部分材料的断裂韧度包含AISI 4340.docx

部分材料的断裂韧度包含AISI 4340.docx

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部分材料的断裂韧度包含AISI4340

ADVANCEDMATERIALSLABORATORY

FractureToughnessMeasurement

Objectives

1.Tomakeavalidmeasurementoftheplanestrainfracturetoughnessofagivenmaterial.

2.Toobserveandunderstandatypicalbrittlefractureprocess.

Background

Thephenomenonofbrittlefracturedealswithsuddenfailureofstructuralcomponentswithoutwarning.Thisisattributedtopresenceofcracksorcrack-likedefectsinthematerialthatappearduringprocessingorduringfabricationandassemblyofthecomponent.Thetheorybehindthisphenomenon,asitappliestomanyengineeringstructures,isreferredtoasLinearElasticFractureMechanics（orLEFM）.Accordingtothistheory,theconditionforbrittlefailurecanbeexpressedas

whereKIiscalledthestressintensityfactorandisdependentonloadingconditionsandtheflawsizeinthematerial,andKICisamaterialpropertyknownastheplanestrainfracturetoughness.Thestressintensityfactorisusuallyexpressedas

whereQisageometrycorrectionfactordependingonthegeometryofthestructuralcomponentandthecrackgeometry,

istheappliedstress,and'a'denotesthecracksize.DefinitionsofthesequantitiesformanytypicalsituationsarepresentedinanAppendixattheendofthishandoutforyourconvienience.FinallynotethatKIandKIChavedimensionsofstress

（i.e.Mpa

orksi

）.

Inordertousetheabovecriterionforfracturetwoconditionshavetobemet.Theseare

（i）smallscaleyieldingcondition.Allin-planedimensionsofthecomponentaswellasthecracksizeshouldbelargerthanfifteentimesthecriticalplasticzonesize（rIC）,whichisdefinedas

where

istheyieldstrengthofthematerial.

（ii）planestraincondition.Thethicknessofthesampleshouldalsobelargerthanfifteentimesthecriticalplasticzonesize（rIC）.

ExperimentalProcedure

TheASTMstandard（E399）forplanestrainfracturetoughnesstestingprovidesaprocedureforcalculatingvaluesofKICformetallicmaterials.Thetestpermitsthreedifferentspecimenshapes:

abendspecimen,aC-shapedspecimen,andacompacttestspecimen（CTS）.TheCTSwillbeusedinthislaboratory.

TheprocedureformeasuringKICwithaCTSisasfollows:

1.MakeaguessoftheexpectedvalueofKIC.Thisenablesyoutocalculateanestimatedcriticalplasticzonesize.

2.Toensurethatonlysmall-scaleyieldingoccursatthecracktip,thelength,a,ofthecrackandtheremainingligament,（W-a）,shouldbegreaterthanorequalto15rIC.

a,（W-a）"15rIC.

3.Toensureplanestrain,thethickness,B,oftheCTSshouldbegreaterthanorequalto15rIC.

B"15rIC.

4.OnceaCTSismachined,accordingtothedimensionscalculatedabove,asharpcrackisintroducedattherootofthemachinednotch.Thisisaccomplishedbyfatiguepre-crackingthespecimen.Thisproceduresinvolvesimposingatime-varyingtensileloadontheCTStocauseasharpcracktoinitiateandslowlygrowattherootofthemachinednotch.Themaximumfatigueloadshouldbelessthan0.6timesthevalueoftheestimatedfinalfractureload:

Pfmax"0.6PQ.

5.Thefatigue-generatedprotionofthecrackshouldbeatleast1.2mmlong.

6.Onceasharpcrackexists,theactualKICtestcanbeperformed.Thetestconsistsofincreasingthetensileload,P,onthespecimenslowlywhilemeasuringthecrackopeningdisplacement,Æ.PlottingthePversusÆproducesacurvesimilartotheoneshowninFigure1.Fastfractureisindicatedbyagrossnonlinearityintheload-displacementrecord.

7.TocalculatetheKIc,firstcalculateaconditionalKQusing

Thegeometricvariablesa,W,andBaredefinedinthesampleschematicinthedatasheet.Determine,a,bymeasuringtheinitialcracklength（notchplusfatiguepre-crack）.PQisdeterminedbyprojectingalinewhoseslopeisfivepercentlessthantheoriginalslopeoftheP-Æcurve.PQistheloadcorrespondingtotheintersectionofthislinewiththeP-Æcurve.SeeFigure1.

8.TheratioPmax/PQshouldbelessthan1.10,wherePmaxisthemaximumloadencounteredinthetest.

Pmax/PQ<1.10.

9.Ifcondition8holds,thencalculate

（KQ/σy）2.Ifthisquantityislessthanthespecimenthickness,B,thecracklength,a,andtheremainingligament（W-a）,thenKQisequaltoKIc.OtherwisethetestisnotavalidKIctest.

Figure1.Schematicofthetypicalload-CODplotobtainedinafracturetoughnessexperiment.

References

W.T.Matthews,PlaneStrainFractueToughness（KIc）DataHandbookforMetals,AMMRCMS73-6,U.S.ArmyMaterialsandMechanicsResearchCenter,Watertown,MA,1973.

DamageTolerantDesignhandbook,MetalsandCeramicsInformationCenter,BatteleColumbusLaboratories,Columbus,Ohio,1975.

C.M.HudsonandS.K.Seward,"CompendiumofSourcesofFractureToughnessandFatigueCrackGrowthDataforMetallicAlloys,"InternationaljournalofFracture,V.14,1978,pp.R151-R184.

C.Hudson,S.K.Seward,"ACompendiumofSourcesofFractureToughnessandFatigueCrackGrowthDataforMetallicMaterials,PartII,"Int.J.Frac.,V.20,1982,pp.R59-R117.

DATASHEET

Material:

B=

mm

Pmax=

N

a=

mm

Pmax/PQ<1.10?

PQ=

N

B,a,（w-a）>

（KQ/σy）2?

σy=

MPa

InvalidTest?

KQ=

MPa

ValidTestKIC=KQ=

MPa

（KQ/σy）2

mm

Pfmax=

N

RepresentativedataforKIcforseveralmetalsaregiveninTable1,alongwiththevaluesofcorrespondingcriticalplasticzonesizes.AlsoincludeinTableIisthecracklengthL*=2a°whichinaGriffithcrackconfigurationwouldcausefractureinitiationatanappliedstressof（1/2）σy.NotethatL*isessentiallythecharacteristiclengthdimensionwhichspecimencracklength,remainingligamentandthicknessmustexceedinordertoobtainavalidKIcvalueformaterial.ThecombinationofhighKIcandlowσyleadstorelativelylargevaluesofcriticalplasticzonesize,andratherlongcracksarerequiredbeforeinitiationwilloccuratstresslevelswhicharesomefractionofthegeneralyieldstress.

Table1.Typicalvaluesofplanestrainfracturetoughness,KIC,atroomtemperature（forillustrationpurposesonly）

MATERIALS

E（GPa）

σy（MPa）

KIC（MPa

）

rIC（mm）

L*（mm）

Steels

Mediumcarbon（AISI-1045）

210

269

50

55

88.0

PressureVessel（ASTM-A5330-B）

210

483

153

16.0

256.0

HighStrengthAlloy（AISI-4340）

210

1,593

75

0.4

6.4

MaragingSteel（250-Grade）

210

1,786

74

0.3

4.8

AluminumAlloys

2024-T4

72

330

34

1.7

27.2

7075-T651

72

503

27

0.5

8.0

7039-T651

72

338

32

1.4

22.4

TitaniumAlloys

Ti-6AL-4V

108

1,020

50

0.4

6.4

Ti-4Al-4Mo-2Sn-05Si

108

945

72

0.9

14.4

Ti-6Al-2Sn-4Zr-6Mo

108

1,150

23

0.1

1.6

Polymers

PS

3.25

0.6-2.3

PMMA

3.-4.

1.2-1.7

PC

2.35

2.5-3.8

PVC

2.5-3.

1.9-2.5

PETP

3

3.8-6.1

Ceramics

Si3N4

310

4.-5.

SiC

410

3.4

Al203

350

3.-5.

Soda-LimeGlass

73

0.7

WC-15wt%Co（cermet）

570

16.-18.

ElectricalPorcelain

-

1.