ConductivityConductivity is the property that enables a metal tocarry heat or electricity. The heat conductivity of a metalis especially important in welding, because it governsthe amount of heat that will be required for properfusion. Conductivity of the metal, to a certain extent,determines the type of jig to be used to control expansionand contraction. In aircraft, electrical conductivity mustalso be considered in conjunction with bonding, whichis used to eliminate radio interference. Metals vary intheir capacity to conduct heat. Copper, for instance, hasa relatively high rate of heat conductivity and is a goodelectrical conductor.Contraction and ExpansionContraction and expansion are reactions producedin metals as the result of heating or cooling. A highdegree of heat applied to a metal will cause it to expandor become larger. Cooling hot metal will shrink orcontract it. Contraction and expansion affect the designof welding jigs, castings, and tolerances necessary forhot-rolled material.QUALITIES OF METALSThe selection of proper materials is a primaryconsideration in the development of an airframe and inthe proper maintenance and repair of aircraft. Keepingin mind the general properties of metals, it is nowpossible to consider the specific requirements thatmetals must meet to be suitable for aircraft purposes.Strength, weight, and reliability determine therequirements to be met by any material used in airframeconstruction and repair. Airframes must be strong andas light in weight as possible. There are very definitelimits to which increases in strength can be accompaniedby increase in weight. An aircraft so heavy that it couldnot support more than a few hundred pounds ofadditional weight would be of little use. All metals, inaddition to having a good strength/weight ratio, must bethoroughly reliable, thus minimizing the possibility ofdangerous and unexpected failures. In addition to thesegeneral properties, the material selected for definiteapplication must possess specific qualities suitable forthe purpose. These specific qualities are discussed in thefollowing text.StrengthThe material must possess the strength required bythe demands of dimensions, weight, and use. There arefive basic stresses that metals may be required towithstand. These are tension, compression, shear,bending, and torsion. Each was discussed previously inthis chapter.WeightThe relationship between the strength of a materialand its weight per cubic inch, expressed as a ratio, isknown as the strength/weight ratio. This ratio forms thebasis of comparing the desirability of various materialsfor use in airframe construction and repair. Neitherstrength nor weight alone can be used as a means of truecomparison. In some applications, such as the skin ofmonocoque structures, thickness is more important thanstrength; and in this instance, the material with thelightest weight for a given thickness or gauge is best.Thickness or bulk is necessary to prevent buckling ordamage caused by careless handling.Corrosive PropertiesCorrosion is the eating away or pitting of the surfaceor the internal structure of metals. Because of the thinsections and the safety factors used in aircraft design andconstruction, it would be dangerous to select a materialsubject to severe corrosion if it were not possible toreduce or eliminate the hazard. Corrosion can bereduced or prevented by using better grades of basemetals; by coating the surfaces with a thin coating ofpaint, tin, chromium, or cadmium; or by anelectrochemical process called “anodizing.” Corrosioncontrol is discussed at length in Aviation MaintenanceRatings Fundamentals, and it is not covered in detail inthis TRAMAN.Working PropertiesAnother significant factor to consider in theselection of metals for aircraft maintenance and repairis the ability of material to be formed, bent, or machinedto required shapes. The hardening of metals bycold-working or forming is called work hardening. Ifa piece of metal is formed (shaped or bent) while cold,it is said to be cold-worked. Practically all the work youdo on metal is cold-work. While this is convenient, itcauses the metal to become harder and more brittle.If the metal is cold-worked too much (that is, if it isbent back and forth or hammered at the same place toooften), it will crack or break. Usually, the moremalleable and ductile a metal is, the more cold-workingit can withstand.1-24
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