Article
Reference
ADM guidance-ceramics: Fatigue principles and testing
KELLY, J.R., et al.
Abstract
Background. Clinical failure of dental ceramics is usually reported as partial fracture of therestoration (chipping) or as catastrophic fracture of the whole structure. In contrast to met-als, ceramics are linear-elastic, brittle materials exhibiting extremely low damage toleranceto failure. Well documented clinical and lab reports have shown this fracture event oftenoccurs at loads far below their fracture strength due to intrinsic fatigue degradation via slowcrack growth or cyclic fatigue mechanisms. The presence and development of surface flawshave a dominant role in damage accumulation and lifetime reduction of ceramic structures.Aims. This ADM guidance document aims to summarize the aspects related to fatigue degra-dation of dental ceramics, reviewing the concepts of fatigue testing and furthermore aimsto provide practical guidance to young scientists entering into fatigue related research.The description of fatigue strength is always accompanied by a clear understanding of theunderlying fracture mechanisms.
KELLY, J.R., et al . ADM guidance-ceramics: Fatigue principles and testing. Dental Materials , 2017, vol. 33, no. 11, p. 1192-1204
PMID : 29017761
DOI : 10.1016/j.dental.2017.09.006
Available at:
http://archive-ouverte.unige.ch/unige:99756
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Review
ADM guidance-ceramics: Fatigue principles and testing
J.R. Kelly
a, P.F. Cesar
b, S.S. Scherrer
c, A. Della Bona
d, R. van Noort
e, M. Tholey
f, A. Vichi
g, U. Lohbauer
h,∗aDepartmentofReconstructiveSciencesandCenterforBiomaterials,UniversityofConnecticutHealthCenter, Farmington,USA
bDepartmentofBiomaterialsandOralBiology,SchoolofDentistry,UniversityofSãoPaulo,Brazil
cDivisionofFixedProsthodonticsandBiomaterials,UniversityClinicofDentalMedicine,UniversityofGeneva, Geneva,Switzerland
dPost-GraduatePrograminDentistry,DentalSchool,UniversityofPassoFundo,CampusI,BR285,99052-900 PassoFundo,RS,Brazil
eSchoolofClinicalDentistry,UniversityofSheffield,Sheffield,UK
fResearchandDevelopmentDepartmentVITAZahnfabrik,BadSaeckingen,Germany
gDepartmentofMedicalBiotechnologies,UniversityofSiena,Siena,Italy
hResearchLaboratoryforDentalBiomaterials,DentalClinic1,UniversityofErlangen-Nuernberg,Erlangen, Germany
a r t i c l e i n f o
Articlehistory:
Received2August2017 Accepted15September2017
Keywords:
Staticfatigue Dynamic Cyclicloading Slowcrackgrowth Ceramicfracture Degradation Lifetime SPTdiagram S–Ncurve
a bs t r a c t
Background.Clinicalfailureofdentalceramicsisusuallyreportedaspartialfractureofthe restoration(chipping)orascatastrophicfractureofthewholestructure.Incontrasttomet- als,ceramicsarelinear-elastic,brittlematerialsexhibitingextremelylowdamagetolerance tofailure.Welldocumentedclinicalandlabreportshaveshownthisfractureeventoften occursatloadsfarbelowtheirfracturestrengthduetointrinsicfatiguedegradationviaslow crackgrowthorcyclicfatiguemechanisms.Thepresenceanddevelopmentofsurfaceflaws haveadominantroleindamageaccumulationandlifetimereductionofceramicstructures.
Aims.ThisADMguidancedocumentaimstosummarizetheaspectsrelatedtofatiguedegra- dationofdentalceramics,reviewingtheconceptsoffatiguetestingandfurthermoreaims toprovidepracticalguidancetoyoungscientistsenteringintofatigue relatedresearch.
Thedescriptionoffatiguestrengthisalwaysaccompaniedbyaclearunderstandingofthe underlyingfracturemechanisms.
©2017TheAcademyofDentalMaterials.PublishedbyElsevierLtd.Allrightsreserved.
∗ Correspondingauthor.
E-mailaddresses:[email protected](J.R.Kelly),[email protected](P.F.Cesar),[email protected](S.S.Scherrer),[email protected] (A.DellaBona),r.vannoort@sheffield.ac.uk(R.vanNoort),[email protected](M.Tholey),[email protected](A.
Vichi),[email protected](U.Lohbauer).
https://doi.org/10.1016/j.dental.2017.09.006
0109-5641/©2017TheAcademyofDentalMaterials.PublishedbyElsevierLtd.Allrightsreserved.
Contents
1. Introduction...1193
2. Generalconsiderations...1194
3. Probabilisticnatureoffatigue...1196
4. Microstructuralconsiderations...1197
5. Generalapproachestofatiguetesting...1198
6. Slowcrackgrowthparameters...1198
7. Staticmethod...1199
8. Dynamicmethod ... 1199
9. Cyclicmethod...1199
10. SPTdiagrams(strength-probability-time)...1200
11. Thresholdconcepts...1201
12. Fatiguetestingofnon-clinicalspecimensunderconditionsreproducingclinicalfailure...1201
13. Fatiguetestingofrealisticprosthesestofailure ... 1201
14. Strengthdegradationundercyclicloading...1202
Acknowledgment...1202
References...1202
1. Introduction
Themechanicalperformance ofceramicmaterialsiscom- monly approached by measuring the fracture strength or toughnessusingsimplifiedbarordiscspecimens.Suchmeth- odsreflectthe static,inert behaviorofmaterialsatcritical loads,focusingonfractureasthefinalevent.Asfractureisthe ruptureofthebonds,fracturestrengthofceramicsisknown to be inversely proportional to the largest or critical flaw presentintheloaded volume,asdescribedbyGriffith’s law [1].Onecanfinddetailedinformationonfracturestrengthand toughnessin the corresponding ADMguidance documents (www.academydentalmaterials.org).
Anycomponentinnormalserviceisloadedfarbelowits critical load either continuouslyor under repetitive condi- tions.Therelatedmechanicalphenomenoniscalled“fatigue”, which is often defined as the degradation (weakening) of astructuralcomponentunderthe influenceofmechanical, chemicalorbiologicalstress–andinmostcases–acombina- tionofthem.
ThefatigueprogressionovertimeisshowninFig.1.Atcer- tainserviceloads(belowthefracturestrength),flaws(defects, cracks)tendtogrow.Asthestressintensityatthecracktip increaseswithgrowingflawsize,therelationbetweenflaw sizeandservicelifebecomesexponential.Dependingonthe level ofapplied service loads, the material strength drops significantlyfromtheinertstrength andafatiguefailureis expected.However,atlowserviceloads,fatigue(orendurance) limits may exist ata stress below which no further crack growth happens and failure willnot occur no matter how manyloadingcyclesareinvolvedorhowlongacomponent isstaticallyloaded(thresholdvalue).
Indentistry,onecouldthinkofacyclicloadingscenario in a compressive or bending configuration combined with the influence ofwaterthat simulates,in vitro, the clinical conditionsofmastication.Degradationofpropertiesalways occurs overtime, sothefatigueparameteractuallyreflects thetime-dependencyofmaterialperformanceandintheend determinesthelifetimeofarestoration.Whileinertstrength measurementsinvestigatefastfracture,fatigueinvestigations
Fig.1–Relationshipbetweenflawsize,timeinserviceandresidualfracturestrength.Inducedbyacombinationofapplied stressinacorrosiveaqueousenvironment,aflawgrowswithtimetoacriticalsizebringingaboutfatiguefailureata reducedstresslevelcomparedtotheinertfracture.
dealwithcrackinitiationandtheslowgrowthofcracksunder theinfluenceoftheenvironment.Thefastfracturecriterionis termed“critical”whereastheslowgrowthofcracksiscalled
“sub-critical”crackgrowth(SCG)[2].
Thedefinitionoffatigueatambienttemperaturesmostly involvestwomajor,relevantmechanisms,arisingeitherfrom stresscorrosion(SCG)(chemically-assistedbywater)and/or from additional cyclic effects [3,4]. While SCG has been demonstrated 70 years ago [5], inthe past it was believed thattherewasnoadditionaleffectfromcyclicloadinginthe fatigue behavior ofbrittle ceramics. Extensive research on thefatigueofmetallicmaterials,showingthatcyclicfatigue playsadominantrole,alsoledtoinsightsinto thedamag- ingeffectofcyclicloadingforceramics.Inbrief,whileSCG mightoccur inacomparablerateindependentfrom static, dynamicor cyclicloading, cycliceffects arisefrom friction andhydrolyticpressureduringcrackclosing.Today,thereisa commonunderstandingthatcycliceffectscontributetoover- alldegradationofbrittleceramics,althoughtolesserextent comparedtoSCG[3].
Clinically,fatiguedegradationovertimeisalwaysassoci- atedwithprogressivesurfacewear(abrasionand attrition).
During wear, an extended damage accumulation zone is formedonthesurfacewiththelargestdefectsfurtherpro- gressingto fatiguecrack growth. A specificADM guidance documentreviewedthemechanismsinvolvedintheintraoral wearprocessthatcontrolsmechanicalstrengthdegradation (www.academydentalmaterials.org).
This document seeks to provide an introductory guid- ancetothe fieldofdental ceramics fatigue.Theprinciples and mechanismspresentedhere are –withinlimitations– expandabletodentalresin-basedcomposites.Forthoseread- ersinterested inlearningmoreabouttheprinciplesbehind slowcrackgrowth,wesuggestliteraturethatprovidesmore comprehensivecoverageofthesubject.Forageneraloverview, thereisaneasy-to-readbookrecommendedfromAshbyand Jonesonpropertiesandapplicationsofengineeringmateri- als.PartsDandEofthisbookintroducetheprinciplesoffast fracture,fracturetoughnessandfatigueandanswersthemost basicquestions[6].Fundamentalstudiesonglassfatiguewere publishedbyCharlesandco-workers[5,7,8].Furtherreading especiallyonthefracture mechanicsbackgroundoffatigue crackgrowthcanbefoundinDavidBroek’sbookintitled“The practicaluseoffracturemechanics”orinDieterMunzand TheoFett’sbook“Ceramics”[9,10]. Amorerecent,compre- hensivereviewon“FractureofCeramics”waspublishedby Danzeretal.[3].Theycomprehensivelyreviewedtheconcept ofstresscorrosionversuscyclicfatigueeffects.Focusingon theaspectsofceramicfatiguerelatedtodentistry,thebook fromKelly[11]isrecommendedaswellasthemorerecent andclinicallyorientedreviewfromZhangetal.[12].Typical fracturemodes,and fatiguemechanismsinclinicalservice aredescribedanddiscussed.Foranin-depthanalysisofthe fatigueresponsesofceramics andconstitutivemodelspro- vidinginsightsinto fatigueprocessesthebookfromSuresh ishighlyrecommended[13].Theprinciplesandmechanisms responsibleforfatigueofresincompositecanbefoundelse- where[14].
Based on ISO and ASTM standards, fatigue of metallic materialsiswelldescribedbutonlylittleguidanceisavailable
onhowtoperformfatigueexperimentsonbrittlematerials.
A Japanese standard introduces the static bending fatigue methodforfineceramics[15].TheASTM-C-1368standardisa comprehensivedocumentdescribingtheconstantstress-rate methodforevaluatingslowcrackgrowthparameters[16].A comparableapproachondynamicfatigueisdescribedinthe EuropeanstandardEN843-3[17].Theonlyadvicerelatedto dentistrycanbefoundinISO14801wherecyclicfatiguetesting ofdentalimplantsisdescribed[18].
Of course, this guidance document cannot comprehen- sively cover all fields related to fatigue degradation, such as fracture toughnessor increase oftoughnesswithgrow- ing defects (R-curve behavior) [19,20]. Also, the influence of internal stresses on toughness and strength as well as relatedaspectsofmultilayeredorgradedcomponentsarenot addressedhere.FurtherreadingisprovidedbyADMguidance documents on fracture toughness and multilayered dental ceramics(www.academydentalmaterials.org)[2,21].
2. General considerations
Damageaccumulationaftermultiplecyclesatlowloadscan alterthedurabilityofceramicparts,reducingtheirservicelife (Fig.1).Thisisespeciallytrueforceramicpartsoperatingin wetenvironments.Chemically-assistedcrackgrowth(SCG)is probablythemostimportant(andmoststudied,eitherdirectly orindirectly)fatiguemechanismaffectingalldentalceram- ics(seeTable1).Thismechanisminvolvestheslowgrowthof cracksatstressesandcracktipstressintensitieswellbelow those associated withcatastrophicfracture. Thehydrolytic principle leadingtocorrosivebondruptureandcleavagein glassesandceramicsisshowninFig.2.
Slowcrackgrowthinvolvesexistingflawsandinfluences the behavior of especially feldspathic porcelains, glass- ceramicsandpolycrystallineceramics.Forallceramics,there arestressintensitiesbelowwhichcrackswillnotgrow(stress intensity factor threshold,KI0).From afracture mechanics standpoint,fatiguecrackgrowthcanbedescribedastherela- tionshipbetweenthecrackvelocity,v,andtheappliedstress intensityKIatthecracktip.Fig.3showstheslowcrackgrowth parameters“n”and“A”,whicharederivedfromfatiguecrack growthexperiments[10].Withincreasingstressintensity,the curve showsalinearrelationshipwiththeincreasingcrack velocity(regionI),describedinthesocalledParislaw[22]:
v= da
dt =A·KnI (1)
Certain polycrystallineceramics,suchastransformation toughened zirconia and possibly some glass-ceramics can experience additional damage accumulation involving the development of internal volume flaws. For example, dis- tributed microcracking along grain facetsand/or interfaces has been documented. Innon-cubic singlephase ceramics (e.g.,alumina,zirconia)andcomposites(e.g.,glass-ceramics) residualstressesmaydevelopatgrainboundariesandcom- posite phase interfaces during cooling that can lead to nucleationofmicrocrackingundertheinfluenceofexternal stress. In transformation toughened ceramics (e.g., Y-TZP) microcracking occursduringthestress-inducedmartensitic
Table1–Fatiguemeasuringtechniquesandtheirapplication.
Principle Advantages Disadvantages
Crackgrowth experiments(direct method,pre-cracked components)
Visualcrackgrowth tracking[20,63–67]
• DirectassessmentofR-curvebehavior
• Certaintyofinitialcracksize
• Requiresfewerspecimens
• Allowsidentificationoftoughening mechanisms
• Extremelydifficulttoobservehighspeed cracksinabrittlematerial
• Usuallyrestrictedtocyclicexperiments
• Mightrequireunloadingforregularvisual inspection
• Usuallyrunsunderfixedstressamplitudes
• Useoflong,less-relevantcracksizes
• Cracksizeatthesidesofthespecimen misrepresenttherealcracksizeinthebulk
Crackgrowth trackingvia compliance[20,68]
• DirectassessmentofR-curvebehavior
• Certaintyofinitialcracksize
• Requiresfewerspecimens
• Mayusesurfacepre-cracksthataremore strengthrelevant
• Givespreciseaccountoffirstincrementsof cracksize
• MaybeconductedunderconstantKappl
• Takenasthegoldstandardofv-K experiments
• Extremelydifficulttodetecthighspeed cracksinabrittlematerial
• Requiresveryrigidequipmentwithprecise displacementcontrol
• Requiresextensivecalibrationandmore complexprogramming
Crackgrowth
experiments(indirect method,naturalflaw population)
Staticfatigue [7,8,69–72]
• Usesnaturalflaws
• Noneedtoproduceanartificialsharp pre-crack
• Easytoconduct
• Needssimpleequipment
• Sensitivetosurfaceresidualstresses
• Usefulforconstructing
strength-probability-time(SPT)diagrams
• Requiresalargeamountofspecimens
• Reliesontheuncertaintyofregression procedures
• Usuallyignoresthereal(Weibull)strength distributionduetoreducedamountof specimensperstressrange
• Uncertaintyofinitialcracksize
• Sensitivetosurfacequality
• Stress-controlledinsteadofK-controlled
Dynamicfatigue [36–39,42,73–76]
• Sameasstaticfatigue
• Fasterthanstaticfatigueexperiments
• Itisthemostappliedfatigueapproach
• Sameasforstaticfatigue(datascattering ismuchlower)
Cyclicfatigue [19,27,40,70,72,77]
• Accountsforcycliceffectsthatmay degradetougheningmechanisms
• Importantparameterssuchasfrequency andstressamplitudemaybevaried
• Morerelevantforrealworldapplications
• MaybedevisedtoaccountforR-curve effects
• Difficulttoestablishtheinitialstresslevel fortesting
• Usuallyencompassesonlyonestresslevel
• Requiresappropriateequipment
• Morecomplexstatisticaltreatmentto accountforearlyfracturesandrun-out specimens
Phenomenological approaches
S–Ncurve[41,72] • Doesnotrequiresacrificialspecimensto determineanappropriatestresslevel
• Encompassesmultiplestresslevels
• Givesinsightontheexistenceandlocation ofanendurancelimit
• Providesafatigueparameter“n”likewise incrackgrowthexperiments
• AbletodistinguishbetweenLCFandHCF
• Requiresalargenumberofspecimensto betestedatdifferentstresslevels
• Usuallylimitednumbersofspecimensare testedforeachstresslevel
• Reliesontheuncertaintyofregression procedures
Staircaseapproach [78–85]
• Canbeperformedwithfewerspecimens
• Providesaccurateestimationsofthemean fatiguestrength
• Providesameanfatiguestrengthfora predefinednumberofcycles
• Inaccurateinestimatingthescatterofthe fatiguestrength
• Giveslittleinsightatextremefailure probabilities
• Informationstemmingfromrun-out specimensisdisregarded
• Formaterialcomparisonsonly
–Table1(Continued)
Principle Advantages Disadvantages
Step-stressapproach [50,86]
• Optimizesthetimeoftesting
• Incorporatesrun-outsintheanalysis
• Employsvaryingstressamplitudes
• Maybeusedtoestimatelongerlifetimes
• Requiresananalysisaccountingfor cumulativedamage
Fig.2–Thedegradinginfluenceofwateronslowcrackgrowthisexplainedbyacorrosive,diffusioncontrolledattackof watermoleculesatthetipofacrack,hydrolyzingsiloxanebonds(Si–O–Si).UndermechanicalloadstheSi–O–Sibondsare strained,whichfurtheracceleratesthehydrolyticreaction(adoptedfromRefs.[5,7,8,31]).
Fig.3–Aprincipalv-KI(crackvelocityversusstress intensity(inmodeI))plot.Thecurveshowstheonset threshold(KI0)ofcrackinitiation,astable,linearcrack extensioninterval(regionI)aplateau(regionII)andthe approximationtowardsfastfractureattheactualfracture toughness(regionIII,KIc,adoptedfromRef.[4]).
transformation creating the transformation zone around stressedcracks.Graphicexamplesofvarioustypesofsurface flawsarepresentedinaADMguidancedocumentonclinical fractography(www.academydentalmaterials.org)[23].
3. Probabilistic nature of fatigue
Thewell-knownfactthatceramicstrengthissensitivetosur- facedefectsandtheirsubsurfaceextension–thelargerthe flaw,thelowerthestrengthofaceramic–directsourattention towardsthestatisticaldistributionofflaws.Unfortunately,the specificdistributionofaflawpopulationinaloadedvolume oftenresultsinhigh scatterofthe experimentaldata. Sur- faceoptimization(polishing)certainlyprovidesmorereliable data(andnarrowdistribution)but arobuststatisticaltreat- mentofdataremainsmandatory.TheuseofWeibullstatistics isbyfarthe mostapplicableprocedureforbrittleceramics [24].Underfatigueconditions,surfacedefectsareinducedto growslowlyanddatascatteringgetsevenworse.Especially cycles-to-failureasafunctionofloadexperiments(S–Ncurve) exhibitnon-normalfailuredistributionsand arecommonly treatedbylog-normalorextremevaluedistributions,orhave been knowntofollow Weibulldistributions.If the strength ofamaterialisdistributedaccordingtotheWeibulldistribu- tiononecoulddeviatea“timeWeibulldistribution”withthe relationof
m∗= m
n−2 (2)
The time Weibullmodulus m* takes the inert flaw dis- tribution (representedbythe strength Weibullmodulusm) as well asthe SCGsusceptibility (representedbythe crack growth exponentn)intoaccount[3,10]. Sucharelationship furtherallowsthedesignandpresentationoffatiguedatain astrength-probability-time(SPT)diagram.
Fig.4–Themajortougheningmechanismseffectiveinceramicsarecrackdeflection,zoneshieldingorcontactshielding illustratedby(a)crackwake,(b)bridging,(c)transformation,and(d)wedgetoughening.Furtherreadingonintrinsicand extrinsictougheningmechanismsisadvised[2,27].
Awaytoovercomehighlyscattereddataistoobservereal crackextensionsfromartificiallyproducedsharpnotchedor pre-crackedspecimenssuchasdouble-torsionspecimens.A singledefinedcrackisintentionallypreparedtostarttheslow crackgrowthandtoexcludetheprobabilisticnatureofanatu- ralflawpopulation.Suchapre-crackismuchdeepercompared tothecommonnaturalflawsandthusdeterminestheonset ofcrackgrowth.Thistypeofexperimenthoweverisbasedon thedirectobservationofcrackvelocities,whichgivesriseto glaringdrawbacksregardingtheexperimentalprocedure(see Table1).
4. Microstructural considerations
Thesmallestflawsize ina partiallycrystalline material is thesinglemicrostructuralunit,e.g.,grainorcrystallitesizes [25,26]. Smaller microstructuralunits (grain sizereduction) would account for a narrow flaw distribution and thus a lowscatteringofthedata,butincontrasttheywould limit the crack resistance ofa material. Thefracture toughness ofa ceramic is determined by the size of the microstruc- turalunitandinconsequencewilldeterminetheslowcrack growth resistance(seeFig. 2). Especially inhigh-crystalline (lithiumsilicates)orpolycrystallineceramics(aluminaandzir- conia),inwhichcracksareforcedtodeflectaroundcrystallites orgrains, cyclicdegradationofstrengthoccurs asresultof frictionbetweenopposingwallsofacrackarisingfromthe roughfractureplanes.Forsuchmaterialsthestressamplitude appliedincyclicloadingtestshasastrongerinfluence,since
lowstressamplitudesinducelittlecrackopening,whilehigh amplitudesresultinhigherfrictionandstrengthdegradation [27].Loosedebris,usuallyfromdeteriorationofcrackbridges, canfurthergetwedgedbetweenthetwocracksurfacesand alsocontributetodegradation[28].Anoverviewofrelevant tougheningmechanismsinbrittleceramicsisshowninFig.4.
Fatigue crack extension is generally driven by intrin- sic versus extrinsic microstructural toughening mecha- nisms. While intrinsic mechanisms are determined by the microstructureaheadofanadvancingcrack,the latteracts inthewakebehindthecracktip.Ritchieprovidesaprofound insightintocompetitivetougheningmechanismsrelevantfor brittleceramics[28].
Fig. 5 shows an example of crack deflection and zone shielding toughening mechanisms in a lithium disilicate glass-ceramic. Elongated Li2Si2O5 crystals account for an effective crack deflection and twisting of a crack front, therebysubstantiallydissipatingfractureenergy.Lithiumdis- ilicate exhibitsa fracture toughness from 2MPam0.5 up to 3.5MPam0.5[29,30].
Fatigueexperimentsusinguncrackedspecimens(natural flaw distribution) are generally understood as “accelerated testing”that use loadsmuchhigherthan those seen clini- cally tocreaterealistictestingtimes.Aslong asthefailure mechanisms (origin, flaw type, damage) under accelerated conditions aresimilar tothose reportedforclinical service failures,acceleratedtestsarevalid.Cyclingfrequenciescan alsobe acceleratedaboveclinical values(approx. 0.5–1Hz).
When fatigue effects primarily involve chemically-assisted
Fig.5–Tougheningmechanismsinthelithiumdisilicatemicrostructure:microcrackingandcrackdeflection/meandering areeffectiveinleadingtoasuperiorfracturetoughnessamongsilicabaseddentalceramics.
crackgrowth,cyclingfrequencyorwaveformgenerallyhave noeffectonlifetimes[13].
When damageaccumulationinvolves nucleationofvol- umedefects(e.g.,intergranularmicrocracking)highercycling frequenciescanbemoredamaging.Therecanalsobeben- eficialeffectsofcyclic versusstatic loadingwheredamage accumulationinvolves microcrackdeflection orcrack wake bridging and conceivably also in the case of incremental toughnessincreaseswithtetragonaltomonoclinictransfor- mation.Suchmaterialsareconsidered“damagetolerant”[13].
5. General approaches to fatigue testing
Sensitivitytodamageaccumulationcanbetestedasamate- rialparameter(e.g.,staticfatigue),asamaterial/environment response(e.g.strengthdecreasefollowingcyclicloading)and additionallyasaceramicsdesignissue,i.e.,developingrobust designstominimize fatiguestrengthdegradation.Boththe design (influencing stress concentrations, development of compressiveversustensile stressesduringservice)andthe processingoftheceramic(involvingevery stageinthefab- rication process from powder formation, powder packing, sintering,tomachiningandfinishing)haveaprofoundinflu- enceonthestressdistributionduringserviceandtheinherent flawdistributions.
It has to be mentioned that in a variety of materials mechanical fatigueis counteracted by an increasingresis- tancetocrackgrowth(R-curveeffect)[10].TheR-curveeffect istypicallyfoundinpolycrystallineceramics (e.g.,zirconia) orhighvol%crystallizedglass-ceramicsand hasanoverar- chingeffectonfatiguedegradation.Tougheningmechanisms responsiblefortheR-curve[28]alsogetdegradedandaccount forthefatigueinthesematerials.TheR-curveisnotpartofthis guidancedocumentandfurtherreadingisreferenced[19,20].
Awidevarietyofapproachestofatiguetestinghasbeen developedforceramics,someoriginatingfromthecommunity ofengineeringceramicsandsomeoriginatingwithindentistry especiallyinrelationtothetestingofwholeprosthesesortheir components:
• Standardizedfatiguetesting.
• Slowcrackgrowthexperimentsoninvitrospecimen.
Staticmethod.
Dynamicmethod.
Cyclicmethod.
• SPT diagrams (stress-probability-time) coupling crack growthexponentswithWeibullstatisticalanalysisofstatic failureprobabilitiessoastoextrapolatefailureprobabilities toclinicallifetimes(alsoincorrelationwithclinicaldata).
• Thresholdconcepts.
• Clinicallyrelevantstructuraltesting.
• Fatiguetestingofnon-clinicalspecimensunderconditions reproducingclinicalfailure.
• Fatiguetestingofrealisticprosthesestofailure.
• Strengthdegradationundercyclicloadingand“fatiguechal- lenge”toprostheses(“aging”)priortostatictesting.
6. Slow crack growth parameters
Ithasalreadybeenshowninthelate1950sthatbrittlesolids such as glasses or ceramics tend to degrade mechanically underexternalloading[5].Eitherwatervapororahumidenvi- ronmentcansignificantlyacceleratethechemicalcorrosion processdirectlyatthecracktipofacriticalmaterialdefect.
Thisoccurspreferentiallyinsilicatebaseglasses,whichare present inmany dentalceramics,and resultsinbondrup- ture.Evenmoisturelevelsaslowas0.02%relativehumidity areknowntocausestresscorrosion[19,31].
BasedontheGriffithfailurecriterion[1]forbrittleceram- ics(KI>KIc)thecrackgrowthrateda/dtcanbeexpressedasa powerfunctionoftheappliedstressintensityKIasshownin Eq.(1).ThesubcriticalcrackgrowthparametersnandAchar- acterizethegrowthrateofflawsinceramics[10,32,33].These parametersarecommonlyappliedineitherdirectorindirect measurementsunderstatic,dynamicorcyclicloadingcondi- tions,assummarizedinTable1andshowninFig.6forindirect measurementtechniques.
Fig.6–Thethreetypesofloadingcommonlyusedforfatigueexperimentsanddeterminationofslowcrackgrowth parametersnandA:(a)staticloading:aconstantfatigueloadissustaineduntilfracture.(b)Dynamicloading:theloadis increasedbyafixedrateuntilfracture.(c)Cyclicloading:loadingandunloadingtakeplaceatafixedfrequencyandload amplitude(R-value).
7. Static method
Thestaticmethod isatest withconstant stress overtime [15,34]. Theexperiment determinesthetime-to-failureofa specimenorstructuralcomponent. Inprinciple,a seriesof experimentsatdecreasingconstant loadingswouldexhibit increasingstaticlifetimesofthematerialunderinvestigation.
Thecalculatedstaticlifetimesshowastrongdependencyon theappliedstress level,especiallyforhighlyglassy silicate basedceramicswithalowcrackgrowthexponentn[10].The thresholdvalueKI0(belowwhichnocrackgrowthisexpected) forslowcrackgrowthcanbeadequatelyapproachedusingthe staticfatiguemethod.Modificationssuchastheinterrupted staticfatiguetesthavealsobeenproposed forKI0determi- nation[34,35].Thisapproachhoweversuffersfromgreatdata variability.
8. Dynamic method
Thismethodusesdifferentconstantstressratesduringflex- ural strength testing todetermine subcriticalcrack growth parameters[36–38].Stressratesaregenerallywidelyseparated overordersofmagnitude,i.e.,0.1,1.0,10and100MPa/s.The graphicalsolutionofatypicaldynamicexperimentandcon- siderationsonthe appliedevaluationprocedureare shown inFig.7.OnecouldpossiblyinferfromFig.7bthatthereli- abilityofan-valuepredictionismaximizedbyusingWeibull
scaleparametersfortheapproximationastheykeepthen- valuedeviationtoaminimum[39].Inoneanalyticalmethod, the slopesofln(fracturestress)versusln(stressingrate)are usedtodeterminecrackgrowthparameters.Goodexamples ofthisprotocolcanbefoundinbasicresearchondynamic fatigue[32].Relevantstandardsforthedynamicmethodare ASTMC1368andEN843-3[16,17].Anothermethodplotsthe log(fracturestress)versuslog(averagetimetofailure)foreach stressrate[33].Inanalternativetothis,discswerecyclically stressedat4Hzinbiaxialflexuretothreemaximumstresslev- els.Theslopeofthelog(maximumstress)tolog(timetofailure) plotwasusedtocalculatecrackgrowthparameters[40].
9. Cyclic method
Themostclinicallyrelevantfatigueapproachhoweveristhe cyclicmethod.Despitethefactthattheseexperimentsareby farthemosttimeconsuming,theyproducethebestinsight in the material response for a complete service life. The mostcomprehensiveapproachisthedeterminationofstress- cycles-to-failureplots(S–N,Wöhlercurve).Theprinciplesand theloadingvariablesareshowninFig.8.
A schematic of typical S–N (Wöhler) curves for differ- ent cyclic fatigue degradation patterns is shown in Fig. 9.
Bothcurves showaconsiderabledegradationathighstress amplitudes. The material is intended to fail with a low number of cycles (LCF, low-cycle-fatigue) whereas at low
Fig.7–Dynamicfatiguemethod:evaluationprocedureandtheinfluenceofthestatisticaltreatmentonthen-value calculation.Then-valuecalculationbasedonWeibullscaleparametersseemstopresentthebestreliability[38,39].
Fig.8–Loadingregimeinatypicalcyclicexperiment simulatingoralmastication,outlinedbythemeanstress m=(min−max)/2,thestressamplitudea=(min+max)/2, andthestressratioR.Thestressratiobetweenalowerand upperstress(R-value=min/max)atacertainmeanstress mdefinestheexperimentalloadingconditions,e.g.,the compressiverepeatedloadingwithmaxandminbeing compressive(−1<R<∞).Aspecialtypewouldbea completeunloadingbetweencycles(max=0,m=a,and R=0).
stressamplitudes materialsshow afatigue limit(curve A), commonlyreachedbeyond105–106loadingcycles(HCF,high- cycle-fatigue).Fatigueorendurancelimitsimplythatthereis astressbelowwhichfailurewillnotoccurunderanassumed upperlimitofcycles.S–Ncurves use specimensforwhich analyticalstresssolutionsexistsuchas3-pointand4-point bendbars orbiaxial flexurediscs.Data isplottedasstress versuslog(cycles).Fracturesurfaceanalysisisrecommended forall specimens toensure that failure occurred from the locationassumedintheanalyticalstresssolution.Onevery goodexample ofsuchtestingofY-TZPzirconiainwateris seen in [41]. The application of fractography on fractured specimensisdescribedinanotherADMguidancedocument (www.academydentalmaterials.org)[23].
Fig.9–schematicS–N-graphsshowingthestrength degradation(S)fortwomaterialswithincreasingload cycles(N).Ingeneral,differentmaterialbehavioraccounts foreitherafatigue(orendurance)limit(curveA)oran ongoingdegradationatlowstressamplitudes(curveB).
Fig.10–SPT-diagramforatypicalCAD/CAMfeldspathic dentalceramic.Therelationshipbetween
(fracture-)strength,(failure-)probabilityand(life-)timecan bederivedfromslowcrackgrowthparametersnandA, andisplotteddependingontheunderlyingloading condition.Estimationsoflifetimebecomepossible,e.g.,the initialstrengthof98MPaat5%failureprobabilityisfound reducedafterapredisposedstaticloadingofoneyear[38].
Moreefficientapproaches(intermsoftimeandeffort)are based onstatistical proceduressuchas thestaircase,step- stress,orboundaryapproaches(seeTable1).Thedrawbackof thosemethodsismostlyrelatedtothelimitationtheyimpose onanyphenomenologicalinsightastothematerial’sbehavior.
Thosemethodsprovidealimitedpictureofthemorecomplete S–Ncurve.Slowcrackgrowthparameters(nandA)cannotbe estimatedusingthosemethods.
10. SPT diagrams (strength-probability-time)
Themaingoaloftheuseofthetechniquesdescribedabove howeveristoapproximateanddesignthelifetimeofaceramic component. For this reason, knowledgeofthe relationship betweenstrength andtimeismandatory(determinationof slowcrackgrowthparametersnandA).Thecombinationof amaterial’sfatiguebehavioranditsstatisticaltreatmentof fracturestrength(namelyWeibulldistribution)allowsforan extrapolationoflifetimes.Therelationshipbetween(fracture) Strength,(failure)Probabilityand(life)Timecanbeillustrated foraceramicmaterialinSPT-diagrams(seeFig.10).
Inmorecomplexwork,datafromWeibullparametersand dynamiccrackgrowthmeasurementswerecombinedtoplot SPTdiagrams toextrapolateforlifetimepredictions[37,42].
Weibulldistributionswereusedinexaminingcyclicflexureas wellascyclictorsionofY-TZPspecimensinairandwater[43].
Bothloadingscenariosexhibitedsimilarcrackgrowthparam- eters inair andwater. However,threshold valuesKI0 were
lowerandcrackgrowthrateswerehigherinwater,reflecting theinfluenceofstresscorrosionatthecracktip.KI0forcrack propagationinwaterwassignificantlylowerthanthecritical KIc(−50%)[43].SPTdiagramswerealsousedtoinvestigatethe influenceofthemicrostructureoffivedifferentceramicson theirlifetimeestimates[36].Acorrelationofinvitromeasured lifetimeswithclinicaloutcome from prospectivelong-term studiesofcoursewouldbeanultimategoal.However,there areseveralboundaryconditionsandsimplificationsregarding thespecimengeometryinvolvedinSPTlifetimepredictions, restrictingthetransferabilitytoclinicalfindings.Estimative approachesaredescribedintheliteraturewithindicationsfor aninvitro/invivocorrelation[44].
11. Threshold concepts
Anotherimportantconceptonthe“otherend”ofthefatigue phenomenonforceramicsisthatforsomeceramicstherecan beastressintensitybelowwhichcrackgrowthdoesnotoccur [45,46].Thisimpliesthatathresholdintensityfactorcanexist asalowerboundaryforcrackpropagation.Thresholdinten- sitieshavebeenmeasuredfor11dentalceramics,andthey generallyarebelow1MPam0.5toaround2MPam0.5forpoly- crystallineceramics[45].Itisinterestingthatthethresholdfor aluminaandzirconiaarenearlyidentical.
Inanother method the crack growth rate was observed arisingfromadvancingedgecrackscreatedbyVickersinden- tations[46].Thisrepresentsaninverseapproachsincecracks arefollowedfromthesurfacetotherespectivecracktips.
Astudyon3Y-TZPemployingthedouble-torsionmethod hascomparedcrackgrowthvelocitiesunderstaticandcyclic loadingconditionsand hasfoundahighersusceptibility to slowcrackgrowthunderconstantloadingconditions.Onthe otherhand,thisstudyhasalsoshownadecreaseinthreshold valueKI0duetorepetitivecyclicloading[27].
Ingeneral,duetotheinabilityofmostfatiguemachines to“handle”zeroload,testingisdonefrom10Nor20Ntothe targetloadorfrom10%to100%ofthetargetload.Ceramicsare generallytestedinwaterduetotheirsensitivitytochemically- assisted,orslowcrackgrowth.Thestartingloadforfatigue testingisoften30–60%ofthemeanmonotonicfailureloads.
12. Fatigue testing of non-clinical specimens under conditions reproducing clinical failure
Thekeytodoingthisinameaningfulfashionistocreatethe samecracksystemasseeninbulkclinicalfailure.Thismeans creatingthesamestresssystemanddrivingfailurefromflaw typesencountered in clinical specimens. Thistype oftest involvescyclicloadingofcementeddiscs/tabsorflatcrowns, eithermonolithicorbilayered,withabluntpistonsimulating loadingatwearfacets[47].Thissetupinvokesfailuredueto radialcrackformationfrom theintagliosurface,which has beenidentifiedasthefractureoriginsiteinastudybyKelly etal.[48]andThompsonetal.[49]onclinically-failedcrowns.
Cracksareusuallydetectedbytransilluminationfollowinga certainnumberofcyclesandtheup-downorstaircasemethod ofstatistical designisused toobtain means and standard
deviations[50].Loadsareusuallycompared,sincegoodana- lyticalsolutionsdonotexistforbluntloadingwherethepiston radiusexceedstheceramicthickness[51].Itisthereforecriti- calthatallspecimensareofthesamethickness.Stressescan becalculatedusingnumericalsolutions.Whilecrownshaving normalanatomycouldbetested,thisbecomesexperimentally cumbersomeandisreallynotneededsincethestressonthe intagliosurfacewouldsimplybeatrigonometricratioofthe loadappliedtoaflatsurface.
Another type of mechanical fatigue, highly relevant in dentistry, is contact fatigue, resulting from repetitive con- tact between two bodies (tooth–tooth, tooth-restoration or restoration–restoration). Contactbetweenteeth/restorations takeplaceonroundandflatsurfacesoftheocclusalsurface (cusp–cusp,cusp-inclineorincline–incline),resultinginlocal stressconcentrationsfollowingthefundamentalsofcontact mechanics.Thecontactbetweenaroundindenterandaflat surface usuallygeneratesacrack typecalledacone crack, whichbeginsasaringcrackonthesurfaceandextendtowards theinteriorofthematerialatanangle,formingaconegeom- etry[52]. Conecracksformaround thecontactarea,where tensilestressesareformed.Theinitiationofringcracksand growthofconecracksdependmainlyontheelasticmodulus andfracturetoughnessofthecontactingmaterial,suchthat thelowertheseproperties,thelowerthenecessaryloadfor crackformation[53]. Becausejawmovement occursduring mastication,frictiondevelopsatthecontactarea,increasing thelocalstressconcentration.Anewtypeofcrackisformed duetosliding,partialconecracks,formingatrailbehindthe movingindenter[53].
13. Fatigue testing of realistic prostheses to failure
Inadditiontoinsitutestingofsingle-unitcrowns,research on multi-unit prostheses is often a challenge,with failure mostfrequentlyoccurringfromcracksoriginatingfrom the gingivalsideofconnectors[43,54].Manyaspectsofthecon- nectordesigncontrolfailureloads,includingconnectorheight (squared), connector width (linear), connector radius and whether theconnectorisveneered[43,55].Since“strength”
oftheconnectorisdependentupontheheightsquaredand islinearwithwidth,connectorareaisnotagoodcriterionto predictfatiguebehaviorofdentalprostheses[56,57].Stresses areconcentratedinconnectorsduetotheveryslighttippingof abutmentteeth,sosomemethodofreplicatingthisisneeded, e.g.,anartificialperiodontalligamentmadeofpoly(vinylsilox- ane) [58,59]. Considering all the above, it is important to fabricateconnectorsthatareasidenticalaspossiblewithin theentireexperiment.Onceagain,failureloadsaregenerally usedforstatisticalcomparisons.Theempiricallydetermined startingloadforfatiguetestingisoftenbetween30%and60%
ofthemeanmonotonicfailureload.Averyhelpfultoolfor thistypeofexperimentistouseCAD/CAMtoproducepros- theses,sincedesignanddimensionscanbekeptconstantfor apopulationoftestspecimens.
Thistypeofsimulated“proof-testing”aimstoinvestigate theclinicalservicelifeofanindividualrestorationmadeof a certain ceramic material [10]. Beyond an extensive data
basisforthematerialstested,theunderlyingfracturemechan- ics principles remain the same and found application in the analysis of the individual components. Essential tools forpredictingtheperformanceofcomplicatedstructuresare numericalsimulations. Thosemethodsallowfor optimiza- tionofdesignandfunctionandmightextendtheirvalueto fracturestatisticsaswellasforlifetimepredictions.Valuable informationonimperfectprocessing,improperuse,ordesign issuescanbefurtheridentifiedbyaclosefractographicexam- inationofthefracturedfragments.AseparateADMguidance documentpresentsfractographictechniquesandapplication (www.academydentalmaterials.org)[23].
14. Strength degradation under cyclic loading
There is an unfortunate trend towards “aging” prostheses andspecimenspriortostatictesting.Forexamplespecimens may beloaded to50Nforonemillion cycles,and perhaps eventhermalcycled,beforesingleload-to-failuretesting.The assumptionisthatsome“realistic”damageaccumulationis occurring.Thisassumptionofdamageaccumulationiscom- monlyapproachedinchewingsimulationstudies.Chewing simulatorsaretypicallyusedtosimulatetheclinicallymas- ticatory process and to produce relevant long-term cyclic fatigue resistancedata from non-clinical specimens. How- ever the experimental settings in such an approach need to be carefully adjusted in order at least to create some damage accumulation [60,61]. If not, the investigators are wasting their time and then misleading readers as well.
Hence, if one wantsto measure strength degradation,the testingconditionsshouldnotbearbitrarilychosenbutdeter- minedaprioriinapilotstudy.Additionally,“aging”conditions shouldnotbesosevereastobeclinicallyunrealistic.Further insightintodifferentchewingsimulation approaches,tech- niquesandindividualmachinesaswellasrecommendations towards reliable pre-clinical testing of individual prosthe- sesaresummarizedinaseparateADMguidancedocument (www.academydentalmaterials.org)[62].
Acknowledgment
Theauthorsare gratefultothe AcademyofDental Materi- alsforsupportduringthedevelopmentandwritingofthese guidancedocuments.
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