CHAPTER 14 Fatigue....................................................................................
FatiguehasbeendiscussedbrieflyinSection10.7whenweexaminedthepropertiesofmaterialsand
also in Section 12.2 as part of the chapter on airworthiness. We shall now look at fatigue in greater
detail and consider factors affecting the life of an aircraft including safe life and fail safe structures,
designingagainstfatigue,thefatiguestrengthofcomponents,thepredictionofaircraftfatiguelife,and
crackpropagation.
Fatigueisdefinedastheprogressivedeteriorationofthestrengthofamaterialorstructuralcomponent
duringservicesuchthatfailurecanoccuratmuchlowerstresslevelsthantheultimatestresslevel.As
we have seen, fatigue is a dynamic phenomenon which initiates small (micro) cracks in the material
orcomponentandcausesthemtogrowintolarge(macro)cracks;these,ifnotdetected,canresultin
catastrophicfailure.
Fatiguedamagecanbeproducedinavarietyofways.Cyclicfatigueiscausedbyrepeatedfluctuating
loads.Corrosionfatigueisfatigueacceleratedbysurfacecorrosionofthematerialpenetratinginward
so that the material strength deteriorates. Small-scale rubbing movements and abrasion of adjacent
partscausefrettingfatigue,whilethermalfatigueisproducedbystressfluctuationsinducedbythermal
expansions and contractions; the latter does not include the effect on the material strength of heat.
Finally, high-frequency stress fluctuations, due to vibrations excited by jet or propeller noise, cause
sonicoracousticfatigue.
Clearly an aircraft’s structure must be designed so that fatigue does not become a problem. For
aircraftingeneral,therequirementsthatthestrengthofanaircraftthroughoutitsoperationallifeshall
besuchastoensurethatthepossibilityofadisastrousfatiguefailureshallbeextremelyremote(i.e.,
theprobabilityoffailureislessthan10−^7 )undertheactionoftherepeatedloadsofvariablemagnitude
expectedinservice.Alsoitisrequiredthattheprincipalpartsoftheprimarystructureoftheaircraft
besubjectedtoadetailedanalysisandtoloadtestswhichdemonstrateasafelifeorthatthepartsof
the primary structure havefail-safecharacteristics. These requirements do not apply to light aircraft
providedthatzinc-richaluminumalloysarenotusedintheirconstructionandthatwingstresslevels
arekeptlow—thatis,providedthata3.05m/supgustcausesnogreaterstressthan14N/mm^2.
14.1 SafeLifeandFail-SafeStructures.................................................................
Thedangerofacatastrophicfatiguefailureinthestructureofanaircraftmaybeeliminatedcompletely
or may become extremely remote if the structure is designed to have a safe life or to be fail-safe. In
theformerapproach,thestructureisdesignedtohaveaminimumlifeduringwhichitisknownthatno
Copyright©2010,T.H.G.Megson. PublishedbyElsevierLtd. Allrightsreserved.
DOI:10.1016/B978-1-85617-932-4.00014-2 403