A New Design Methodology for High Temperature Structural Components made of ContinuousFiber Ceramic Composites exhibiting thermally induced Anisotropic DamagebyD! "lachos#$ %& 'appas#$ ( De Stefano) and " *ostopoulos#1Applied Mechanics Laboratory, University of Patras and ICE-HT!"#TH,$#-%&' (( Patras, $reece2 )*ovo Pi+none, ,ia !- Matte*cci %, '(1%. !lorence - ItalyA+S(TACT Continuous Fiber Ceramic (Matrix) Composites (CFCCs) have found during the last decade numerousindustrial applications in a variety of technological areas, where structural components aresubjected to high temperature combined with significant mechanical loading.he present wor! deals with the application of innovative design methodologies for the developmentof an industrial gas turbine combustor chamber made of oxide"oxide composite materials.#xide"oxide composites offer high$temperaturestructural stability without theneed of any !indofoxidationprotectionandthuspermit theincreaseofthewor!ingtemperatureofthegasturbines,increasing the efficiency of the system and decreasing the need for cooling air and %#x emissions.&ince, oxide"oxidecomposites degradetheir structural properties as afunctionof theoperatingtemperature (for temperature higher than '(((o C) and the exposure time, an incremental approach hasbeen introduced for the solution of the problem and each increment represents a thermalexposurestage. hestructurehasbeendividedintemperature)onesandameanwor!ingtemperaturewasconsidered for each )one. he data set re*uired for the application of the present design methodology,was obtained through an extensive material characteri)ation program based on the measurement of theanisotropic properties of oxide"oxide composites using ultrasonic techni*ues.,NT(-D.CT,-N+as turbines for both simple and combined cycles had in the last five years a mar!et of ,( +-"annumandtheexpectationsarefor.(+-"annum, fortherunningdecade.-ithinthismar!et thereisatremendous commercial pressure on gas turbine manufacturers to increase plant efficiency and reducespecific costs, whilst continuing to meet the ever more stringent emission limits. From several !eytechnology areas, all the gas turbine manufacturers have identified CFCCs as the !ey material to fulfilthe imposed targets and conse*uently CFCCs are aggressively pursued with aim of full engine test inthe very near future /'0. he basic parameter, which supports the use of CFCCs in high temperature engineering components,istheir improvedfracturetoughness, comparedtomonolithicceramics, duetotheactivationofvarious stress redistribution mechanisms in the material structure /10. hus CFCCs have the ability towithstand damage without immediate catastrophic rupture.he present wor! deals with the application of innovative design methodologies for the developmentof an industrial gas turbine combustor chamber made of oxide"oxide composite materials /,0.#xide"oxide composites offer high$temperaturestructural stability without theneed of any !indofoxidationprotectionandthuspermit theincreaseofthewor!ingtemperatureofthegasturbines,increasing the efficiency of the system and decreasing the need for cooling air and %#x emissions.2owever, in the long run oxide"oxide composites indicate a reduction of their structural performance,as a function of the wor!ing temperature and the duration of high temperature exposure, fortemperature higher than '(((o C. 3mong the limitations in applying CFCCs in structural components is the uncertainty in using well$documented design methodologies. 4p to now, the most of the existing design approaches suffer fromthepresenceof astrongmetal cultureonthefinal products. 5nother words, somebasicdesignmethodologies, based on a database of life and stiffness data, have been used together with a strain ortime"cycle based life prediction philosophy /,0. hen, after conducting a thermal analysis,a linearstatic stress analysis follows, using stiffness information for the given composite system. he obtainedstresses can be used to find the life under a given thermo$mechanical loading. he proposed procedure, in the present wor! design, involves stress redistribution and this is achievedthroughthedevelopment of alamina stiffness evolutionlawthat includes al themajor factorscontributingtostiffness reductionas aconse*uenceof thedevelopedanisotropicdamage. hisapproach, certainly more accurate as it allows for stress transferring of the more damaged parts of thestructure where stiffness is lower, involves6 Completely new design of the combustor chamber which ma!es use of the material capacity andta!es into consideration the manufacturing limitations hermo$mechanical analysis of the component and calculation of the temperature and the stressprofiles for the given thermo$mechanical loading and boundary conditions.3t thispreliminaryphase, anoperational lifeof'7((hdurationisdemandedforthecombustorchamber.&ince, oxide"oxide composites indicate anisotropic degradation of their structural properties /.0, anincremental approachhas beenintroducedfor the solutionof the problemandeachincrementrepresents a thermal exposure stage. he structure has been divided in temperature )ones and a meanwor!ingtemperature was consideredfor each)one.8uringeach incrementthe materialpropertieswere considered constant and they changed through the successive increments. For the analysis of the problem the commercially available F9 code 3%&:& was used /70. hedataset re*uiredtheapplicationof theabove$describedapproachwas obtainedthroughanextensivematerialcharacteri)ationprogram.hematerialcharacteri)ationprocessinvolvestypical*uasi$static tests and an extensive set of ;stop and go< thermal exposure$fatigue experiments of theoxide"oxide composite, at different temperature levels. 3t each stage of the thermal exposureexperiments, advancedultrasonicmeasurementsofthecompositestiffnessmatrixwereconducted.hen, all the elements of the stiffness matrix of oxide"oxide composite were !nown as functions of theappliedtemperatureandthethermal exposureduration, andthesepropertiesweretransformedtolamina stiffness evolution law.he stresses and the displacements results obtained from the combustor chamber made of oxide"oxidecomposites bythe applicationof the proposeddesignmethodologywere comparedagainst theconventional design of the same component and the results are discussed analytically.MAT!(,A/ CHA(ACT!(,&AT,-Nhe complete set of the mechanical properties of the single lamina, which are necessary for the designof the laminate structure, are not available since unidirectional material is very difficult tomanufacture and its properties do not represent the actual lamina properties within the laminate.For the purpose of the present wor! the following strategy has been adopted6 Calculation of the laminate mechanical properties for a given laminate using ultrasonic velocitymeasurements. 5nversionof thelaminate mechanical properties for the determinationof the actual laminaproperties. he determination of the elastic properties of a laminate structure can be achieved by calculating thecoefficientsof thepropagatione*uationofanelasticplanewave, fromaset of properlychosenvelocity measurements along !nown material directions. hese measurements are carried out using aspecial experimental setup that is described below /=0.3 thin specimen with plan parallel faces is immersed in an acoustically coupling fluid (water). hespecimen is placed between a transmitter (9) and a receiver (>), which are rigidly connected to eachother through a supporting arm. ?oth the specimen and the arm, holding the two probes, can rotate asseeninFig. '. 4singappropriatesignal processingdevices, thephasevelocities of eachof thepropagatedwaves withinthespecimenarecalculatedfor anumber of specimen"probes relativeorientations.8epending on the angle of incidence, the pulse sent by transmitter 9 is refracted within the material inone, two or three bul! waves (one *uasi longitudinal wave @A, one *uasi transverse wave @, or two*uasi transverse waves @', @1) that propagate in the anisotropic solid at different phase velocitiesand in different directions.Figure '6 9xperimental setup for the ultrasonic velocity measurementsFigure 16 ypical geometry of the specimen used for ultrasonic velocity measurementshe difference in the propagation time of each the waves and the propagation time of the emittedsignal in the coupling fluid without the specimen, is measured.he evaluation procedure is based on the measurement of the time difference of the longitudinal andone or both transverse waves, and is only valid when the @A and the @ waves are appropriatelyseparated.he components of the elasticity tensor are obtained from the propagation velocities through a leasts*uare regression analysis, which minimi)es the residuals of the wave propagation e*uations.3fter the calculation of the components Cijof the elasticity tensor, the engineering constants of thelaminate structure (orthotropic in general) are determined by the following relations6&''B(C11CC,,$C11,)CC$'&..BC..$'&11B(C''CC,,$C1',)CC$'&77BC77$'&,,B(C11CC''$C1'1)CC$'&==BC==$'(')&'1B(C',CC1,$C'1CC,,)CC$'&',B(C'1CC1,$C',CC11)CC$'&1,B(C'1CC',$C1,CC'')CC$'where6CBC''CC11CC,,$C''CC11,$C11CC1',$C,,CC1'1D 1CC'1CC1,CC',and finally9'B&''$'+'1B&==$''1B $&'1C9'91B&11$'+1,B&..$'1,B $&1,C91(1)9,B&,,$'+',B&77$'',B $&',C9,able 'shows the conditions under whichthe ultrasonic tests were performed, while able 1summari)es the calculated laminate properties for the 3l1#,"3l1#, laminate of /(,E(0=& configuration.MaterialThic0ness(mm)Density(!g"m,)1ater Temperature(C)Fre2uency(M2))3l1#,"3l1#,1.7= 1,', 1, 7able '6 4ltrasonic test conditions and material geometry!#(+Fa)!)(+Fa)!3(+Fa)4#)(+Fa)4#3(+Fa)4)3(+Fa)5#)5#35)3G7.H G,.1 '. '..1 G.7 G.1 (.(H (.1= (.17able 16 9ngineering constants of /(, E(0 =& 3l1#,"3l1#, laminate.3s it is expected due to the symmetric structure of the laminate, the modulus of elasticity is almostidentical inthedirections'and1, whileismuchlowerinthe,rddirection(normal totheplatethic!ness). 5n addition these values are very close to the ones measured under *uasi$static loading.his testingprocedurewas appliedtoaseries of material samples that hadbeenexposedtoasimulatedturbineengineenvironment insideaspeciallydesignedfurnace. 8ifferent test durationswere used up to 7(( hours, allowing for the measurement of the degradation of the elastic propertiesof the oxide"oxide material, due to the sustained microstructural damage under operating conditions.able , shows the lamina properties of 3l1#,"3l1#, at the various steps"time of the thermal exposureas they have been calculated by inverting the measured laminate properties to the respective laminaones for a given laminate structure.For the purpose of the present analysis it was assumed that the developed damage does not affect thedegreeof anisotropy. 3ll thethermal exposureexperimentswereconductedat ''((oCandtheevolution laws of the various stiffness components, with no lac! of generality, were assumed constantwith respect to temperature. hus, after an initial thermal analysis step, where the actual temperatureof the component was identified, the combustor was divided to temperature )ones and the stiffnessdegradation laws were applied to the temperature )ones that exceed '(((o C. M-D!/ D!SC(,'T,-Nheouter liner of thecombustionchamber wasmodelledusing..'., H$node&29AAEElayeredelements /70. >educed modelling was used since the component exhibits rotational symmetry. hus,only the '"'= of the whole chamber was introduced in the model along with the appropriate symmetryconditions, in order to reduce run time. he liner is held on its supports with '= screws. his conditionwas transferred into the numerical model using appropriate nodal displacement constrains on the areaof contact between the liner and the supports. 3 '7 bar pressure was uniformly applied on the liner. Figure ,6 Iiew of the geometric model using symmetry expansion.Figure .6 he finite element mesh.hermal loadswereneglectedat thisstageinordertosimplifytheanalysisandtofocusontheinfluence of the anisotropic stiffness degradation on the mechanical response of the combustor liner. he lay up se*uence used for the layered material was (("J.7"E()1s. his results to a .mm thic!nessfor the component. he material properties used were calculated from a series of tests conducted onthe 3l1#,"3l1#, ceramic material as described in the previous section.en sets of material properties were used in e*ual analyses to depict the degradation sustained by thematerial, due to the microstructural damage developed during its exposure to the extreme temperatureenvironment of the combustion chamber.-peration Time(hours)!#(+Fa)!)6!3(+Fa)4#)64#3(+Fa)4)3(+Fa)n#)6n#3n)3( ''H.G=H 1(.E7E 1(.((( H.((((.'7 (.1717 ''1.==( 'E.HH' 'E..(( G.G=(7( '(H.7HH 'E.'=, 'H.H(( G.71(G7 '(..H77 'H.7(. 'H.,7( G.,.('(( '('.H(' 'G.E=7 'H.((( G.1(('7( E=.G'' 'G.(=G 'G.7(( G.(((1(( E,.,'H '=..=H 'G.1(( =.HH(,(( E(.GG, '=.('E '=.E(( =.G=(.(( HE.(G= '7.G'E '=.H(( =.G1(7(( HH.11H '7.7G( '=.G(( =.=H(able ,6 Material properties used in the analysisN.M!(,CA/ '(-C!D.(!he stiffness degradation of the ceramic material cannot be simulated with standard material models.5n order to avoid the complicated procedure of developing a new material model, a simple procedurewas selected for the analysis. he analysis was conducted in ten steps in order to simulate the non$linear behaviour of the material in the presence of microstructural damage. he overall procedure isbased on a %ewton$>aphson approach for the solution of the non$linear problem, being applied usingmultiple analysis runs. For each analysis step the deformed geometry resulting from the previous step,was usedtoupdate the model tothe current deformationcondition. 5nthis way, displacementcontinuity was achieved. he results of the last analysis step show the condition of the liner at the endof the 7(($hour operation time.(!S./TS8isplacement and stress data were recorded for each analysis step. he evolution of the maximumdisplacement in the form of radial expansion is presented in Figure 7 against the operation time inhours.3 second curve thatrepresents the variationof 9'stiffness component ofthesingle laminaversus the operation time in hours is also given in the same Figure. For each stress component, theevolutionof themaximumvalues, withrespect totheengineoperationtime, is plottedfor therespective critical locations. Figure = shows the critical nodes for which the stress components will begiven in the next. he use of layered elements in the analysis allows for the computation of the stressdistribution through the laminate thic!ness. Figure G presents the through thic!ness variation of the axial stress component at the node ',(1G,which is the node were the maximum values for the axial stresses appear.Figure H plots the variation of the maximum axial stress, which has been monitored at the outer layer((o degree layer) versus the operation time of the gas turbine.Figure 76 >adial expansion and stiffness degradation ofthe liner versus engine operation time.Figure =6 Aocations of maximum stress3s it is shown, there is a '(K increase of the maximum axial stress experienced by the material at thesaid point, due to the anisotropic degradation of the lamina properties caused by thermally induceddamage. Figure G6 3xial stress distribution through thelaminate at node ',(1. Figure H6 3xial stress at node ',(1. vs. engineoperation time Figure E6 2oop stress distribution through thelaminate at node H,,1 Figure '(6 2oop stress at node H,,1 vs. engineoperation timeFigure E shows the through thic!ness variation of the hoop stress component at the node H,,1, whichis the node were the maximum hoop stresses appear.Figure'(presents thevariationof themaximumhoopstress versus theoperation timeofthe gasturbine. 5n this case, the decrease of the maximum hoop stress appears to be of ,K, as a result of thethermally induced damage within the material structure and the increase of the axial monitored stress.Figure'' shows thethrough thic!nessvariationof theshear stresscomponentatthe node ',(GE,which is the node were the maximum shear stresses appear.Figure '1 presents the variation of the maximum shear stress, which has been monitored at the outerlayer ((odegree layer) versus the operation time of the gas turbine. 5n this case an increase of themaximum shear stress of about 7K appears as a result of the thermally induced damage within thematerial structure.Figure ''6 5n plane shear stress distributionthrough the laminate at node ',(GEFigure '16 5n plane shear stress at node ',(GE vs.engine operation timeFigure',showsthedeformableshapeoftheanalysedcomponent andthecorrespondingdisplacement contours of the outer lamina, while Figure '. presents the axial stress contoursthat correspond to the outer$critical lamina. 4nder the given loading conditions the maximummonitored displacement is of the level of (.'G mm.Figure ',6 8isplacement contour Figure '.6 3xial stress contour at lamina '=Finally, Figure '7 shows the hoop stress contours monitored at the lamina . while the shearstress contours developed at the lamina '7 are given in Figure '=.Figure '76 2oop stress contour at lamina . Figure '=6 5n plane shear stress contour atlamina '7C-NC/.S,-NS3n #xide"#xide composite material was used for the design and the manufacturing of the combustorliner of an industrial gas turbine system. 3nalytical characteri)ation of the material was conducted at ambient and high temperature. 3dditionalcharacteri)ation of the material at ambient temperature was also made using ultrasonic waves. &incethe exposure of the #xide"#xide composite at hightemperature (over '(((oC) results tothedevelopment of anisotropic damage within the material structure, it concludes to anisotropicdegradation of the composite properties. 4sing ultrasonic waves, the complete characteri)ation of the#xide"#xide lamina properties at different high temperature exposure time is available.For thedesignof thecombustor liner aseriesof F9analysiswasappliedusingagive, easytomanufacturestac!ingse*uencefor thelaminatedstructure. >esultsshowedthat the#xide"#xidecompositefulfilsthedesignre*uirementsforhightemperaturestructural applicationsandmaybeconsidered as a very promising candidate material in future application. 5n addition, for design of hightemperature components using CFCCs, innovative design methodologies that ta!e into considerationthe anisotropic damage, which is developed within the material structure under operation conditions,is necessary.Futurewor!that hasbeenalreadyplannedincludescoupledthermo$mechanical analysis andtheintroductionof material models that permit anisotropicdamage andproperties degradationwithrespect to exposure time and wor!ing temperature.(!F!(!NC!S/'0 Aamic*, F., ?oury, 8., LCeramic MatrixComposite Farts 8esignM, 3+3>8>9F#>GE7L5ntroduction of Ceramics into 3erospace &tructural CompositesM, %ov. 'EE,./10 Nostopoulos, I., Fappas, :.O., Loughening Mechanisms in Aong Fiber Ceramic MatrixCompositesM, 5n LComprehensive Composite MaterialsM Iol. ., 9lsevier &cience Atd, 9d. 3. Nelly PC. Oweben, 1(((/,0 &ygulla, 8., MQhlrat)er, 3gathovic, F., L5ntegrated 3pproach in Modelling, esting and 8esign of+radient$CI58erivedCMCComponentsM3+3>8>9F#>GE7L5ntroductionofCeramicsinto3erospace &tructural CompositesM, %ov. 'EE,./.0 ?aste, &., 3udoin, ?., L#n 5nternal Iariables in 3nisotropic 8amageM, 9uropean Rournal of Mech.,3"&olids, vol. '(, =,'EE'./70 3%&:&, #peration Manual, 3%&:& >elease 7.., %ov. 'EEG/=0 C9%echnical Committee 'H., &ubCommittee ', S3dvancedechnical Ceramics,$ CeramicMatrix Composites $ Mechanical Froperties at >oomemperature $ 8etermination of 9lasticFroperties by an 4ltrasonic echni*ueS, -5 ''., March 1(('.