M.L. Antti et al. Journal of the European Ceramic Society 24(2004)565-578 or a woven. conunuous o xide fibre reinforced composite Efficiency and Renewable Energy under contract DE with a porous oxide matrix, the stress-strain behaviour AC05-00OR22725 with UT-Battelle, LLC. Financial being evaluated on centre-hole notched plates support from the Swedish Research Council for Engi- In the as-received condition the 0/90 orientation of neering Sciences (TFR) is also acknowledged. The the composite exhibited non-brittle behaviour and authors thank Dr. Michael Lance for performing the moderate notch sensitivity at room temperature, 1000 RAMAN experiment and Siemens-Westinghouse and and 1100C. The non-brittle behaviour and low notch Composite Optics for providing the material used in this sensitivity could be attributed to both extensive fibre- study. Dr. Oleg Babushkin is acknowledged for per- matrix debonding and shear damage in the matrix. The forming the XRD experiments +45 orientation also had low notch sensitivity. At 1000C it exhibited a significantly higher fracture stress and fracture strain than at room temperature. effect can be attributed to simultaneous sintering References creep of the matrix occurring during the test Thermal exposure at 1000 and 1100C caused a pro- W ange, F. F. and Evans, A. G, Concept for a gressive degradation of the composite indicated, in the amage-tolerant ceramic composite with"strong" interfaces. 0/90 composite, by a fall in room-temperature strength Am. Ceran.Soc.,1996,79,417-424. nd strain to failure for a given sample geometry (a/w 2. Levi, C. G. Yang J.Y. Dalgleish, B J, Zok, F.w. and Evans, ratio). This degradation could be described in terms of a A. G, Processing and performance of an all-oxide ceramic com- Larson-Miller plot which consequently permitted the posite. J. Am. Ceram. Soc., 1998, 81, 2077-2086 3. Kelly, A, Zweben, C, ed, Comprehensive Composite Materials. prediction of the strength loss for a given exposure Elsevier Science Ltd. London. 2000 emperature and time combination within the investi 4. Antti, M.-L. and Lara-Curzio, E, Effect of notches, specimen gated experimental range. Thermal exposure of the +45 size and fiber orientation on the monotonic tensile behavior of composite led to an increase in its room-temperature composites at ambient and elevated temperatures. Ce Sci. Proc.2001,22,643-650 racture strength but also a marked embrittlement 5. The embrittlement of the 0/90 material could be deformation and failure in carbon-matrix composites subject to successfully characterised in terms of an effective frac- ensile and shear loading.. dm. Ceram. Soc. 1995.78. 1841- (co) derived on the basis of the simple model of 6. Heredia, F. E spearing. s. M, Mackin,. He MY, Evan Waddoups et al. These parameters were constant (i.e matrix composites. J. A. Ceram. Soc., 1994, 77, 2817-2827. independent of a/w) for a given thermal exposure but 7. Heredia, F.E., Spearing, S M, Evans, A. G, Mosher, P and decreased consistently with thermal exposure tempera- Curtin, W.A. Mechanical properties of continuous-fiber-rein ture and time forced carbon matrix composites and Exposure for 100 h at 1100C led to almost"com- properties. J. Am. Ceram. Soc., 1992. 75. 3017-3025 8. McNulty, J. C, Zok, F. W, Genin, G. M. and Evans, A. G, Notch sensitivity of fiber-reinforced ceramic-matrix composites fracture was brittle and the values of Kc and co fell to effects of inelastic straining and volume-dependent strength. J. values characteristic for a monolithic oxide while the Am. Ceran.Soc,1999,82,1217-1228 notch sensitivity was much greater than for the lower 9. Buchanan, D.J., John. R. and Zawada. L. P, Notched fracture thermal exposures. The fact that the +45 samples behavior of oxide/ ci.Proc,2000,21,s8l-588 exhibited similar stress-strain behaviour after the same 10. Heathcote, J.A., Gong, X.Y., Yang, J.Y., Ramamurty, Uand exposure is consistent with this interpretation. Zok, F. w, In-plane mechanical properties of an all-oxide cera- Microstructural and fractographic examination indi cated that the property degradation was caused by a I1. Kramb. v. A, John. R. and Zawada. L. P, Notched fract combination of matrix densification and increased fibre/ behavior of an oxide/oxide ceramic-matrix composite. J.4m. matrix bonding. Some degradation of fibre properties 12. Mackin, T J and Roberts, M. C, Evaluation of damage evolu could also have been a contributory factor. Formation on in ceramic-matrix composites using thermoelastic stress nalysis. J. Am. Ceram. Soc., 2000, 83, 337-343 at 1100C. This was deemed to have occurred locally in 13. Jurt,R. A and Butner, S C, Advances in oxide -oxide cmc in ne matrix and not to have contributed directly to the Indianapolis, IN, 1999 property degradation Berger, M.H., Jeulin, D. and Bunsell, A.R. Acknowledgements This work was sponsored in part by the Us Depart 16. Milz. C. Goering J and Schneider, H. Mechanical and micro- ment of Energy, Assistant Secretary for Energy structural properties of Nextel 720 relating to its suitabilityof a woven, continuous oxide fibre reinforced composite with a porous oxide matrix, the stress–strain behaviour being evaluated on centre-hole notched plates. In the as-received condition the 0/90
orientation of the composite exhibited non-brittle behaviour and moderate notch sensitivity at room temperature, 1000 and 1100
C. The non-brittle behaviour and low notch sensitivity could be attributed to both extensive fibre￾matrix debonding and shear damage in the matrix. The 45 orientation also had low notch sensitivity. At 1000
C it exhibited a significantly higher fracture stress and fracture strain than at room temperature. This effect can be attributed to simultaneous sintering and creep of the matrix occurring during the test. Thermal exposure at 1000 and 1100
C caused a pro￾gressive degradation of the composite indicated, in the 0/90
composite, by a fall in room-temperature strength and strain to failure for a given sample geometry (a/w ratio). This degradation could be described in terms of a Larson–Miller plot which consequently permitted the prediction of the strength loss for a given exposure temperature and time combination within the investi￾gated experimental range. Thermal exposure of the 45 composite led to an increase in its room-temperature fracture strength but also a marked embrittlement. The embrittlement of the 0/90
material could be successfully characterised in terms of an effective frac￾ture toughness (KC) and effective damage zone length (c0) derived on the basis of the simple model of Waddoups et al.21 These parameters were constant (i.e. independent of a/w) for a given thermal exposure but decreased consistently with thermal exposure tempera￾ture and time. Exposure for 100 h at 1100
C led to almost ‘‘com￾plete’’ degradation of the composite in the sense that the fracture was brittle and the values of KC and c0 fell to values characteristic for a monolithic oxide while the notch sensitivity was much greater than for the lower thermal exposures. The fact that the 45 samples exhibited similar stress–strain behaviour after the same exposure is consistent with this interpretation. Microstructural and fractographic examination indi￾cated that the property degradation was caused by a combination of matrix densification and increased fibre/ matrix bonding. Some degradation of fibre properties could also have been a contributory factor. Formation of cristobalite was observed in samples treated for 100 h at 1100
C. This was deemed to have occurred locally in the matrix and not to have contributed directly to the property degradation. Acknowledgements This work was sponsored in part by the US Depart￾ment of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy under contract DE. AC05-00OR22725 with UT-Battelle, LLC. Financial support from the Swedish Research Council for Engi￾neering Sciences (TFR) is also acknowledged. The authors thank Dr. Michael Lance for performing the RAMAN experiment and Siemens-Westinghouse and Composite Optics for providing the material used in this study. Dr. Oleg Babushkin is acknowledged for per￾forming the XRD experiments. References 1. Tu, W.-C., Lange, F. F. and Evans, A. G., Concept for a damage-tolerant ceramic composite with ‘‘strong’’ interfaces. J. Am. Ceram. Soc., 1996, 79, 417–424. 2. Levi, C. G., Yang, J. Y., Dalgleish, B. J., Zok, F. W. and Evans, A. G., Processing and performance of an all-oxide ceramic com￾posite. J. Am. Ceram. Soc., 1998, 81, 2077–2086. 3. Kelly, A., Zweben, C., ed., Comprehensive Composite Materials. Elsevier Science Ltd, London, 2000. 4. Antti, M.-L. and Lara-Curzio, E., Effect of notches, specimen size, and fiber orientation on the monotonic tensile behavior of composites at ambient and elevated temperatures. Ceram. Eng. Sci. Proc., 2001, 22, 643–650. 5. Turner, K. R., Speck, J. S. and Evans, A. G., Mechanisms of deformation and failure in carbon-matrix composites subject to tensile and shear loading. J. Am. Ceram. Soc., 1995, 78, 1841– 1848. 6. Heredia, F. E., Spearing, S. M., Mackin, T. J., He, M. Y., Evans, A. G., Mosher, P. and Brondsted, P., Notch effects in carbon matrix composites. J. Am. Ceram. Soc., 1994, 77, 2817–2827. 7. Heredia, F. E., Spearing, S. M., Evans, A. G., Mosher, P. and Curtin, W. A., Mechanical properties of continuous-fiber-rein￾forced carbon matrix composites and relationships to constituent properties. J. Am. Ceram. Soc., 1992, 75, 3017–3025. 8. McNulty, J. C., Zok, F. W., Genin, G. M. and Evans, A. G., Notch sensitivity of fiber-reinforced ceramic-matrix composites: effects of inelastic straining and volume-dependent strength. J. Am. Ceram. Soc., 1999, 82, 1217–1228. 9. Buchanan, D. J., John, R. and Zawada, L. P., Notched fracture behavior of oxide/oxide NextelTM720/AS composite. Ceram. Eng. Sci. Proc, 2000, 21, 581–588. 10. Heathcote, J. A., Gong, X.-Y., Yang, J. Y., Ramamurty, U. and Zok, F. W., In-plane mechanical properties of an all-oxide cera￾mic composite. J. Am. Ceram. Soc., 1999, 82, 2721–2730. 11. Kramb, V. A., John, R. and Zawada, L. P., Notched fracture behavior of an oxide/oxide ceramic-matrix composite. J. Am. Ceram. Soc., 1999, 82, 3087–3096. 12. Mackin, T. J. and Roberts, M. C., Evaluation of damage evolu￾tion in ceramic-matrix composites using thermoelastic stress analysis. J. Am. Ceram. Soc., 2000, 83, 337–343. 13. Jurf, R. A. and Butner, S. C., Advances in oxide–oxide CMC. In: Proc. Int. Gas Turbine and Aeroengine Congress and Exhibition, Indianapolis, IN, 1999. 14. Personal Communication with E. Carelli, Siemens Westinghouse, Pittsburgh, PA. 15. Dele´glise, F., Berger, M. H., Jeulin, D. and Bunsell, A. R., Microstructural stability and room temperature mechanical properties of the Nextel 720 fibre. Journal of the European Cera￾mic Society, 2001, 21, 569–580. 16. Milz, C., Goering, J. and Schneider, H., Mechanical and micro￾structural properties of NextelTM 720 relating to its suitability for M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578 577