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Effects of temperature and of oxidation on the interfacial shear stress between fibres and matrix in ceramic-matrix composites
Pergamon PII:
Acta mafer. Vol. 46, No. 7, pp. 2461-2469, 1998 c 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 S1359-6454(97)00404-7
EFFECTS OF TEMPERATURE AND OF OXIDATION ON THE INTERFACIAL SHEAR STRESS BETWEEN FIBRES AND MATRIX IN CERAMIC-MATRIX COMPOSITES P. REYNAUD,
D. ROUBY and G. FANTOZZI
d’Etudes de Mktallurgie Physique et de Physique des Mattriaux G.E.M.P.P.M. UMR CNRS No. 5510, Institut National des Sciences Appliquies de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne cedex, France
Groupe
Abstract-Under cyclic loading, the mechanical behavior of ceramic matrix composites (Sic/Sic, Sic/ MAS-L) changes with the number of applied cycles, as shown by life-time diagrams and shape evolutions of stress/strain loops. According to these observations, a shear-lag model has been developed where the cyclic fatigue effect is attributed to an interfacial wear between fibres and matrix. At high temperature under inert atmosphere, since physical and chemical changes are inhibited, the main effect of temperature on cyclic fatigue of ceramic matrix composites is the release of radial thermal residual stresses. But under vacuum when the temperature is high, fibre/matrix interfaces can be removed due to chemical instabilities. Hence, after ageing under vacuum at high temperature, cyclic fatigue at room temperature of Sic/Sic composites exhibits an increase followed by a decrease in mechanical hysteresis. This can be explained by a decrease in the interfacial shear stress due to the previous heating at high temperature under vacuum. For such treated composites, an original stiffening effect is also observed during cyclic fatigue. This original phenomenon is attributed to a contribution of cracks in transversal yarns. 0 1998 Acta A4etallurgica Inc. R&um&Sous sollicitation cyclique le comportement mCcanique des composites B matrice cCramique (Sic/ Sic, SiC/MAS-L) t-value avec le nombre de cycles appliquks comme le montrent les diagrammes d’endurante et la variation de la forme des boucles contrainte/dkformation. En accord avec ces observations, un modkle micromtcanique de transfert de charge a &t&d&elopp& dans lequel l’effet de fatigue cyclique a &tk attribut $ une usure progressive des interfaces fibre/matrice. A hautes temptratures sous atmosphere inerte, tant que les constituants sont physiquement et chimiquement stables, l’effet principal de la temptrature sur le comportement en fatigue cyclique des composites $ matrice ciramique correspond g la relaxation des contraintes thermiques rtsiduelles. Cependant, sous vide, dts que la temptrature est suffisante pour rendre le composite chimiquement instable, les interfaces entre fibres et matrice peuvent &tre &limin&es. Dans ce cas, un traitement prkliminaire sous vide i chaud d’un composite Sic/Sic modifie son comportement en fatigue cyclique car l’hystirksis mtcanique augmente puis diminue en fonction du nombre de cycles. Ce phinomtne peut s’expliquer par le fait que le traitement B chaud sous vide provoque une diminution de la contrainte de cisaillement interfacial entre fibres et matrice. Lorsque les composites ont subi ce traitement thermique sous vide, leur comportement en fatigue cyclique montre kgalement que le module klastique moyen augmente avec le nombre de cycles appliquks. Ce phknomine original a &t&attribut g un effet 1% & la fissuration des torons transversaux. 0 1998 Acta Metallurgica Inc.
1. INTRODUCTION In
long-fibre-reinforced
ceramic-matrix
(fibre, interfacial
composites,
is not only controlled by the individual failure of the constituents (fibres and matrix), but also by the interaction between these constituents. That is the reason why such materials exhibit a non-brittle mechanical behavior, even if the constituents are themselves brittle. This phenomenon is due to the interfaces between the constituents, and for example to the friction between fibres and surrounding matrix during the sliding of the fibre in this matrix [l]. In composites with a simple reinforcement arrangement (unidirectional or bidirectional composites), the mechanical hysteresis measured during a loading-unloading sequence is dependent on the fibre/ matrix interfacial shear stress [l-l 11. Hence, changes in the microstructure of the constituents the
macroscopic
mechanical
behavior
matrix, shear
interfaces) stress,
esis can be sensitive
may and
the
modify mechanical
the
actual hyster-
to these modifications.
The purpose of this paper is to present a micromechanical analysis of the mechanical hysteresis during cyclic fatigue in ceramic matrix composites, in order to underscore the effects of the microstructural evolutions induced by a cyclic loading on the macroscopic mechanical behavior, and to point out the influence of the testing environment (temperature and atmosphere) on this behavior. It can be seen that cyclic fatigue, testing temperature and heat treatment at high temperature under vacuum are different routes to modify fibre/matrix interfaces, and that calculation of the relation between mechanical hysteresis and interfacial shear stress can explain the evolutions of the mechanical hysteresis with the number of cycles which are observed
246 1
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INTERFACIAL
SHEAR STRESS IN CERAMIC
during cyclic fatigue in the case of various materials and testing conditions. The experimental work was carried out on two ceramic matrix composites: (i) a cross-weave Sic/ SIC composite unprotected against oxidation (twodimensional Sic/Sic GS4C) provided by the Societe Europeenne de Propulsion (France), and (ii) a cross-ply [0, 901s SiC/MAS-L laminate (12 plies) provided by Aerospatiale (France). Both composites have been studied within the framework of the scientific association entitled: “Thermomechanical behavior of fibrous ceramic-ceramic composites”.
2. MICRO MECHANICAL
MODELLING OF FATIGUE HYSTERESIS
Modelling the macroscopic mechanical behavior of a material by a micro-mechanical approach implies one must build up a schematic description of its microstructure in order to identify and separate all the mechanisms involved. A previous attempt in this field was made on unidirectional SiC/Si3N4 composites [2], in order to describe the microstructural origin of hysteresis under cyclic fatigue. In this model, the composite damaged by fatigue is assumed to be a serial set of damaged zones (with a matrix crack bridged by broken and surviving fibres) and undamaged zones. Under cycling, the mechanical hysteresis increases as the fraction of broken fibres increases due to cyclic fatigue. This model shows that the mechanical hysteresis is narrow if the interfacial shear stress is very low or very high, and that the mechanical hysteresis is maximum for intermediate levels of the interfacial shear stress. In cross-weave Sic/Sic composites and cross-ply laminates SiC/MAS-L, fibres are set in two perpendicular directions. Hence, the first hypothesis needed to describe the mechanical behavior of these composites is to separate the effect of longitudinal fibers from the effect of transversal fibres [&ll]. In longitudinal yarns the loading is parallel to the fibres, and the behavior of these yarns is like an unidirectional composite with a brittle matrix and parallel fibers [5,6]. During a cyclic fatigue test, if the maximum stress applied is higher than the stress at which the first crack appears in the matrix, multi-cracking of the matrix occurs in transversal and longitudinal yarns, and can be assumed achieved at the end of the first loading-unloading cycle. Then, all further damage in longitudinal yarns due to cyclic fatigue will appear mainly on the fraction of broken fibres in the vicinity of a matrix crack, and therefore on the shape of the stress profile of surviving fibres. During cycling, all the surviving fibers are sliding in the matrix with an alternate slip due to the repeat of loading-unloading steps. Hence this alternate sliding creates at the interface a progressive wear of the fibre and of the matrix [3-l 11.
AOf
Maximum
I i-
II-
MATRIX
loaded
neighbouring
Lm1oaaea
Fig. 1. Theoretical evolution analysis) of the stress profile vicinity of matrix cracks sequence - effect of the
COMPOSITES
composite
matrix
cracks
i
cozuposite (established from a shear-lag (udx)) along the fibers in the during a loading/unloading interfacial shear stress (r).
The analysis of the stress profile along a surviving fibre (see Fig. l), and its evolution during a loading-unloading cycle allows an analytical calculation of the stress-strain loops [l l] that can point out the dependence of the mechanical hysteresis on the interfacial shear stress in a ceramic-matrix composite damaged by cyclic fatigue. For a unidirectional composite, the results of the analytical calculation of the mechanical hysteresis (A W/We) as a function of the interfacial shear stress (r) are [ll]: if :>l,
(1)
where W, =$Y2/Ex, c( = E,v,,,/Epf, and 7* = arE$/ 2dE,, with S: the maximum stress applied, r: the radius of the fibres, d: the mean distance between two neighboring matrix cracks, Ef, vf, E, and v,: the Young’s modulus and the volumic fraction of fibre and of matrix, respectively, and E,: the mean elastic modulus of the composite (Ex = Epf+ Emv,). The parameter r* corresponds to the limit value of interfacial shear stress between a local and a total sliding of the fibres. This specific value is not only dependent on the characteristics of the material but is also dependent on the amplitude of the mechanical loading (S). The theoretical calculation of the stress/strain loop area relative to the elastic strain energy (AW/ We) in terms of the interfacial shear stress (T), shows (see Fig. 2) that as r increases from zero to infinity, the mechanical hysteresis A W/We increases from zero to a maximum value (maximum which has no specific physical meaning), then decreases and tends to zero as 7 increases to infinity. This relation comes from the fact that one of the two
REYNAUD
Mechanical
et al.:
INTERFACIAL
hysteresis
SHEAR
STRESS
(Aw/We)
OS4 -
0.3
--
ona
--
O,l--
04
Interfacial
I/
!
0
1
stress
shear
(z/2*) I
2
3
4
Fig. 2. Theoretical effect of the interfacial shear stress (5) on the mechanical hysteresis (A IV) of long-fibre-reinforced ceramic-matrix composite calculated from a shear-lag model (with IV,: the elastic strain energy, and z*: the limit value of interfacial shear stress between a local and a total sliding of the fibers).
following situations occurs in a brittle-matrix composite (see Figs 1 and 2): - if the interfacial shear stress (7) is high (r 2 r*), the size of the fibre sliding zone in the vicinity of a matrix crack is lower than the distance between two matrix cracks (case of 2D Sic/Sic composite). In that case, the hysteresis increases hyperbolically as the interfacial shear stress (z) decreases due for example to fatigue (local sliding of fibers); - if the interfacial shear stress (7) is low (z I z*), the sliding of the fibers in the matrix occurs along all the fibres (case of SiC/MAS-L [0, gOIs). In that case, the mechanical hysteresis depends parabolically on the interfacial shear stress (T), and if the running point (A IV, z) is on the left side of the maximum of the curve, the mechanical hysteresis decreases as the shear stress (z) decreases (total sliding of fibres). This analytical calculation is consistent with the results obtained on unidirectional composites by Kotil et al. [2].
3. EFFECTS
OF TEMPERATURE
Temperature effects on the cyclic fatigue behavior of ceramic matrix composites are not well known at present time. Nevertheless, since the temperature of the composite is low enough to keep the material physically and chemically inert, the predominant mechanism of cyclic fatigue is interfacial wear, plus a dependence of the testing temperature on the thermal residual stresses. But, when the temperature is high enough to enable structural changes in the components (evolution of fibres or matrix), new mechanisms (possibly depending on time) occur and possibly hide the effects of the interfacial wear observed at lower temperatures.
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Just after processing, the initial interfacial shear stress is the sum of all the interactions found in the interfaces. All these interactions can be classified as being one of two types: (i) short range interactions, and (ii) long range interactions. During a cyclic fatigue test at a given temperature under inert atmosphere, the short range interactions are progressively removed by a progressive wear of the fibre/matrix interfaces, and therefore the interfacial shear stress decreases from an initial value (7,J to a non zero limit value (7,). On the other hand, the long range interactions remain constant during a cyclic fatigue test, and only the thermal residual stresses are modified by the temperature of the cyclic fatigue test. Effectively, because thermal residual stresses are due to the mismatch between the thermal expansion of the fibres and of the matrix, these stresses are released until the testing temperature (7’) reaches the processing temperature (7’s). The stresses increase as long as the testing temperature is higher than the processing temperature. This effect acts on the contribution of the radial thermal residual stresses on the current level of the interfacial shear stress. According to the relative value of radial thermal expansion coefficients of the fibre and of the matrix, two cases have to be analyzed. If the radial thermal expansion coefficient of the matrix is higher than the coefficient of the fibres (case of two-dimensional Sic/Sic composite), at a testing temperature (7’) lower than the processing temperature (To), the radial thermal residual stresses are compressive. Assuming that between fibre and matrix a Coulombian friction is effective, the interfacial shear stress (T.,) due to the long range interactions is given by: 7 00 =
7LR
+p
Ic(f - &&To - T) A
(3)
with p: the friction coefficient, c(, and nr: the radial thermal expansion coefficients of the matrix and the fibers, respectively, T: the testing temperature, To: the processing temperature, A: a constant depending on the elastic characteristics of matrix and fibres [12], and rta: the effects of all the long range interactions other than the radial thermal expansion effect. In this case, the interfacial shear stress decreases as the testing temperature increases up to the processing temperature. On the other hand (as in the case of the Sic/ MAS-L composite) the thermal residual stresses are tensile. After fibre-matrix debonding, a gap appears between the fibre and the matrix, this gap (6) is given by: 6 = r(cq - cc,)(T(J - T)
(4)
with r: the radius of the fibres, tl, and af: the radial thermal expansion coefficients of respectively the matrix and the fibres, T: the testing temperature, and To: the processing temperature. This gap
2464
REYNAUD et al.:
INTERFACIAL
Mechanical
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
Hysteresis(kPa)
30
--1ooooc -8OOOC
25
-6OOOC
Number 0 lE+OO
I I lE+Ol
, I lE+02
1 1 1E+03
of cycles I I
I I lE+04
lE+05
I lE+06
Fig. 3. Experimental effect of temperature on the evolution of mechanical hysteresis during cyclic fatigue of two-dimensional Sic/Sic GS4C composite under inert atmosphere.
decreases as the testing temperature increases. Therefore, because of the roughness of fibres and of matrix, the interfacial shear stress increases when the testing temperature increases. According to the dependence of the mechanical hysteresis on the interfacial shear stress, the effects of the testing temperature on the interfacial shear stress should be observed also on the mechanical hysteresis. In order to confirm this hypothesis, fatigue tests have been conducted on two-dimensional Sic/Sic composites and on SiC/MAS-L [0, 901s
Mechanical
composites with the same mechanical loading (2130 MPa for Sic/Sic and O/l10 MPa for Sic/ MAS-L, 1 Hz) but at various temperatures and under inert atmosphere (argon). These experiments (see Figs 3 and 4) exhibit the evolutions of the mechanical hysteresis in relation to the number of cycles applied for both material at various temperatures (from 600°C to 1000°C). It can be seen that the mechanical hysteresis increases with the number of cycles applied for the 2D Sic/Sic composite, and decreases for the SiC/MAS-L composite. It can
hysteresis
(kPa)
40 -1ooooc
35
-8OOOC 30
-6OOOC
25
lE+OO
lE+Ol
1E+O2
lE+03
lE+04
Number
lE+05
lE+06
of cycles
Fig. 4. Experimental effect of temperature on the evolution of mechanical hysteresis during cyclic fatigue of two-dimensional SiC/MAS-L composite under inert atmosphere.
REYNAUD et al.:
Mechanical
INTERFACIAL
hysteresis
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
From this theoretical analysis, it can be seen that the main behavior of ceramic-matrix composites observed during cyclic fatigue at high temperature under inert atmosphere can be explained by a combination of (i) a progressive interfacial wear due to the cyclic loading at constant amplitude, and (ii) the effect of the actual radial residual thermal stresses on the interfacial shear stress, residual stresses which are obviously dependent on the testing temperature.
(AW/W,)
0,4
T 0.3
0.2
II
T3 SiC!/MAS-L T2
0.1
-
VT1
l-3
2D
SiC/SiC
T2 T,
Interfacial shear
stress
(z/Z')
I
0
1
a
3
2465
4
Fig. 5. Theoretical effect of interfacial wear during cyclic fatigue and of radial residual thermal stresses on hysteresis of stress/strain loops in long-fibre-reinforced ceramic-matrix composites.
be also observed that these increases and decreases are more important at high temperature than at low temperature. In fact, in the two-dimensional Sic/Sic composite the initial interfacial shear stress (ra) is high, hence the mechanical hysteresis is low according to the (A W/ We, 7/7*) diagram (see Fig. 5), and during a cyclic fatigue test at constant amplitude the mechanical hysteresis increases with the number of cycles applied due to the interfacial wear. In addition, when the testing temperature is high, the initial interfacial shear stress is lower than at low temperature due to the release of the radial thermal residual stresses. Hence, at high temperature the mechanical hysteresis is higher than at low temperature, and at high temperature the increase of the mechanical hysteresis during cyclic fatigue is also more important than at low temperature (see Figs 3 and 5) On the other hand, in SiC/MAS-L composite the initial interfacial shear stress (ra) is very low. Hence, the associate mechanical hysteresis is very low according to the (AW/W,, t/z*) diagram (see Fig. 5). Moreover, during a cyclic fatigue test at constant amplitude the mechanical hysteresis decreases with the number of cycles applied due to the interfacial wear. This is because for this material the characteristic point (A W/ We, r/7*) is placed on the left side of the theoretical (AW/W,, z/z*) diagram. In addition, in this material, when the testing temperature is high, the initial interfacial shear stress is higher than at low temperature due to the release of the radial thermal residual stresses. Therefore, because the characteristic point is running on the left side of the (A W/ We, z/z*) diagram, at high temperature the mechanical hysteresis is higher than at low temperature, and at high temperature the decrease of the mechanical hysteresis during a cyclic fatigue test is more important than at low temperature (see Figs 4 and 5) because the interfacial wear is more important.
4. CYCLICFATIGUEATROOMTEMPERATURE AFTERAGEINGATHIGHTEMPERATUREUNDER VACUUM In unidirectional and bidirectional composites, fibre/matrix interfacial shear stress and macroscopic mechanical hysteresis during a loading-unloading sequence are directly related (see Fig. 2). Hence, any microstructural change in fibres, matrix or interfaces leads to a modification of the interfacial shear stress, and these changes are exhibited by the mechanical hysteresis. That is the reason why the effects of a progressive wear of the interface during cyclic fatigue, or the effects of temperature on the radial residual thermal stresses can be observed from the evolutions of the mechanical hysteresis. However, another way to modify the level of the interfacial shear stress is to modify the microstructure of fibres, matrix or interfaces by the activation of chemical instabilities in the materials. This is, for example, possible by heating under vacuum an as received composite before running a cyclic fatigue test. That operation initiates a slight chemical degradation of the fibre-matrix interface, and leads to a small decrease of the interfacial shear stress [13,14]. These experiments have been conducted on the two-dimensional Sic/Sic GS4C composite, by heating the specimens 50 h under low vacuum at a constant temperature (from 800°C to 1OOOC). Then each specimen had been subjected to a cyclic tension/compression loading (+130 MPa, 1 Hz) at room temperature under air. The mechanical behaviors obtained (see for example Fig. 7) are compared to the behavior obtained by the same loading (+130 MPa, 1 Hz) of a pristine composite (see Fig. 6). For the as received material (see Fig. 6) it can be observed that: (i) the mechanical hysteresis increases continuously during the cyclic fatigue test (see Figs 6 and 8), and (ii) the tensile mean elastic modulus decreases monotonically whereas the compressive mean elastic modulus remains constant (see Figs 6, 10 and 11). On the other hand, when the composite is subjected to a previous heating under vacuum before running a cyclic fatigue test at room temperature, the mechanical behavior during cyclic fatigue (see Fig. 7) exhibit: (i) an increase followed by a decrease of the mechanical hysteresis (see Figs 7
2466
REYNAUD et al.:
INTERFACIAL
150~
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
Stress(WPa)
Strain(%)
Fig. 6. Experimental evolution of stress/strain loops under tension/compression cyclic loading at room temperature of pristine two-dimensional Sic/Sic GS4C composite (run out after 500000 cycles, +130 MPa, 1 Hz).
and 8) and (ii) an original increase of the tensile mean elastic modulus during cyclic fatigue (see Figs 7 and lo), where the tensile mean elastic modulus reaches the value of the compressive mean elastic modulus (see Fig. 11). Concerning the evolution of the mechanical hysteresis during cyclic fatigue of a previously-heated composite, the (AWjW,, T/T*) diagram can explain (see Fig. 9) why the mechanical hysteresis goes to a maximum value then decreases when the number of cycles applied increases. This phenomenon is due to
the fact that the previous heating under vacuum of the composite lowers the initial interfacial shear stress (re) due to chemical reactions in fibre/matrix interfaces (may be a small diffusion of the surrounding residual oxygen of Nicalon NLM202 SIC fibers to the interfacial pyrocarbon layer [13, 141). Hence, as shown by the (AW/W,,z/z*) diagram (see Fig. 9), at the beginning of the cyclic fatigue test, because the initial interfacial shear stress of previously-heated composites is lower than for nonheated composites, the mechanical hysteresis is
150 Stress(MPa)
T
I
-0,l
-
Fig. 7. Experimental evolution of stress/strain loops under tension/compression temperature
cyclic loading at room of two-dimensional Sic/Sic GS4C composite after a previous ageing of 50 h under vacuum at 800°C (run out after 500 000 cycles, +130 MPa, 1 Hz).
REYNAUD et al.:
INTERFACIAL
Mechanical
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
hysteresis
-.1ooooc
lE+OO
lE+Ol
-9oooc
lE+02
(kPa) -8OOOC
lE+03
2467
lE+04 Number
-- 23OC
1E+05
lE+06
of cycles
Fig. 8. Experimental evolutions of mechanical hysteresis during cyclic fatigue at constant amplitude of two-dimensional Sic/Sic GS4C previously aged 50 h at various temperatures under vacuum.
higher for previously-heated composites than for non-heated composites. Moreover, for previouslyheated composites the starting point on the (AW/ We, t/z*) diagram is closer to the maximum of the (A W/W,, s/a*) curve than for non-heated composites. Hence, because during cyclic fatigue the interfacial shear stress decreases due to the interfacial wear, the mechanical hysteresis increases enough to reach the maximum value of the (AW/ We, 7/7*) curve and then decreases according to this curve. On the other hand, concerning the stiffening effect observed during cyclic fatigue of previously-
ye4chanical hysteresis (AW/W,) I 2D SiC/SiC after under
O-3
ageing vac!Ilum
n
Bear
:::i/
stress
(7/T*)
0
I 0
1
2
3
heated composites, the origin of this unusual phenomenon is not well established yet. The experimental measurement of the tensile mean elastic modulus exhibits that this modulus increases up to the value of the compressive mean elastic modulus (see Figs 10 and 11). Moreover the theoretical calculation of the tensile mean elastic modulus [l I] indicates that this modulus decreases when the interfacial shear stress decreases due to the interfacial wear. Hence, the stiffening effect can not be attributed to the longitudinal yarns. But, the twodimensional Sic/Sic composite is a bidirectional composite, and this stiffening effect can be rather attributed to a physical phenomenon in transversal yarns. For example, during cycling, the cracks in transversal yarns are opened during the loading step, and closed during the unloading step. Then, if the opening of the crack under the maximum loading is high, a bad closure of these cracks during cycling may occur. This hypothesis is indeed consistent with the fact that (i) the tensile mean elastic modulus increases and reaches the compressive mean elastic modulus, (ii) the residual strain increases with the number of cycles applied, and (iii) the evolution of the mechanical hysteresis of the composite during cyclic fatigue is mainly due to the interfacial wear of longitudinal fibres. At present time, this hypothesis is just proposed, and further investigations are needed to confirm.
4
Fig. 9. Theoretical effect of interfacial removal due to ageing at 800°C under vacuum on the evolution of hysteresis of stress/strain loops in long-fibre-reinforced ceramic-matrix composites during a cyclic fatigue test at constant amplitude.
5. CONCLUSIONS Studies at room temperature of the mechanical behavior of ceramic matrix composites under cyclic
2468
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et al.:
INTERFACIAL
SHEAR
STRESS
IN CERAMIC
Mean elastic modulus
MATRIX
COMPOSITES
1E+05
1E+06
(GPa)
250
200
150
100
50 lE+OO
lE+Ol
lE+02
1E+03 lE+04 Nknber
of
cycles
Fig. 10. Experimental evolutions of mean tensile (&) and compressive (EC) elastic moduli during cyclic fatigue at constant amplitude of two-dimensional Sic/Sic GS4C previously aged 50 h at various temperatures under vacuum.
fatigue pointed out a major effect of a progressive wear at the fibre/matrix interfaces in longitudinal yarns due to repeated loading/unloading sequences. A preliminary and qualitative approach of cyclic fatigue at high temperatures indicates that this interfacial wear is mainly modified by the dependence of the radial thermal residual stresses with the testing temperature. All these phenomena have been observed from the evolutions of the mechanical hysteresis in relation to the number of cycles
Normalized 1
T
applied. Another way to modify the interfacial properties is to activate some chemical instabilities. For example, heating under vacuum leads to a decrease of the interfacial shear stress that leads to an increase until a maximum value then a decrease of the mechanical hysteresis. This treatment leads also to an original stiffening of the composite during cyclic fatigue. This new phenomenon is not well understood at present time, and further investigations are still required.
elastic modulus
-lOOO°C
-900°C
-800°C
(Et/E,) -- 23OC
0,s 018 Of? On6 0,5 0,4 lE+OO
lE+Ol
lE+02
1E+03
lE+04
Number
lE+05
lE+06
of cycles
Fig. 11, Experimental evolutions of the ratio (tensile mean elastic modulus (&)/compressive tic modulus (EC)) during cyclic fatigue at constant amplitude of two-dimensional Sic/Sic viously aged 50 h at various temperatures under vacuum.
mean elasGS4C pre-
REYNAUD
et al.:
INTERFACIAL
SHEAR
Acknowledgements-This work has been carried out with the support of the Centre National de la Recherche Scientifique, the Ministere de 1’Enseignement Superieur et de la Recherche, the Direction des Recherches et Etudes Techniques, the Centre National d’Etudes Spatiales, and the companies: Aerospatiale and Societe Europeenne de within association Propulsion, the scientific “Thermomechanical behaviours of fibrous ceramicceramic composites”. Thanks are particularly due to M. Bourgeon, F. Abbe and J. P. Richard from the Societe Europeenne de Propulsion, and to P. Peres from Airospatiale, for their interest on this work and for their support.
STRESS
5. 6. I. 8, 9, 10. 11.
REFERENCES Aveston, J., Cooper, G. A. and Kelly, A., in Proceedings of Conference on the properties of jibre composites of the national physical laboratory, Vol. 4, Surrey: IPC Sci. Technol. Press, 1971, pp. 15-26. Kotil, T., Holmes, J. W. and Comninou, M., J. Am. Ceram. Sot., 1990, 73, 1879. Cho, C., Holmes, J. W. and Barber, J. R., J. Am. Ceram. Sot., 1991, 74, 2802. Holmes, J. W., in Ceramics and Ceramic Matrix Composites, ed. S. R. Levine, Vol. 3 in Flight-Vehicle Materials, Structures and Dynamics Assessment
12. 13. 14.
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and Future Directions, ASME, New York, 1992, pp. 193-238. Rouby, D. and Reynaud, P., Compos. Sci. Technol. , 1993, 48, 109. Evans, A. G. and Zok, F. W., J. Mater Sci., 1994, 29, 3857. Reynaud, P., Rouby, D. and Fantozzi, G., Ser. Metall. Mater., 1994, 31, 1061-1066. Evans, A. G., Zok, F. W. and McMeeking, R. M., Acta MetaN. Mater., 1995, 43, 859. Reynaud, P., Compos. Sci. Technol. , 1996, 56, 809. Kerans, R. J. and Parthasarathy, T. A., J. Am. Ceram. Sot., 1991, 74, 1585-1596. Reynaud, P., Etude du comportement en fatigue des materiaux composites a matrice ceramique suivi par emission acoustique, These de Doctorat, INSA de Lyon, 1992. Brun, M. K. and Singh, R. N., Adv. Ceram. Mater. , 1988, 3, 506. Abbe, F., Fluage en flexion d’un composite Sic-Sic 2D. These de Doctorat, Universite de Caen. 1990. Labrugtre, C., Influence de l’evolution physico-chimique des fibres et de la zone interfaciale fibreimatrice sur le comportement mtcanique des composites Sic/ C/Sic et SiC/MAS-L apres vieillissement thermique sous atmosphere control&e, These de Doctorat, Universite de Bordeaux I, 1993.
Acta mafer. Vol. 46, No. 7, pp. 2461-2469, 1998 c 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 S1359-6454(97)00404-7
EFFECTS OF TEMPERATURE AND OF OXIDATION ON THE INTERFACIAL SHEAR STRESS BETWEEN FIBRES AND MATRIX IN CERAMIC-MATRIX COMPOSITES P. REYNAUD,
D. ROUBY and G. FANTOZZI
d’Etudes de Mktallurgie Physique et de Physique des Mattriaux G.E.M.P.P.M. UMR CNRS No. 5510, Institut National des Sciences Appliquies de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne cedex, France
Groupe
Abstract-Under cyclic loading, the mechanical behavior of ceramic matrix composites (Sic/Sic, Sic/ MAS-L) changes with the number of applied cycles, as shown by life-time diagrams and shape evolutions of stress/strain loops. According to these observations, a shear-lag model has been developed where the cyclic fatigue effect is attributed to an interfacial wear between fibres and matrix. At high temperature under inert atmosphere, since physical and chemical changes are inhibited, the main effect of temperature on cyclic fatigue of ceramic matrix composites is the release of radial thermal residual stresses. But under vacuum when the temperature is high, fibre/matrix interfaces can be removed due to chemical instabilities. Hence, after ageing under vacuum at high temperature, cyclic fatigue at room temperature of Sic/Sic composites exhibits an increase followed by a decrease in mechanical hysteresis. This can be explained by a decrease in the interfacial shear stress due to the previous heating at high temperature under vacuum. For such treated composites, an original stiffening effect is also observed during cyclic fatigue. This original phenomenon is attributed to a contribution of cracks in transversal yarns. 0 1998 Acta A4etallurgica Inc. R&um&Sous sollicitation cyclique le comportement mCcanique des composites B matrice cCramique (Sic/ Sic, SiC/MAS-L) t-value avec le nombre de cycles appliquks comme le montrent les diagrammes d’endurante et la variation de la forme des boucles contrainte/dkformation. En accord avec ces observations, un modkle micromtcanique de transfert de charge a &t&d&elopp& dans lequel l’effet de fatigue cyclique a &tk attribut $ une usure progressive des interfaces fibre/matrice. A hautes temptratures sous atmosphere inerte, tant que les constituants sont physiquement et chimiquement stables, l’effet principal de la temptrature sur le comportement en fatigue cyclique des composites $ matrice ciramique correspond g la relaxation des contraintes thermiques rtsiduelles. Cependant, sous vide, dts que la temptrature est suffisante pour rendre le composite chimiquement instable, les interfaces entre fibres et matrice peuvent &tre &limin&es. Dans ce cas, un traitement prkliminaire sous vide i chaud d’un composite Sic/Sic modifie son comportement en fatigue cyclique car l’hystirksis mtcanique augmente puis diminue en fonction du nombre de cycles. Ce phinomtne peut s’expliquer par le fait que le traitement B chaud sous vide provoque une diminution de la contrainte de cisaillement interfacial entre fibres et matrice. Lorsque les composites ont subi ce traitement thermique sous vide, leur comportement en fatigue cyclique montre kgalement que le module klastique moyen augmente avec le nombre de cycles appliquks. Ce phknomine original a &t&attribut g un effet 1% & la fissuration des torons transversaux. 0 1998 Acta Metallurgica Inc.
1. INTRODUCTION In
long-fibre-reinforced
ceramic-matrix
(fibre, interfacial
composites,
is not only controlled by the individual failure of the constituents (fibres and matrix), but also by the interaction between these constituents. That is the reason why such materials exhibit a non-brittle mechanical behavior, even if the constituents are themselves brittle. This phenomenon is due to the interfaces between the constituents, and for example to the friction between fibres and surrounding matrix during the sliding of the fibre in this matrix [l]. In composites with a simple reinforcement arrangement (unidirectional or bidirectional composites), the mechanical hysteresis measured during a loading-unloading sequence is dependent on the fibre/ matrix interfacial shear stress [l-l 11. Hence, changes in the microstructure of the constituents the
macroscopic
mechanical
behavior
matrix, shear
interfaces) stress,
esis can be sensitive
may and
the
modify mechanical
the
actual hyster-
to these modifications.
The purpose of this paper is to present a micromechanical analysis of the mechanical hysteresis during cyclic fatigue in ceramic matrix composites, in order to underscore the effects of the microstructural evolutions induced by a cyclic loading on the macroscopic mechanical behavior, and to point out the influence of the testing environment (temperature and atmosphere) on this behavior. It can be seen that cyclic fatigue, testing temperature and heat treatment at high temperature under vacuum are different routes to modify fibre/matrix interfaces, and that calculation of the relation between mechanical hysteresis and interfacial shear stress can explain the evolutions of the mechanical hysteresis with the number of cycles which are observed
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during cyclic fatigue in the case of various materials and testing conditions. The experimental work was carried out on two ceramic matrix composites: (i) a cross-weave Sic/ SIC composite unprotected against oxidation (twodimensional Sic/Sic GS4C) provided by the Societe Europeenne de Propulsion (France), and (ii) a cross-ply [0, 901s SiC/MAS-L laminate (12 plies) provided by Aerospatiale (France). Both composites have been studied within the framework of the scientific association entitled: “Thermomechanical behavior of fibrous ceramic-ceramic composites”.
2. MICRO MECHANICAL
MODELLING OF FATIGUE HYSTERESIS
Modelling the macroscopic mechanical behavior of a material by a micro-mechanical approach implies one must build up a schematic description of its microstructure in order to identify and separate all the mechanisms involved. A previous attempt in this field was made on unidirectional SiC/Si3N4 composites [2], in order to describe the microstructural origin of hysteresis under cyclic fatigue. In this model, the composite damaged by fatigue is assumed to be a serial set of damaged zones (with a matrix crack bridged by broken and surviving fibres) and undamaged zones. Under cycling, the mechanical hysteresis increases as the fraction of broken fibres increases due to cyclic fatigue. This model shows that the mechanical hysteresis is narrow if the interfacial shear stress is very low or very high, and that the mechanical hysteresis is maximum for intermediate levels of the interfacial shear stress. In cross-weave Sic/Sic composites and cross-ply laminates SiC/MAS-L, fibres are set in two perpendicular directions. Hence, the first hypothesis needed to describe the mechanical behavior of these composites is to separate the effect of longitudinal fibers from the effect of transversal fibres [&ll]. In longitudinal yarns the loading is parallel to the fibres, and the behavior of these yarns is like an unidirectional composite with a brittle matrix and parallel fibers [5,6]. During a cyclic fatigue test, if the maximum stress applied is higher than the stress at which the first crack appears in the matrix, multi-cracking of the matrix occurs in transversal and longitudinal yarns, and can be assumed achieved at the end of the first loading-unloading cycle. Then, all further damage in longitudinal yarns due to cyclic fatigue will appear mainly on the fraction of broken fibres in the vicinity of a matrix crack, and therefore on the shape of the stress profile of surviving fibres. During cycling, all the surviving fibers are sliding in the matrix with an alternate slip due to the repeat of loading-unloading steps. Hence this alternate sliding creates at the interface a progressive wear of the fibre and of the matrix [3-l 11.
AOf
Maximum
I i-
II-
MATRIX
loaded
neighbouring
Lm1oaaea
Fig. 1. Theoretical evolution analysis) of the stress profile vicinity of matrix cracks sequence - effect of the
COMPOSITES
composite
matrix
cracks
i
cozuposite (established from a shear-lag (udx)) along the fibers in the during a loading/unloading interfacial shear stress (r).
The analysis of the stress profile along a surviving fibre (see Fig. l), and its evolution during a loading-unloading cycle allows an analytical calculation of the stress-strain loops [l l] that can point out the dependence of the mechanical hysteresis on the interfacial shear stress in a ceramic-matrix composite damaged by cyclic fatigue. For a unidirectional composite, the results of the analytical calculation of the mechanical hysteresis (A W/We) as a function of the interfacial shear stress (r) are [ll]: if :>l,
(1)
where W, =$Y2/Ex, c( = E,v,,,/Epf, and 7* = arE$/ 2dE,, with S: the maximum stress applied, r: the radius of the fibres, d: the mean distance between two neighboring matrix cracks, Ef, vf, E, and v,: the Young’s modulus and the volumic fraction of fibre and of matrix, respectively, and E,: the mean elastic modulus of the composite (Ex = Epf+ Emv,). The parameter r* corresponds to the limit value of interfacial shear stress between a local and a total sliding of the fibres. This specific value is not only dependent on the characteristics of the material but is also dependent on the amplitude of the mechanical loading (S). The theoretical calculation of the stress/strain loop area relative to the elastic strain energy (AW/ We) in terms of the interfacial shear stress (T), shows (see Fig. 2) that as r increases from zero to infinity, the mechanical hysteresis A W/We increases from zero to a maximum value (maximum which has no specific physical meaning), then decreases and tends to zero as 7 increases to infinity. This relation comes from the fact that one of the two
REYNAUD
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et al.:
INTERFACIAL
hysteresis
SHEAR
STRESS
(Aw/We)
OS4 -
0.3
--
ona
--
O,l--
04
Interfacial
I/
!
0
1
stress
shear
(z/2*) I
2
3
4
Fig. 2. Theoretical effect of the interfacial shear stress (5) on the mechanical hysteresis (A IV) of long-fibre-reinforced ceramic-matrix composite calculated from a shear-lag model (with IV,: the elastic strain energy, and z*: the limit value of interfacial shear stress between a local and a total sliding of the fibers).
following situations occurs in a brittle-matrix composite (see Figs 1 and 2): - if the interfacial shear stress (7) is high (r 2 r*), the size of the fibre sliding zone in the vicinity of a matrix crack is lower than the distance between two matrix cracks (case of 2D Sic/Sic composite). In that case, the hysteresis increases hyperbolically as the interfacial shear stress (z) decreases due for example to fatigue (local sliding of fibers); - if the interfacial shear stress (7) is low (z I z*), the sliding of the fibers in the matrix occurs along all the fibres (case of SiC/MAS-L [0, gOIs). In that case, the mechanical hysteresis depends parabolically on the interfacial shear stress (T), and if the running point (A IV, z) is on the left side of the maximum of the curve, the mechanical hysteresis decreases as the shear stress (z) decreases (total sliding of fibres). This analytical calculation is consistent with the results obtained on unidirectional composites by Kotil et al. [2].
3. EFFECTS
OF TEMPERATURE
Temperature effects on the cyclic fatigue behavior of ceramic matrix composites are not well known at present time. Nevertheless, since the temperature of the composite is low enough to keep the material physically and chemically inert, the predominant mechanism of cyclic fatigue is interfacial wear, plus a dependence of the testing temperature on the thermal residual stresses. But, when the temperature is high enough to enable structural changes in the components (evolution of fibres or matrix), new mechanisms (possibly depending on time) occur and possibly hide the effects of the interfacial wear observed at lower temperatures.
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Just after processing, the initial interfacial shear stress is the sum of all the interactions found in the interfaces. All these interactions can be classified as being one of two types: (i) short range interactions, and (ii) long range interactions. During a cyclic fatigue test at a given temperature under inert atmosphere, the short range interactions are progressively removed by a progressive wear of the fibre/matrix interfaces, and therefore the interfacial shear stress decreases from an initial value (7,J to a non zero limit value (7,). On the other hand, the long range interactions remain constant during a cyclic fatigue test, and only the thermal residual stresses are modified by the temperature of the cyclic fatigue test. Effectively, because thermal residual stresses are due to the mismatch between the thermal expansion of the fibres and of the matrix, these stresses are released until the testing temperature (7’) reaches the processing temperature (7’s). The stresses increase as long as the testing temperature is higher than the processing temperature. This effect acts on the contribution of the radial thermal residual stresses on the current level of the interfacial shear stress. According to the relative value of radial thermal expansion coefficients of the fibre and of the matrix, two cases have to be analyzed. If the radial thermal expansion coefficient of the matrix is higher than the coefficient of the fibres (case of two-dimensional Sic/Sic composite), at a testing temperature (7’) lower than the processing temperature (To), the radial thermal residual stresses are compressive. Assuming that between fibre and matrix a Coulombian friction is effective, the interfacial shear stress (T.,) due to the long range interactions is given by: 7 00 =
7LR
+p
Ic(f - &&To - T) A
(3)
with p: the friction coefficient, c(, and nr: the radial thermal expansion coefficients of the matrix and the fibers, respectively, T: the testing temperature, To: the processing temperature, A: a constant depending on the elastic characteristics of matrix and fibres [12], and rta: the effects of all the long range interactions other than the radial thermal expansion effect. In this case, the interfacial shear stress decreases as the testing temperature increases up to the processing temperature. On the other hand (as in the case of the Sic/ MAS-L composite) the thermal residual stresses are tensile. After fibre-matrix debonding, a gap appears between the fibre and the matrix, this gap (6) is given by: 6 = r(cq - cc,)(T(J - T)
(4)
with r: the radius of the fibres, tl, and af: the radial thermal expansion coefficients of respectively the matrix and the fibres, T: the testing temperature, and To: the processing temperature. This gap
2464
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SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
Hysteresis(kPa)
30
--1ooooc -8OOOC
25
-6OOOC
Number 0 lE+OO
I I lE+Ol
, I lE+02
1 1 1E+03
of cycles I I
I I lE+04
lE+05
I lE+06
Fig. 3. Experimental effect of temperature on the evolution of mechanical hysteresis during cyclic fatigue of two-dimensional Sic/Sic GS4C composite under inert atmosphere.
decreases as the testing temperature increases. Therefore, because of the roughness of fibres and of matrix, the interfacial shear stress increases when the testing temperature increases. According to the dependence of the mechanical hysteresis on the interfacial shear stress, the effects of the testing temperature on the interfacial shear stress should be observed also on the mechanical hysteresis. In order to confirm this hypothesis, fatigue tests have been conducted on two-dimensional Sic/Sic composites and on SiC/MAS-L [0, 901s
Mechanical
composites with the same mechanical loading (2130 MPa for Sic/Sic and O/l10 MPa for Sic/ MAS-L, 1 Hz) but at various temperatures and under inert atmosphere (argon). These experiments (see Figs 3 and 4) exhibit the evolutions of the mechanical hysteresis in relation to the number of cycles applied for both material at various temperatures (from 600°C to 1000°C). It can be seen that the mechanical hysteresis increases with the number of cycles applied for the 2D Sic/Sic composite, and decreases for the SiC/MAS-L composite. It can
hysteresis
(kPa)
40 -1ooooc
35
-8OOOC 30
-6OOOC
25
lE+OO
lE+Ol
1E+O2
lE+03
lE+04
Number
lE+05
lE+06
of cycles
Fig. 4. Experimental effect of temperature on the evolution of mechanical hysteresis during cyclic fatigue of two-dimensional SiC/MAS-L composite under inert atmosphere.
REYNAUD et al.:
Mechanical
INTERFACIAL
hysteresis
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
From this theoretical analysis, it can be seen that the main behavior of ceramic-matrix composites observed during cyclic fatigue at high temperature under inert atmosphere can be explained by a combination of (i) a progressive interfacial wear due to the cyclic loading at constant amplitude, and (ii) the effect of the actual radial residual thermal stresses on the interfacial shear stress, residual stresses which are obviously dependent on the testing temperature.
(AW/W,)
0,4
T 0.3
0.2
II
T3 SiC!/MAS-L T2
0.1
-
VT1
l-3
2D
SiC/SiC
T2 T,
Interfacial shear
stress
(z/Z')
I
0
1
a
3
2465
4
Fig. 5. Theoretical effect of interfacial wear during cyclic fatigue and of radial residual thermal stresses on hysteresis of stress/strain loops in long-fibre-reinforced ceramic-matrix composites.
be also observed that these increases and decreases are more important at high temperature than at low temperature. In fact, in the two-dimensional Sic/Sic composite the initial interfacial shear stress (ra) is high, hence the mechanical hysteresis is low according to the (A W/ We, 7/7*) diagram (see Fig. 5), and during a cyclic fatigue test at constant amplitude the mechanical hysteresis increases with the number of cycles applied due to the interfacial wear. In addition, when the testing temperature is high, the initial interfacial shear stress is lower than at low temperature due to the release of the radial thermal residual stresses. Hence, at high temperature the mechanical hysteresis is higher than at low temperature, and at high temperature the increase of the mechanical hysteresis during cyclic fatigue is also more important than at low temperature (see Figs 3 and 5) On the other hand, in SiC/MAS-L composite the initial interfacial shear stress (ra) is very low. Hence, the associate mechanical hysteresis is very low according to the (AW/W,, t/z*) diagram (see Fig. 5). Moreover, during a cyclic fatigue test at constant amplitude the mechanical hysteresis decreases with the number of cycles applied due to the interfacial wear. This is because for this material the characteristic point (A W/ We, r/7*) is placed on the left side of the theoretical (AW/W,, z/z*) diagram. In addition, in this material, when the testing temperature is high, the initial interfacial shear stress is higher than at low temperature due to the release of the radial thermal residual stresses. Therefore, because the characteristic point is running on the left side of the (A W/ We, z/z*) diagram, at high temperature the mechanical hysteresis is higher than at low temperature, and at high temperature the decrease of the mechanical hysteresis during a cyclic fatigue test is more important than at low temperature (see Figs 4 and 5) because the interfacial wear is more important.
4. CYCLICFATIGUEATROOMTEMPERATURE AFTERAGEINGATHIGHTEMPERATUREUNDER VACUUM In unidirectional and bidirectional composites, fibre/matrix interfacial shear stress and macroscopic mechanical hysteresis during a loading-unloading sequence are directly related (see Fig. 2). Hence, any microstructural change in fibres, matrix or interfaces leads to a modification of the interfacial shear stress, and these changes are exhibited by the mechanical hysteresis. That is the reason why the effects of a progressive wear of the interface during cyclic fatigue, or the effects of temperature on the radial residual thermal stresses can be observed from the evolutions of the mechanical hysteresis. However, another way to modify the level of the interfacial shear stress is to modify the microstructure of fibres, matrix or interfaces by the activation of chemical instabilities in the materials. This is, for example, possible by heating under vacuum an as received composite before running a cyclic fatigue test. That operation initiates a slight chemical degradation of the fibre-matrix interface, and leads to a small decrease of the interfacial shear stress [13,14]. These experiments have been conducted on the two-dimensional Sic/Sic GS4C composite, by heating the specimens 50 h under low vacuum at a constant temperature (from 800°C to 1OOOC). Then each specimen had been subjected to a cyclic tension/compression loading (+130 MPa, 1 Hz) at room temperature under air. The mechanical behaviors obtained (see for example Fig. 7) are compared to the behavior obtained by the same loading (+130 MPa, 1 Hz) of a pristine composite (see Fig. 6). For the as received material (see Fig. 6) it can be observed that: (i) the mechanical hysteresis increases continuously during the cyclic fatigue test (see Figs 6 and 8), and (ii) the tensile mean elastic modulus decreases monotonically whereas the compressive mean elastic modulus remains constant (see Figs 6, 10 and 11). On the other hand, when the composite is subjected to a previous heating under vacuum before running a cyclic fatigue test at room temperature, the mechanical behavior during cyclic fatigue (see Fig. 7) exhibit: (i) an increase followed by a decrease of the mechanical hysteresis (see Figs 7
2466
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150~
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
Stress(WPa)
Strain(%)
Fig. 6. Experimental evolution of stress/strain loops under tension/compression cyclic loading at room temperature of pristine two-dimensional Sic/Sic GS4C composite (run out after 500000 cycles, +130 MPa, 1 Hz).
and 8) and (ii) an original increase of the tensile mean elastic modulus during cyclic fatigue (see Figs 7 and lo), where the tensile mean elastic modulus reaches the value of the compressive mean elastic modulus (see Fig. 11). Concerning the evolution of the mechanical hysteresis during cyclic fatigue of a previously-heated composite, the (AWjW,, T/T*) diagram can explain (see Fig. 9) why the mechanical hysteresis goes to a maximum value then decreases when the number of cycles applied increases. This phenomenon is due to
the fact that the previous heating under vacuum of the composite lowers the initial interfacial shear stress (re) due to chemical reactions in fibre/matrix interfaces (may be a small diffusion of the surrounding residual oxygen of Nicalon NLM202 SIC fibers to the interfacial pyrocarbon layer [13, 141). Hence, as shown by the (AW/W,,z/z*) diagram (see Fig. 9), at the beginning of the cyclic fatigue test, because the initial interfacial shear stress of previously-heated composites is lower than for nonheated composites, the mechanical hysteresis is
150 Stress(MPa)
T
I
-0,l
-
Fig. 7. Experimental evolution of stress/strain loops under tension/compression temperature
cyclic loading at room of two-dimensional Sic/Sic GS4C composite after a previous ageing of 50 h under vacuum at 800°C (run out after 500 000 cycles, +130 MPa, 1 Hz).
REYNAUD et al.:
INTERFACIAL
Mechanical
SHEAR STRESS IN CERAMIC MATRIX COMPOSITES
hysteresis
-.1ooooc
lE+OO
lE+Ol
-9oooc
lE+02
(kPa) -8OOOC
lE+03
2467
lE+04 Number
-- 23OC
1E+05
lE+06
of cycles
Fig. 8. Experimental evolutions of mechanical hysteresis during cyclic fatigue at constant amplitude of two-dimensional Sic/Sic GS4C previously aged 50 h at various temperatures under vacuum.
higher for previously-heated composites than for non-heated composites. Moreover, for previouslyheated composites the starting point on the (AW/ We, t/z*) diagram is closer to the maximum of the (A W/W,, s/a*) curve than for non-heated composites. Hence, because during cyclic fatigue the interfacial shear stress decreases due to the interfacial wear, the mechanical hysteresis increases enough to reach the maximum value of the (AW/ We, 7/7*) curve and then decreases according to this curve. On the other hand, concerning the stiffening effect observed during cyclic fatigue of previously-
ye4chanical hysteresis (AW/W,) I 2D SiC/SiC after under
O-3
ageing vac!Ilum
n
Bear
:::i/
stress
(7/T*)
0
I 0
1
2
3
heated composites, the origin of this unusual phenomenon is not well established yet. The experimental measurement of the tensile mean elastic modulus exhibits that this modulus increases up to the value of the compressive mean elastic modulus (see Figs 10 and 11). Moreover the theoretical calculation of the tensile mean elastic modulus [l I] indicates that this modulus decreases when the interfacial shear stress decreases due to the interfacial wear. Hence, the stiffening effect can not be attributed to the longitudinal yarns. But, the twodimensional Sic/Sic composite is a bidirectional composite, and this stiffening effect can be rather attributed to a physical phenomenon in transversal yarns. For example, during cycling, the cracks in transversal yarns are opened during the loading step, and closed during the unloading step. Then, if the opening of the crack under the maximum loading is high, a bad closure of these cracks during cycling may occur. This hypothesis is indeed consistent with the fact that (i) the tensile mean elastic modulus increases and reaches the compressive mean elastic modulus, (ii) the residual strain increases with the number of cycles applied, and (iii) the evolution of the mechanical hysteresis of the composite during cyclic fatigue is mainly due to the interfacial wear of longitudinal fibres. At present time, this hypothesis is just proposed, and further investigations are needed to confirm.
4
Fig. 9. Theoretical effect of interfacial removal due to ageing at 800°C under vacuum on the evolution of hysteresis of stress/strain loops in long-fibre-reinforced ceramic-matrix composites during a cyclic fatigue test at constant amplitude.
5. CONCLUSIONS Studies at room temperature of the mechanical behavior of ceramic matrix composites under cyclic
2468
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INTERFACIAL
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IN CERAMIC
Mean elastic modulus
MATRIX
COMPOSITES
1E+05
1E+06
(GPa)
250
200
150
100
50 lE+OO
lE+Ol
lE+02
1E+03 lE+04 Nknber
of
cycles
Fig. 10. Experimental evolutions of mean tensile (&) and compressive (EC) elastic moduli during cyclic fatigue at constant amplitude of two-dimensional Sic/Sic GS4C previously aged 50 h at various temperatures under vacuum.
fatigue pointed out a major effect of a progressive wear at the fibre/matrix interfaces in longitudinal yarns due to repeated loading/unloading sequences. A preliminary and qualitative approach of cyclic fatigue at high temperatures indicates that this interfacial wear is mainly modified by the dependence of the radial thermal residual stresses with the testing temperature. All these phenomena have been observed from the evolutions of the mechanical hysteresis in relation to the number of cycles
Normalized 1
T
applied. Another way to modify the interfacial properties is to activate some chemical instabilities. For example, heating under vacuum leads to a decrease of the interfacial shear stress that leads to an increase until a maximum value then a decrease of the mechanical hysteresis. This treatment leads also to an original stiffening of the composite during cyclic fatigue. This new phenomenon is not well understood at present time, and further investigations are still required.
elastic modulus
-lOOO°C
-900°C
-800°C
(Et/E,) -- 23OC
0,s 018 Of? On6 0,5 0,4 lE+OO
lE+Ol
lE+02
1E+03
lE+04
Number
lE+05
lE+06
of cycles
Fig. 11, Experimental evolutions of the ratio (tensile mean elastic modulus (&)/compressive tic modulus (EC)) during cyclic fatigue at constant amplitude of two-dimensional Sic/Sic viously aged 50 h at various temperatures under vacuum.
mean elasGS4C pre-
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INTERFACIAL
SHEAR
Acknowledgements-This work has been carried out with the support of the Centre National de la Recherche Scientifique, the Ministere de 1’Enseignement Superieur et de la Recherche, the Direction des Recherches et Etudes Techniques, the Centre National d’Etudes Spatiales, and the companies: Aerospatiale and Societe Europeenne de within association Propulsion, the scientific “Thermomechanical behaviours of fibrous ceramicceramic composites”. Thanks are particularly due to M. Bourgeon, F. Abbe and J. P. Richard from the Societe Europeenne de Propulsion, and to P. Peres from Airospatiale, for their interest on this work and for their support.
STRESS
5. 6. I. 8, 9, 10. 11.
REFERENCES Aveston, J., Cooper, G. A. and Kelly, A., in Proceedings of Conference on the properties of jibre composites of the national physical laboratory, Vol. 4, Surrey: IPC Sci. Technol. Press, 1971, pp. 15-26. Kotil, T., Holmes, J. W. and Comninou, M., J. Am. Ceram. Sot., 1990, 73, 1879. Cho, C., Holmes, J. W. and Barber, J. R., J. Am. Ceram. Sot., 1991, 74, 2802. Holmes, J. W., in Ceramics and Ceramic Matrix Composites, ed. S. R. Levine, Vol. 3 in Flight-Vehicle Materials, Structures and Dynamics Assessment
12. 13. 14.
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and Future Directions, ASME, New York, 1992, pp. 193-238. Rouby, D. and Reynaud, P., Compos. Sci. Technol. , 1993, 48, 109. Evans, A. G. and Zok, F. W., J. Mater Sci., 1994, 29, 3857. Reynaud, P., Rouby, D. and Fantozzi, G., Ser. Metall. Mater., 1994, 31, 1061-1066. Evans, A. G., Zok, F. W. and McMeeking, R. M., Acta MetaN. Mater., 1995, 43, 859. Reynaud, P., Compos. Sci. Technol. , 1996, 56, 809. Kerans, R. J. and Parthasarathy, T. A., J. Am. Ceram. Sot., 1991, 74, 1585-1596. Reynaud, P., Etude du comportement en fatigue des materiaux composites a matrice ceramique suivi par emission acoustique, These de Doctorat, INSA de Lyon, 1992. Brun, M. K. and Singh, R. N., Adv. Ceram. Mater. , 1988, 3, 506. Abbe, F., Fluage en flexion d’un composite Sic-Sic 2D. These de Doctorat, Universite de Caen. 1990. Labrugtre, C., Influence de l’evolution physico-chimique des fibres et de la zone interfaciale fibreimatrice sur le comportement mtcanique des composites Sic/ C/Sic et SiC/MAS-L apres vieillissement thermique sous atmosphere control&e, These de Doctorat, Universite de Bordeaux I, 1993.