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Aviation History
1926
1926 - 0715.PDF
SEPTEMBER 30, 1926 81 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT which when stressed will develop into destructive cracks. This means that Duralumin that is normally stressed will not fail in consequence of the enlargement of the discontinuities into serious and dangerous flaws. Actually discontinuities do not spread at all when exposed to normal stress, and there- is no reason whatever to believe that the discontinuities reduce the effective strength of the material when working under normal conditions. When the material is stressed abnormally the discontinuities do not exert any effect so long as the principal stresses are applied parallel to the length of the discontinuities, i.e. parallel to the grain. Where, however. a stress which produces plastic deformation of the material is applied in a direction at right angles to the plane of the dis- continuities, the fracture occurs with greater ease across the plane of the discontinuity than through the surrounding material. This unquestionably is the reason of the somewhat lower transverse strength of Duralumin that has been pointed out above. So far as strength is concerned, therefore, the discontinuities in Duralumin only have effect if the material is stressed in such a way as to produce plastic deformation. Such stresses never are applied to the material in ordinary engineering practice and service. Static stresses that are within the elastic limit, or dynamic stresses within the fatigue range do not produce plastic deformation and, therefore, under either conditions the material is not affected by the discontinuities. Such stresses are, of course, the rule in engineering service, and it is safe, therefore, to say that Duralumin is none the worse as an engineering material because of the existence of what are called discontinuities. The method of fracture by bending does, of course, produce those conditions which tend to produce severe distortion at right angles to the grain, and it is in consequence of this method of fracture that the facets become visible. Brieflv then it mav be summarised that the discontinuities in Duralumin are the result of the macro-crystalline formation in the cast material that is characteristic of light alloys of this type. They are not internal defects, and they are not definite potential sources of weakness or fracture in material that is exposed to stresses inferior to the elastic limit and the fatigue limit. From what has been said it is evident that discontinuities are net likely to constitute a source of trouble in forgings, drop forgings, bars, and such like bulky material. They may, however, be a source of trouble in sheets of medium thickness when such materials are cold worked. Troubles are not frequently experienced even with such material, though it can readily be understood that where the metal is subject to a considerable amount of plastic deformation, rupture may be started at a discontinuity which would not develop in the body of the material. When the metal is being cold-shaped, particularly by such an operation as pressing or spinning, it is inevitable that in certain parts of the material stresses are applied that tend to rupture the metal at right angles to the plane of the discontinuities. Under these circumstances the material is liable to fracture more easily where a discon- tinuity is located than through the surrounding crystals. This is actually what does happen sometimes in practice. If, however, the material has not fractured during plastic working there is no reason at all to fear that it will subsequently fracture during the normal stressing that is incidental to engineering service. (To be continued). LUG DESIGN. By W. S. HOLLYHOCK. The design of lugs, although very simple mathematically, is a somewhat tedious process, occupying in the aggregate a considerable amount of time unless it is conveniently stan- dardised and condensed. Re \i II •* \i II L li 1 i I I 1 i • • Y X li i 1 4 £ 1 2 • •4- 5 \i I 1 1 • i 8 STE Ro If - li it li • I* i 5 4 I 2 8 4 • - :EL c 9 32 I 4 232 3 16 s. 3Z l"8 .i32 i 32 > D li 1 IS 16 i & S a i - t • 1 3 16 i 5 1 ii - - ' J / 0 DO / : / ' / / - / / / / (Y-I-25X) / / / : • 2-4 -6-8 I-O l;2 1-4 l;6 l;8 2 A - 2 7 8 i ii " if ti - IS 1 7 1 " 1 5 3 8 i - 8 —LT—'—i—1—H 1 i r"1 r 1 .1 i- 3 4 li- l-fc |i 2 4 2 *• ' I* 12 '» e- W INCHES OR CENTIMETRES 2 • 7 3 16 5. i 8 2 52 5 Ro X - - - - - - - - - 2* 28 2 7 8 4- I iJL H li " is l 7 8 I 1 i s a i -IC D 3 4 E 2. 8 i 1 e RALUM Y _ - - - 3 14- 8 I 7 8 1 1 5 8 8 in Rc - - - - - - zk 28 2 i 4- £8 2 •I IJ l§ 1 7 8 4 5. 8 5 1 • 4 8 Fig. 3. 636c
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