10.6 Composite Materials 331Newfabricationprocesses(e.g.,superplasticformingcombinedwithdiffusionbonding)enablelarge
andcomplexcomponentstobeproduced,resultinginareductioninproductionman-hoursandweight.
Typical savings are 30 percent in man-hours, 30 percent in weight, and 50 percent in cost compared
withconventionalrivetedtitaniumstructures.Itispredictedthatthenumberoftitaniumcomponents
fabricated in this way for aircraft will increase significantly and include items such as access doors,
sheetforareasofhotgasimpingement,andsoforth.
10.4 Plastics................................................................................................
Plain plastic materials have specific gravities of approximately unity and are therefore considerably
heavierthanwood,althoughofcomparablestrength.Ontheotherhand,theirspecificgravitiesareless
thanhalfthoseofthealuminumalloyssothattheyfindusesaswindowsorlightlystressedpartswhose
dimensions are established by handling requirements rather than strength. They are also particularly
useful as electrical insulators and as energy-absorbing shields for delicate instrumentation and even
structureswhereseverevibrationsuchasinarocketorspaceshuttlelaunchesoccurs.
10.5 Glass...................................................................................................
Themajorityofmodernaircrafthavecabinspressurizedforflightathighaltitudes.Windscreensand
windowsarethereforesubjectedtoloadsnormaltotheirmidplanes.Glassisfrequentlythematerialused
forthispurposeintheformofplainorlaminatedplateorheat-strengthenedplate.Thetypesofplate
glassusedinaircrafthaveamodulusofelasticitybetween70000and75000N/mm^2 ,withamodulus
ofruptureinbendingof45N/mm^2 .Heat-strengthenedplatehasamodulusofruptureofaboutfourand
ahalftimesthisfigure.
10.6 CompositeMaterials.................................................................................
Compositematerialsconsistofstrongfiberssuchasglassorcarbonsetinamatrixofplasticorepoxy
resin,whichismechanicallyandchemicallyprotective.Thefibersmaybecontinuousordiscontinuous
butpossessastrengthverymuchgreaterthanthatofthesamebulkmaterials.Forexample,carbonfibers
haveatensilestrengthoftheorderof2400N/mm^2 andamodulusofelasticityof400000N/mm^2.
A sheet of fiber-reinforced material is anisotropic—in other words, its properties depend on the
direction of the fibers. Generally, therefore, in structural form, two or more sheets are sandwiched
togethertoformalay-upsothatthefiberdirectionsmatchthoseofthemajorloads.
In the early stages of the development of composite materials, glass fibers were used in a matrix
of epoxy resin. This glass-reinforced plastic (GRP) was used for radomes and helicopter blades but
foundlimiteduseincomponentsoffixedwingaircraftduetoitslowstiffness.Inthe1960s,newfibrous
reinforcementswereintroduced;Kevlar,forexample,isanaramidmaterialwiththesamestrengthas
glassbutisstiffer.Kevlarcompositesaretoughbutpoorincompressionanddifficulttomachine,sothey
wereusedinsecondarystructures.Anothercomposite,usingboronfiberanddevelopedintheUnited
States,wasthefirsttopossesssufficientstrengthandstiffnessforprimarystructures.