Fracture mechanics CTOD Crack Tip Opening Displacement

Fracture Mechanics .Whilst the Crack Tip Opening Displacement (CTOD) test was developed for the characterisation of metals it has also been used to determine the toughness of non-metallics such as weldable plastics.
The CTOD test is one such fracture toughness test that is used when some plastic deformation can occur prior to failure - this allows the tip of a crack to stretch and open, hence 'tip opening displacement

1. Fracture Mechanics
Davalsab.M.L
M.Tech(Machine Design), Reva university
[email protected]
8904567449
India

2. CTOD testing
WhilsttheCrackTipOpeningDisplacement(CTOD)
testwasdevelopedforthecharacterisationofmetals
ithasalsobeenusedtodeterminethetoughnessof
non-metallicssuchasweldableplastics.
TheCTODtestisonesuchfracturetoughnesstest
thatisusedwhensomeplasticdeformationcan
occurpriortofailure-thisallowsthetipofacrackto
stretchandopen,hence'tipopeningdisplacement
Therearetwobasicforms-asquareorarectangular
crosssectionspecimen.Ifthespecimenthicknessis
definedas'B',thedepth(W)willbeeitherBor2B
withastandardlengthof4.6W.Anotchismachined
atthecentreandthenextendedbygeneratinga
fatiguecracksothatthetotal'defect'lengthishalf
thedepthofthetestpiece-seeFig.1.Atestona
100mmthickweldwillthereforerequireaspecimen
measuring100mmthick,200mmwideand920mm
long-anexpensiveoperation,thevalidityofwhich
canonlybedeterminedoncethetesthasbeen
completed.

3. Thetestisperformedbyplacingthespecimen
intothreepointbendingandmeasuringthe
amountofcrackopening.Thisisdonebymeans
ofastraingaugeattachedtoaclipplaced
betweentwoaccuratelypositionedknifeedges
atthemouthofthemachinednotch
Asbendingproceeds,thecracktipplastically
deformsuntilacriticalpointisreachedwhen
thecrackhasopenedsufficientlytoinitiatea
cleavagecrack.Thismayleadtoeitherpartialor
completefailureofthespecimen.Thetestmay
beperformedatsomeminimumtemperatureeg
theminimumdesigntemperatureor,more
rarely,atarangeoftemperatures.
Thevaluesthatarerequiredforthecalculation
oftoughnessarefirstlytheloadatwhich
fractureoccursandsecondlytheamountby
whichthecrackhasopenedatthepointofcrack
propagation

4. Sincethelengthofthecrackandtheopening
atthemouthofthenotchareknownitisa
simplemattertocalculatethecracktip
openingbysimplegeometry.Whilstthetestis
inprogresstheresultsarerecorded
automaticallyonaload/displacementchart.
Thisillustratesthevariousshapesofcurvethat
maybeproduced–
(a)Itisatestwherethetestpiecehas
fracturedinabrittlemannerwithlittleor
noplasticdeformation.
(b)exhibitsa'pop-in'wherethebrittlecrack
initiatesbutonlypropagatesashort
distancebeforeitisarrestedintougher
material-thismayoccurseveraltimes
givingthecurveasawtoothappearance
orafterthisonepop-indeformationmay
continueinaductilemannerasin
(c)whichshowscompletelyplastic
behaviour.

6. J-test
ASTME1820hastwoalternativemethodsforJtests:
thebasicprocedureandtheresistancecurve
procedure.Thebasicprocedureentailsmonotonically
loadingthespecimentofailureortoaparticular
displacement,dependingonthematerialbehavior.
Theresistancecurveprocedurerequiresthatthecrack
growthbemonitoredduringthetest.TheJintegralis
calculatedincrementallyintheresistancecurve
procedure.Thebasicprocedurecanbeusedto
measureJatfractureinstabilityorneartheonsetof
ductilecrackextension.
Becausecrackgrowthisnotmonitoredasapartofthe
basictestprocedure,amultiple-specimentechniqueis
normallyrequiredtoobtainaJ-Rcurve.Insuchcases,
aseriesofnominallyidenticalspecimensareloadedto
variouslevelsandthenunloaded.Differentamounts
ofcrackgrowthoccurinthevariousspecimens.The
crackgrowthineachsampleismarkedbyheattinting
orfatiguecrackingafterthetest.Eachspecimenis
thenbrokenopenandthecrackextensionis
measured.

9. Fracture testing of Weldments

10. Weldment’s test
consideration
Weldedjoints,however,havedecidedlyheterogeneous
microstructuresand,inmanycases,irregularshapes.
Weldmentsalsocontaincomplexresidualstress
distributions.Mostexistingfracturetoughnesstesting
standardsdonotaddressthespecialproblems
associatedwithweldmenttesting
Number of factors need special consideration.
•Specimen design and fabrication are more difficult
because of the irregular shapes and curved surfaces
associated with some welded joints.
•The heterogeneous microstructure of typical
weldments requires special attention to the location
of the notch in the test specimen.
•Residual stresses make fatigue precrackingof
weldment specimens more difficult.
•After the test, a weldment may be sectioned and
examined metallographicallyto determine whether
or not the fatigue crack sampled the intended
microstructure.

11. Specimen Design and
Fabrication
TheunderlyingphilosophyoftheBritishStandardstestprocedureon
specimendesignandfabricationisthatthespecimenthicknessshouldbe
asclosetothesectionthicknessaspossible.Largerspecimenstendto
producemorecrack-tipconstraint,andhencelowertoughness
Achievingnearlyfull-thicknessweldmentoftenrequiressacrificesinother
areas.Forexample,ifaspecimenistobeextractedfromacurvedsection
suchasapipe,onecaneitherproduceasub-sizerectangularspecimen
thatmeetsthetolerancesoftheexistingASTMstandards,orafull-
thicknessspecimenthatiscurved.
Ifcurvatureordistortionofaweldmentisexcessive,thespecimencanbe
straightenedbybendingoneithersideofthenotchtoproducea‘‘gull
wing’’configuration,whichisillustratedinFigure7.34.Thebendingmust
beperformedsothatthethreeloadingpoints(inanSE(B)specimen)are
aligned

12. Notch location and
orientation
Weldmentshaveahighlyheterogeneousmicrostructure.Fracture
toughnesscanvaryconsiderablyoverrelativelyshortdistances.Thus,it
isimportanttotakegreatcareinlocatingthefatiguecrackinthecorrect
region.Ifthefracturetoughnesstestisdesignedtosimulateanactual
structuralflaw,thefatiguecrackmustsamplethesamemicrostructure
astheflaw
Oncethemicrostructureofinterestisidentified,anotchorientation
mustbeselected.Thetwomostcommonalternativesareathrough-
thicknessnotchandasurfacenotch,whichareillustratedinFigure7.35.
Sincefull-thicknessspecimensaredesired,thesurface-notched
specimenshouldbeasquaresection(B×B),whilethethrough-
thicknessnotchwillusuallybeinarectangular(B×2B)specimen.
Forweldmetaltesting,thethrough-thicknessorientationisusually
preferablebecauseavarietyofregionsintheweldaresampled.
However,theremaybecaseswherethesurface-notchedspecimenis
themostsuitablefortestingtheweldmetal.Forexample,asurface
notchcansampleaparticularregionoftheweldmetal,suchastheroot
orcap,orthenotchcanbelocatedinaparticularmicrostructure,such
asunrefinedweldmetal.
IntypicalC–Mnstructuralsteels,lowtoughnessisusuallyassociated
withthecoarse-grainedheat-affectedzone(HAZ)andtheintercritically
reheatedHAZ.Amicrohardnesssurveycanhelpidentifylowtoughness
regionsbecausehighhardnessisoftencoincidentwithbrittlebehavior.

13. Heat affected zone NotchlocationintheHAZoftendependsonthetypeofweldment.Ifweldsare
producedsolelyformechanicaltesting,forexample,aspartofaweld
procedurequalificationoraresearchprogram,
theweldedjointcanbedesignedtofacilitateHAZtesting.Figure7.36illustrates
theKandhalf-Kpreparations,whichsimulatedouble-Vandsingle-Vwelds,
respectively.Theplatesshouldbetiltedwhentheseweldmentsaremade,to
havethesameangleofattackfortheelectrodeasinanactualsingle-ordouble-
Vjoint.Forfracturetoughnesstesting,athrough-thicknessnotchisplacedin
thestraightsideoftheKorhalf-KHAZ.
Inmanyinstances,fracturetoughnesstestingmustbeperformedonanactual
productionweldment,wherethejointgeometryisgovernedbythestructural
design.Insuchcases,asurfacenotchisoftennecessaryforthecracktosample
sufficientHAZmaterial.Themeasuredtoughnessissensitivetothevolumeof
HAZmaterialsampledbythecracktipbecauseoftheweakest-linknatureof
cleavagefracture
Anotherapplicationofthesurface-notchedorientationisthesimulationof
structuralflaws.Figure7.37illustratesHAZflawsinastructuralweldanda
surface-notchedfracturetoughnessspecimenthatmodelsoneoftheflaws.
Figure7.37demonstratestheadvantagesofallowingarangeofa/Wratiosin
surface-notchedspecimens.Ashallownotchisoftenrequiredtolocateacrack
inthedesiredregion,butmostexistingASTMstandardsdonotallowa/Wratios
lessthan0.45.Shallow-notchedfracturetoughnessspecimenstendtohave
lowerconstraintthandeeplycrackedspecimens,as

14. Posttest Analysis
Correctplacementofafatiguecrackinweldmetalisusuallynotdifficult
becausethisregionisrelativelyhomogeneous.Themicrostructureinthe
HAZ,however,canchangedramaticallyoververysmalldistances.Correct
placementofafatiguecrackintheHAZisoftenaccomplishedby
trialanderror.Becausefatiguecracksareusuallyslightlybowed,the
preciselocationofthecracktipinthecenterofaspecimencannotbe
inferredfromobservationsonthesurfaceofthespecimen.Thus,HAZ
fracturetoughnessspecimensshouldbeexaminedmetallographically
afterthetesttodeterminethemicrostructurethatinitiatedfracture.In
certaincases,aposttestexaminationmayberequiredinweldmetal
specimens.

15. Thank you