Variable Amplitude and Multiaxial Fatigue.pdf

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1、4VariableAmplitudeandMultiaxialFatigue4.1IntroductionEarlyfatiguetestsonstructures,componentsandtestspecimenswereallcar-riedoutusingconstantamplitudefatigueloading.However,manystructuresandcomponentsaresubjectedinservicetovariableamplitudefatigueload-ing(orva

2、riableamplitudeloading),withvariationsfollowingeitheraregularorarandompattern(Frostetal.1974).Variableamplitudefatigueloadingscandividedintotwobroadclasses(Pook1979):thoseinwhichindividualloadcyclescanbedistinguished,suchasnarrowbandrandomloading(Fig-ure4.1),

3、andthoseinwhichindividualloadcyclescannotbedistinguished,suchasbroadbandrandomloading(Figure4.2).Modernfatiguetestingequipmentmakesitpossibletoapplyvirtuallyanydesiredloadhistory,andnarrowandbroadbandrandomloadinghavebeenusedextensivelyforsometime,especiallyi

4、nstructuraltesting(Pook1983a,Marsh1988).Theuseofbroadbandrandomloadingcanmaketheanalysisoftestresultsdifficult(Smith1965).Conceptstakenfromrandomprocesstheory(Papoulis1965,BendatandPiersol1971,2000)areusedinthecharacterisationofrandomloadhistories(seeSectionB.

5、2.1).Conventionsusedinthefatiguetestinglit-eraturesometimesdifferfromthoseusualinrandomprocesstheory.Althoughreferredtoasrandom,loadhistoriesusedintestsareusuallypseudorandominthattheyrepeatexactlyafterareturnperiod.Manyfatiguetestsontestspecimens,componentsa

6、ndstructuresarecar-riedoutusingasimple,uniaxialloading.Morecomplexfatigueloadingsareknownasmultiaxialfatigueloadings(ormultiaxialloadings).Manycom-ponentsandstructuresaresubjectedtomultiaxialfatigueloadinginservice.Amultiaxialfatigueloadingmaybedefinedasoneinw

7、hichmorethanonestresssystemisproducedatspecificpointsintheobjectbeingloaded.This36Chapter4:VariableAmplitudeandMultiaxialFatigueFigure4.1.Narrowbandrandomloading,frequency∼100Hz,irregularityfactor∼0.99(Pook1987a).ReproducedunderthetermsoftheClick-UseLicence.Fi

8、gure4.2.Broadbandrandomloading,irregularityfactor0.410(Pook1987a).ReproducedunderthetermsoftheClick-UseLicence.maybebecausemorethanonefatigueloadisapplied,orbecauseofthenatureoftheload.Wh