NASA Technical Reports Server (NTRS) 19980237335: Room Temperature Tensile Behavior and Damage Accumulation of Hi-Nicalon Reinforced SiC Matrix Composites

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by  NASA Technical Reports Server (NTRS)

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ROOM TEMPERATURE TENSILE BEHAVIOR AND DAMAGE
ACCUMULATION OF HI-NICALON REINFORCED SIC MATRIX COMPOSITES
G.N. Morscher*, Case Western Reserve University, Cleveland, OH
J.Z. Gyekenyesi ° ,Cleveland State University, Cleveland, OH
ABSTRACT
Composites consistingofwoven Hi-Nicalon fibers,BN interphases, and different
SiC matrices were studied intensionat roomtemperature. Composites with SiC matrices
processed byCVI and meltinfiltrationwere compared. Monotonic and load/unload/reload
tensile hysteresis experiments were performed. A modal acousticemission (AE) analyzer
was usedto monitordamage accumulation duringthe tensiletest. Post test polishingof
the tensilegage sectionswas performed todetermine the extent ofcracking. The
occurrence andlocationofcrackingcouldeasily bedetermined usingmodal AE. The loss
ofmodulus couldalsoeffectivelybedetermined from thechange inthe velocity of sound
across thesample. Finally,the stresses where cracks appear tointersect the load-
bearing fibers correspondwithhightemperature lowcyclefatigue runout stresses for
these materials.
INTRODUCTION
The hightemperature mechanical propertiesofSiC/SiC composites are limited
duetooxidation reactionswhichoccur inthe presence ofspecies suchas oxygen and
water vapor. What enables suchreactionstooccuratthe fiber/interphase/matrix region
ofacomposite isenvironmental ingressthrough pores inthe matrixand/or through matrix
cracks. For SiC/SiC composites where chemical vapor infiltration(CVI) isthe matrix
processing route,thefiber-matrix interphase regionshouldbe sealed offfrom the
environment even though significantmatrixporosityexists. Therefore, the environmental
reactions are dependent on matrixcracking, althoughahighly porous matrix willallow the
environment access toinnerregionsofa compositewhichwouldnot beaccessible ifthe
matrixwas dense. The compositefabrication methoddictates the amount of porosity
which exists inthe matrix. The loadinghistoryofthecomposite, coupled withthe flaw
distributioninthe matrix, dictatethe amount and natureofcracking that exists inthe
matrix.
An understanding ofthe damage accumulation inacomposite is criticalin
determiningwhen matrixcrackingoccursand thenature ofmatrix cracking, i.e. local
microcracks orthrough-thickness cracks. Guillaumat and Lamon[1.2h1ave characterized
"Resident Research Associates atNASA Lewis Research Center; Cleveland, OH
the damage accumulation inaNicalon #,C interphase,CVI SiC matrixcompositeas a
TM
function ofstrain. They foundthat
1. Microcracks emanate from macropores between differentfiberplies and
propagate acrossthe plies(s=0.025% to0.12%)
2. Cracks then form andpropagate intransverse (90°) bundlesand dense matrix
regions (s=- 0.12% to0.2%)
3. Finally, cracksform inthe matrixsurrounding0° bundles(s> 0.2%)
The microcracks formedat largepores inthe matrixandin90° tows are
sometimes referred toas tunnelcracksp'_. A slightreduCtioninelasticmodulus wouldbe
expected from suchcracking. When the microcracksintersect anddebond the
fiber/matrix interphases ofthe load-bearingfibers, significantelasticmodulus reduction
and unload/reload hysteresis behaviorwas observed161.
This studywillcompare thestress-strainbehavior ofwoven Hi-Nicalon_u_,BN
interphase, SiC matrix compositeswithmeltinfiltrated(MI) and CVI SiC matrices. Modal
acoustic emission_]willbeused tomonitorthedamage accumulation andunload/reload
hysteresis loopswill be performedtodetermine theonset ofdamage inthe 0° bundles as
well asthe interfacial propertiesofthe two compositesystems. The findingswillthen be
related tothe elevated temperature propertiesofthese twocomposite systems.
EXPERIMENTAL
Table I:Woven Hi-Nicalon TM. BN-intemhase. SiC Matrix ComPosites
Composite Composite Processing Weave vf
Details (loading
direction)
CVI SiC CVD BN interphase* 8 HS 0.15
CVI SiC matrix*
MI SiC CVD BN-interphase* 5 HS 0.17
2pm CVI SiC overcoat*
Si meltinfiltratedSiC matrix**
allcompositeswere0/90layupswith8plies(17endsperinch)
*DupontLanxide:BNinterphase;CVISiC **Carborundum:MeltInfiltrationSiC
- specificprocessingdetailsareproprietary
Table Idescribes the processingapproaches, vendors, andwoven architectures
ofthe twocomposites compared inthisstudy. The tensile sample dimensionsand
mounting arrangement are shown inFigure 1. The samples were cutfrom 152mm x229
mm plates intodogbone tensilespecimens where the lergth was 203 mm, the widthatthe
ends was 12.7 mm andthewidthatthe gagesectionwas -10.2 mm. The plate
thicknesses were nominally2.35 and2.2 mmforthe CVI SiC and MI SiCcomposites,
respectively. Tensile testswere performedwitha screw-driven universal testing machine#.
Glass fiberreinforced epoxy tabs (1.5 mmthick) were mounted on bothsides ofthe
' Nicalon andHi-Nicalon producedby Nippon Carbon, J_pan
TM TM
' InstronModel 8562, InstronLtd.,Canton,Mass.
sample inthe gripregions. The specimens were grippedwith rigidly mounted
hydraulically actuated wedge grips. Aclipon straingage, with a range of2.5% strain and
25.4 mmgage lengthwas used tomeasure strain.
Wide-band (50 kHz to2.0
MHz)==, highfidelitysensors (9.2 mm
indiameter) were placedjustoutside
the tapered regionofthe dogbone
sample to recordacoustic emission
events. Vacuum grease was usedas
acouplant and electricaltape was
transducer
usedto mountthe sensors tothe
wsaemreplree.coArcdoeudsutiscinegmaiss2i-ocnhawnanveel,forrns 25mmI "_--Clip-on-_ =65ram
Extensometer
Fracture Wave Detector_ (FWD). The
FWD consisted ofapersonal
computer(Pentium, 120 MHz) witha
12-bit, 30 MHz analogto digital
acquisition board. Each sensorwas
connected to apreamplifier andfilter
triggermodule whichwas fed intothe
computer. The preamplifier was setat
20 dB, the filtersignalwas amplified 3
riB,andthe filtertriggerwas amplified point of lead break
21 dB. The loadand strainwere also
recorded withthe FWD computer. Figure1:Specimen configuration.
The post-test analysis was performed onWave DetectorTM software providedbythe FWD
manufacturer.
RESULTS AND DISCUSSION
Typical monotonic tensile stress-straincurvesare shown inFigure 2for the two
composites. The stress-strain behaviorisverysimilartothat shown by otherresearchers
for Nicalon/SiC woven compositesIs'l°].Bothcompositesfailed atsimilarstresses (~380
MPa); however, the MI composites always failed inthe gripswhereas the CVI composites
always failed inthe gage section.The"knee" inthe stress straincurve occursata lower
stress for the CVI SiC composites andthestrain tofailureofthe CVI SiCcomposites is
greater than the strain tofailure ofthe MI SiC composite. The mechanical properties of
the two composites are listedinTable II.
Figures 3a and bshowmonotonic andhysteresis stress-strain curves ofthe two
composite systems. The monotonic andhysteresisstress-strain curvesare almost
identicalfor bothcomposite systems. The hysteresis stress-strain curveshave slightly
larger strainsfor thesame stress compared tothe monotonic stress-strain curves. For
the hysteresis experiments, AE activityoccurred inthe gage section upon initialunloading
and atthe endofthe subsequent reloading. This indicatesa greater amount of damage
inthehysteresis specimensm]andcould accountfor the increased strainincurredfor
hysteresis tensile tests.
Digital Wave Corporation, Englewood, CO
3
The maximum loopwidthwas
determined for each unload/reload hysteresis =_
loopat thestress whichwas halfthe _
maximum stress ofthe specificloop(Figure _ =,
4). The onset of0°cracking isconsidered to _ =_
....... -
ogcrecautrewr thheannthzeeroh.ysTtherisesoiscclouorspawtid~th11is0 MPa i ',".
for CVI SiCcomposites and ~ 140 MPa for ,
the MI SiC composites. Itisalsointeresting
0 ...J .... , .., ..i .... ,.. , .... ,,,i, 010
tonote thatthe loop widthofthe MI SiC 0 0-1 0.2 0.0 OA _l LI 0.7 _0
8_kl, %
compositeswere greater than thatoftheCVI
SiC compositesathigher stresses. Itwas
observed from polishedsectionsoffailed Figure 2: Monotonic stress-strain
composites that CVI SiC composites had curves for MI SIC and CVI SIC
almost threetimes the numberofcracks of composites.
4OO
40
,, MI(did not fail in gag_ .... .o CV] Std. :::....
...-3.;_/
soo ..=:::;.._
2J
/ /_. d/t'./" 1_ ,,-'_/"'.-/.:_,_.-'"
"-l//,;s:d-'"
/ +,"/ 100 " / _'/_"""
co,.i ,__//_
i_.'/_ "_ ............................
0 .,. 4 .... , ,i,..i .... L....
• I/'_'O[ t O.2 O`S O.4 O.S O`S O.7 0.0 0 0.1 01 0.3 0.4 0.0 0.1 O.T 0.0 0.0
Strain, % Straht, %
(a) (b)
Figure 3: Monotonic and unload/reload hysteresis tensile stress-strain curves for
MI (a) and CVI (b) composites.
MI SiC (Table II). 0._IS
The modal AE analysis was
0.04 .
usedtodiscernthe locationofevents
O.03S
which occurred inthe gage sectionof
the composites[lq. This isan important i0.0+25
factor todetermine because thenumber
0.02
of events recorded for the monotonic
0.045
experiments was ~ 7000 and 9000 for 0.01
the MI and CVI SiC composites, 0.006
respectively, whereas only- 1500 and
1250 events actually occurred inthe
ComlpOelte Stress, MPa
gage sectionfor thetwo composites, Figure 4: Hysterssis loop width for the
respectively. Modal AE analysis, as two composites versus the peak stress of
the hysteresis loop.
4
opposed to traditional AE", involvesthedigitizationofthe realAE waveforms andanalysis
ofthe "modes", i.e. extensional (longitudinal)andflexural, of soundtravelthat make up
the individualwaveforms.
Inordertodetermine the location ofthe events, the change inthe speed ofsound
must be knownfor the stress-straincondition. This was done, as described inref. 11, for
the extensional wave from AE eventswhich occurred outside ofthetwo sensors during
the tensiletest. Ifthespeed ofsoundisknown as afunction ofstrain,then, the elastic
modulus asafunction ofstrain can bedetermined. The speed of soundofthe
extensional wave, C,, isrelated toelasticmodulus, Eo,by:
1 'lh_ • normce(al)
C. =[Eo/ p (1-v2)]lr2 (1)
• 0.9 O norm Eo (MI)
t.)
where p isthe densityand v is 0.8 _rQ • norm, Co (CVI)
Poisson's ratio. The Eoltzata 001_ I (_ • a norm Eo (CVI)
givenstrain can be normalized _,,o 0.7
II ° goo
byEo1_ats=0 and Ceata _ 0.6
o
given straincan be normalized z 0.5
by C, ats =0. The normalized
0.4 ] _ t I I
Eolr2andnormalized C,1r2
0 0.2 0.4 0.6 0.8 1
shouldhave the same
relationshipas shownin Strain,%
Figure 5for the unload/reload
hysteresis tensile testswhere Figure 5: Normalized Eo'rz,{Eo(s)lEo(c=O)}'=, and
Eowas determined from the normalized C., C.(s)IC.(s=O), plotted versus
tangent modulus upon initial composite strain for the two composites from
unloadingofthe hysteresis unload/reload hysteresis tensile experiments.
loopt6].There isexcellent agreement between the reductionin modulusand the reduction
inthe speed ofsound for bothcomposites. Infact the modulusand speed ofsound
decreases are the same relative tothe initialmodulus (Table II)and speed ofsoundofthe
undamaged material for bothcomposites, respectively. This isnot toosurprisingsince
both compositesappear tohave reached ornearly reached matrixcrack saturation and
bothcomposites failedat similarstress levelswhichwould result ina similaroverallstress
distributionon the fibersoverthe entire gage length(assuming aconstant interfacial
shear strength).
The specificgage events were identifiedfrom (i) the difference intime ofarrival
between the two sensors (Figure 1)for agiven event and (ii)the change inthe speed of
soundasthe damage inthe material increases[11].Figure 6 showsthe AE data for thetwo
composites tested monotonicallyintensiontofailure. The AE data showthe same
general trend for thetwo composites. Initially,rapidAE activity isoccurring uptothe
"knee" inthe stress straincurve. The rate ofAE activitythen decreases substantially.
Forthe CVI SiC composite, an increase inthe rate ofAE activityoccurredjustpriorto
"In traditional AE, the real sound wave is fittoa damped sine curve and AE parameters such as
counts, amplitude, duration, energy, etc.., are tabulated and saved. Resonant frequency
transducers are used withtraditionalAE which cannot detect the wide frequency range of the real
waveform thatisproduced by a fractureevent. See references 7 and 11forgreater detail
concerning the differences between the two types of AE analysis.
failure; whereas thisdid nothappen for the MI material. TheonsetofAE activityinthe
gage sectionoccurred atlower stresses andstrainsfor the CVI composite(Table II).
However, thetotal amount ofAE events recorded was greater for the MI composite. It
shouldalsobe notedthat the =loudest"orhighestenergy events occur predominantlyat
the lower strainswhere the rate ofAE activitywas mostrapid["].
Table I1: Results from Tensile Tests
Composite Ultimate Elastic 1"tAE 1't0° Cracking Avg. Avg.
[#tests] Strength Modulus Noise Stress (MPa) Crack
I[
(MPa) (GPa) Stress Spacing (MPa)
(MPa) [LCF stress]# (mm)
CVl Std. 370 + 5 195+5 55 - 110 0.34 36+8
[2m, 2h] [103]
MI* >390_+5 214+6 74 ~ 140 0.87 11+3
[4m, 2h] [143]
#LowCycle Fatigue (2hrhold, unload/reload cycle) runout stress(500 hours)
*did notfail ingage section
The tapering offoftheAE activityis _.
mostlikelydueto thedecrease inmatrix =.
_°°,-°°
crackingormatrixcrack saturation (- 0.35% _,
strain). The inflectioninthe stress-strain | °°°°'*_ _ Std
curveisalsoindicativeof matrixcrack == ooo..Oo-' "=!
saturation. The occurrence ofAE activity |
i
nearthe ultimatestrength for theCVI
material isprobablydue tothe initiationof
fiberfailures inthe gage sectionm]. This did ' ..... ,....... .i .... J,,, o
o.1 o.,1 o.,1 0.4 o.S o.$ o.7 o.II o.il
nothappen for the MI material because itdid 81nd..%
notfail inthegage. Figure 6:AE activity for monotonic
Also notedon Figure6 isthe point stress strain curves of two composites.
on the stress-strainandAE event-strain
curveswhere theonsetof0°crackingwas determined (Figure 4). Note that ~350 AE
events occurred priortothisstressinthe CVI material and~200 AE events occurred
priortothisstress inthe MI material. Forthe CVI material,the number ofAE events
occurringpriortothe onset of0°cracks accounts for ove_25% ofthe total numberofAE
events occurringinthe gagesection. Evidently,a significantnumberofmicrocracks in
the matrixand900bundles were formed inthese materials priortotheextension or
formation ofcracks which significantlyintersectthe load-nearing fibers.
The gage sectionsofthe failed composites were cutand polishedlongitudinallyin
thegage section todetermine theamount andextent ofcracking(Figures 7and 8). For
bothsystems, each crack couldessentially be tracedcor_pletelythrough the composite
from one edge orsideofthecomposite tothe other. The cracksappeared to be
transverse withinaply,but could deviate upordown between plies. The numberof
crackswere counted over the lengthofthe gage section.ca;ndthe average crack spacing
was found tobe 0.34 mm and0.87 mmfor the CVI and MI composites, respectively. In
6
otherwords,nearlythreetimesasmanycracksoccurredinthegagesectionfor the CVI
composites compared tothe MI composites.
The most striking
difference between the two
composites isthe porosityofthe
CVI matrixcomposite. Inthe CVI
composites, similartorefs. 1and2,
crackingappeared toemanate
primarilyfrom the large,jagged
regions (figure7b) ofthe pores
whichexisted between 90°tows
andin-between pliesoron the
surface atthe intersectionofa 0
and90°tow. Forthe MI
composites, suchpores andjagged
regionsdidnot existand itwas
impossibletodiscernwhere the
originofthe cracks occurred.
Presumablythey occurredat
surface flaws, large matrixregions,
and/or inthe 90° bundles.
The interfacial shear (a) (b)
stress,_,was determined from the Figure 7: Lower magnification (a) and higher
hysteresis loopwidth,6s,,=, and magnification (b) optical micrographs of polished
average matrixcrack spacing, longitudinal section of CVI composite tested intension
to failure. Arrows indicate cracks.
dc,fOrthe composites from the
relationshipl6]:
= (Gp=/28s,,=x[)bz(1-alv_)=(RJdc)/{4v_E_](2)
where, apisthe hysteresis loop peak stress,b=andat are coefficientsgiven by
HutchinsonandJensen112]R,fisthe average fiber diameter (13 microns), and Emisthe
elasticmodulus ofthe matrix(400 GPa). Inordertoapproximate dcfor each hysteresis
loop,the average crack spacing was assumed tobe indirectlyproportionalto theAE
activity(similarto Figure 6). Forthe apof agivenhysteresis loop,the finaldcmeasured
afterfracture was multipliedby the finalnumberofAE events inthegage anddivided by
the numberofAE events which occurred inthe gage up toGp. The larger86.==anddefor
the MI composites resulted ina• value lessthan one thirdthat ofthe CVI composites.
Thiscouldbe dueto differingresidual stressstatesfrom the different matrixprocessing
routesused tofabricate the composites. However, itis mostlikelythat thedifference in
was due totheformation ofa carbon layerbetween the fibers andthe BN interphase in
the MI composites. The processingtemperature for MI SiC isat leastthat ofthe melting
temperature of Si(~ 1410°C). The Hi-NicalonTMfibers undergosome decomposition
above about 1400°C which results intheformation ofa carbonaceous layeratthe surface
ofthefiber_131T.ransmission electron microscopystudies on similarcomposites have
shownthata thincarbon layerdoes existbetween the fiberand theBN for Hi-
7
Nicalon_/BN/MSIiCcompositeasndhasnot been observedfor Hi-Nicalon_U/BN/CVI SiC
compositesI"1.
The nature ofdamage
accumulation isexpected to
influence theelevated temperature
lifepropertiesofCMC's. Some
data existszls-_6f)or low-cycle
fatigue, LCF, tests performed on
these same types ofcompositesat
815°(3 inair. The LCF cycle
consisted ofa loadingstep tothe
peak stress,a2 hour holdatthe
peak stressfollowed byan
unloadingstep down toaminimum
stress value. The run-outcondition
for these experiments was setat
250 cycles(500 hoursatthe peak
stress). Itwas found that the run-
out stressesfor the CVI and MI
compositeswere 103 MPa and 143 (a) (b)
MPa respectively. These are Figure 8: Lower magnification (a) and higher
nearly identicalwiththe onset of0° magnification (b) optical micrographs of polished
crackingasdetermined from the longitudinal section of MI composite tested in
hysteresistensile tests (Table II). tension to failure. Arrows indicate cracks.
Evidently,for thiscomposite life
criteria,the stresswhere the onsetof crackingoccurswas notlifelimiting. Instead, itwas
the stresswhere crack growthorcrack formation resultedinthe intersectionofthe load-
bearing fibers. The LCF peak stress was about twicethe:stress atwhich crackingwas
firstdetected inthegage sectionofthese composites. Tqisofcourse assumes thatthe
onsetofcrackingat815°C isthe same asat roomtemperature. Lipetzkyetaitl°] have
shown thatfor aNicalon-CVI SiC composite themodulus,proportionallimitanddamage
regime slopeofthe tensile stress-straincurve upto 1200°C isessentially unchanged.
Since these properties do notchange itisreasonable toassume thatthe onsetof
cracking isessentiallythe same overthistemperature ra_lge.
CONCLUSION
The mechanical propertiesanddamage accumU,ationofHi-Nicalon, BN
interphase woven composites withCVI SiCand MI SiC rrlatriceswere determined and
compared. The ultimateand elasticpropertiesofthe twocompositeswere similar;
however, the onsetofcrackingoccurred atlower stresses andstrainsfor the CVI
compositesystem. This wasdue tothe large,jagged poreswhichexist inthe CVI SiC
matrixandwere the site ofinitialcrackingfor these composites. MI SiC matrix
composites didnot have such pores. Modal acousticemissionwas found tobe auseful
techniqueto monitortheonset andcontinuum ofdamage which occurredinthegage
section ofthe tensilespecimens tested. The decrease inthe speed ofsoundas
increasingdamage occurredwithincreasingstress couldbe usedtodetermine the elastic
moduluswithincreasingdamage accumulation.
Fewer cracks occurred inthe gage sectionfor MI compositesdueto the lower
interfacialshear strengthofthese composites. Itwaspresumed thatthe lower interfacial
strengthofthe MI composites was duetothe formation ofacarbon layer atthefiber
matrix interphasedue tothe decompositionofthe fibersduringprocessing.
Finally,the stresses for bothcomposite systemswhere cracks intersectedthe
load-bearing bundles (0°cracking) couldbecorrelated tothe life-limitingstresses from
8
elevatedtemperaturleow cycle fatiguetests performed inairat 815°C; even though
crackinghad occurred inthe compositesatstresses whichwere halfthe stressfor 0°
cracking.
ACKNOWLEDGEMENT
This work was supported by the HITEMP program at NASA Lewis Research
Center, Cleveland, OH.
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