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Aviation History
1949
1949 - 1742.PDF
FLIGHT. 13 October 1949 SAFE OUTLOOK Current Developments by Triplex in Glass and Perspex for Aircraft EVER since the days when Hollywood introduced theniceties of gangster technique to a wider public,"bullet-proof glass" has ceased to be something that savours of a paradox in terms. Nevertheless, the business of making safety glass is technically a complex matter—not, perhaps, in terms of what is done so much as how it is done. Although glass is a substance with which all civilized peoples are unthinkingly familiar, it is, nevertheless, a remarkably curious material. For example, unlike metals, it is non- crystalline and is virtually structureless; it shows no sharp change in physical properties when heated or cooled and does not even have a sharp melting point, but simply begins to flow under its own weight at about 600 deg C. Most materials yield continuously when loaded beyond their elastic limit, but glass has no elastic limit and remains unchanged right up to the actual moment of fracture. It may be broken by loading in the usual variety of ways—by impact, tension, twist, com- pression, shear and so forth—but there is seemingly no con- nection between the numerical values of breaking strength. The general assumption is that glass always breaks in tension and that, no matter what the type of loading, fracture will occur only when deformation has produced the necessary ten- sile stress at some point in the specimen. Of the two most usual types of safety glass in use to-day, toughened glass is based on the principle discovered three centuries ago by Prince Rupert, Duke of Bavaria and a nephew of Charles I. A great military commander, and yet more than ordinarily interested in the sciences and the arts, Rupert found that, when molten glass was poured into cold water, it formed pear-shaped droplets of immense strength— "Prince Rupert's Drops." The other form of safety glass— laminated—owes its invention to an accident. Edouard Bene- dictus, a French chemist, accidentally knocked a flask from a shelf. The flask had contained a collodian solution, which had dried with so great an adhesive strength that no piece of glass was detached from the flask. Benedictus did some hard thinking and, at the end of 24 hours, with the aid of an ordinary letterpress, had produced a piece of laminated safety glass. It was on this basis that, in 1909, he started the first commercial manufacture of safety glass by founding the Societe de Verre Triplex. The English company was founded in 1912, and to-day its products are fitted to practically every British cai on the roads and, moreover, to practically every British aircraft. During recent visits to the Triplex factories' at King's Norton, Birmingham, and Willesden, London, we were able to witness the manufacture both of laminated and toughened safety glasses, and also the fabrication of aircraft components in Perspex. Perhaps the most telling illustration of specifica- tion stringency which Triplex are called upon to meet is in The heated sheet of Perspex is draped over the form tool for edge-clamping. Edge-grinding a bullet-proof screen for a fighter: a tricky job, in that the weight ma/ be as much as 60 Ib. • connection with front screens for modern fighter aircraft. In the first place, the screen must possess optical qualities of a very high standard in order to permit use of a reflector gun- sight, and, additionally in this connection, the percentage of light transmission must also be high (the specification calls for 78 per cent, and the Company provides 81 per cent). These qualities in themselves would not occasion any great difficulty of provision, but when they must be allied to the ability to withstand the impact of a 20 mm. cannon shell at a range of 200 yd the matter does become somewhat more complex. In point of fact, it devolves on the physical properties of the material used as an interlayer in the laminae. Superior Synthetic The first forms of laminated safety glass employed celluloid as an interlayer, but, as this had the disadvantage that it readily turned brown and became brittle and blistered, it was supplanted by cellulose acetate. It was, however, still neces- sary to seal the edges of the glass with pitch to prevent the ingress of moisture and to retain within the sandwich the slightly volatile assembly liquid. Then research into the field of synthetic materials resulted in the discovery of polyvinyl butyral, a synthetic resin which, with a tensile strength of about 3,000 lb/sq in, can be stretched approximately 400 per cent before breaking. This material, which is known as Vinal, as well as adhering to glass by heat and pressure alone and needing no edge-sealing, also gives to the interlayer a toughness and shock-resistant quality never before achieved. The process of using Vinal interlayers developed in this country by the Triplex Safety Glass Co. is claimed to yield the safest safety glass ever made. In its natural sheet form, Vinal is a flexible, creamy-white opaque material. After having been cut to size, the interlayeT is placed between the glass components and preliminary adhesion obtained by heating the loose sandwich and passing it between rubber-covered rollers. This process, as well as forming the sandwich as such, also reduces the opacity of the interlayer, but, in order completely to clarify the Vinal, it is necessary for the sandwich to be subjected to a pressure of at least 100 lb/sq in and a temperature of at least no deg C for a period of at least 15 minutes. The whole process, which is performed in an autoclave, takes approximately an hour and three-quarters (for the heating and cooling must be rela- tively gradual) and on emergence the sandwich has every appearance of a soKd homogeneous structure, although natur- ally, the lamince are visible at the edges. In order to stop a 20 mm shell from 200 yd a seven-p!y B IO
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