12.2 Load Factor Determination 377Forsomematerials,suchasmildsteel,thecurve(usuallyknownasanS–Ncurveordiagram)isasymp-
totictoacertainminimumvalue,whichmeansthatthematerialhasanactualinfinite-lifestress.Curves
for other materials, for example, aluminium and its alloys, do not always appear to have asymptotic
valuessothatthesematerialsmaynotpossessaninfinite-lifestress.Weshalldiscusstheimplications
ofthisalittlelater.
Priortothemid-1940s,littleattentionhadbeenpaidtofatigueconsiderationsinthedesignofaircraft
structures. It was felt that sufficient static strength would eliminate the possibility of fatigue failure.
However,evidencebegantoaccumulatethatseveralaircraftcrasheshadbeencausedbyfatiguefailure.
The seriousness of the situation was highlighted in the early 1950s by catastrophic fatigue failures
of two Comet airliners. These were caused by the once-per-flight cabin pressurization cycle which
producedcircumferentialandlongitudinalstressesinthefuselageskin.Althoughthesestresseswere
wellbelowtheallowablestressesforsinglecycleloading,stressconcentrationsoccurredatthecorners
ofthewindowsandaroundrivetswhichraisedlocalstressesconsiderablyabovethegeneralstresslevel.
Repeatedcyclesofpressurizationproducedfatiguecrackswhichpropagateddisastrously,causingan
explosionofthefuselageathighaltitude.
Several factors contributed to the emergence of fatigue as a major factor in design. For example,
aircraftspeedsandsizesincreased,callingforhigherwingandotherloadings.Consequently,theeffect
ofturbulencewasmagnifiedandthemagnitudesofthefluctuatingloadsbecamelarger.Incivilaviation,
airlinershadagreaterutilizationandalongeroperationallife.Thenew“zinc-rich”alloys,usedfortheir
highstaticstrengthproperties,didnotshowaproportionalimprovementinfatiguestrength,exhibited
highcrackpropagationratesandwereextremelynotchsensitive.
Despite the fact that the causes of fatigue were reasonably clear at that time, its elimination as a
threattoaircraftsafetywasadifferentmatter.Thefatigueproblemhastwomajorfacets:theprediction
of the fatigue strength of a structure and a knowledge of the loads causing fatigue. Information was
lackingonbothcounts.TheRoyalAircraftEstablishment(RAE)andtheaircraftindustry,therefore,
embarkedonanextensivetestprogramtodeterminethebehaviorofcompletecomponents,joints,and
otherdetailpartsunderfluctuatingloads.TheseincludedfatiguetestingbytheRAEofsome50Meteor
4tailplanesatarangeoftemperatures,plusresearch,alsobytheRAE,intothefatiguebehaviorofjoints
andconnections.Furtherworkwasundertakenbysomeuniversitiesandbytheindustryitselfintothe
effectsofstressconcentrations.
Inconjunctionwiththeirfatiguestrengthtesting,theRAEinitiatedresearchtodevelopasuitable
instrument for counting and recording gust loads over long periods of time. Such an instrument was
developed by J. Taylor in 1950 and was designed so that the response fell off rapidly above 10Hz.
Crossingsofgthresholdsfrom0.2to1.8gat0.1gintervalswererecorded(notethatsteadylevelflight
is1gflight)duringexperimentalflyingattheRAEonthreedifferentaircraftover28000km,andthebest
techniquesforextractinginformationfromthedataestablished.Civilairlinescooperatedbycarrying
theinstrumentsontheirregularairservicesforanumberofyears.Eightdifferenttypesofaircraftwere
equippedsothatby1961recordshadbeenobtainedforregionsincludingEurope,theAtlantic,Africa,
India,andtheFarEast,representing19000hoursand8millionkmofflying.
Atmospheric turbulence and the cabin pressurization cycle are only two of the many fluctuating
loadswhichcausefatiguedamageinaircraft.Ontheground,thewingissupportedontheundercarriage
andexperiencestensilestressesinitsuppersurfacesandcompressivestressesinitslowersurfaces.In
flight,thesestressesarereversedasaerodynamicliftsupportsthewing.Also,theimpactoflandingand
groundmaneuveringonimperfectsurfacescausestressfluctuationswhile,duringlandingandtake-off,