US2429235A - Stiff structural sheet - Google Patents

Description

0 1947- 4 R. E. MISKELLY EI'AL 2,429,235

STIFF STRUCTURAL SHEET Filed March 13, 1942 a& 4. Irma Patented Oct. 21, 1947.

STIFF STRUCTURAL SHEET Raymond E. Miskelly and Ralph L. Drew, Plymouth, Mass., assignors to Plymouth Cordage Company, North Plymouth, 'Mass., a corporation of Massachusetts ApplicationMarch 13, 1942, Serial No. 434,552

This invention relates to improvements in still structural sheets.

More particularly it provides composite, sheets in the general nature of plywood, which, by the invention, can be so made as to afi'ord strength substantially exceeding what has been heretofore obtainable with plywood for resisting tension, bending, impact, and edgewise compression.

The composite sheets of the invention are useful in aeroplanes, automobiles, houses and many other structures.

The term "sheet is here employed in its ordinary sense to signify a thin article having dimensions of length and breadth which are very great relative to the dimension of its thickness. Thickness of the stiff structural sheet of the invention is composed of laminae which are also sheets.

According to the invention, one or more compacted thin sheets of parallel fibres of long and strong variety enclose one or more sheets of wood veneer, or are themselves enclosed by veneer sheets. Interfibrous spaces are filled with a. strong and strongly adhesive solid substance which bonds the fibres firmly to each other and to the veneer sheets. The compacting of the fibres having been effected while the filler-binder was liquid, and the fibres being immovably encased and held by the solidification of this liquid, the total sheet may have thickness like plywood and yet may possess vastly greater strength.

Distinctive results thus gained are indicated by physical tests made in comparison with sheets of ordinary plywood similar in dimensions and in kind of wood. Specimen sheets thus tested have shown marked superiority, (a) in resistance to impact on the side of the sheet,'(b) in resistance to tension applied in the direction of the long fibre; (c) in resistance to compression edgewise of the sheet, and also (d) in magnitude of bending pressure withstood before bending begins, as well as (e) in modulus of rupture in bending.

The invention is herein illustratively described with reference to aeroplane manufacture, this being a field in which the requirements that sheets have stiffness, strength, and lightness of weight are so severe that sheets made of cloth, plywood, steel, metal alloy, and plastic have each in turn been recognized as having merit and yet have each been found not altogether satisfactory. Steel sheets, for example, if thick enough to afford necessary stifi'ness have too much weight; textile cloth, agreeably light, lacks stiffness: spars and ribs to support cloth are fragile, or else too heavy; plywood, having edgewise stiffness, has low limits of resistance to lateral stress; and

4 Claims. (Cl. 154-453) the light metals and plastics are open to objections in various respects, two of which are that they are very expensive and that they are available only in limited supply.

The specification, that the fibres in each fibresheet are long, hard and approximately parallel to each other, excludes sheets of paper, and of woven fabric, for in such the fibres are not parallel; and it excludes thin sheets of wood, as veneer, for in wood the fibres are not long. Also it excludes fibres of varieties known in the cordage industryas "soft fibre among which is cotton. However, fibres of flax, hemp, andoframie can be used, for each of these has suflicient of both length and strength to be usable with merit, notwithstanding that they are not equal to hard fibre." I

Fibres known in the cordage industry as hard fibre" are acceptable and are in general preferred over other long hard fibres, such as those of glass, that have high strength-weight ratio. Fibres of the class called "hard fibre" include manila, sisal, Java, New Zealand and .very many other varieties of fibre of vegetable origin, from the leaf, leaf stem or best of the plant, which individually have length of several feet, ranging from two or three feet up to eighteen or so; and which individually are very strong. Manila, for example, has been found to have a strength-weight ratio comparable to that of steel. Fibre selected for use in the present invention is optimum when it has a relatively low ratio of extensibility under tension, and a relatively high tensile strength. In general, varieties of fibre of vegetable origin classified industrially as "hard fibre will be found acceptable, and will also be superior to the long filaments of synthetic plastics, except those having a high tensile strength. Hard fibre, so called, is at present available in great abundance at low cost.

For a simple specific illustration, to which however the invention is by no means limited, it may be assumed that a sheet of Honduras mahogany veneer is enclosed between two broad, thin, compacted sheets of manila fibres in which the fibres are laid approximately in parallelism. Preferably the grain crosses the lengthwise direction of the manila fibre. The spaces between the manila fibres are filled with a solid phenol-formaldehyde resin that has been set by heat and pressure, adhering to the wood and to the manila fibres. And there may be, if desired, thin sheets of veneer on both faces of the assembly thus made. Dimensions of the sheet may be whatever is desired and found feasible; the total thickness and the numher of laminae may be whatever is planned by the maker; and the ratio of thickness of fibre sheet to wood sheet may be chosen at will. As an illustration of practicable dimensions, the thickness overall may be one-tenth of an. inch, occupied approximately equally by the three plies, respectively two of fibre and one of wood, thus making each ply be one-thirtieth of an inch thick; and this composite sheet may be a few feet wide and a few feet long.

Assuming that such a three-ply structural sheet has been made, and that applied forces tend to bend one end of the sheet upward about the other end, as in the Wing of an aeroplane, it can be understood that the upmost ply of fibre will come into compression as it becomes concave; and that the lowest ply will come into tension as it becomes convex. The fibre in this is below the neutral axis, and the inextensibility and great longitudinal tensile strength possessed by manila are thus available in the lower ply to resist the bending. The superior tensilestrength of the fibre. makes the resistance to bending be much greater than it would be if all three plies were of wood.

Such a composite sheet will hold against breakage by bending until there is failure of either of the other plies or the synthetic filler, or the adhesion. The fact that the wood ply has lower limits of strength becomes relatively inconsequential because the wood is at the neutral axis. If a sheet composed in this way is to be exposed to air flow in an aeroplane, the superficial crevices between manilla fibres should be well filled, and the filler be made smooth, in order to reduce air friction. The adding and bonding of thin veneers of wood, on both surfaces-provides on one face a smooth surface for air now and on the other face a surface which can be strongly bonded to ribs, spars or other sub-structure by known adhesives.

A long sheet becomes able to act both as a sheet and as a beam. if embodying long hard fibre lengthwise of the curve. on that side of its neutral axis which the bending force tends to make convex. Considered as a beam, such a sheet will benefit by the combination between length of individual fibres and length of beam, because an initial bending stress is instantly resisted by each fibre that extends integrally through the sheet length, or through a long part of it. Considering a fibre several feet long, and assuming that l at each end it is held to the wood element and to the adhesive filler-binder element of a sheetbeam, and that this composite sheet is to receive force tending to deflect it by bending, the fibres on the outside of the bend must undergo elongation if they accompany the beam in bending. But because of the integrality and the inherent inextensible nature of that fibre, and the inability of that fibre to change its position laterally to eliminate any minor wrinkles, or lack of perfect parallelism to other fibres, the fibre resists extension and so resists bending with whatever strength that fibre has. This is in contrast with the situation if the same length of beam were covered by a succession of short fibres, held together through the length of sheet only by being woven or matted, or held together by the relativel weak organization of fibres which obtains in wood veneer. The tensile strength through the length of any such sheet would be'only the strength of the medium occupying the spaces between fibres. The invention brings the powerful pull of the long strong fibre into action to resist the bending.

In the sheet construction of the invention the strength of such fibres is most fully'available if each is straight and parallel to the others. Parallelism of fibres to each other can be approximated by having the fibres held straight under mild tension while the plastic adhesive becomes solidified around them; and that holding is in turn facilitated by the great length of the fibres. If the fibres are twisted in small yams their departure from parallelism is not great, and the advantage is had that all of the yarns can be equally gripped at each end for tensioning, and can be held equally under tension, so that full resistance to bending will from the outset be exerted by all of them. The inherent ruggedness of surface of long hard fibres of vegetable origin 'makes them secure against slippage through the solidified filler; and vegetable fibresare superior in this respect to smooth fibres of steel, glass or plastic, unless such smooth fibres are twisted together.

The invention can be embodied in structures having any desired number of intervening or covering veneer sheets of wood, or of whatever other solid material may be substituted for wood to sustain the compression, alternating with sheets of long hard fibre. The laminae can differ among themselves in thickness, according as engineering design may indicate would be wise for the particular service for which the composite sheet is designed, If strength is needed to resist bending in crosswise direction, more than the wood veneers afiord, one or more suitable sheets of parallel long hard fibres can be laid crosswise. In that case the width of the sheet, as well as the length, becomes stiff as above explained with regard to the length.

It is intended that the patent shall cover, by suitable expression in the appended claims, whatever features of patentable novelty exist in the invention disclosed, except that the herein disclosed structural sheets embodying fibres of syn thetic material are claimed in our co-pending application Serial No. 734,044, filed March 12, 1947. r The accompanying drawings illustrate some of the many forms in which the invention may be embodied, dimensions being exaggerated.

Referring to the drawings:

Figure l is a diagrammatic perspective indicating a fragment of a mass of long hard fibre loosely assembled together with the fibres lying approximately in parallelism.

Figure 2 is a diagrammatic perspective showing a fragment of the same mass after being filledwith a liquid resin, squeezed, and released, being still in loose assembly; showing also fragments of wood laminae which are to be assembled under compression with the long hard fibre;

Figure 3 shows a fragment of a completed stiff structural sheet embodying the invention, with wood veneers enclosing a sheet of long strong fibres;

Figure 4 shows a fragment of another embodiment of the invention, in which laminae of fibres enclose a lamina of wood; and

Figure 5 shows a fragment of still another embodiment, in which three laminae of wood are combined with four of fibres.

In planning for the stiff sheet of the invention, a designer may ordinarily first determine what total strength of fibre is to constitute each of the fibre laminae, or at least decide what kind and quantity of fibre is to be used per unit of width of the lamina; and then an assemblage will be made of this fibre in loose parallel form, as by putting together a sufficient number of slivers, on top of each other and beside each other.

For the kind of fibre, hard fibre, which is of vegetable origin, is at present preferred, this being long, strong, and having a surface character suitable for the making offa strong bond with the filler-binder substances herein indicated. Each so-calied individual fibre is really a bundle of fibrillae, affording multitudinous roughnesses, and crevices of microscopic dimension inwhich the filler-binder can engage. It is superior in this respect to fibres of glass or nylon or other synthetic fibres, which however can be used if strong filaments thereof are twisted together, making contlines; or, without twisting, if the bonding agent is sumciently adhesive.

Measurement of quantity of hard fibre, used illustratively herein, can be expressed in weight per unit of length, or in terms such as are used in the cordage industry for designating quantity of fibre'in a cross-section of rope. The present arrangement will be in a fiat mass; and such an assemblage is represented in Figure 1, marked ID.

A particular variety of heat-and-pressurehardening adhesive filler having been selected, the mass it) is to be impregnated with that. If the filler is in liquid form, and is a solution, the mass should then be squeezed, as between the rolls of a wringer, to remove excess. Upon release from this step the body of fibre will expand somewhat, while the filler is still liquid, and then it is advisable to eliminate the solvent, as by eva-poration. If the filler is one that was applied in aqueous solution it is suflicient merely to allow the squeezed mass to dry. In Figure 2 the numeral i2 indicates an assemblage in this stage, and the large dots is indicate the liquid added to the bare fibre l seen in Figure 1.

The laminae which are to be combined with the fibre laminae may be thin sheets of wood, selected of any suitable kind, made as veneer sheets are made for manufacture into plywood; or may be of whatever suitable material may be preferred, made as may be appropriate. In Figure 2 fragments of sheets of veneer I l, it which are to enclose and to be united with the fibre assemblage [2 are seen, arranged with. grain running across the lengthwise direction of the long hard fibres H. The inner surface of each sheet of veneer is to be well coated as at l5 with the same kind of liquid as is impregnating the mass of fibres; and this may be allowed to dry.

The ultimate sheet is made by compressing the fibre l2 and its filler l3 and the veneer Hi, It, all together under heat and pressure sufiicient for compacting the fibres to desired density and thickness of lamina and converting the filler from liquid to solid state, usually by the chemical process known as condensation, the result being that indicated in Figure 3 where the compacted fibre embedded in solidified resin is marked it.

The body I0 shown in Figure 1 can if preferred be built up by depositing thin sheets, like veils, of fibres on top of each other, until the desired quantity is assembled. The slivers or veils which are to be thus assembled to make the body l0 can be produced, with their contained fibres in approximate parallelism, by drawing and combing machines of types well known in the cordage industry for working in hard fibre. The liquid with which this mass is to be impregnated may be chosen among any of numerous synthetic resins which have a liquid state convertible by heat and pressure, or otherwise, into a firm solid state, of which resins the heat-hardening water-soluble varieties are examples. In general, any of the binding substances, in liquid or powder form,

which are known in the plywood industry may be used, examples of which are urea formaldehyde and phenol-formaldehyde bonding resins, as well as thermo-plastic resins whenever these are applicable. These have compressive strength exceeding or close enough to equalling, that of wood, to serve satisfactorily, as distinguished from natural gums and mucilages which are not so strong, and as distinguished from glues that are subject to disintegration by action of water or by fungus growth. If preferred the impregnating material may be applied to the veils or slivers before their assembly in the whole thick mass Hi.

In the example above assumed for illustration, wherein the long hard fibre lamina in a completed sheet is one-thirtieth of an inch thick, the thickness assumed by the specified quantity while in the initial loose assembly H3 has been found to be in the vicinity of three-eighths of an inch; but this is not a material factor. and may vary. However, all thicknesses, including the final thickness, as well as otherdimensions, are matters of choice,

according to what is wanted in strength or size.

And so also is the determination as to whether there shall be two laminae of wood enclosing one lamina of long hard fibre, as in Figure 3; or be one lamina of woodenclosed by a plurality of laminae of long hardfibre, as in Figure 4; or be a plurality of each kind as in Figure 5; and so also the determination whether the long hard fibres in a plurality of fibre laminae shall all run in one direction, of which Figure 4 is an instance, or shall run across each other, of which Figure 5 is an illustration, is a matter of engineering design. Greater benefit, of the long hard fibre for resisting bending, is had when a. lamina of that fibre relied on to resist the bending is at or close to the surface that will be convexed by the undesired bending, it being there farthest from the neutral axis. If it is not covered by a smooth veneer, airflow surfaces of the fibre lamina can be made smooth by having the filler solidify, in the compressing mechanism, in contact with smooth plate faces.

The covering of the fibre laminae with wood veneer is the method at present preferred for getting sheet surfaces that serve well for low air friction and also can be strongly bonded, with adhesives, to other parts of an aeroplane, as to the substructure, ribs and spars. I

In a composite sheet thus constituted the thickness may in general be any within a considerable range, according to the use and service desired, but the thickness should be sufiicient so that, under bending stress, tension in the long hard fibre and compression in the said solid substance will coact to give plate rigidity; If used in an aeroplane for wing covering, and compared with plywood as heretofore similarly used. the total sheet has greater strength per unit of weight, because the long hard fibre has greater strength than a similar weight of wood. And the greater strength gives greater plate rigidity, less tendency to buckle, because of its distance from the neutral axis.

The improvement in rigidity, over plywood, may

be availed of for reducing the frame structure in the design of an aeroplane, and perhaps also the weight.

While the drawing illustrates the invention as it may be applied in a plate having extension in width and length, and only relatively small thickness, advantages of the invention can be enjoyed by applying it in structures in the nature of spars, which would be strengthened by mounting long hard fibre along the top edge, and along the bottom edge, of a spar for use in the wing of an aeroplane, thereby stifiening the spar against upward and downward bending, and attaining any specified strength for resistance with less vertical dimension of thickness of the spar. Likewise the principle can be applied against rearward bending of a spar, by intimately securing a thin layer of fibres of the long hard variety along the forward edge of the spar which is to be thus strengthened.

Also, for the front edge of the wings, where plywood is sometimes used, the making of a sheet of plywood in form having the necessary sharp bend gives trouble by tending to break those veneers which are bent across their grain; but the sheet of the inevntion can be constituted in the desired rather sharply curved form with less trouble where long hard fibre is to undergo the lengthwise bending. Also the fibre lends itself readily to molding processes.

Sheets embodying the invention are useful in many places where plate rigidity is desired, affording a greater stiffness than would be had in a suitable metal plate. In general the new product is applicable in all places where plywood is now used and where greater strength would be useful.

11, for example, a thin sheet of Honduras mahogany, which has a specific gravity of about .6, is combined with manila -.or other fibres which have specific gravity of about 1.5, and with a suitable resin having 1.0 to 2.0 specific gravity, a sheet'can be made in which the low density of the wood contributes bulk, i. e. thickness, which is useful in producing stiffness. In the composite result, the long fibres contribute a tensile strength which in a sense is greater than that of steel; and as the resin, within which those fibres are firmly embedded, has a compression strength which is greater than that of either wood or fibre, so that individual fibres cannot buckle in the resin, this makes the individual fibres available, as solid bodies for resistance to endwise compression or crushing. The great length of the long fibres also makes a contribution to this combination of resistance abilities, for tensile stresses arising over distance of many feet, as by the tendency of aeroplane wings to bend when supporting the body of the plane in the air, as well as in cases where tension is applied directly as tension, are to an extent carried directly by the strong fibres. They are carried even more efiiciently than if those same fibres were embodied in a rope of equal length, because of the straight parallelism of the fibre to the direction of stress, without the deduction due to torsion, to which fibres in a rope under tension are always subject.

Application of a, bending stress to the sheet structure of the invention, whether it be one of impact or of sustained pressure, tends to convex one side of the bend; and this involves tension on the convex side, and brings into action the great tensile strength of the long fibre. The concave side of the bend, if the fibres there run par allel to those on the convex side, has fibres that individually cannot buckle, as above indicated, and so the great strength which hard fibre has to resist endwise crushing supplements the strength of the resin to resist the endwise compression that is incidental to bending stresses. Thus, as compared with plywood, there is superior resistance both to tension, and to endwise compression,

and to bending, and also to lateral impact of any sort that would produce a bending.

We claim as our invention:

1. A still. structural sheet comprising a plurality of laminae of fibres, these laminae standing in mutual sequence in the direction or the thin ness of the sheet, and each of these laminae being in length and breadth parallel to and co-extensive with the structural sheet; at least one of these laminae consisting of a multiplicity of strong fibres selected from the group consisting of vegetable hard fibre, ramie, fiax and hemp; these fibres of this lamina being individually several feet long extending substantially straight in approximate mutual parallelism and being compact together in a thin mass whose thickness is only a minor fraction of the thickness of mass which those fibres would occupy if they were in loose assemblage; said structural sheet also comprising at least one lamina of a coherent integral solid substance set between said laminae of fibres and co-extensive therewith; there being, throughout the extent of the said structural sheet, means bonding together the fibres of each lamina of fibres and also bonding those fibrelaminae together with the said coherent laminae.

2. A stifi structural sheet which comprise a plurality oi laminae, mutually in sequence in the direction of the thinness of the heet, and co-extensive with the sheet in length and breadth; said laminae being bonded firmly throughout the extent of the sheet; at least one of said bonded laminae consisting of a multiplicity of strong fibres selected from the group consisting of vegetable hard fibre, ramie, flax and hemp; the fibres of said one lamina bein individually several feet long, extending substantially straight, in approximate mutual parallelism, and being compact together in a Ehin mass whose thickness is only a minor fraction of the thickness or mass which those fibres would occupy if they were in loose assemblage; and another of said plurality of laminae being of a coherent integral solid substance; there being, for the said bonding, an adhesive binding substance :in the voids between and beside the individual fibres.

3. A stifi structural sheet as in claim 2, further characterized in that at least one or the laminae is 01 wood.

4. A stiff structural sheet as in claim 2, further characterized in that, in the said one lamina of fibres, the said strong feet-long fibres are taut.

' RAYMOND E. MISKELLY. RALPH L. DREW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 453,354 Hanmore June 2, 1891 1,851,177 Harvey et al Mar. 29, 1932 1,926,560 Redman Sept. 12, 1933 2,305,817 Sukohl Dec. 22, 1942 2,205,600 Payzant June 25, 1940 FOREIGN PATENTS Number Country Date 626,828 France May 21, 1927 11,753 Great Britain May 20, 1913

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