376 CHAPTER 12 Airworthiness
componentstoverifydesignestimates.Theproblemofstructuraldeteriorationduetocorrosionandwear
maybelargelyeliminatedbycloseinspectionduringserviceandtheapplicationofsuitableprotective
treatments.
12.2.3 Variation in Structural Strength
To minimize the effect of the variation in structural strength between two apparently identical com-
ponents, strict controls are employed in the manufacture of materials and in the fabrication of the
structure.Materialcontrolinvolvestheobservanceofstrictlimitsinchemicalcompositionandclose
supervisionofmanufacturingmethodssuchasmachining,heattreatment,rolling,andsoon.Inaddition,
the inspection of samples by visual, radiographic, and other means, and the carrying out of strength
testsonspecimensenablebelow-limitbatchestobeisolatedandrejected.Thus,ifasampleofabatch
ofmaterialfallsbelowaspecifiedminimumstrength,thenthebatchisrejected.Thismeans,ofcourse,
thatanactualstructurealwayscomprisesmaterialswithpropertiesequaltoorbetterthanthoseassumed
fordesignpurposes,anaddedbutunallowedfor“bonus”inconsideringfactorsofsafety.
Similarprecautionsareappliedtoassembledstructureswithregardtodimensiontolerances,quality
of assembly, welding, and so on. Again, visual and other inspection methods are employed and, in
certaincases,strengthtestsarecarriedoutonsamplestructures.
12.2.4 Fatigue
Althoughadequateprecautionsaretakentoensurethatanaircraft’sstructurepossessessufficientstrength
towithstandthemostsevereexpectedgustormaneuverload,therestillremainstheproblemoffatigue.
Practically,allcomponentsoftheaircraft’sstructurearesubjectedtofluctuatingloadswhichoccura
great many times during the life of the aircraft. It has been known for many years that materials fail
underfluctuatingloadsatmuchlowervaluesofstressthantheirnormalstaticfailurestress.Agraph
of failure stress against number of repetitions of this stress has the typical form shown in Fig. 12.2.
Fig.12.2
Typical form ofS−Ndiagram.