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Aviation History
1943
1943 - 1903.PDF
JULY 29TH, 1943 FLIGHT 119 Plywood and Plastics—II Moulded Plywood Construction : Characteristics of Wood Veneer i Effects of Moisture Content : Qlue Spread By W. NICHOLS, A.R.Ae.S. (Continued from page 102, July 22nd, 1943) MORE recent developments have introduced anentirely new type of plywood, different in appear-ance and application and manufactured under widely different methods. This class of manufactured wood is known as laminated or impregnated wood. Whereas the grain of the plies in plywood are arranged- at 90 deg. or 45 deg. to each other, in laminated wood the grain of the successive veneers run parallel to each other, or nearly so. Laminated wood is usually produced in plank form and not sheet form as with plywood. For the structurally important parts of aircraft the British industry uses plywood of the latter variety, manufactured AD B.S. Specification V3, which calls for plies of birch ^European or sweet or yellow) or rock maple. Strength ' properties of this material are given in Table 5, compared with those for a similar plywood prior to the use of modern synthetic adhesives; the superior characteristics of the modern material should be noted. . A further specification, D.T.D.427, covers the ordinary commercial qualities of plywood made from various timber for lightly stressed aircraft parts. In addition, there is also a B.S. Specification V.34 for plywood for use on unstressed parts, much more elastic in choice of timber. TABLE 5.—TENSILE STRENGTH OF BIRCH 3-PLY, Ib./sq. in. Material Plywood cemented with organic glues. aJ^Twood cemented Jtmih Tego filmglue. Ult. tensile stressElastic modulus Ult. tensile stressElastic modulus Parallel to Grain 10,000-12,0001.2x10" 15,000-16,0001.7x10' Normal to Grain 6,500-7,500 0.7 X10' 8,000-9,000 0.95 X106 *5deg. toGrain 4,000-4,5000.3x10" 6,000-6,5000.5x10' Unfortunately, three-ply wood is far from being isotropic. The pronounced grain structure of the wood precludes this. From Table V it will be noted that the tensile strength of three-ply wood at 45 deg. to the grairi is less than half that parallel to the grain. Actually, in the former case failure always occurs due to shear along the grain. Modern applications to plywood structures can, to some extent, overcome this difficulty, and will be referred to later. A plywood structure, however, distributes the fibre strength of the wood, greatly reduces swelling and shrinkage and eliminates to a large extent the possibility of the material splitting. The durability of such a material is largely dependent upon the bonding medium. Phenolic and urea resins have been applied to the veneers as dry powders, in solution or emulsion, and, more recently, in "film'' form. Laminated Compressed Wood An important factor in obtaining maximum adhesion with synthetic resins in plywood manufacture is the control of the moisture content before hot-pressing. This point is dealt with under the characteristics of wood veneers. Developments in the use of cold-curing synthetic resins as the bonding medium are also taking place. After plywood, the next step in improving natural wood was to minimise the effect of the grain structure on its strength properties. The compressive strength of most woods is approximately half the tensile strength, a charac- teristic which is due to instability of the grain fibres under compressive loads. .Consequently, it was conceived that if the pores and cells in the wood could be filled with a sub- stance having a good compressive strength/weight ratio, some degree of uniformity in strength might result. One of the many types of synthetic resins available was chosen, and much of this early research work was conducted in Germany. Impregnation of solid timber was not a success, due to the difficulty of inducing the resin to penetrate into the timber more than a few hundredths of an inch below the surface. The idea was then conceived of im- pregnating thin veneers with resin. These experiments eventually led to the production of laminated compressed wood, which in this country is manufactured to Specification D.T.D.370, birch veneer being the only timber allowed. In Table 6 representative strength characteristics of natural birch wood, cresol synthetic resin and laminated birch wood compressed and impregnated are compared. It will be noticed that the natural wood has a high ratio of tensile to compressive strength, and that the cresol syn- thetic resin has nearly the same ratio inverted. The effect when combining the two materials is therefore to produce a third, having compressive. and tensile strengths sub- stantially equal to unity. By thoroughly impregnating seasoned wood veneers with 'phenol-formaldehyde resin, drying and hot pressing to- gether, the material produced results in both the wood and the resin losing their separate identities, and the result is a new product of interesting properties. After preliminary seasoning, the wood veneers are pro- cessed under heat in a press to remove "free" water. A batch of dried veneers are then loaded into metal cages and placed within an impregnating vessel. The vessel is next evacuated in order to complete the removal of any free moisture left in the wood. An alcohol solution of phenolic resin is added and pressure applied to the vessel. After thorough impregnation, the veneers are removed and sub- mitted to action of heat and vacuum in another vessel in order to remove the alcohol. Balanced Properties Due to Resin Used Hot pressing is carried out in multiple daylight steam- heated presses, the required number of veneers being packed between the platens. Hot-pressing reduces the thickness by about 30 per cent, or more, and the tem- perature and pressure used are carefully controlled in order Properties Specific gravity Density (lb./cu. ft.) ... Ult tensile stress (lb./sq. in.) Ult. compress stress(lb./sq. in.) Ult. shear stress(!b./sq. in.) Elastic modulus E ... Modulus of rupture(Ib./sq. in.) Impact value (ft./Ib.) . Tensile/sp. grCompress/sp. gr E/sp. gr Natural Bircf TABLE Solid 15 per cent. Moisture Content 0.70 44 PlrallalriiaHl toGrain 21,000 8,500 2,500- 1.8 xlO6 15,000— 3.0x10*1.21 X 10* 2.57 x 10« Manualnvrmip toBrain — 1,500 1,500 11.5— — 6 Impregnated and Compressed Laminated Birch Pamtlillto Grain 22,000 21,000 4,000 3xlO« 25,000 —2.20x10* 2.10 X104 3.0 X 10* (Synthetic Resin) 1.0 62 Normal to Grain Normal toVeneers — 8,000 — — 6.5 — — Parallel toVeneers 2,000 7,500 2,000 — 7.5 — — Cresol Syn- thetic Resin 1.2 75 5,000 22,000 . 0.7x10* — — .——
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