10.7 Properties of Materials 347metals, such as zinc and lead, creep occurs over a relatively short period of time, whereas materials
suchasconcretemaybesubjecttocreepoveraperiodofyears.Creepoccursinsteeltoaslightextent
atnormaltemperaturesbutbecomesveryimportantattemperaturesabove316◦C.
Closelyrelatedtocreepisrelaxation.Whereascreepinvolvesanincreaseinstrainunderconstant
stress,relaxationisthedecreaseinstressexperiencedoveraperiodoftimebyamaterialsubjectedto
aconstantstrain.
10.7.5 Fatigue
Structuralmembersarefrequentlysubjectedtorepetitiveloadingoveralongperiodoftime.Forexample,
themembersofabridgestructuresuffervariationsinloadingpossiblythousandsoftimesadayastraffic
moves over the bridge. In these circumstances, a structural member may fracture at a level of stress
substantially below the ultimate stress for nonrepetitive static loads; this phenomenon is known as
fatigue.
Fatigue cracks are most frequently initiated at sections in a structural member where changes in
geometry,suchasholes,notches,orsuddenchangesinsection,causestressconcentrations.Designers
seek to eliminate such areas by ensuring that rapid changes in section are as smooth as possible. At
re-entrantcorners,forexample,filletsareprovided,asshowninFig.10.19.
Otherfactorswhichaffectthefailureofamaterialunderrepetitiveloadingarethetypeofloading
(fatigueisprimarilyaproblemwithrepeatedtensilestressesdue,probably,tothefactthatmicroscopic
cracks can propagate more easily under tension), temperature, the material, surface finish (machine
marksarepotentialcrackpropagators),corrosion,andresidualstressesproducedbywelding.
Frequently, in structural members, an alternating stress,σalt, is superimposed on a static or mean
stress,σmean,asillustratedinFig.10.20.Thevalueofσaltisthemostimportantfactorindeterminingthe
numberofcyclesofloadthatproducefailure.Thestressσaltthatcanbewithstoodforaspecifiednumber
ofcyclesiscalledthefatiguestrengthofthematerial.Somematerials,suchasmildsteel,possessastress
levelthatcanbewithstoodforanindefinitenumberofcycles.Thisstressisknownastheendurancelimit
ofthematerial;nosuchlimithasbeenfoundforaluminumanditsalloys.Fatiguedataarefrequently
presentedintheformofanS–ncurveorstress–endurancecurve,asshowninFig.10.21.
Fig.10.19
Stress concentration location.