10.7 Properties of Materials 333A composite (fiberglass and aluminum) is used in the tail assembly of the Boeing 777, while the
leading edge of the Airbus A310–300/A320 fin assembly is of conventional reinforced glass fiber
construction, reinforced at the nose to withstand bird strikes. A complete composite airframe was
producedfortheBeechcraftStarshipturbopropexecutiveaircraft,which,however,wasnotacommercial
successduetoitscanardconfigurationcausingdragandweightpenalties.
Thedevelopmentofcompositematerialsiscontinuingwithresearchintotheremovalofstrength-
reducingflawsandlocalimperfectionsfromcarbonfibers.Othermatrices,suchaspolyetheretherketone
whichabsorbsmuchlessmoisturethanepoxyresin,hasanindefiniteshelflife,andperformswellunder
impact, are being developed; fabrication, however, requires much higher temperatures. Metal matrix
compositessuchasgraphite–aluminumandboron–aluminumarelightweightandretaintheirstrength
athighertemperaturesthanaluminumalloysbutareexpensivetoproduce.
Generally,theuseofcompositesinaircraftconstructionappearstohavereachedaplateau,particu-
larlyincivilsubsonicaircraft,wherethefractionofthestructurecomprisingcompositesisapproximately
15percent.Thisisduelargelytothegreatercostofmanufacturingcompositescomparedwithaluminum
alloystructures,sincecompositesrequirehandcraftingofthematerialsandmanualconstructionpro-
cesses.Theseincreasedcostsareparticularlyimportantincivilaircraftconstructionandarebecoming
increasinglyimportantinmilitaryaircraft.
10.7 PropertiesofMaterials..............................................................................
In Sections 10.1 to 10.6, we discussed the various materials used in aircraft construction and listed
someoftheirproperties.Weshallnowexamineinmoredetailtheirbehaviorunderloadandalsodefine
differenttypesofmaterial.
Ductility
Amaterialissaidtobeductileifitiscapableofwithstandinglargestrainsunderloadbeforefracture
occurs. These large strains are accompanied by a visible change in cross-sectional dimensions and
thereforegivewarningofimpendingfailure.Materialsinthiscategoryincludemildsteel,aluminum,
andsomeofitsalloys,copper,andpolymers.
Brittleness
Abrittlematerialexhibitslittledeformationbeforefracture,thestrainnormallybeingbelow5percent.
Brittlematerialsthereforemayfailsuddenlywithoutvisiblewarning.Includedinthisgroupareconcrete,
castiron,high-strengthsteel,timber,andceramics.
Elastic Materials
Amaterialissaidtobeelasticifdeformationsdisappearcompletelyonremovaloftheload.Allknown
engineeringmaterialsare,inaddition,linearlyelasticwithincertainlimitsofstresssothatstrain,within
theselimits,isdirectlyproportionaltostress.