Thethermal and mechanical propertiesofcarbon,glassand Kevlar fibrereinforcedepoxy compositesarediscussed,wi thparticular referencetothebehaviourofthesematerialsatcryogenictemperatures. The effectsofproductiontechniquesand variousfibrearrangementsare determined. Fibre-epoxycompositesatlowtemperatures* G.Har t wi gandS.Knaak Keywords: low temperaturetechniques,composite materials, fibre-epoxy composites Nomencl at ur eaucUl t i mat ecompressi vestress cr/crUTFat i gueendur ancelimit(tensilet hreshol d) EYoung' smodul usV]LSI nt er l ami nar shearstrength FFi brevol umefract i ontoFi breangle f i , L / L Integralt her mal expansi on RTRoomt emper at ur eIndices tRelativet hi cknessofangle-plyMMat ri x Coeffi ci entoft her mal expansi onFFibre eStrainCComposi t e eUTUl t i mat etensilestrain XTher mal conduct i vi t y /xPoi sson' sratioPolymers crStressPCPol ycar bonat e cruTUl t i mat etensilestressPSUPol ysul fone Fi brecomposi t esareattractivealternativestotureductilet hermopl ast i cswilli mprovethemechani calmetalsbecauseoft hei rhighspecificstrengthorproperties. stiffnessor t hei rexcel l ent fatiguebehavi our. TheyareaCryogeni cappl i cat i onsofpol ymeri cfibre necessarysuppl ement tometalsbecauseoft hei rlowcompositesaremai nl yinsuperconductivity, space electricalandt her mal conductivities, thelatterbei ngt echnol ogyandhandl i ngofliquefiedgases.They relatedtostrengthorstiffness.Thei r di sadvant agei ncl udesuper conduct i nggeneratorsorpul sedmagnet s arisesfromt heweakpol ymeri cmat ri xandresultsinforfusionreactorswhi chcallformaterialsresistantto lowi nt er l ami nar shear strengthandlowtransversefatigueandelectricallyresistivetoeddycurrents. strength.Atlowt emperat uressomepropertiesoftheSupport el ement sforliquefiedgascont ai nershaveto het erogeneousfi bre-mat ri xsystemaresuper i mposedbybeopt i mi zedwithregardtolowt hermal conduct i vi t y suchpeculiaritiesasaddi t i onal t her mal resistanceatandmechani cal strength.Tr anspor t vesselsand boundar i es(Kapi t zaeffect) bywhi cht het her mal component susedinspacet echnol ogyshoul dbelight conduct i vi t yisreduced. Inthecourseofcool i ngtheweightandresistanttofatigue.Inseveralappl i cat i ons di fferent t her mal cont ract i onsof fibreandmat ri xgivelowt hermal cont r act i onisnecessary.Thecryogenic, risetot her mal residualstressesandstrainswhi chmechani cal andt her mal propertiesofi mpor t anti nfl uencemost ofthemechani cal properties.Atlowpol ymeri cfibrecomposi t eswillbepresented.t emperat urest hemaj ori t yofcurrent l yusedmatricesThedependenceof mechani cal propert i esonproduct i on arebrittleanddonot allowrel axat i onofresidualt echni queswasstudiedforcar bonfibrecomposites.stressesorstressconcent r at i onstotakeplace.Infut ure devel opment st heappl i cat i onof special,lowt empera-Mat er i al s *DedicatedtoProfessorDr.W.Heinz ontheoccasionofThefibresusedinthisworkwereE-glassfibres, his60t hbirthday,highmodul uscar bonfibres(M40,Toray), hightensile 0 0 1 1 - 2 2 7 5 / 8 4 / 0 1 1 6 3 9 - 0 9 $03. 001984Butterworth~tCo(Publishers)Ltd. CRYOGENICS.NOVEMBER1984639 Tabl e1.Fibrepropertiesfract urest rai nofBUTM ~ 4-6%at4.2ICTheyare candi dat esformat ri cesof futurecomposi t esfor F i b r e s oU T II,8UTII,EF I I ,EF *, cryogeni cappl i cat i ons? GPa%GPaGPaFor allmat ri cesnotonl yt hei rcryogeni c propert i esaredecisivebut atRTaddi t i onalT3 0 0 3. 51.52 3 0 ~24r equi r ement smust befulfilled.Thi sappl i esmai nl yto CarbonfibreM4 0 A2. 40. 64 0 0 thecreepcharact eri st i cof composi t eswhi chshoul dbe AS- 42. 82 1 0 sufficientlylow.However, inmost cryogeni cmagnetFibregl ass(E-glass)3. 0- 3. 43.17070appl i cat i onshi ghl oadsareappl i edonl yatlow Kevlar492. 82.11 4 0 ~1 1t emperat ures, wherecreepprocessesarenegligible.At RTt heyhavetowi t hst andtheintrinsicweightl oad * - a t T =4.2Konl y(egst ruct ural part sforsuper conduct i ngmagnet s andgenerators).car bonfibres(T300.Tor ayandAS4,Hercul es)andFor one-andt wo- di mensi onal l oadcondi t i ons Kevlar-49fibres( DuPont).Thepr i nci pal fibrethefollowingfibrear r angement swerechosen: propert i esareshowninTabl e1.uni di rect i onal (UD)layers;uni di rect i onal ( UDW)Mat eri al sampl eswerepr oducedbyf i l ament madef r omnearl yuni di rect i onal fabricswithafillofwinding, fortubes,aswellaswetl ay-upandpr epr egt hi nglasst hreadsforfixation;angl e-pl y(0 ,___ 45 ,90 ) t echni ques. Thr eeepoxyresinsyst emswereusedtomadefromUDpliesor0 and90 fabricsandcross- formthepol ymer i cmatrices. Allt hreearebasedonply(0 ,90 )madefromweaves. bi , phenol - Aandmanuf act ur edbyCi baGeigy.The syst emCy221/ Hy979isaflexiblel ami nat i ngresin:Ani sot ropyof composite properties thetwoothers, Ly556/ Ht 972andMy740/ Hy917,areMost propert i esof fibrecomposi t esareof a rigidatr oomt emper at ur eandareusedinpr epr egt ensori al nature.Theani sot r opyisduetothefibre t echni ques.ar r angement , theintrinsicfibreani sot r opy( car bonand Epoxyresinsarecross-l i nkedpol ymer sandshowKevl arfibres)andthef i br e- mat r i xinterfacialbond.brittlebehavi our atlowt emperat ures. Thetensile fract urest rai nforflexibilizedl ami nat i ngresinsisofFibreanisotropy theor der ofBUTM ~2%at4.2K.Thesyst emsusedin pr epr egt echni quesaremuchmor ebrittleandexhi bi t Fi breglassconsistsof ani sot ropi cmat eri al with anevenlowerfract urestrain.Sometypesof st rongcoval ent bondi ngbet weenat omswhi chresults t her mopl ast i cs(egPC,PSU)areductileint hei rinahi ghstrengthint hreedi mensi ons. Car bonfibres behavi our evenatlowt emper at ur esandshowaareani sot r opi candhavest rongcoval ent bondi ngonl y Table2.Propertiesofmatri xandfibrecomposi tes OUT,UT,E,Cuc,~t'lLS,dr/O'uT'b G Pa%G PaG PaM Pa% MAT RI XAT 4. 2K Epoxyr esi nCy2 2 1 / Hy 9 7 9 0 . 1 8 2. 28-0 . 3 7 -4 0 at 77K F I BRE- EPOXYCOMPOSI T ES( F I BRECONT ENT 6 0 VOL%) AT 4. 2K U ni di r ec t i onal II Wi t hcar bonT3 0 0 2. 01. 51 4 0 0 . 8 - 1 . 3 0 . 3 2 1 1 0 8 5fibreM4 0 A 1. 50. 52 4 0 -0 . 3 2 9 0 8 5AS4--1 3 0 --1 4 0 - Wi t hf i br eglass (E-gl ass)1. 1- 1. 81. 5- 1. 84 5 0 . 7 - 1 . 0 -1 7 0a- Wi t hKevl ar 4 9 1. 3- 1. 51. 2- 1. 31 0 0 0 . 3 2 -3 8a6 5U ni di r ec t i onal 1 Wi t hcarbon fibreT3 0 0 0 . 0 5 - 0 . 0 6 0 . 4 - 0 . 5 13-0 . 0 7 -- Wi t hKevl ar 4 9 --9. 5-0. 1-- Angl e- pl y ( 0 =,4-4 5,9 0) Wi t hcar bonT3 0 0 0. 51.15 3 -O. 31-6 5fibreM4 0 A I O7)(tensilethreshold)Thermalconductivity 85*/085%~ Asurveyoft her mal conduct i vi t y ofUDfibre lillisin6.andKevl arcomposi t esgivenFig.Fibre-glass s~*/,65*/,fibrecomposi t esdonot showagreatt emperat ure IIII 5~"~dependenceoft her mal conduct i vi t i es) H3t hei rvalues b50"1, 5o_NI ~bei ngfivetot entimeshi ghert hant hat oftheepoxy 4o*/,Tmatrix.Bycontrast,car bonfibrecomposi t esshowa zs*/,|very larget emperat uredependenceduetofreezingoutM,.ofel ect ront her mal fibreconduct i vi t yatlow temperatures.oInFig.7t het her mal conduct i vi t y ofcar bonfibre E-GlossT300M40AKevlor49composi t esisshownwithdifferenttypesand rovingCorbonfibresarrangement soffibres.AtRTallUDcomposi t eswith Fig.5Tensi l e strengthGUT, sti f f ness EII andfati gueendurancel i mi ta / %T HMfibres(M40A)showahi gherconduct i vi t y ofUD and (0"; :t: 45*, 90*) fibre composites at77K t hreshol dtestsat77Kt heval ueis85%. Theval uesofot herUDandangleplycomposi t esareshownin Fig,5.Testsundercompressivefatigueloading yieldlower valuesbecauseofmi crobuckl i ng, especiallyforfibresComposites?11.~.__19140A whi chareweakint hetransversedirection.Thedat a(so v o l%)/ - ~areveryscarce.Somei nf or mat i ononfibreglass composi t eshasbeenpubl i shed?/(Steel ) O nei mpor t ant quest i onist hedegr adat i onof Io1~~~ epropert i esint hecourseoffatiguecycling.For fibrepoxy. . . ~ . - - T3 0 0g l a s s a n d c a r b o n f i b r e c r o s s - p l i e s i t wa s s h o wn t h a t / , / " ! / ' / " x,,_ _stiffnessdoesnot degradebymor et han15%under Kevlor/epoxy uni axi al tensilefatiguecondi t onsat77Kunt i l fracture' l [ ] / _. . . . - ~ - -occurs.Under bi axi al fatiguecycling withcombi nedI t orsi onandt ensi onamuchhi gher andcumul at i ve. . . . . . _ ~_ Gloss/epoxy degradat i onhasbeenfound.1 E.......- ~~~_ _ - - - - Epoxy Interlaminarshearstrength(ILS)~io-I I /For shear l oadi ngofcomposi t est hedecisive limitingpar amet er ist heILS.I fthisval ueisexceeded,del ami nat i onofcomposi t elayersoccurs.Fi bre breakageoccursatl oadsdefi nedbybendi ngstrength. TheILSmeasur ement sareper f or medinshort beam162 tests.Theval uesofILSaredet er mi nedmai nl yby failuresoft hematrix,i nt erfaci al bondorfibres. Thequest i onoft heupper l i mi t ofILSarisesand anot her quest i oniswhi chcomponent sconst i t ut ethe l i mi t at i onsofthecomposi t esconsidered. Thelow t emper at ur eshear strengthofepoxyresins( andmost i63IIIIIIIIIIIII~~iii~iI pol ymers)liesbet ween150and200MPaatT ~77Kotoozoo3o0 andconstitutest heupper limitofcomposi t escon-Temperoture,K t ai ni ngepoxyresins.For fibreglasscomposi t est he high,nearl yi sot ropi cfibrestrengthandverylikelytheFig.6Survey of t hermal conduct i vi t i esof UDf i brecomposi t esversus bondstrengthsdonot constitutet helimitingfactors,t em p er a t ur e 6 4 4 CRYOGENI CS. NOVEMBER19 8 4Carbonf i br es/ epoxyKevl arf i bre-composi t e 60vol%Fi bre content60vol % 150 M40AIIxl O - 4~-E.~ox~yr ~~M40A_+ 450--~ ' ~ , ~ ~, , ~r a n s v e r se oo o o oo i / / / , " " ~T 3 0 0 I I -~" "~ /f/T SO0 0", + -45" , 90 50_~""... "-. / / / ~" / f ~ T : 500 45~EIm~ 0 IIIII - , o~ / / / ~ ' / / / Epoxy resinxt O" 5-- I0- I~ / / f ~ . / ~ CY221HY979- I 00- 6 4 5 tI[II~IIIIIIIII 0I 00200300 16 2IIIII I II I IIIII 20406080IO0150200250500Tempereture, K Fig.8Longi t udi nal andt r ansver sei nt egral t her mal expansi onf or UD 7", KKevl arf i br ecomposi t esver sust emper at ur e Fig.7T her mal conduct i vi t yof di f f er ent car bonf i br ecomposi t esver sus t em per at ur ethefibredirectionsduetostretchingvibrationsin covalentbondi ngpotentials.Thenegativeexpansi onof compar edtothosewithHTfibres.TheelectricKevlarfibresisassumedtoarisefrombendi ng conductivityof HMfibresatRTisabout 2.5timesvibrationsperpendi cul artothealignedmol ecul ar highert hanthatof HTfibres.Thisisduetolargerchainsoffibres.Incont r ast theirtransverseexpansi on graphi t i zedareasof HMfibres,islargeandlargert hanthatof mostepoxyresins.This Atverylowtemperaturesthisdifferencevani shesis, duetovibrationsintheweakasymmet ri cVander becausewavelengthswhicharecharacteristicof thatWaalsorhydrogenbondpotentials.Thus,alow temperaturerange( domi nant phononwavelength)arethermalexpansi onisonl yavailableinthefibre toolargetbrresolvingthemicrostructuresof fibres.Adirection.Thisisdemonst rat edinFig.8forthe similarconsi derat i onholdsforthel ongi t udi nal andthermalexpansi oninthelongitudinalandtransverse transversecomponent softhefibreconductivitytensor,directionsofKevlarfibreUDcomposites./3For car bon AtRTitisrat herani sot ropi candgetsmoreisotropicfibrecomposi t essimilarresultsapply. atlowtemperaturesduetotheincreaseoftheInangle-pliesamedi umlowt hermal cont ract i on domi nant phononwavelengths.Inadditionthecar boncanbeachievedinplanebymeansof residualstresses fibreconduct i vi t ydecreaseswithdecreasingbetweenplies.Howeveralarget hermal cont ract i on temperaturesandgetscompar abl etoorlowert hanthatexistsinthedirectionperpendi cul artothelaminates. oftheepoxymatrix.Thecoefficientofthermalexpansi onofUD AsseenfromFig.7thedifferenttypesofcar boncompositesatc IIcanbecalculated 5 fromthoseofthe fibrecomposi t esconsi deredvarybyafactorof50atfibreso~F IIandthematrixo~M by RT.At7Kthisfactorreducesto1.5. Forthet hermalconduct i vi t ytransversetothefibredirectionat hermal C~M--~rll boundar yresistancebyphononmi smat ch(KapitzaSell=erll+(8) resistance)mayberelevantatverylowtemperatures.1 +1.1F/ (1-1. 1/ 7)(Evl p / EM)Thismeansthatatlowtemperaturesthet hermalconductivityofcar bonfibrecompositesisrathersmallwhereFisthefibrecont ent pervolume:E M and andnearlyi ndependent offibretypeandarrangement.Thust hermal conductivityimposesnomaj or restric-EF IIarethemodul i ofmatrixandfibre,respectively. Forcalculatingt hermal expansi onofangle-pliesseea tionsif onewantstoopt i mi zeot hercomposi t epreviouspubl i cat i on? propertiesatlowtemperatures.Thet hermal expansi oncanbevariedbythefibre T h e r ma l e x p a n s i o n cont ent andarrangement orbydifferentthicknessesofangle-plies. Forseveralappl i cat i onsincryogenic t emperat ureExperi ment al valuesofintegralthermal t echnol ogylowcontractivematerialsareofadvantage,expansi on Duetotheani sot ropi ct ensoroft hermal expansi onthis isnotpossibleforallfibrecomposi t esinalldirections.4.2 Fibreglasshasalowcoefficientoft hermal A L / L =(e cd Texpansi on(at r~4.810-nK -1)whichisnearlyisotropic inalldirections.Thus,thecont ract i onofthose293 composi t essi mul t aneousl ycanbemadesmallinall threedirections,onangle-plyfibreglasscomposi t esareshowninFig.9. Car bonandKevlarfibresexhibitaverysmallorTheparamet erofthecurvesisthefibreangle__.to evennegativecoefficientoft hermal expansi ononl yinrelatedtothedirectionofmeasur ement )4 C R Y OGE N I C S . N OV E MB E R 1 9 8 4 6 4 5Fibregloss/epoxy Fibregloss/epoxy7 0-',c_5vol % I ~ ' ~ x 70 -+ 3vol % Directionof~ _ :500Directionof+ ~ _ ~ _ + ~= 90 measurement+