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Aviation History
1968
1968 - 2418.PDF
670 THE NEW STEEL . . . with 14in lengths of carbon fibre for use in experimental compressor blades. While this had been going on in the United Kingdom, the Americans were working on both boron and carbon fibre composites. Their main effort was on boron, but this suffers from a high manufacturing cost and from the fact that only one type of boron fibre is possible, compared with the wide variety possible with carbon. The cost of carbon fibre in the United States, at £200 per lb, is also considerably higher than in the United Kingdom, where it is £35 per lb. Morganite manufacture two types of carbon fibre based on polyacrylonitrile. Type 1 has a high Young's Modulus (60X lO^lb/sq in) and a moderate strength, while Type 2 has a high strength (^XlO^lb/sq in) and a medium modulus. The different moduli in the two types derive from the degree of orientation of their graphite-based planes; the more nearly these are parallel to the fibre axis, the greater the modulus. A high consistency of manufacture of the fibres is achieved and this makes possible composites of uniform and predictable characteristics when used to reinforce a resin, metal or ceramic matrix. The matrix allows the fibres to be ordered in the desired pattern and directions and transmits loads both to and between the fibres. Resins are the normal matrices at ordinary temperatures and epoxy or polyester resins are commonly used, although other types are selected for particular applications. In terms of specific flexural modulus, carbon fibre resin composites are over four times better than most other structural materials, including wood, the majority of metals and alloys and glass-reinforced plastics. A second important design feature of composite materials is the ratio of their strength to modulus, that is their strain at failure. To make full use of the potential of a material, its maximum strength should be reached before its extension becomes unacceptable for the particular component. Thus, glass fibres may be stronger than carbon fibres but they reach a given extension under a much smaller load. Their usable strength may therefore be lower than that of the weaker but stiffer carbon fibres. Theoretically, the tensile strength and modulus of unidirec- tional fibre composites are proportional to the volume of the reinforcing fibres. In practice, the degree of orientation of the fibres, their length-to-diameter ratio, and particularly the degree of adhesion between them and the matrix, can all cause differences from the theory. With the early carbon fibres, adhesion was poor and a special surface treatment was developed whereby the interlaminar shear strength was increased to the level where it was no longer critical in a correctly designed composite structure. However, the impact strength of such structures appears to fall as the interlaminar shear strength increases, so that an infinite increase in adhesion is to be avoided. Commercial production of fibres Morganite started producing carbon fibres in metre lengths in tonnage quantities during 1967 and plan to produce soon even greater quantities in continuous lengths. Immediate applications are in the aerospace industries in Europe and the United States but many others await development. Courtaulds produce carbon fibres processed into unidirectional warp sheets, random mats, preferred orientation tape and moulding powders. The warp sheet is claimed to be the thinnest pre-impregnated carbon material in the world and is currently being evaluated by the aerospace industries. Sheets O.OOlin, 0.005in and O.OlOin thick are being developed, the first for specific satellite applications. The thinnest material will be worked by vacuum bag or autoclave processes, the thicker by matched metal moulding. The aerospace industries are mainly interested in pre-impregnated materials, which are being developed in continuous form as well as unidirectional sheets. Courtaulds should soon be able to supply off-the-shelf continuous filament pre-impregnated with an epoxy novolac resin and, a little further ahead, pre-impregnated with other high-temperature resins. Rolls-Royce have taken a prominent part in the development of carbon-fibre-reinforced plastics and are using this material FLIGHT International, 24 Octofcer I in the fan and fan stator of the RB.211 engine for Lockheed L-1011. The 1968 New Scientist Award (second pri™ had gone to these Rolls-Royce engineers, J. Whitney, M. R Rowland and S. G. Jones, for their work on the development of the continuous process system of manufacture which has been so important to mass production of the fibre and to reductions in its cost of manufacture. This system involves bundles of 10,000 fibres being pressed into sheets and passed between graphite rollers through which an electric current is passing. The fibres act as the conductor and as the whole furnace does not have to be raised to the required temperature of 2/3,000°C, the energy required is low Fibre composite components are made in three ways: (1) Moulding The fibres form a random mat in the resin matrix. This is the method used for most glass-fibre com- ponents, such as motor car bodies, boat hulls and some structural components used in the building industry. It is cheap and requires only unskilled labour but is not a very efficient use of the fibre reinforcement. (2) Wound continuous filaments Filaments are wound on with resin as a binder and matrix. This method, which gives high strength and stiffness in the required directions, is particu- larly suited to circular or conical sections or pressure vessels but is more difficult and requires specialised machinery for complicated shapes. (3) Lamination. Thin laminations are cut from undirec- tional sheet. Each sheet of parallel fibres is pre-impregnated with resin and is sufficiently sturdy for the sheets to remain together once arranged in correct order and position. Then, as a complete component shape, the laminates are placed in a matched die. The die is closed and subjected to a carefully controlled cycle of pressure and temperature to form the required shape and finally to cure the resin by a chemicafly irreversible process. This means that the component can later be taken back to the curing temperature yet will retain mosi of its strength and stiffness. The component is finished when removed from the die, except for the removal of a slight "flash" at the closing surfaces. Establishment of the correct pre-form shapes and the moulding cycle is a highly skilled procedure but, once fixed, manufacture is semi-automatic. Compressor-blade applications This latter method is used by Rolls-Royce for all its high- duty composite (Hyfil) components. A feature of the design of these components is the use of the concept of cross-bracing whereby components are stiffened in a torsional sense by arranging the filaments of successive laminates at carefully determined angles to each other. Varying the cross-bracing alters the moduli of the material. By selecting the correct relationship of the two principal moduli, the frequencies of vibration of compressor blades can be controlled and much higher bending frequencies achieved than with conventional materials. Similar techniques are used to control torsional vibration. As a result, Hyfil blades give considerably greater freedom from flutter than blades in titanium or more conventional materials. In addition, by altering the lay-up of the sheets, without altering the external shape of the blade, its resonance frequencies can be changed. Attachment of blades to the disc was originally done with a "bulb" root in which continuous fibres went from the blade, round an attachment pin and back to the blade. However, there were difficulties in manufacturing this design and sub- sequently it was altered to a simple wedge shape which slots into the disc. Initially a single wedge was used, but more recently this has been replaced by a multiple wedge which is two or three times stronger. It is impossible at this point to foresee the almost nm'^ future application of this "new steel." As with glass-reinforced plastics and earlier advanced materials, carbon-fibre-reinforced plastics will initially be used by the, aerospace industry However, it is already possible to see important applications i chemical engineering, as a bearing material, in thermoplasuc applications, for competition yachts and racing craft, > racing cars, for radar scanners and masts, in submarines a° underwater research craft, in the glass industry, for ovcr^L power lines and even—for those who can afford sophistication—for model aeroplanes and man-powered aircran
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