Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

10.2 Steel 329

magnesium,0.6%manganese,withtheremainderaluminumandappearstobeasatisfactorycompromise
betweenthevariousimportant,butsometimesconflicting,mechanicalproperties.
Interest in aluminum–magnesium–silicon alloys has recently increased, although they have been
in general use in the aerospace industry for decades. The reasons for this renewed interest are that
theyarepotentiallycheaperthanaluminum–copperalloysand,beingweldable,arecapableofreducing
manufacturingcosts.Inaddition,variants,suchastheISO6013alloy,haveimprovedpropertylevels
and,generally,possessasimilarhighfracturetoughnessandresistancetocrackpropagationasthe2000
seriesalloys.
Frequently, a particular form of an alloy is developed for a particular aircraft. An outstanding
example of such a development is the use of Hiduminium RR58 as the basis for the main structural
material, designated CM001, for use previously in the Concorde. Hiduminium RR58 is a complex
aluminum–copper–magnesium–nickel–ironalloydevelopedduringthe1939to1945warspecifically
forthemanufactureofforgedcomponentsingasturbineaeroengines.Thechemicalcompositionof
theversionusedintheConcordewasdecidedonthebasisofelevatedtemperature,creep,fatigue,and
tensiletestingprogramsandhasthedetailedspecificationof

%Cu %Mg %Si %Fe %Ni %Ti %Al

Minimum 2.25 1.35 0.18 0.90 1.0 – Remainder

Maximum 2.70 1.65 0.25 1.20 1.30 0.20

Generally,CM001isfoundtopossessbetteroverallstrength/fatiguecharacteristicsoverawiderange
oftemperaturesthananyoftheotherpossiblealuminumalloys.
The latest aluminum alloys to find general use in the aerospace industry are the aluminum–
lithiumalloys.Ofthese,thealuminum–lithium–copper–manganesealloy,8090,developedintheUnited
Kingdom, is extensively used in the main fuselage structure of GKN Westland Helicopters’ design
EH101; it has also been qualified for Eurofighter 2000 (now named the Typhoon) but has yet to be
embodied.IntheUnitedStates,thealuminum–lithium–copperalloy,2095,hasbeenusedinthefuse-
lage frames of the F16 as a replacement for 2124, resulting in a fivefold increase in fatigue life and
a reduction in weight. Aluminum–lithium alloys can be successfully welded, possess a high fracture
toughness,andexhibitahighresistancetocrackpropagation.

10.2 Steel...................................................................................................

Theuseofsteelforthemanufactureofthin-walled,box-sectionsparsinthe1930shasbeensupersededby
thealuminumalloysdescribedinSection10.1.Clearly,itshighspecificgravitypreventsitswidespread
useinaircraftconstruction,butithasretainedsomevalueasamaterialforcastingsforsmallcomponents
demandinghightensilestrengths,highstiffness,andhighresistancetowear.Suchcomponentsinclude
undercarriagepivotbrackets,wing-rootattachments,fasteners,andtracks.
Although the attainment of high and ultra-high tensile strengths presents no difficulty with steel,
it is found that other properties are sacrificed and that it is difficult to manufacture into finished