332 CHAPTER 10 Materials
Thesecompositeshavenowbeenreplacedbycarbon-fiber-reinforcedplastics(CFRP),whichhave
similar properties to boron composites but are very much cheaper. Typically, CFRP has a modulus
of the order of three times that of GRP, one and a half times that of a Kevlar composite, and twice
thatofaluminumalloy.Itsstrengthisthreetimesthatofaluminumalloy,approximatelythesameas
thatofGRP,andslightlylessthanthatofKevlarcomposites.CFRPdoes,however,sufferfromsome
disadvantages.Itisabrittlematerialandthereforedoesnotyieldplasticallyinregionsofhighstress
concentration.Itsstrengthisreducedbyimpactdamagewhichmaynotbevisible,andtheepoxyresin
matricescanabsorbmoistureoveralongperiod,whichreducesitsmatrix-dependentproperties,such
asitscompressivestrength;thiseffectincreaseswithincreaseoftemperature.Further,theproperties
ofCFRParesubjecttomorerandomvariationthanthoseofmetals.Allthesefactorsmustbeallowed
forindesign.Ontheotherhand,thestiffnessofCFRPismuchlessaffectedthanitsstrengthbythe
preceding,anditislesspronetofatiguedamagethanmetals.Itisestimatedthatreplacing40percent
ofanaluminumalloystructurebyCFRPwouldresultina12percentsavingintotalstructuralweight.
CFRPisincludedinthewing,tailplane,andforwardfuselageofthelatestHarrierdevelopment,is
usedintheTornadotaileron,andhasbeenusedtoconstructacompleteJaguarwingandenginebay
doorfortestingpurposes.TheuseofCFRPinthefabricationofhelicopterbladeshasledtosignificant
increasesintheirservicelife,wherefatigueresistanceratherthanstiffnessisofprimaryimportance.
Figure10.1showsthestructuralcomplexityofaSeaKinghelicopterrotorblade,whichincorporates
CFRP,GRP,stainlesssteel,ahoneycombcore,andfoamfilling.Anadditionaladvantageoftheuse
ofcompositesforhelicopterrotorbladesisthatthemoldingtechniquesusedallowvariationsofcross
sectionalongthespan,resultinginsubstantialaerodynamicbenefits.Thisapproachisbeingusedinthe
fabricationofthemainrotorbladesoftheGKNWestlandHelicoptersEH101.
Fig.10.1
Sectional view of helicopter main rotor blade (courtesy Royal Aeronautical Society,Aerospacemagazine).