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Aviation History
1951
1951 - 1061.PDF
FLIGHT 8 JUNE 195I About Aluminium... MECHANICAL PROPERTIES • tensile ALUMINIUM alloys offer a very wideJi\ range of properties, from those of thc commercially pure metal on the one handto those of advanced heat treatable alloys on the other. The range between isadequately covered by a large number of alloys, most of which are available inseveral conditions or tempers. Behaviour under tension is generallyconsidered the first yardstick of an engin- eering material, and the figure at the footof the page shows typical tensile stress/ strain curves for three different aluminiumalloys and compares them with that for mild steel. The alloys are : commerciallypure aluminium (Noral 2S) in the fully annealed state suitable for deep pressing;a 5% magnesium aluminium alloy (Noral A56S) after strain-hardening, by rolling,to the " half-hard" temper; a zinc- magnesium - copper aluminium alloy,known as Noral M75S and used in aircraft construction, after solution treatment andageing. Modulus of Elasticity — It will at once be obvious that the strainper unit increase of stress is much higher for the light metal than for steel, in factthe Elastic Modulus is about one third, being between 9J and io| million lb. persq. in. for most aluminium alloys. If, to effect a weight saying of 66%, asteel structural member is replaced by one of identical form in an aluminiumalloy of equal strength, elastic deflection will be about three times as large. Shouldthis be objectionable, a redesigned mem- ber of equal stiffness will weigh abouthalf as much as the steel one instead of a third. This relatively low stiffness is an advan-tage under shock or impact loads, which, by virtue of the large elastic deformation,are absorbed well by the aluminium alloys. Proof Stress No clearly denned elastic limit or yield point is to be seen on stress/strain curves This article is one of a series by which it is hoped to make students more familiar with the nature and properties of aluminium and its alloys. Only the barest outlines can be presented, but these may serve as an introduction to this major structural material. for aluminium alloys, so the point ofdeparture from the elastic range has to be denned arbitrarily. For convenience inroutine testing, a point is chosen at which the permanent deformation is easilymeasured: in this country, a permanent set of 0.1 °0 of the original gauge length,abroad more usually 0.2%. To mark the limit of proportionality closely, 0.01% setis sometimes used. The stress at which a 0.1% set isobserved is called the 0.1% proof stress, and, because it reveals the onset of plasticmovement, is often of more value to the designer than is the ultimate stress. s Elongation - per cent 13The figure above shows how itobtained from a stress/strain diagram. Some alloys, notably the heavily strain-hardened ones, have a high ratio of proof strength to ultimate strength; in 2SH forexample the 0.1% proof stress is 9 tons per sq. in. and the ultimate stress 10 tonsper sq. in. Generally the ratio of proof to ultimate varies from 35% for softtempers to 90% for the hardest; in the fully heat treated alloys it is about 80%.Although a high proof stress is in itself an advantage, a high proof stress/ultimatestress ratio implies a low ductihty. JO c s; v° I ft 10L .—•—"" _—• — _—.——- — * — . " ^ Al -ft — __ _— Ay ALLOY, ""•- Al Zr. .— ROLLED T My Cu ALL • " D ''2 HARD Or . fULLV w TEMPER HEAT TR AT£O ILO STEEL . NORMALISED CALCD Elongation Elongation - per cent The amount of permanent stretch atthe instant of breaking is a useful guide to the ductility of a metal, and a minimumvalue is usually demanded by light alloy specifications. It is not, however, an in-fallible index of workability, and selection of an alloy for forming operations shouldnever be made on this basis alone. Elongation may be found by clampingthe pieces of a broken test specimen together and measuring between marksapplied before starting the test. It is gen- erally expressed as a percentage of theoriginal gauge length of the test specimen. Elongation is not equal everywhere in thespecimen but is greatest around the fracture; the gauge length chosen willtherefore affect the value very much, and is always specified. A gauge length of 2 inches is a commonstandard in this country. For better com- parison of different sized specimens, thelength may be referred to the original cross-sectional area; thus 4VArea isfrequently used, especially for round sections. In Europe, gauge lengths of11.3\/Area and 5.65\/Area (equivalent to 10 and 5 diameters for round speci-mens) have long been standard. Typical elongation values for alumin-ium alloys vary from 35 „ (on two inches) in annealed material to as little as 3% infully strain-hardened metal. The heat treated alloys, even the strongest, possessa fair elongation, ranging from 9% to 20%. Strength/weight ratio — It will be seen that the strength of analuminium alloy may exceed that of mild steel. If the weight of the two materialshas to be considered, aluminium is able to compete with the high-tensile steels. A rough indication of the structuralmerit of a material, at least under tension, is given by its specific 0.1% proof stress.This empirical figure, useful only for comparison, is obtained by dividing the0.1% proof stress by the specific gravity. A similar idea is sometimes expressedas the length of uniformly cross-sectioned rod of a material that could be suspendedbefore it broke under its own weight. On this basis, a commonly used alloy ofthe heat treatable type, Noral 26S, is over three times as effective as mild steel. The fifth article in the series will touch on some other mechanical properties. ORTHERN ALUMINIUM COMPANY LIMITED TECHNICAL DEVELOPMENT DIVISION BANBURY, OXON
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