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 speci￾men 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, mod￾els3,4,21–28 of frictional sliding at debonded, unoxidized inter￾phases 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 rela￾tionship between fiber displacement and force predicts crack growth kinetics that are similar to those experimentally observed in inert (nonoxidizing) environments, whereas a linear relation￾ship, 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 compos￾ites. 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 Fibre￾Reinforced 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 Ceramic￾Matrix 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 High￾Temperature, Environmentally Assisted Embrittlement in Ceramic Matrix Compos￾ites,” 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,” Com￾posites: 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. Eldridge, “Desktop Fiber Push-Out Apparatus,” NASA Technical Memoran￾dum, No. 105341, December 1991. 18L. Filipuzzi, G. Camus, R. Naslain, and J. Thebault, “Oxidation Mechanisms and Kinetics of 1-D-SiC/C/SiC Composite Materials: II, Modeling,” J. Am. Ceram. Soc., 77 [2] 467–80 (1994). 19C. F. Windisch Jr., C. H. Henager Jr., G. D. Springer, and R. H. Jones, “Oxidation of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite,” J. Am. Ceram. Soc., 80 [3] 569–74 (1997). 20A. 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 [4] 972–80 (1995). 21D. B. Marshall, B. N. Cox, and A. G. Evans, “The Mechanics of Matrix Cracking in Brittle-Matrix Fiber Composites,” Acta Metall., 33 [11] 2013–21 (1985). 22L. N. McCartney, “Mechanics of Matrix Cracking in Brittle-Matrix Fibre￾Reinforced Composites,” Proc. R. Soc. London, A409, 329–50 (1987). 23F. Zok and C. L. Hom, “Large Scale Bridging in Brittle Matrix Composites,” Acta Metall Mater., 38 [10] 1895–904 (1990). 24R. J. Kerans and T. A. Parthasarathy, “Theoretical Analysis of the Fiber Pullout and Pushout Tests,” J. Am. Ceram. Soc., 74 [7] 1585–96 (1991). 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). 26D. A. W. Kaute, H. R. Shercliff, and M. F. Ashby, “Modelling of Fibre Bridging and Toughness of Ceramic Matrix Composites,” Scr. Metall. Mater., 32 [7] 1055–60 (1995). 27C. H. Hsueh, “Matrix Cracking with Frictional Bridging Fibres in Continuous Fibre Ceramic Composites,” J. Mater. Sci., 30, 1781–89 (1995). 28J. Lamon, F. Rebillat, and A. G. Evans, “Microcomposite Test Procedure for Evaluating the Interface Properties of Ceramic Matrix Composites,” J. Am. Ceram. Soc., 78 [2] 401–405 (1995). 29C. A. Lewinsohn, C. H. 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