Finite element simulation of stress evolution in thermal barrier coating systems [Elektronische Ressource] / vorgelegt von Piotr Bednarz

Piotr Bednarz

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• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2006
• Nombre de lectures: 41
• Langue: English
• Poids de l'ouvrage: 19 Mo

Extrait

FiniteElementSimulationofStress
EvolutioninThermalBarrier
CoatingSystems
VonderFakultat¨ fur¨ MaschinenwesenderRheinisch Westf alischen¨ Technischen
HochschuleAachenzurErlangungdesakademischenGradeseinesDoktors
derIngenieurwissenschaftengenehmigteDissertation
vorgelegtvon
PiotrBednarz
aus
Biłgoraj,Polen
Berichter: Univ. Prof. Dr. Ing. LorenzSingheiser
apl. Prof. Dr. rer. nat. FlorianSchubert
Tagdermundlichen¨ Prufung:¨ 26. Juni2006
DieseDissertationistaufdenInternetseitenderHochschulbibliothekonlineverfugbar¨“Dreamscometrueforthose
whoworkwhiletheydream.
Sweetdreams.”
byH.JacksonBrownJr.Acknowledgements
This thesis is the product of years of work, procrastination, changing minds and
opinions. It is a delight to acknowledge those who have supported me over the last
years.
Firstly, I wish to express my deepest gratitude to my supervisor Prof. Dr. Lorenz
Singheiser for his excellent concern and endless support during the course of my re
searchstudy.
ToDr. PatrickMajerus,Iowethanksbeyondmeasureforhistremendoushelp,for
hismanyastutecomments,stimulatingdiscussionsandcritiqueofthiswork. Thanks
for beeing always ready to share your wide knowledge, and also for your wise yet
flexibleapproach. Ialsowishtothankmylastminuteproofreader,Mr. PhilipJ.Ennis
wasalifesaver;withtheeyeofaknowledgeabletechnicalscientificwriter.
MyveryspecialthanksgotoLineoMakheleforbeingaconstantsourceofencour-
agement and support in times of crisis, thanks for the your huge smiles and patience
whichkeptmegoingdespiteallthefrustrationsoftheselastthreeyears. Thanksalso
forefforttoproofreadvariouschaptersofthisthesis.
IgladlyacknowledgeDr. IrynaMarchuk,whohasalwaysbeenunderstandingand
supportive. Thanksforbeingmyfriend. ItgivesmegreatpleasuretothankDr.Teresa
Majerusforputtingupwithmeandthisthesis. ManythanksgotoDr. TatyanaKashko
fortheinterestingdiscussionsaboutscienceandothersecrettopics.
I must surely thank Dr. Roland Herzog for the contribution that he has made to
myhardwork. Whatwouldthisthesisbelikewithoutyourremarks?
Finally,theencouragementprovidedbymyparents,familyandclosefriends,dur-
ingthisworkhasbeenaconstantformofsupportandmotivation. Iacknowledgemy
brother Dr. Eugeniusz Bednarz for the funny time in the childhood as well as now.
Personal thanks are due tonumerouspeoplearound the world, toeveryonewho has
madethepastthreeyearsmemorable,interestingandenjoyable.Abstract
FiniteElementSimulationofStressEvolutioninThermalBarrierCoatingSystems
byPiotrBednarz
Gasturbinematerialsexposedtoextremehightemperaturerequireprotectivecoat
ings. To design reliable components, a better understanding of the coating failure
mechanisms is required. Damage in Thermal Barrier Coating Systems (TBCs) is related
to oxidation of the Bond Coat, sintering of the ceramic, thermal mismatch of the ma
terial constituents, complex shape of the BC/TGO/TBC interface, redistribution of
stresses via creep and plastic deformation and crack resistance. In this work, exper-
imental data of thermo mechanical properties of CMSX 4, MCrAlY (Bond Coat) and
APS TBC(partiallystabilizedzirconia),wereimplementedintoanFE modelinorder
to simulate the stress development at the metal/ceramic interface. The FE model re
produced the specimen geometry used in corresponding experiments. It comprises
a periodic unit cell representing a slice of the cylindrical specimen, whereas the peri
odiclengthoftheunitcellequalsanidealizedwavelengthoftheroughmetal/ceramic
interface. Experimental loading conditions in form of thermal cycling with a dwell
time at high temperature and consideration of continuous oxidation were simulated.
By a stepwise consideration of various material properties and processes, a reference
model was achieved which most realistically simulated the materials behavior. The
influences of systematic parameter variations on the stress development and critical
sites with respect to possible crack paths were shown. Additionally, crack initiation
andpropagationatthepeakofasperityatBC/TGOinterfacewascalculated. Itcanbe
concludedthatarealisticmodelingofstressdevelopmentinTBCsrequiresatleastre
liabledataofi)BCandTGOplasticity,ii)BCandTBCcreep,iii)continuousoxidation
including in particular lateral oxidation, and iv) critical energy release rate for inter-
faces(BC/TGO,TGO/TBC)andforeachlayer. Themainresultsfromtheperformed
parametric studies of material property variations suggest that porosity in the TBC
shouldbeincreasedandsinteringdecreased,inordertopreventorhindercontinuous
paths of tensile stresses above the valleys in the TBC. It was shown that variations of
creep rates in the BC influence marginaly stress values in TBCs . Therefore neither a
positivenoranegativeinfluenceonthelifetimecanbeextrapolated. Itwasshownthat
highercreepratesintheTBClayerledtoalowerstresslevel. Theextremevariations
of thermal expansion coefficient (±50%) help in better understanding of these vari
ations on stress development. The creep of base material only slightly affects stress
field development, under pure thermal cycling and can therefore be neglected in this
case. Asthetensilestressesincreasewitharelativelyhighfractionoflateraloxidation
not only the out of plane oxidation kinetics, but also its lateral component should be
low. Themodificationofamplitudeandwavelengthoftheasperityshowedthatwith
increasing roughness a continuous radial tensile path in the TBC and partially in the
TGO was formed already after 161 cycles. The variations of wavelength, amplitude
and shapes improve the understanding of stress development. The large variety of
parametricvariationsstudiedbythepresentworkinahighlycomplexandratherre
alisticFEmodelcontributesignificantlytoanenhancedunderstandingofTBCs. This
is supported by the final conclusion, that the set of crucial parameters could be re
duced to the time dependent deformation behavior of TBC and TGO, the oxidation
kinetics,includinglateraloxidationandtheshapefunctionoftheinterfaceasperity.Contents
Nomenclature xi
1Introduction 1
2Literatureoverview 5
2.1 Analyticalstudy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Numericalstudy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3Methods 11
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1 Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.2 Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.3 Straindecomposition . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Governingequationsofsolidbodydeformation. . . . . . . . . . . . . . . 18
3.2.1 Totalpotentialprincipleforlinearmechanics. . . . . . . . . . . . 19
4Modelingapproachandmaterialsdata 23
4.1 Geometryofspecimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2 Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3 Materialdata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 LoadandBoundaryconditions . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4.1 ThermalLoads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.4.2 Displacementconditions . . . . . . . . . . . . . . . . . . . . . . . . 27
5Results 31
5.1 Basicinfluenceofmaterialpropertiesonstressresponseandstressevo
lution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.1.1 InfluenceofTGOgrowthstresses(caseA) . . . . . . . . . . . . . 31
5.1.2 of BC plasticity on elastic TBCs including continuous
oxidation(caseD) . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.1.3 Influence of TGO plasticity on elastic TBCs including continu
ousoxidation(caseE) . . . . . . . . . . . . . . . . . . . . . . . . . 42
5.1.4 InfluenceofBCandTGOplasticity(caseF) . . . . . . . . . . . . . 42
5.1.5ofBCcreep . . . . . . . . . . . . . . . . . . . . . . . . . . 45
iCONTENTS
5.1.6 InfluenceofTBCcreep . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.1.7 Stressdevelopmentduringthefirsttwocycles . . . . . . . . . . . 53
5.2 VariationofMaterialpropertiesandinterfaceshape . . . . . . . . . . . . 58
5.2.1 InfluenceofThermalexpansioncoefficient . . . . . . . . . . . . . 58
5.2.2oftheElasticmodulusonthestressresponse . . . . . . 69
5.2.3 VariationofBCcreeprates . . . . . . . . . . . . . . . . . . . . . . 72
5.2.4 InfluenceofTBCcreeprates . . . . . . . . . . . . . . . . . . . . . . 74
5.2.5ofTGOcreep . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2.6 Influenceofbasematerialcreep. . . . . . . . . . . . . . . . . . . . 82
5.2.7oflateralTGOgrowth . . . . . . . . . . . . . . . . . . . 82
5.2.8 Influenceofroughnessamplitudeandwavelength . . . . . . . . 85
5.2.9ofdifferentshapesoftheinterface . . . . . . . . . . . . 92
5.2.10 Longtermstressdevelopment . . . . . . . . . . . . . . . . . . . . 96
5.3 DamagesimulationsatthepeakoftheTGO/BCinterface . . . . . . . . . 98
6DiscussionandConclusions 101
AppendixA 111
References 121
ii