404 CHAPTER 14 Fatigue
catastrophicdamageoccurs.Attheendofthislife,thestructuremustbereplacedeventhoughthere
maybenodetectablesignsoffatigue.Ifastructuralcomponentisnoteconomicallyreplaceablewhen
itssafelifehasbeenreached,thecompletestructuremustbewrittenoff.Alternatively,itispossiblefor
easilyreplaceablecomponentssuchasundercarriagelegsandmechanismstohaveasafelifelessthan
thatofthecompleteaircraft,sinceitwouldprobablybemoreeconomicaltouse,say,twolightweight
undercarriagesystemsduringthelifeoftheaircraftratherthancarryaheavierundercarriagewhichhas
thesamesafelifeastheaircraft.
Thefail-safeapproachreliesonthefactthatthefailureofamemberinaredundantstructuredoes
not necessarily lead to the collapse of the complete structure, provided that the remaining members
areabletocarrytheloadshedbythefailedmemberandcanwithstandfurtherrepeatedloadsuntilthe
presenceofthefailedmemberisdiscovered.Suchastructureiscalledafail-safestructureoradamage
tolerantstructure.
Generally,itismoreeconomicaltodesignsomepartsofthestructuretobefail-saferatherthanto
have a long safe life, since such components can be lighter. When failure is detected, either through
a routine inspection or by some malfunction such as fuel leakage from a wing crack, the particular
aircraft may be taken out of service and repaired. However, the structure must be designed and the
inspectionintervalsarrangedsuchthatafailure—forexample,acrackthatistoosmalltobenoticedat
oneinspection—mustnotincreasetoacatastrophicsizebeforethenextinspection.Thedetermination
ofcrackpropagationratesisdiscussedlater.
Somecomponentsmustbedesignedtohaveasafelife;theseincludelandinggear,majorwingjoints,
wing–fuselagejoints,andhingesonall-movingtailplanesoronvariablegeometrywings.Components
whichmaybedesignedtobefail-safeincludewingskinswhicharestiffenedbystringersandfuselage
skinswhicharestiffenedbyframesandstringers;thestringersandframespreventskincracksspreading
disastrouslyforasufficientperiodoftimeforthemtobediscoveredataroutineinspection.
14.2 DesigningAgainstFatigue..........................................................................
Various precautions may be taken to ensure that an aircraft has an adequate fatigue life. We have
seeninChapter10thattheearlyaluminum–zincalloyspossessedhighultimateandproofstressesbut
weresusceptibletoearlyfailureunderfatigueloading;choiceofmaterialsisthereforeimportant.The
naturallyagedaluminum–copperalloyspossessgoodfatigueresistancebutwithlowerstaticstrengths.
Modernresearchisconcentratingonalloyswhichcombinehighstrengthwithhighfatigueresistance.
Attentiontodetaildesignisequallyimportant.Stressconcentrationscanariseatsharpcornersand
abruptchangesinsection.Filletsshouldthereforebeprovidedatre-entrantcorners,andcut-outs,such
aswindowsandaccesspanels,shouldbereinforced.Inmachinedpanels,thematerialthicknessshould
be increased around bolt holes, while holes in primary bolted joints should be reamered to improve
surface finish; surface scratches and machine marks are sources of fatigue crack initiation. Joggles
inhighlystressedmembersshouldbeavoided,whileasymmetrycancauseadditionalstressesdueto
bending.
Inadditiontosoundstructuralanddetaildesign,anestimationofthenumber,frequency,andmag-
nitude of the fluctuating loads an aircraft encounters is necessary. Thefatigue load spectrumbegins
when the aircraft taxis to its take-off position. During taxiing, the aircraft may be maneuvering over
unevengroundwithafullpayloadsothatwingstresses,forexample,aregreaterthaninthestaticcase.
Also,duringtake-offandclimb,anddescentandlanding,theaircraftissubjectedtothegreatestload