J. dn. Ceram Soc..84]866-6802001 ournal Measuring Interphase Recession by Fiber Push-In Testing Charles A. Lewinsohn. Charles H. Henager Jr. and Russell H. Jones Pacific Northwest National Laboratory Richland, Washington 99352 Jeffrey I. Eldridge NASA Glenn Research Center. Cleveland Ohio 4413 A novel technique for measuring interphase recession in IL. Experimental Method ceramic-matrix composites(CMCs) due to oxidation is de- scribed. The technique involves fiber push-in testing and The materials were fabricated by chemical vapor infiltration were conducted on carbon-coated Hi-Nicalon SiC fibers in a (Cvi) of a preform of seven layers of a two-dimensional plain- CVI SiC matrix, where the carbon interphase had recessed due to oxidation. Estimates of interphase recession distances from thick pyrolitic carbon interphase. The resulting composite was also coated with an - 2 um thick Cvd SiC layer intended for oxidation analysis of fiber push-in tests are in reasonable agreement with protection. Samples for push-in testing that were -4 mm X 4 measurements made by optical microscopy. Besides measuring mm x 4 mm were cut from plates of CVI material the recession distance the fiber push-in test can be used to For push-in tests on unoxidized specimens, the CVD SiC investigate environmental effects on fiber bridging oxidation protection coating was removed from one of the faces of imen by grinding on an abrasive wheel. To ensure that the surfaces were smooth and flat, the exposed face and the face L. Introduction opposite to it were polished with diamond paste. For studying effects of oxidation, the face of the specimen that was to be tested Fa IBER-REINFORCED, non-oxide ceramic-matrix composites was polished before oxidation, after oxidation, but before push-in 3(CMCs)typically employ an interphase material between the testing, the face opposite the test face was polished until both faces and the matrix to allow fiber/matrix debonding, which were parallel. Specimens were oxidized for 1.5, 3.6, 5.4, and 7.2 ks 25, 60, 90, and 120 min)at 1073 K in air. This temperature was on-oxide CMCs occurs at elevated temperatures. In some in- tances failure is due to the growth of oxidation products at the and polished with diamond paste for optical microscopy. Micro- interfaces between the fibers and the matrix 8-2 In environments graphs of the specimens were created and the recession distance containing low oxygen concentrations where oxide growth is vas measured with digital micrometers, with a resulting accuracy limited, such as those anticipated in fusion energy systems, failure of 100 nm may be due to interphase recession. -I6 Therefore, methods for measuring interphase recession distances provide essential infor- (2) Fiber Push-In Testing mation for developing models that can predict CMC lifetimes. This Fiber p ests were performed using a desktop fiber study introduces an experimental approach employing fiber push-in apparatus described previously. iNdividual fibers were push-in testing to measure the interphase recession rate due to pushed in with a 70-included-angle conical diamond indenter with a 10 um diameter flat on the bottom. Because of the conical oxidation. This new approach is compared with the measurement shape of the indenter, push-in distances were limited to a few of recession distances by optical microscopy. micrometers. Loading was accomplished using a vertical transla- tion stage that moved the specimen at a speed of I um/s. All tests were performed in room-temperature air (25%60% relative ( Fiber Push-In Test Analysis, Recession Interface recession lengths were determined by analysis of the J. D. Cawley--contributing editor relationship between fiber end displacement and applied force. The first step was to make a compliance correction(based on tests where the force was applied to only the Sic matrix) to the measured displacement data so that the corrected displacement Based, in part, on work presented a the ziny /9, 199, aproaenceramie sce and where there is no debonding along the unoxidized interface and no represented the fiber end displacement. For the simplest case contact between fiber and matrix along the recessed length, the Contract no. be-aco6-76rlo 1830 with Pacific northwest national Laboratory, which is operated for the DOE by Battelle. Additional support was provided by the NASA HITEMP Program. of Energy by Battelle Memorial Institute under 小中 on Carbon com tokwturead by DuPont Lanxide Composites, Newark, DE. 866
Measuring Interphase Recession by Fiber Push-In Testing Charles A. Lewinsohn,* Charles H. Henager Jr.,* and Russell H. Jones* Pacific Northwest National Laboratory,† Richland, Washington 99352 Jeffrey I. Eldridge* NASA Glenn Research Center, Cleveland, Ohio 44135 A novel technique for measuring interphase recession in ceramic-matrix composites (CMCs) due to oxidation is described. The technique involves fiber push-in testing and analysis of the load–displacement curves. Fiber push-in tests were conducted on carbon-coated Hi-Nicalon SiC fibers in a CVI SiC matrix, where the carbon interphase had recessed due to oxidation. Estimates of interphase recession distances from analysis of fiber push-in tests are in reasonable agreement with measurements made by optical microscopy. Besides measuring the recession distance, the fiber push-in test can be used to investigate environmental effects on fiber bridging. I. Introduction FIBER-REINFORCED, non-oxide ceramic-matrix composites (CMCs) typically employ an interphase material between the fiber and the matrix to allow fiber/matrix debonding, which provides higher toughness and greater reliability relative to unreinforced ceramics.1–7 It is well known that rapid failure of many non-oxide CMCs occurs at elevated temperatures. In some instances failure is due to the growth of oxidation products at the interfaces between the fibers and the matrix.8–12 In environments containing low oxygen concentrations where oxide growth is limited, such as those anticipated in fusion energy systems, failure may be due to interphase recession.13–16 Therefore, methods for measuring interphase recession distances provide essential information for developing models that can predict CMC lifetimes. This study introduces an experimental approach employing fiber push-in testing to measure the interphase recession rate due to oxidation. This new approach is compared with the measurement of recession distances by optical microscopy. II. Experimental Method (1) Materials The materials were fabricated‡ by chemical vapor infiltration (CVI) of a preform of seven layers of a two-dimensional plainweave (0/90°) cloth of Hi-Nicalon®§ fibers coated with a 1 mm thick pyrolitic carbon interphase. The resulting composite was also coated with an ;2 mm thick CVD SiC layer intended for oxidation protection. Samples for push-in testing that were ;4 mm 3 4 mm 3 4 mm were cut from plates of CVI material. For push-in tests on unoxidized specimens, the CVD SiC oxidation protection coating was removed from one of the faces of each specimen by grinding on an abrasive wheel. To ensure that the surfaces were smooth and flat, the exposed face and the face opposite to it were polished with diamond paste. For studying effects of oxidation, the face of the specimen that was to be tested was polished before oxidation; after oxidation, but before push-in testing, the face opposite the test face was polished until both faces were parallel. Specimens were oxidized for 1.5, 3.6, 5.4, and 7.2 ks (25, 60, 90, and 120 min) at 1073 K in air. This temperature was chosen so as to minimize microstructural changes in the fibers. Some specimens were cut with a diamond saw, mounted in resin, and polished with diamond paste for optical microscopy. Micrographs of the specimens were created and the recession distance was measured with digital micrometers, with a resulting accuracy of 100 nm. (2) Fiber Push-In Testing Fiber push-in tests were performed using a desktop fiber push-in apparatus described previously.17 Individual fibers were pushed in with a 70°-included-angle conical diamond indenter with a 10 mm diameter flat on the bottom. Because of the conical shape of the indenter, push-in distances were limited to a few micrometers. Loading was accomplished using a vertical translation stage that moved the specimen at a speed of 1 mm/s. All tests were performed in room-temperature air (25%–60% relative humidity). (3) Fiber Push-In Test Analysis, Recession Length Determination Interface recession lengths were determined by analysis of the relationship between fiber end displacement and applied force. The first step was to make a compliance correction (based on tests where the force was applied to only the SiC matrix) to the measured displacement data so that the corrected displacement represented the fiber end displacement. For the simplest case where there is no debonding along the unoxidized interface and no contact between fiber and matrix along the recessed length, the J. D. Cawley—contributing editor Manuscript No. 189152. Received August 19, 1999; approved January 2, 2001. Based, in part, on work presented at the 22nd Annual Cocoa Beach Conference and Exposition of the Engineering Ceramics Division of the American Ceramic Society, January 1998. Supported, in part, by Basic Energy Sciences under U.S. Department of Energy (DOE) Contract No. DE-AC06-76RLO 1830 with Pacific Northwest National Laboratory, which is operated for the DOE by Battelle. Additional support was provided by the NASA HITEMP Program. *Member, American Ceramic Society. † Operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC06-76RLO 1830. ‡ Composites were manufactured by DuPont Lanxide Composites, Newark, DE. § Nippon Carbon Co., Tokyo, Japan. 866 journal J. Am. Ceram. Soc., 84 [4] 866–68 (2001)
April 2001 Communications of the American Ceramic Society analysis reduces to the problem of a spring under uniform Hence, the modified fiber end displacement become compression. Since the axial strain in an unbonded fiber, E(), is AEr TrEt 验(m it follows that I 2Tr/ This is still a linear relationship between fiber force and displace nent with the same slope(independent of Ts )as Eq (2). Therefore all recession length determinations were made by a least-squares F甲p< Fubon linear regression fit of Eq.(4)to the compliance- push-in loading curves(omitting nonlinear regions at low loads) where 8 is the fiber end displacement, Iox is the interface recession For long recession lengths, when there was too mucl ength, Fapp is the applied force, Debond is the force required to from debonding events during initial loading, the recession length initiate debonding along the remaining unoxidized interface, rr is was determined by fitting to t the first reloading curve instead of the the fiber radius, and er is the fiber modulus. initial loading curve While Eq(2)is valid for short recession lengths, at longer recession lengths most fibers tended to move to one side of the gap left by interphase oxidation(Fig. 1(a)) so that there was at least Il. Results intermittent contact between the fiber and matrix producing some sliding friction. It was assumed that this localized sliding friction (Optical Microscopy could be treated as an equivalent uniform sliding friction, Ts, that The recession of the carbon interphase from the surface of counteracted the axial applied force, Fapp, on the fiber, ecimens was readily apparent after oxidation at 1073K(Fig I(b)). The interphase recession distance increased linearly with F(2)=F甲p-2mr (3) time, up to between 5.4 and 7.2 ks. These observations are in agreement with those of other investigators who have studied the oxidation behavior of carbon interphases. 8-20 The recession distance measured at 7.2 ks, by either the optical or push-in techn ques, was er than that predicted by the linear rate observed for times up to 5.4 ks. Although the reason for this discrepancy could not be determined, it is possibly due to the formation of interconnected pathways within the material that could provide a shortcut for oxygen transport relative to the channel caused by oxidation of the interphase (2) Push-In Testing Typical fiber push-in load-displacement curves for oxidation times,at1073K,of1.5,3.6,5.4,and7.2ks(25,60,90,andl20 min) are shown in Fig. 2. Consistent with Eq. (4), these curves are linear(except at low loads for long recession lengths) with slopes that decrease with longer oxidation times. Table i summarizes the recession lengths determined using Eq. (4)for the different oxidation times(9 to 18 tests per specimen) and compares these values with the optical measurements IV. Discussion The interphase recession distances calculated from push-in testing exhibited more scatter than those made optically. There ar two likely contributions to this scatter. As shown in Fig. 1(a), the gap due to the oxidized interphase is relatively large, hence the oxidized fibers are not laterally restrained and may experience buckling under the push-in load. This could result in a component of the nter phase Fig. 1. Examples of interphase oxidation: (a) SEM micrograph of a the distinction between oxidized and unoxidized interphases( the typical Fig. 2. Typical fiber push-in initial loading curves for various oxidation
analysis reduces to the problem of a spring under uniform compression. Since the axial strain in an unbonded fiber, ε(z), is ε~z! 5 F AfEf 5 F prf 2 Ef (1) it follows that d 5 E 0 lox ε~z! dz 5 S lox prf 2 Ef DFapp (2) for Fapp , Fdebond where d is the fiber end displacement, lox is the interface recession length, Fapp is the applied force, Fdebond is the force required to initiate debonding along the remaining unoxidized interface, rf is the fiber radius, and Ef is the fiber modulus. While Eq. (2) is valid for short recession lengths, at longer recession lengths most fibers tended to move to one side of the gap left by interphase oxidation (Fig. 1(a)) so that there was at least intermittent contact between the fiber and matrix producing some sliding friction. It was assumed that this localized sliding friction could be treated as an equivalent uniform sliding friction, ts, that counteracted the axial applied force, Fapp, on the fiber, F~z! 5 Fapp 2 2prftsz (3) Hence, the modified fiber end displacement becomes d 5 E 0 lox ε~z! dz 5 S lox prf 2 Ef DFapp 2 tslox 2 rfEf 5 S lox prf 2 Ef DFapp 2 d0 (4) for 2prftslox , F , Fdebond 1 2prftslox This is still a linear relationship between fiber force and displacement with the same slope (independent of ts) as Eq. (2). Therefore, all recession length determinations were made by a least-squares linear regression fit of Eq. (4) to the compliance-corrected fiber push-in loading curves (omitting nonlinear regions at low loads). For long recession lengths, when there was too much interference from debonding events during initial loading, the recession length was determined by fitting to the first reloading curve instead of the initial loading curve. III. Results (1) Optical Microscopy The recession of the carbon interphase from the surface of specimens was readily apparent after oxidation at 1073 K (Fig. 1(b)). The interphase recession distance increased linearly with time, up to between 5.4 and 7.2 ks. These observations are in agreement with those of other investigators who have studied the oxidation behavior of carbon interphases.18–20 The recession distance measured at 7.2 ks, by either the optical or push-in techniques, was larger than that predicted by the linear rate observed for times up to 5.4 ks. Although the reason for this discrepancy could not be determined, it is possibly due to the formation of interconnected pathways within the material that could provide a shortcut for oxygen transport relative to the channel caused by oxidation of the interphase. (2) Push-In Testing Typical fiber push-in load–displacement curves for oxidation times, at 1073 K, of 1.5, 3.6, 5.4, and 7.2 ks (25, 60, 90, and 120 min) are shown in Fig. 2. Consistent with Eq. (4), these curves are linear (except at low loads for long recession lengths) with slopes that decrease with longer oxidation times. Table I summarizes the recession lengths determined using Eq. (4) for the different oxidation times (9 to 18 tests per specimen) and compares these values with the optical measurements. IV. Discussion The interphase recession distances calculated from push-in testing exhibited more scatter than those made optically. There are two likely contributions to this scatter. As shown in Fig. 1(a), the gap due to the oxidized interphase is relatively large; hence the fibers are not laterally restrained and may experience buckling under the push-in load. This could result in a component of the Fig. 1. Examples of interphase oxidation: (a) SEM micrograph of a push-in test fiber from an oxidized specimen (5.4 3 103 s at 1073 K, in air) showing a fiber leaning to one side of gap, and (b) a micrograph showing the distinction between oxidized and unoxidized interphases (the typical fiber diameter is 12 mm). Fig. 2. Typical fiber push-in initial loading curves for various oxidation times at 1073 K in air. April 2001 Communications of the American Ceramic Society 867
Communications of the American Ceramic Sociery ol.84.No.4 Table I. Comparison of Recession Distance providing needed information for crack growth models was Estimates provided Oxidation time (m) knowledgement 33±1 3±38 We thank Professor R. Bordia, of Washington, and Dr. J. Lamon, 266±10 803±18 688±278 LCTS, for stimulating discussions. The assistance of Ms. S. Carlson and Mr. N. Saenz 7.2 3810 for preparing polished specimens is gratefully acknowledged. 3620±72 References A. G. Evans and F. W. Zok, "Review: The Physics and Mechanics of Fibre- tiber end displacement not associated with compressive strain of Rein oe.Marshal Mad r. nomos le ater re 2 B isle Mat i cor underestimate of recession length can occur if there is intermittent In pence of Fber strength: Ac a Metal i 26sl-di9( 1987). ontact along the recessed interface, due to incomplete interphase experiments in Brittle xidation, bonding between the fiber and matrix(SiO, growth), or en,"Models of Fiber Debonding and Pullout in displacement of unsupported fibers leading to contact with the t SN. Chawla, I.W. Holmes, and R. A. Lowden, "The role of Interfacial Coatings While both techniques require significant specimen preparation, 35( 12)1411 preparation of push-in test specimens is performed before speci J. R. Pachalis, J. Kim, and T -w. Chou, "Modeling of Aligned Short-Fiber Reinforced Ceramic Composites, Compos. Sci. TechnoL, 37, 329-46(1990). A.c.H. Henager Jr, C.A. Lewinsohn, and R. H. Jones, "The Influence of Fiber and specimens with unbonded fibers, after oxidation, is more likely to Matrix Composites, "submitted to Acta Mater perature Failure in Continuous Fiber C cause damage that can interfere with accurate measurements N. Frey, R. Molins Boussuge, "Oxidizing Ageing Effects on SiC-SiC Push-in tests also offer the advantage of allowing oxidation and opposites,". Mater. Sci., 27, 5084-90(1992)- retesting of the same fibers, which is impossible with optical Temperature, Environmentally Assisted Embrittlement in Ceramic Matrix Compos. He of Oxidative Degradation in a SiC-SiC an be difficult to follow an individual fiber, which may not Composite, J.Anm Ceram Soc., 81[112777-84(1998). remain in the exposed plane of observation. In contrast, the push-l E. Lara-Curzio, "Analysis of Oxidation-Assisted Stress-Rupture of Continuous tests can be performed across the full width of the Fiber-Reinforced Ceramic Matrix Composites at Intermediate Temperatures,Com- An additional advantage of the fiber push-in tests is that they are H. Henager Jr, C. A. Lewinsohn, and C. F. Windisch, In " Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites, ndicative of the mechanical behavior of fibers bridging cracks in J. Am. Ceram Soc, 83[8]1999-2005(2000) effective compliance of the fiber increases(Fig. 2). The effective Composites at Intermediate Temperatures,"J Mater. Sci. Lett, 16, 23-26(1997) compliance of a fiber with an oxidized interface that is bridging a C. A. Lewinsohn, C. H. Henager Jr, and R. H. Jones, "Environmentally Induced crack is, from Eq (2), given by Time-Dependent Failure Mechanisms in CFCCs at Elevated Temperatures, Ceram A. Lewinsohn, J. I. Eldridge, and R. H. Jones, "Techniques for Measuring Ceram. Eng. Sci. Proc., 19 [3]19-26(1998 an, C H. Henager Jr, and R. H. Jones, "Environmentally Induced where p is the fiber compliance in units of m/N. Since the Failure. Mec rictional sliding resistance, Ts, along the recessed interphase is ns, Vol. 96, Advances in Ce small, neglecting the do term in Eq(4)is a good approximation Edited by J. P. Singh and N. Bansal. American Ceramic Society, Westerville, O The fiber compliance is linear with respect to lox, and the fiber end 17J. 1. Eldridge, "Desktop Fiber Push-Out Apparatus, "NASA Technical Memoran displacement, 8, is linearly proportional to F. In contrast, mod els 4, 21-28 of frictional sliding at debonded, unoxidized inter L. Filipuzzi, G. Camus, R. Naslain, and J. Thebault, "Oxidation Mechanisms and Kinetics of l-D-SiC/C/SiC Composite Materials: Il, Modeling, "J Am. Ceram. Soc. ases show that fiber displacement is proportional to the force 77[2]467-80(1994 quare. Results from a discrete, micromechanical model of C F, Windisch Jr, C. H. Henager Jr. G. D. Springer, and R. H. Jones, "Oxidation dynamic subcritical crack growth indicate that a quadratic rela- of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite tionship between fiber displacement and force predicts crack J Am Ceram Soc., 80[3569-74(1997). rowth kinetics that are similar to those experimentally observed 2 A J. Eckel, J. D. Cawley, and T. A. Parthasarathy, " Oxidation Kinetics of a Continuous Carbon Phase in a Nonreactive Matrix, "J. Am Ceram. Soc., 78(41 inert(nonoxidizing) environments, whereas a linear relation- ship, as indicated here, predicts crack growth kinetics under D B. Marshall, B N Cox, and A G. Evans,"The onditions of interphase recession. Even in situations where in Brittle-Matrix Fiber Composites, Acta Metall, 33 [11 2013- oxidation does not result in a simple relationship such as Eq (2), Reinforced Composites, " Proc. R Soc. London, A 409, 329-50(1987). the push-in results provide an experimental relationship between F Bridging in Brittle Matrix Composites, and 8 that can be used to investigate the effects of interphase Acta Metall Mater, 38[10] 1895-904(1990) development on subcritical crack growth and other mechanical 2R. J Kerans and T.A. Parthasarathy, "Theoretical Analysis of the Fiber Pullo properties of CMCs with reaction-prone interphases Soc,741585-96(19 25T. A. Parthasarathy, D B. Marshall, and R. J. Kerans, "Analysis of the Effect of Interfacial Roughness on Fiber Debonding and Sliding in Brittle Matrix Composites, Acta Metall. Mater, 42 [11]3773-84(1994) v. Summary and Conclusio and To gh ness of ce最Mm Interphase recession due to oxidation was microscopy and fiber push-in testing, on CV u secs re via optical 27C. H. Hsueh, "Matrix Cracking with Frictional Bridging Fibres in abre Ceramic Composites,J. Mater. Sci., 30, 1781 tes. Although the results of the push-in technique ha mon, F, Rebillat, and A. G. Evans, "M -c oss Test pro atter,the test technique also provided information related Properties of Ceramic Matrix Composites,"J. closure stresses exerted by crack-bridging fibers. The recession Soc,78[2]401-405(199 ate was linear with respect to time and in agreement with other 2C. A. Lewinsohn, C. H. Henager Jr, and R. H. Jones, "Time-Dependent Crack rowth in Ceramic posits: From results. An example of the utility of fiber push-in studies in Sci. Proc, 21 [3]415-22(2000)
fiber end displacement not associated with compressive strain of the fiber, leading to an overestimate of recession length. An underestimate of recession length can occur if there is intermittent contact along the recessed interface, due to incomplete interphase oxidation, bonding between the fiber and matrix (SiO2 growth), or displacement of unsupported fibers leading to contact with the matrix. While both techniques require significant specimen preparation, preparation of push-in test specimens is performed before specimen oxidation, whereas preparation is performed after specimen oxidation for optical measurements. Sectioning and polishing on specimens with unbonded fibers, after oxidation, is more likely to cause damage that can interfere with accurate measurements. Push-in tests also offer the advantage of allowing oxidation and retesting of the same fibers, which is impossible with optical measurements. In addition, the optical measurements are restricted to the plane exposed on sectioning; for long recession distances, it can be difficult to follow an individual fiber, which may not remain in the exposed plane of observation. In contrast, the push-in tests can be performed across the full width of the specimen. An additional advantage of the fiber push-in tests is that they are indicative of the mechanical behavior of fibers bridging cracks in CMCs. The results indicate that, as oxidation progresses, the effective compliance of the fiber increases (Fig. 2). The effective compliance of a fiber with an oxidized interface that is bridging a crack is, from Eq. (2), given by F 5 d F 5 lox prf 2 Ef (5) where F is the fiber compliance in units of m/N. Since the frictional sliding resistance, ts, along the recessed interphase is small, neglecting the d0 term in Eq. (4) is a good approximation. The fiber compliance is linear with respect to lox, and the fiber end displacement, d, is linearly proportional to F. In contrast, models3,4,21–28 of frictional sliding at debonded, unoxidized interphases show that fiber displacement is proportional to the force squared. Results from a discrete, micromechanical model of dynamic subcritical crack growth indicate that a quadratic relationship between fiber displacement and force predicts crack growth kinetics that are similar to those experimentally observed in inert (nonoxidizing) environments, whereas a linear relationship, as indicated here, predicts crack growth kinetics under conditions of interphase recession.29 Even in situations where oxidation does not result in a simple relationship such as Eq. (2), the push-in results provide an experimental relationship between F and d that can be used to investigate the effects of interphase development on subcritical crack growth and other mechanical properties of CMCs with reaction-prone interphases. V. Summary and Conclusions Interphase recession due to oxidation was measured, via optical microscopy and fiber push-in testing, on CVI SiC matrix composites. Although the results of the push-in technique had greater scatter, the test technique also provided information related to closure stresses exerted by crack-bridging fibers. The recession rate was linear with respect to time and in agreement with other results. An example of the utility of fiber push-in studies in providing needed information for crack growth models was provided. Acknowledgments We thank Professor R. Bordia, University of Washington, and Dr. J. Lamon, LCTS, for stimulating discussions. The assistance of Ms. S. Carlson and Mr. N. Saenz for preparing polished specimens is gratefully acknowledged. References 1 A. G. Evans and F. W. Zok, “Review: The Physics and Mechanics of FibreReinforced Brittle Matrix Composites,” J. Mater. Sci., 29, 1857–96 (1994). 2 D. B. Marshall and B. N. Cox, “Tensile Fracture of Brittle Matrix Composites: Influence of Fiber Strength,” Acta Metall., 35 [11] 2607–19 (1987). 3 D. B. Marshall, “Analysis of Fiber Debonding and Sliding Experiments in Brittle Matrix Composites,” Acta Metall., 40 [3] 427–41 (1992). 4 J. W. Hutchinson and H. Jensen, “Models of Fiber Debonding and Pullout in Brittle Composites with Friction,” Mech. Mater., 9, 139–63 (1990). 5 N. Chawla, J. W. Holmes, and R. A. Lowden, “The Role of Interfacial Coatings on the High Frequency Fatigue Behavior of Nicalon/C/SiC Composites,” Scr. Mater., 35 [12] 1411–16 (1996). 6 J. R. Pachalis, J. Kim, and T.-W. Chou, “Modeling of Aligned Short-Fiber Reinforced Ceramic Composites,” Compos. Sci. Technol., 37, 329–46 (1990). 7 C. H. Henager Jr., C. A. Lewinsohn, and R. H. Jones, “The Influence of Fiber and Interface Properties on High-Temperature Failure in Continuous Fiber CeramicMatrix Composites,” submitted to Acta Mater. 8 N. Frety, R. Molins, and M. Boussuge, “Oxidizing Ageing Effects on SiC–SiC Composites,” J. Mater. Sci., 27, 5084–90 (1992). 9 A. G. Evans, F. W. Zok, R. McMeeking, and Z. Z. Du, “Models of HighTemperature, Environmentally Assisted Embrittlement in Ceramic Matrix Composites,” J. Am. Ceram. Soc., 79 [9] 2345–52 (1996). 10L. U. J. T. Ogbuji, “A Pervasive Mode of Oxidative Degradation in a SiC–SiC Composite,” J. Am. Ceram. Soc., 81 [11] 2777–84 (1998). 11E. Lara-Curzio, “Analysis of Oxidation-Assisted Stress-Rupture of Continuous Fiber-Reinforced Ceramic Matrix Composites at Intermediate Temperatures,” Composites: Part A, 30, 549–54 (1999). 12R. H. Jones, C. H. Henager Jr., C. A. Lewinsohn, and C. F. Windisch, “Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites,” J. Am. Ceram. Soc., 83 [8] 1999–2005 (2000). 13E. Lara-Curzio and M. K. Ferber, “Stress-Rupture of Continuous Fibre Ceramic Composites at Intermediate Temperatures,” J. Mater. Sci. Lett., 16, 23–26 (1997). 14C. A. Lewinsohn, C. H. Henager Jr., and R. H. Jones, “Environmentally Induced Time-Dependent Failure Mechanisms in CFCCs at Elevated Temperatures,” Ceram. Eng. Sci. Proc., 19 [43] 11–18 (1998). 15C. A. Lewinsohn, J. I. Eldridge, and R. H. Jones, “Techniques for Measuring Interfacial Recession in CFCCs and the Implications on Subcritical Crack Growth,” Ceram. Eng. Sci. Proc., 19 [3] 19–26 (1998). 16C. A. Lewinsohn, C. H. Henager Jr., and R. H. Jones, “Environmentally Induced Failure Mechanism Mapping for Continuous-Fiber, Ceramic Composites”; pp. 351–59 in Ceramic Transactions, Vol. 96, Advances in Ceramic Composites IV. Edited by J. P. Singh and N. Bansal. American Ceramic Society, Westerville, OH, 1999. 17J. I. 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Henager Jr., and R. H. Jones, “Time-Dependent Crack Growth in Ceramic Composites: From Single Fibers to Bridged Cracks,” Ceram. Eng. Sci. Proc., 21 [3] 415–22 (2000). M Table I. Comparison of Recession Distance Estimates Oxidation time (3103 s) Optical (mm) Push-in (mm) 1.5 33 6 16 123 6 38 3.6 413 6 10 266 6 102 5.4 803 6 18 688 6 278 7.2 3810 3620 6 727 868 Communications of the American Ceramic Society Vol. 84, No. 4