) foam. These cores 20 are mutually parallel along the X-direction and form a periodic array along the Y- direction. The medial strip 40 of each sheet 30 is sandwiched between two directly adjacent cores 20. In this example with Z-shaped sheets 30, the first lateral strip 36 of each sheet 30 extends in the negative Y-direction across sides of the cores 20 that are located near the first surface 12, whereas the second lateral strip 38 of each sheet 30 extends in the positive Y-direction across opposite sides of the cores 20 that are located near the second surface 14. Preferably, each first and second lateral strip 36, 38 extends across three or more consecutive core elements 20.

[0057] At the side surface 16 of the panel 10, where the core elements 20 terminate in face end surfaces 22, each of the sheets 30 further defines a first lateral flap 42, a second lateral flap 44, and a medial flap 46. The first lateral flap 42 forms an extension of the first lateral strip 36 of this sheet 30, but is folded away from this first strip 36 along folding line 48 downwards in the negative Z-direction. The second lateral flap 44 forms an extension of the second lateral strip 38 of this sheet 30, but is folded away from this second strip 38 along folding line 50 upwards in the positive Z-direction. The medial flap 46 forms an extension of the medial strip 40 of this sheet 30, but is folded away from this medial strip 40 along folding line 52 sideways in the positive Y- direction so that this strip 40 extends along an edge of the second strip 38. These flaps 42-46 form patches of sheet material that extend in the Y- and Z- directions along the side surface 16, and that mutually overlap in a manner explained in more detail below. A similar arrangement of (further) overlapping flaps 42-46 may also be present on a further side surface of the panel 10, which is opposite to side surface 16 viewed along the X-direction (figure 1a).

[0058] Figure 2 schematically shows the side surface 16 of an exemplary panel 10 formed by a sequence of Z-shaped sheets 30 similar to figure 1 b. This side surface 16 is defined by the overlapping arrangement of first lateral flaps 42, second lateral flaps 44, and medial flaps 46. [0059] Figure 3 schematically illustrates a sequence of folding lateral and medial flaps 42, 44, 46 for achieving the arrangement in figure 2.

[0060] In this example, the medial flap 46i of each respective sheet 30i (= a, b, c, etc.) is folded sideways towards the positive Y-direction, and the second lateral flap 44i of the same sheet 30i is folded upwards towards the positive Z-direction so that it directly overlaps a lower portion 64i of the medial flap 46i, without covering an upper portion 60i of medial flap 46i. Each subsequent sheet 30j (= b, c, d, etc.) is placed with its first lateral strip 36j on top of the first lateral strip 36i of directly preceding sheet 30i, and with its second lateral strip 38j on top of the second lateral strip 38i of the preceding sheet 30i. The overlapping second lateral flap 44j and medial flap 46j are placed inwards relative to the overlapping second lateral flap 44i and medial flap 46i of the preceding sheet 30i. In the resulting stack of sheets 30, the pairs of second flaps 44 and lower portions 64 of medial flaps 46 jointly form an imbricated sequence of interleaved second flaps 44 and medial flaps 46 along the side surface 16, whereas the upper portions 60 of medial flaps 46 overlap each other directly.

[0061] The first lateral flaps 42 are folded downwards towards the negative Z-direction. As the first lateral strips 36 project towards the negative Y-direction, each flap 42j directly overlaps the medial flap 46i of its preceding sheet 30i, but does not overlap the medial flap 46j of its own sheet 30j. In the resulting stack of sheets 30, the first lateral flaps 42 directly overlap each other to form another imbricated sequence, which in its entirety covers the layer formed by mutually overlapping upper portions 60 of the medial flaps 46. The direction of overlap in the imbricated sequence of first lateral flaps 42 is reversed and has a shifted periodicity in the Y-direction, relative to the imbricated sequence of second flaps 44.

[0062] Figure 2 further illustrates that the first and second lateral strips 36, 38 of adjacent sheets 30 do not extend perfectly parallel along the respective first and second panel surfaces 12, 14. The smoothness and macroscopic direction of these panel surfaces 12, 14 (and also 16) may for instance be defined by an outer layer of material 24 that envelops the cores 20 and sheets 30. This outer layer 24 may for instance be formed during moulding of the panel 10 (e.g. an outermost portion of resin introduced during RTM/RIM of fibre sheets inside a mould), or applied as a coating in a subsequent coating process.

[0063] Instead, each of the first lateral strips 36 is oriented predominantly along the Y-direction, but with a small tilt towards the Z-direction over a non-zero first angle b1 with respect to the first surface 12. Similarly, each of the second lateral strips 38 is oriented predominantly along the Y- direction, but with a small tilt towards the Z-direction over a non-zero second angle b2 with respect to the second surface 14. The overlapping sheets 30 at the outer panel surfaces 12, 14 may be described as an “oblique layered material”, and the arrangements of cores 20 and sheets 30 together may be described as an “oblique layered composite structure”. The angles pk (k = 1 , 2) are preferably small e.g. 0° < pk < 5° (or even 0° < pk < 2°) to achieve an optimal panel strength. Due to the small tilt angles pk, each row of flaps 42, 44 forms a sequence of rectangular shapes that are individually rotated over angles pk, while the sequence as a whole is periodic along the Y-direction.

[0064] In this embodiment, the first and second lateral sheet angles b1 , b2 are essentially identical i.e. b1 = b2. Nevertheless, other embodiments with non-identical tilt angles (b1 ¹ b2) also fall within the scope. This may for instance occur when the first and second lateral strips 36, 38 have different width Wls and/or sheet thickness.

[0065] In the exemplary folding illustrated in figure 3, the medial flap 46 of a sheet 30 is first folded along line 52 towards the positive Y-direction. In an alternative embodiment, the medial flap 46 may also be folded towards the negative Y-direction. After folding the medial flap 46 of sheet 30, the second lateral flap 44 of sheet 30 is folded along second lateral folding line 50. The flap is folded away from the second lateral strip 38 to extend along the side surface 16 of the panel 10. Only after all medial flaps 46 and second lateral flaps 44 are folded, the first lateral flaps 42 are folded along first lateral folding lines 48. The flap 42 is folded away from the first lateral strip 36 to extend along and at least partially cover the side surface 16 of the panel 10. [0066] Figure 3 further illustrates the dimensional properties of this exemplary panel 10. In this example, the core elements 20 have rectangular cross-sections, with a height approximately equal to Hms and a width of approximately DU. Each first strip 36 has a width Wls in the negative Y-direction, which is approximately three times the distance DU, so that each strip 36 overlaps a sequence of about three adjacent core elements 20. Each second lateral strip 38 extends along the second surface over a similar width Wls, so that each strip 38 also overlaps a sequence of about three adjacent core elements 20, albeit from below and towards the positive Y-direction. [0067] In this example, the medial flap 46 extends from line 52 over a length Lmf in the Y- direction. This length Lmf is approximately three times the interspacing distance DU between consecutive medial strips 40 (Lmf = 3 DU), so that each respective medial flap 46i overlaps at least two adjacent medial flaps 46j of subsequent sheets 30j, and overlaps a sequence of approximately three adjacent core elements 20.

[0068] In this example, the medial strip 40 extends over a height Hms in the Z-direction, and the medial flap 46 extends over a height Hmf in the Z-direction that is almost identical to Hms (Hmf = Hms). In addition, the first and second lateral flaps 42, 44 have rectangular shapes with a width Wlf, and a length Llf. Here, the widths Wlf of lateral flaps 42, 44 are equal to the widths Wls of lateral strips 36, 38. These identical widths Wlf and Wls imply that each first and second lateral flap 42, 44 also overlaps about three adjacent core elements 20. In this example, the length Lmf of the medial flaps 46 is approximately equal to the widths Wlf of the first and second lateral flaps 42, 44, so that the second lateral flap 44 of a sheet 30 fully covers the medial flap 46 over its length Lmf, with neither one protruding beyond the other in the Y-direction.

[0069] In this example, the lengths Llf of the first lateral flap 42 and the second lateral flap 44 are equal, and each length Llf is slightly less than half the height Hms. The combined length 2 -Llf of the first lateral flap 42 and the second lateral flap 44, when folded in negative/positive Z- directions, is slightly less than the height Hms of the medial strip 40 (in this case also less than height Hmf of medial flap 46). As a result, the distal edges 54, 56 of the first and second lateral flaps 42, 44 do not abut, but lie closely along each other to define a joining line 58 along the Y- direction. Due to the small tilt angles pk (k = 1 ,2), the distal flap edges 54, 56 are also rotated over angles pk, such that each sequence of edges 54 or 56 forms a periodic saw-tooth contour along the Y-direction (figure 2).

[0070] Figures 4-6 illustrate an alternative embodiment 110, in which strips and flaps have additional tilted portions and edges. Figure 4 shows a perspective view of part of the panel 110, in which only a subset of three sheets 130 and only part of one core element 120 are shown. Other sheets and core elements are not shown, but should be considered present. Figure 5 schematically shows a top view of the part of the panel 110 in figure 4, when viewed downwards along the Z-direction. Figure 6 schematically shows a cross-sectional view of the panel 110 from figures 4-5, along cross-sectional XZ-plane VI through core element 120 indicated in figure 5. [0071] Features in the panel 110 that have already been described above with reference to the panel 10 in figures 1a-3 may also be present in the panel 110 shown in Figures 4-6, and will not all be discussed here again. For the discussion with reference to figures 4-6, like features are designated with similar reference numerals preceded by 100, to distinguish the embodiments. [0072] In this example, the shapes of the first and second lateral strips 136, 138, first and second lateral flaps 142, 144, and medial flap 146 form non-rectangular quadrilaterals, which are adapted to be folded along the side surface 116 and placed in obliquely overlapping arrangements, yielding a relatively smooth side surface 116.

[0073] In this example, the first and second lateral flaps 142, 144 are planar regions with quadrilateral peripheries. The first flap folding line 148 and the distal edge 154 of the first flap 142 are tilted relative to each other over a non-zero angle. Similarly, the second flap folding line 150 and the distal edge 156 of the second flap 144 are tilted relative to each other over a further nonzero angle. The two rows of flaps 142 and 144 adjoin along these edges 154, 156, thereby forming a relatively narrow seamline 158.

[0074] The folded medial flap 146 deviates from a flat surface, instead forming a developable surface that is composed of two flat portions 160, 164 at slightly different angles, and a curved intermediate portion 162 interconnecting the flat portions 160, 164. The upper flat portion 160 is folded along folding line 152, so that it is oriented an angle 90°- ym towards the positive Y- direction relative to the XZ-plane of the medial strip 140. Angle ym is relatively small, e.g. 0° < ym < 2°, so that the surface normal of this upper portion 160 points largely towards the X-direction but with a slight tilt ym towards the negative Y-direction (figure 5). The lower flat portion 164 is also folded along line 152 away from medial strip 140, but over a slightly smaller angle 90°- y2 towards positive Y-direction, so that the surface normal of lower portion 164 also points largely towards the X-direction but with a larger tilt towards the negative Y-direction. This correction angle y2 is preferably in a range of ym < y2 < 5°.

[0075] The second lateral flap 144 is folded upwards along lateral folding line 150 over an angle of 90° towards positive Z-direction (figure 6). This folding line 150 is tilted at angle y2, so that the second flap 144 obtains a tilt towards the negative Y-direction that is similar to the tilt of lower portion 164 of medial flap 146, thus allowing flaps 144 and 146 to abut smoothly. The first lateral flap 142 is folded downwards along lateral folding line 148 over an angle of 90° towards negative Z-direction. This folding line 148 is tilted at an angle y1 towards the positive Y-direction. In this example, the tilt angle y1 is in a range -2° < y1 < 0° (i.e. similar to ym but in opposite direction).

[0076] Also in this example, the first and second lateral strips 136, 138 may be oriented at small tilt angles b1 , b2 relative to respective panel surfaces 112, 114, similar as in figure 2.

[0077] Sheet element 130 may be folded into the 3D shape depicted in figure 4 while it is being combined with other sheets. However, sheet element 130 may also be pre-formed as a self- supporting 3D element as such, for instance made of pre-folded aluminium sheet or of hardened fibre reinforced plastic. A plurality of such pre-formed elements 130 may then be slid together into the plate-shaped structure 110 in figure 4.

[0078] The intermediate portions 162 of the medial flaps 146 coincide with the seamline 158 defined in-between the distal edges 154, 156 of the lateral flaps 142, 144. The lower portions 164 of the medial flaps 146 overlap with the second lateral flaps 144 in an alternating manner, to form an interleaved overlapping arrangement. The intermediate portions 162 curve from respective upper portions 160 outwards to respective lower portions 164 of corresponding medial flaps 146, thereby effectively sealing the seamline 158 between the distal edges 154, 156 (figure 6).

[0079] To provide sufficient space for accommodating the imbricated layer of overlapping upper portions 160 of medial flaps 146 below the first lateral flaps 142, the end point of folding line 148 near lines 132 and 152 is displaced outwards in X-direction over a distance DC relative to the proximal end point of folding line 152. Because of the tilt y1 of folding line 148, this displacement increases towards the distal end of folding line 148. In the resulting stack of sheets 130, the obliquely imbricated sequence of first lateral flaps 142 is displaced outwards at distance DC relative to the face end surfaces 122 of the cores 120. The width DC of the space in-between the stack of first flaps 142 and the face ends 122 of the cores 120 equals the combined thickness of the stack of upper portions 160 of medial flaps 146. In the example of figure 6, the number of overlapping medial flaps 146 equals three, so the distance DC is approximately three times the thickness of a medial flap 146.

[0080] Figure 7 schematically shows a perspective cross-sectional view of another embodiment of a panel 210, which is composed of a plurality of P-shaped sheets 230. Like features are designated with similar reference numerals preceded by 200. Figure 7 shows only a subset of two sheets 230 and two core elements 220, but further sheets and core elements should be considered present.

[0081] In this panel 210, both the first lateral strips 236 and the second lateral strips 238 extend towards the positive Y-second. The first lateral strips 236 mutually overlap in an obliquely layered sequence along the first panel surface 212, while the second lateral strips 238 mutually overlap in an obliquely layered sequence along the second panel surface 214. Each medial strip 240 extends through the panel 210 and in-between two cores 240, and interconnects corresponding first and second lateral strips 236, 238. In this panel 210, both the first lateral flap 242 and the second lateral flap 244 overlap the medial flap 246 of the same sheet 230, with distal their flap edges extending along each other to define a seamline 258.

[0082] Further features that have already been described above with reference to the panels 10 and 110, like the various tilt angles pk and yk, may also be present in panel 210 and will not be discussed here again.

[0083] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0084] Those skilled in the art and informed by the teachings herein will realize that the folding sequences as discussed for the embodiments are exemplary only. Alternative sequences are also covered by the present invention, as long as the side surface of the structure is formed by overlapping flaps that are extensions of the same sheets that also define the main surfaces of the structure. In some embodiments, the sheets may have only one or two types of such flaps.

[0085] The exemplary embodiments described above were shaped as a panel, but it should be understood that similar structures may be formed with different shapes. The panel structures may for example have a simple concave shape that gradually curves towards the vertical direction Z as a function along the X-coordinate. Alternatively, the panel structure may have a concave curvature towards the Z-direction as a function of transverse coordinate Y. More complex shapes may also be conceived, for example having double curvatures in both the longitudinal and transverse directions and/or curvatures with multiple local minima/maxima and/or inflection points.

[0086] Alternatively or in addition, the plate-shaped structure may omit core elements in some or all of the spaces that are defined in-between the strips. The core elements or internal spaces should also not be considered limited to elongated rectangular cuboid shapes. Core elements or spaces with other shapes would also be possible, for instance with cross-sectional shapes having discrete symmetry for finite rotations about its body axis A along the X-direction (e.g. regular triangular or hexagonal shape), and/or with mirror-symmetry with respect to one or more body planes, or with more general polygonal (e.g. quadrilateral) or curved cross-sectional shapes. The shapes of the cores or spaces may also be varied to create plate structures with non-rectangular quadrilateral outer contours in the XY-directions, for instance a parallelogram or trapezoidal peripheral shape.

[0087] In the above-described exemplary embodiments, the sheet material was composed of resin-impregnated mats of woven continuous fibres. Strength of surface layers in the plateshaped structure may then be adjusted by appropriate choice of fibre directions within the mats. As schematically indicated by cross-shaped markings in figures 1 b-5 and 7, the fibre directions within sheet layers may be parallel to the main (first/second/third) directions, and/or may be oriented at non-right angles (e.g. ±45°) to the main directions. In alternative embodiments, other sheet materials may be used, such as aluminium sheets, steel sheets, thermoplastic sheets, etc. [0088] The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0089] Note that for reasons of conciseness, the reference numbers corresponding to similar elements in the various embodiments (e.g. 110, 210 being similar to element 10) have been collectively indicated in the claims by their base numbers only, without multiples of hundreds. This does not suggest that the claim elements should be construed as referring only to features corresponding to base numbers. The applicability of similar reference numerals will be apparent from a comparison with the figures. List of Reference Symbols

Similar reference numbers that have been used in the description to indicate similar elements (but differing only in the hundreds) should be considered implicitly included.

10 plate-shaped structure (e.g. panel)

12 first surface (e.g. upper surface)

14 second surface (e.g. lower surface)

16 side surface

18 longitudinal surface

20 core element

22 face end

24 outer layer (e.g. hardened resin)

30 sheet (e.g. web or plate material)

32 first sheet folding line

34 second sheet folding line

36 first lateral strip

38 second lateral strip

40 medial strip

42 first lateral flap

44 second lateral flap

46 medial flap

48 first lateral flap folding line

50 second lateral flap folding line

52 medial flap folding line

54 first flap edge

56 second flap edge

58 seamline

60 first portion

62 medial portion

64 second portion

X first direction (longitudinal direction)

Y second direction (transverse direction)

Z third direction (vertical direction)

DC flap displacement

DU interspacing between medial strips

Hmf height of medial flap

Hms height of medial strip

Llf length of lateral flap

Lmf length of medial flap

Wlf width of lateral flap

Wls width of lateral strip b1 tilt angle of first lateral sheet b2 tilt angle of second lateral sheet ym tilt angle of medial flap g1 tilt angle of first lateral flap g2 tilt angle of second lateral flap