材料的动态行为_香港科技大学余同希讲座PPT

CHAPTER3CHAPTER3.DYNAMICBEHAVIOR2012年DYNAMICBEHAVIOROFMATERIALS年10月25日13.1Materials’BehavioratHighStrain-rates3.2DynamicExperimentalTechnique13.3ConstitutiveEquationsatHighStrain-rates3.1MATERIALS’BEHAVIORATGSAASHIGHSTRAIN-RATES22STRAIN-RATEESTIMATIONSTRAINRATEESTIMATIONDefinitionofstrainrate:1,(unit:)dsdtCalculatethestrainratesExample1.Atensilespecimenwithlengthof10cmistddittihitlitf1/extendedinatestingmachineatavelocityof1m/s.1001(unit:)llvmss00,(unit:)0.1stlvlm110s3Ifthetensilespeedis6mm/min,then3110s310sSTRAIN-RATEESTIMATION(CONT’D)CalculatethestrainratesExample2.Acylindricalprojectileimpingingarigidtarget.AthjtildltliltththlthAssumetheprojectiledecelerateslinearlytorestwhenthelengthreducesto2.5cm(inshapeofatruncatedcone)02.55lltt2.5scm522510sst5,510vtstv4152.5105510ssForshockwavesproducedbynucleardetonation,theshockfrontthicknessdecreaseswithincreasingpressure.Strain-ratehbtthtilit5510s8911010mayreach,butthematerialresponseisnotknownIncreeptests,89110~10s7110s410s4SCHEMATICCLASSIFICATIONOFTESTINGTECHNIQUESACCORDINGTOSTRAIN-RATETECHNIQUESACCORDINGTOSTRAIN-RATEstrainrate,s-110-910-810-710-610-510-410-310-210-1100101102103104105106107onalinestersulic,ulicveninesringbartestulic,nes;eterpactfoilaserpactivesConventiostingmachiCreeptestHydrausevo-hydraurscrew-drivstingmachiExpandingrHopkinsonayloranvilcityhydrauaticmachinmplastomecl.plateimpExplodingPulsedlaalplateimpExplositessortesEHTaHigh-velocorpneumacamIncENormaCommontestingmethodsCreepandstressrelaxationQuasi-staticDynamic-lowDynamic-highHighvelocityimpactmethodsDynamicconsideraVisco-plasticresponseofmetalTestswithconstantcross-headvelocityandstressbeingthesamethroughoutlengthofMechanicalresonancePlasticwavepropa-Shockwavepropagationconsidera-tionsresponseofmetalsamethroughoutlengthofspecimenisimportantpropagationpropagationInertialforcesnegligibleInertialforcesimportant5STRAIN-RATEDEPENDENCE1905,B.Hopkinson’sexperimentsonsteel,showtheditthtlttihihitdynamicstrengthwasatleasttwiceashighasitslow-strain-ratestrength.AfterthatmanydynamicexperimentshaveshownAfterthat,manydynamicexperimentshaveshownthattheflowstressofmaterialsdependsoncurrentstrain,currentstrainrate,thestrainhistoryandthetemperature;i.e.,,,history,fTJohnson-Cookequation:**01ln1nmeffeffBcT66COMPRESSIVESTRESS-STRAINCURVESCOMPRESSIVESTRESSSTRAINCURVES7075-T6aluminiumalloydisplaysnoapparentstrainFortitanium6%-Al4%Valloy,considerablestrainrateeffectis7displaysnoapparentstrainrateeffectconsiderablestrainrateeffectisobserved7TENSILESTRESS-STRAINCURVESFORMILDSTEELbothyieldstressandultimatestressincreasewithystrain-rateductility(maximumelongation)reduceswithstrain-rate8ratemildsteelisarate-sensitivematerial8VARIATIONOFDYNAMICUNI-AXIALYIELDSTRESSOFMILDSTEELWITHSTRAINRATE99SHEARSTRESS-STRAINCURVESIn1983,Cliftonreporteddramaticincreasesinflowstressasstrain-ratesof105s-1areattainedfor1100-0Alasstrainratesof10sareattainedfor11000AlThus,somescientistsbelievethatthereisalimitstrain-rate,atwhichthestrengthofthematerialbecomesinfinitegWhenthestrainrateincreases,thedeformationprocesschangesfromdeformationprocesschangesfromfullyisothermaltofullyadiabaticSometimes,adiabaticshearbands,areformedForcertainmaterial,differentconstitutiveequationsshouldbeproposedindifferentrangesofstrainrate10strain-rate103.2DYNAMICEXPERIMENTALTECHNIQUE1111INTERMEDIATESTRAIN-RATEMACHINESINTERMEDIATESTRAINRATEMACHINESCompressed-gasmachineCompressedgasmachine1212INTERMEDIATESTRAIN-RATEMACHINESDropHammerDynatup8250:Dropweight2.5-45.5kg;Impactvelocity0.6-3.6m/s;PneumaticallyassisteduptoNDT-ZCJ2603:Max.energy~6000J;Dropweight=200kg;13.4m/s;Energyupto840J.Impactvelocity3.8~7.7m/s.13Typicalstrain-rate:0.1～103s-113INTERMEDIATESTRAIN-RATEMACHINESRotaryflywheelmachineRotaryflywheelmachine14Typicalstrain-rate:0.1～103s-114SPLITHOPKINSONPRESSUREBAR(SHPB)()OriginalHopkinsonpressurebar:byB.Hopkinsonin1914producesapressurepulseofcontrolledlengthandtoltflaccelerateaflyerSHPB:byDaviesandKolskyinlate1940’sWidltdthiittfWidelyacceptedasthemaininstrumentforintermediatestrain-rate(102~104s-1)testingv0vl2LldetonatorL15flyerpendulumPressurebarexplosive2l15SPLITHOPKINSONPRESSUREBAR(SHPB)(CONT’D)(CONTD)AnelasticpressurepulseisppproducedfromtheimpactendoftheincidentbarOnceitreachesthespecimenbetweenincidentandtransmitterblttfbars,acomplexpatternofreflectionsisproducedThepulsepassesthroughtheThepulsepassesthroughthespecimen(withplasticdeformation),andcontinuesintransmitterbarandcontinuesintransmitterbar1616SPLITHOPKINSONPRESSUREBAR(SHPB)(CONT’D)ThedesignofaSHPBshouldsatisfy:(CONT’D)LprojectileLincidentandLtransmtterLpulse=2LprojectileLspecimenpulseprojectilespecimenTheformerensuresthatthestrainsignalsintheincidentandtransmitterbarsarenotdisturbedbyreflectedsignalsfromthefarendsThelatterensuresthatthestressandstrainstateiniiltifspecimenisalmostuniformFinally,astress-straincurvecanbeconstructedfromthedatagatheredinaSHPBtestthedatag