US3391511A

US3391511A - Lightweight structure having a honeycomb interior

Info

Publication number: US3391511A
Application number: US378168A
Authority: US
Inventors: Harris Leighton Richard; Davey Alan Edwin
Original assignee: Bristol Aeroplane Plastics Ltd
Current assignee: Rolls Royce Composite Materials Ltd
Priority date: 1963-06-28
Filing date: 1964-06-26
Publication date: 1968-07-09
Anticipated expiration: 1985-07-09
Also published as: NL140593B; DE1484135B1; NO124324B; CH432784A; SE340057B; NL6407261A; GB1070927A

Description

y 1968 1.. R. HARRIS ETAL 3,391,511

LIGHTWEIGHT STRUCTURE HAVING A HONEYCOMB INTERIOR Filed June 26, 1964 6 Sheets-Sheet 1 I nvenlor 1 6/(1/754! flea/4a lam 4ml Maw/v Java B ya? pm! A Home a y 1968 L. R. HARRIS ETAL 3,39

LIGHTWEIGHT STRUCTURE HAVING A HONEYCOMB INTERIOR Filed June 26, 1964 6 Sheets-Sheet 2 A ttorney;

1968 R. HARRIS ETAL 3,391,511

LIGHTWEIGHT STRUCTURE HAVING A HONEYCOMB INTERIOR Filed June 26, 1964 6 Sheets-Sheet 5 I 1mm y 1968 L. R. HARRIS ETAL 3,39

LIGHTWEIGHT STRUCTURE HAVING A HONEYCOMB INTERIOR Filed June 26, 1964 6 Sheets-Sheet 4 I nvenlor m-humane; H IMP/216 I 4 aww fi/V/ July 9, 1968 L. R. HARRIS ETAL.

LIGHTWEIGHT STRUCTURE HAVING A HONEYCOMB INTERIOR Filed June 26, 1964 /llI 4 6 Sheets-Sheet 5 July 9, 1968 L. R. HARRIS ETA States Unite ABSTRACT OF THE DISCLOSURE A structure according to this disclosure consists of two skins of sheet material, and a system of interconnected webs extending through the structure from one skin to the other, the webs being constituted by the walls of a plurality of cell units, each cell unit having a wall which forms a closed perimeter and which is perpendicular to the sheets or nearly so, the wall of each cell unit being substantially in contact with, and secured to, the walls of neighbouring cell units, and the ends of the walls of the cell units being secured to the skins, the securing between each cell unit and each skin being distributed over an area several times greater than the cross sectional area of the wall of that cell unit intersected by an imaginary plane lying between the sheets.

This invention concerns structures having a thickness dimension of a lesser order than their other two dimensions. The invention has been developed for such purposes as the making of hulls of boats, and domed structures, but it is of wide potential use.

A method of making a structure according to this invention comprises assembling a plurality of substantially rigid cell units between two sheets, each cell unit having a wall which forms a closed perimeter and which is perpendicular to the sheets or nearly so, the wall of each cell unit being substantially in contact with the walls of the neighbouring cell units, and the ends of the cell units being substantially in contact with the sheets; and securing the cell units to one another and to the sheets at those surfaces which are substantially in contact, the securing between each cell unit and each sheet being distributed over an area several times greater than the cross sectional area of the wall of that cell unit intersected by an imaginary plane lying between the sheets; the walls of the cell units collectively becoming a system of interconnected webs extending through the final structure from one sheet to the other.

A variety of preferred ways of carrying out the invention, and their advantages, will be described with reference to the accompanying drawings, in which:

FIGURE 1 is a perspective view of a structure in course of assembly, using cell units with corrugated walls;

FIGURES 2 and 3 show alternative shapes of cell unit;

FIGURES 4 to 6 show alternative constructions of cell unit;

FIGURE 7 illustrates the use of tapered cell units to make a curved structure;

FIGURE 8 is a view similar to FIGURE 1, showing cell units with tongues and grooves on their walls;

FIGURES 9 to 11 illustrate the formation of structures of single or double curvature, using cell units as shown in FIGURE 8;

FIGURE 12 is a perspective view of a larger cell unit; and

FIGURE 13 shows one way of completing the edge of a structure.

ice

In making the particular structure shown in FIGURE 1, the first stage is to make the cell units 15. Each unit includes an outer component 2 and an inner component 14. These are rigid mouldings of resin-impregnated glass fibre. The perimeter of the outer component, formed by a wall 4, is square, and each face of the wall has corrugations, with flat crests 6, extending from end to end of the cell unit. One end of the outer component (the upper end in FIGURE 1) is open, and the other end is closed by an end wall 10, which is integral with the wall 4. The open end is encircled by an intermittent flange 12. Into the open end of each outer component 2 there is inserted an inverted inner component 14, which is generally similar to the outer component 2, but with a perimeter wall 13 which is plain and sufficiently smaller to fit inside the outer component.

Before insertion, the wall 13 of the inner component 14 is coated with a synthetic resin adhesive, preferably an epoxy resin, for example by dipping, brushing, or spraying. Upon insertion of the inner component 14 into the outer component 2, this adhesive forms a layer in contact with the walls 4, 13 of both components. This gradually cures at atmospheric temperature, or curing may be assisted by heating.

With a supply of these completed substantially rigid cell units 15 available, a support 16 is made ready, with a top surface 17 having the contour required for one face of the structure, in this case fiat. The support 16 may be of concrete.

A sheet 13 of felted glass fibres is soaked in epoxy resin, and is then laid on the support. A plurality of the cell units 15 are coated with epoxy resin and are then assembled on the sheet into close relationship, with the corrugation crests 6 of each cell unit abutting respective crests on neighbouring cell units. A second sheet 20 of felted glass fibre soaked in epoxy resin is laid upon the assembly, and is pressed down by a roller (not shown). Then the components are left while the resin cures, either at atmospheric temperature, or with the application of heat. The resin forms bonds between all those surfaces which are substantially in contact, namely between the lower end walls 10 and the sheet 18, between the upper ends 19 of the components 14 and the sheet 20, between the flanges 12 and the sheet 20, and between the abutting corrugation crests 6. As a consequence, the cell units 15 become bonded to the sheets and to one another, and the walls 4 and 13 collectively become a system of interconnected webs, which irnpart stability to the structure in all directions and which are themselves stiffened by the corrugations.

The resin in the

sheets

18 and 20 may act as the bonding agent between the sheets and the cell units 15, so that it may only be necessary to coat the crests 6 of the corrugations on the cell units before assembly.

A principal feature of the invention is that the area of bonded surfaces between each cell unit 15 and each

sheet

18 or 20 is several times greater than the cross sectional area of the wall of that cell unit which is intersected by an imaginary plane lying between the sheets. This is illustrated in FIGURE 1, where part of the wall 4 of one outer component 2 is shown broken away to the plane A-A, to reveal the cross section 22. The cross section of the wall 13 of the inner component 14 requires to be added to this, and is indicated in chain lines at 24. In this way, an optimum relationship may be Obtained between the weight of the structure as a whole, and the strengths of the webs and of the bonded joints.

Preferably the area of bonded surfaces between each cell unit and each sheet is at least 10 times the cross sectional area of the wall of that cell intersected by an imaginary plane lying between the sheets.

It is also desirable to have a large area of bonded surfaces connecting the Wall 4 of the outer component 2 to the wall 13 of the inner component 14. In effect the joint is in series with the joint between the upper end 19 and the sheet 2t However, it is not essential for the wall 13 to extend the full height of the wall 4. FIGURE 8, described more fully below, shows how the inner component 14 may be replaced by a lid 26, with a depending skirt 28 which fits inside the wall 4 and is bonded to it.

A square perimeter to the cell units is convenient ior many purposes, but many other shapes are possible, provided that they enable the cell units to be assembled with their walls substantially in contact with one another. For example, FIGURE 12 shows a rectangular perimeter, and FIGURES 2 and 3 show examples of regular polygonal perimeters; hexagonal in FIGURE 2 and triangular in FIGURE 3. Preferably all the cell units have the same polygonal perimeter. Alternatively an assortment of cell unit shapes and sizes may be used.

In FIGURE 3, the walls 30 of the outer components 32 are flat, like those of the inner components 34.

Another general way of making the cell units is by bonding the cell wall to upper and lower members (e.g. lids or flanges), and then bonding these members to the sheets. FIGURE 4 shows a cell wall 36 with upper and lower open ends, closed by

lids

38, 40, which have

skirts

42, 44 which fit within the wall 36 and are bonded to it. In this case the area of the flat face 46 of each lid is preferably at least times the cross section of the wall 36.

Each lid has a flange 48 which projects as far as the plane of the corrugation crests 58, but no further.

The cell wall 36 may be formed by cutting lengths from a tube, preferably one produced by extrusion.

If a rather less area of bonding will suffice (but still several times greater than the cross section of the cell wall), a flange 12 as shown in FIGURE 1 may provide the sole connection, the inner component 14 of FIGURE 1 being omitted. Furthermore, this flange 12 may be repeated at the other end of the cell unit, in place of an integral end wall 10. Such flanges may be moulded integral with the wall 4, as shown in FIGURE 1. Alternatively they may be separate components, similar to the

lids

38, 40 in FIGURE 4, but with the parts within the

skirts

42, 44 omitted.

The interiors of the cell units may simply contain air, or other gas, or they may be filled with lightweight matcrial, e.g. polyvinyl chloride foam. If the cell units are open-ended, such material will help to keep the sheets 13 and smooth until they have cured.

FIGURES 5 and 6 illustrate other forms of cell unit construction, in both of which the cell unit is a hollow block. In FIGURE 5, the wall 52 and ends 54 are all parts of an integral moulding formed around a core 55, which may be of lightweight material and be left in place, or may be of low melting point material which is removed through a small hole by heating after curing. In FIGURE 6, the wall 57 is built up of components secured together, and is also secured to end members 5?. This construction is particularly convenient if the cell unit is of metal. The parts of the cell unit may be welded or riveted together, or may be bonded, either by a suitable adhesive, or by a low melting point metal, i.e. by soldering or brazing.

If the structure is to be curved, with single or double curvature, the cell units may be tapered from one end to the other, as shown at 55 in FIGURE 7. By placing the cell units alternately opposite ways up, it is possible to produce flat areas of structure using these same tapered cell units.

A more versatile means of producing curved structures is shown in FIGURES 8 to ll. The outsides of the walls 58 of the cell units have tongues 60 and grooves 62, defined between pairs of ribs 64. These tongues and grooves extend from end to end of the cell unit, and are brought into cooperating engagement with grooves and tongues of neighbouring cell units, and a bonding agent is provided between the engaging tongues and grooves. As shown in FIGURES 9 and 10, it is possible to tilt adjacent units relatively to one another while still retaining a substantial area of bonding between tongues and grooves. The grooves 62 are somewhat wider than the tongues 6i), and in addition the grooves and tongues may be somewhat tapered as shown. This enables adjacent cell units to be twisted somewhat relatively to one another, as indicated by the outline 65 in FIGURE 9. In this way, curved structures, for example as complex as that shown in FIGURE 11, may be made.

This construction has the advantage that all joints between cell units are in double shear, and that dimensional accuracy of the width of the cell units is not critical. The Wall may be moulded, or may be extruded in the manner described with reference to FIGURE 4. The tongues 69 and ribs 64 form parts of the system of interconnected webs in the completed structure. In fact, the remainder of the walls of neighbouring cell units need not come close together. For example, the cell walls may be cylindrical, with tongues and grooves alternately at intervals of around their peripheries.

Another way of making curved structures is by using cell units with corrugated walls as shown in FIGURE 1, but arranged so that the crests on any one unit enter the valleys in adjacent units, and vice versa.

If the structure is of single curvature, the sheets should either be formed to a correct rigid curved shape between being placed in position, or should be of flexible material. If these are substantial double curvature, the sheets should either again be formed to curved shape beforehand, or they should be of material which will stretch. A fibrous material is particularly suitable.

If the cell units are of larger area, as may be the case in flat structures, they may have internal partitions, as shown at 66 in FIGURE 12. The spacing of the walls (and partitions if any) in the cell units, which determines the spacing of the webs in the final structure, should be selected in accordance with the rigidity of the skins in relation to the rigidity of the structure as a whole, and can be chosen to produce an optimum ratio of strength and rigidity to weight of structure. Moreover, the spacing can be made non-uniform, to deal with localised points of high loading of the structure.

At the edges of a structure, as illustrated very diagrammatically in FIGURE 13, fillers 68 of a curable resin/ fibre dough may be used, to produce a required outline while using cell units of a uniform shape, and this outline may be closed in by a cover strip 70.

In a particular example of a structure forming the hull of a boat'of 50 feet overall length, the layers of sheet material are each 0.3 inch thick, of glass fibre reinforced plastic material, and the cell units are as shown in FIG- URE 1, measuring 5 inches square and 3 inches deep. The inner and outer components making up each cell unit are made as pressure mouldings from a glass fibre dough, and have wall thicknesses of 0.1 inch.

A wide variety of materials may be used for the cell units and for the sheets, and a wide variety of means may be used for securing the cell units to one another and to the sheets. For example:

The sheets may be of: fibrous material impregnated with synthetic resin, the fibrous material being of glass, asbestos, sisal, or wire, and being in the form of woven sheet, parallel threads or rovings, felt, or mat; continuous sheets of polyvinyl chloride, nylon, or polythene; or metal, in particular stainless steel.

The cell units may be of materials similar to those of the sheets.

The securing may be by means of an epoxy, polyester, phenol, or rubber based adhesive; welding synthetic resinous materials by a solvent; soldering, brazing, spot welding or puddle welding of metals; or riveting. If an adhesive, a solvent, or a low melting point metal is used, the

securing may be by bonding over the entire area of those surfaces which are substantially in contact. If the securing is by spot welding, puddle welding (i.e. local fushion initiated from one outer face only of two thin components to be joined), or riveting, the securing is at a plurality of points distributed over the area. In all cases, the areas are made sufficiently great to enable the securing of the webs to the sheet to be of the same order of strength as the webs themselves.

A structure according to the invention may be formed from more than one sandwich; for exmple it could comprise three layers of sheet material, two outer and one central, and two intermediate layers of cell units.

We claim:

1. A lightweight sandwich structure and having a honeycomb interior, the structure comprising a first sheet, a plurality of adjacent, individual and substantially rigid cell units of plastic material which are each covered on one side thereof by, and bonded to the said first sheet, and a second sheet which covers, and is bonded to, sides of the said adjacent cell units opposite each said one side; each cell unit comprising a continuous peripheral wall which lies transversely to both said first and second sheets, first and second sheet engaging portions which are substantially wider than the thickness of said peripheral wall and extend substantially perpendicular to said peripheral wall and from opposite ends thereof, said first and second sheets being respectively bonded to said first and second portions; said peripheral wall having at least first and second pairs of surfaces, said first pair of surfaces being adhesively fastened to a complementary pair of surfaces on a first adjacent cell unit, said second pair of surfaces being adhesively fastened to a complementary pair of surfaces on a second adjacent cell unit, the two surfaces in each of the said pairs of surfaces lying substantially parallel to each other but facing in opposite directions, the planes of all the said surfaces intersecting said first and second sheets, the planes of each pair of surfaces intersecting that part of the peripheral wall which lies immediately adjacent to that pair, and the planes of said first pair of surfaces intersecting the planes of said second pair of surfaces.

2. A structure according to claim 1, in which one of the said pairs of surfaces is constituted by the opposite sides of a rib member and the complementary pair of surfaces on the adjacent cell unit is constituted by the two sides of a groove.

3. A structure according to claim 1 in which the peripheral wall is polygonal with substantially straight sides and the first and second pairs of surfaces are each positioned intermediate the lengths of the sides and are perpendicular thereto.

4. A structure according to claim 1, in which at least the first sheet engaging portion of each cell unit is constituted by a lid member which is applied to close one end of the space within the cell unit.

5. A structure according to claim 1, in which each cell unit includes an outer and an inner component, each component comprising a peripheral wall and an end wall, the peripheral wall of the inner component lying within the peripheral wall of the outer component and being secured to it, the inner component being in an inverted attitude relatively to the outer component, so that the end walls constitute the first and second sheet engaging portions, respectively.

6. A structure according to claim 1, in which the sheet engaging portion at at least one end of the peripheral wall of each cell unit comprises an outwardly extending flange.

7. A structure according to claim 6, in which the of each cell unit lying opposite to said flange has an wall.

8. A structure according to claim 1, in which the cell units are rectangular when viewed in the direction perpendicular to the sheets, the peripheral wall having four rectangular faces.

9. A structure according to claim 8, in which each cell unit has a flange at one end constituting the first sheet engaging portion, and a closed end at its other end constituting the second sheet engaging portion, each of the four faces of the peripheral wall has two ribs, each of which ribs presents one of said adhesively fastened surfaces, and each of the said four faces presents two further pairs of closely spaced ribs, the mutually facing surfaces of each pair also constituting two of the said adhesively fastened surfaces.

10. A structure according to claim 1, in which each said cell unit is tilted with respect to at least some adjacent cell unit and said sheets are curved.

end end References Cited UNlTED STATES PATENTS 2,577,120 7/1951 Franz 21171 2,668,327 2/1954 Steele 16168 X 2,959,257 11/1960 Campbell 16168 3,103,460 9/1963 Picket 16169 3,193,434 7/1965 Weiss 16169 X FOREIGN PATENTS 15,900 1890 Great Britain. 998,428 5/1951 France.

EARL M. BERGERT, Primary Examiner.

H. F. EPSTElN, Assistant Examiner.