SYNTHETIC RESIN

Dominion, Volume 30, Issue 45, 17 November 1936, Page 9

SYNTHETIC RESIN

Use in Building Aircraft The use of synthetic resin materials for aircraft construction is being investigated and developed by Aero Research, Limited, a private and independent enterprise whose laboratories and workshops at Whittlesford, near Duxford, England, were formally opened recently. The director is Dr. N. A. de Bruyne, a young scientist who has been engaged in this work for the last year or bo with the financial support of the Aeronautical Research Committee and the Department of Scientific and Industrial Research. Synthetic resin products are familiarly known to most persons as Bakelite, although that is but one of several proprietary names. To-day they are so varied and numerous that nearly every house contains some fittings or furnishings of this material, and a great industry, manufacturing thousands of different articles, has been built up. For a number of years experiments have been in progress in the use of synthetic resin’s in making parts of aeroplanes, but the field of their usefulness here has been limited by the peculiarities of the material. When used aIQDe, synthetic resins have too low a tensile strength for important elements in aeroplane construction. They must, therefore, be reinforced, as concrete is reinforced, so that th.e resin itself is subjected to only compressive forces. Instead of using steel it has been found better to use cellulose, for which some of these resins have a peculiar affinity. Cellulose, the material of cotton and artificial silk, with a density of 1-5, has an ultimate tensile strength of 60 tons to the square inch and an elastic modulus of 15,000.0001 b. to the square inch. Its tensile strength is, in fact, surpassed only by that of high-grade steel, which is far heavier and more expensive. In most of the reinforced resins obtainable commercially the fabric is apparently in a state of permanent tension as a direct result of the process of manufacture, the material being moulded under pressure. The usual moulding pressure, about a ton to the square inch, is applied till the resin has hardened. When this external pressure is removed the fabric tends to contract on to the resin, with the result that there is a state of permanent strain between them of slightly less than a ton to the square ineh. So long as the resin remains compressed there is no tendency for the constituents to part company ; when, under tensile stress, the stage at which this compression ceases is reached the resin “falls out of step.” This is the point at which “creep" begins, and “creep” represents the failure of resin and fabric to adhere together. Dr. de Bruyne and his colleagues have established the important point that, by giving the fabric a large initial tension during manufacture, the stage at which “creep” begins can be substantially raised and the elastic modulus at the same time increased. It is true that these qualities are secured at the expense of the ultimate tensile strength of the material. But the enormous tensile strength of cellulose—and- the ultimate strength of reinforced resin is simply the strength of tl»e fabric—makes it possible to increase the “creep” stress to a valuable extent without reducing the ultimate strength too greatly for many practical purposes. The essence of the process, which is provisionally patented by Aero Research, is to apply a direct external tension to the fabric while the resin is hardening round it. The most striking application of the new process shown was seen in two blades of a controllable-pitch airscrew, made by the De Havilland Company—which is co-operating with Dr. de Bruyne in this work—from material supplied by Bakelite, Limited. These blades have withstood severe tests, including 165 hours of test flying involving continual changes tn pitch, and also 50 backfires—an ordeal capable of breaking an engine crankshaft.

This, it is believed, marks an important advance in the direction of safety, for the material has been found rej markably free from “notch sensitivity” and to have a high fatigue limit, especially against impact; and since these tests the material has been further improved. The simplicity of the attachment between the root end of each blade and its Bakelite socket is also claimed to confirm in practice another advantage attributed to reinforced plastics—that in using them the designer can take liberties that would be impossible with metals. Another exhibit was an aeroplane wing rib built of reinforced plastic materials. The joints were made with a cement produced in these laboratories.