Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

330 CHAPTER 10 Materials

components. To overcome some of these difficulties, types of steel known asmaragingsteels were
developed in 1961, from which carbon is either eliminated entirely or present only in very small
amounts.Carbon,whileproducingthenecessaryhardeningofconventionalhigh-tensilesteels,causes
brittleness and distortion; the latter is not easily rectifiable, as machining is difficult and cold form-
ing impracticable. Welded fabrication is also almost impossible or very expensive. The hardening
of maraging steels is achieved by the addition of other elements such as nickel, cobalt, and molyb-
denum. A typical maraging steel would have these elements present in the proportions: nickel
17 to 19%, cobalt 8 to 9%, molybdenum 3 to 3.5%, with titanium 0.15 to 0.25%. The carbon
content would be a maximum of 0.03%, with traces of manganese, silicon, sulfur, phosphorus,
aluminum,boron,calcium,andzirconium.Its0.2percentproofstresswouldbenominally1400N/mm^2
anditsmodulusofelasticity180000N/mm^2.
The main advantages of maraging steels over conventional low-alloy steels are higher fracture
toughness and notched strength, simpler heat treatment, much lower volume change and distortion
duringhardening,verymuchsimplertoweld,easiertomachine,andbetterresistancetostresscorrosion/
hydrogenembrittlement.Ontheotherhand,thematerialcostofmaragingsteelsisthreeormoretimes
greaterthanthecostofconventionalsteels,althoughthismaybemorethanoffsetbytheincreasedcost
of fabricating a complex component from the latter steel. Maraging steels have been used in aircraft
arresterhooks,rocketmotorcases,helicopterundercarriages,gears,ejectorseats,andvariousstructural
forgings.
In addition to the preceding, steel in its stainless form has found applications primarily in the
constructionofsupersonicandhypersonicexperimentalandresearchaircraft,wheretemperatureeffects
are considerable. Stainless steel formed the primary structural material in the Bristol 188, built to
investigatekineticheatingeffects,andalsointheAmericanrocketaircraft,theX-15,capableofspeeds
oftheorderofMach5–6.

10.3 Titanium...............................................................................................

The use of titanium alloys increased significantly in the 1980s, particularly in the construction of
combat aircraft as opposed to transport aircraft. This increase continued in the 1990s to the stage
where,forcombataircraft,thepercentageoftitaniumalloyasafractionofstructuralweightisofthe
same order as that of aluminum alloy. Titanium alloys possess high specific properties, have a good
fatigue strength/tensile strength ratio with a distinct fatigue limit, and have some retain considerable
strength at temperatures up to 400 to 500◦C. Generally, there is also a good resistance to corrosion
andcorrosionfatigue,althoughpropertiesareadverselyaffectedbyexposuretotemperatureandstress
inasaltenvironment.Thelatterposesparticularproblemsintheenginesofcarrier-operatedaircraft.
Further disadvantages are a relatively high density so that weight penalties are imposed if the alloy
isextensivelyused,coupledwithhighprimaryandhighfabricationcosts,approximatelyseventimes
thoseofaluminumandsteel.
In spite of this, titanium alloys were used previously in the airframe and engines of Concorde,
while the Tornado wing carry-through box is fabricated from a weldable medium-strength titanium
alloy.TitaniumalloysarealsousedextensivelyintheF15andF22Americanfighteraircraftandare
incorporatedinthetailassemblyoftheBoeing777civilairliner.Otherusesincludeforgedcomponents
suchasflapandslattracksandundercarriageparts.