348 CHAPTER 10 Materials
Fig.10.20
Alternating stress in fatigue loading.
Fig.10.21
Stress–endurance curves.
Inmanypracticalsituations,theamplitudeofthealternatingstressvariesandisfrequentlyrandom
innature.TheS–ncurvedoesnot,therefore,applydirectlyandanalternativemeansofpredictingfailure
isrequired.Miner’scumulativedamagetheorysuggeststhatfailureoccurswhen
n 1
N 1+
n 2
N 2+···+
nr
Nr= 1 (10.1)
wheren 1 ,n 2 ,...,nrare the number of applications of stressesσalt,σmean,andN 1 ,N 2 ,...,Nrare the
numberofcyclestofailureofstressesσalt,σmean.
WeshallexaminefatigueanditseffectonaircraftdesigninmuchgreaterdetailinChapter14.