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HENAGER et al. SUBCRITICAL CRACK GROWTH: PART I 3737 cracking offsets this and, fortuitously, brings the 12. Jones, R H, Henisch, H L and Mccarthy, KA,J.Nucl. Mater.,1999,271-272,518 effective crack calculation into agreement witn 13. Lewinsohn, C.A., Jones, R. H, Youngblood, G. E and experiment(Fig. 9a) Henager, C. H. Jr, J. Nucl. Mater., 1998, 258-263(Pt 5. SUMMARY AND CONCLUSIONS 14. Abbe. F. Vicens, J. and Chermant. J. L.J. Mater:. Sci. Let,1989,8(9),1026. 15. Chermant, J. ve a growth of subcritical cracks in argon for Hi-Nicalon and R. W. Evans, 5th ed. Institute of Materials, London and Nicalon -CG fiber-reinforced Cvi silicon-carbide UK,1993,p.371 composites (SiC/SiC) is controlled by fiber creep rocesses. Four major observations support this 76(10),2695 assertion:(1)displacement-time data and crack 17. Grathwohl, G, Meier, B and Wang, P, Key Eng Mater. 1995,108-110(Ce length-time data during crack growth obey Sherby- 18. Evans, A. G and Weber, C, Mater. Sci. Eng, 1996, Dorn(nonlinear)kinetics, similar to fiber creep kin- A208(1),1 etics, for each composite type; (2)activation energies Mumm, D. R, Morris, W, Dadkhah, M.S. and Cox, B for crack growth agree with independent measure- in Thermal and Mechanical Test Methods and Behavior of Continmuous-Fiber Ceramic Composites, ASTM STP 1309 nents of fiber-creep activation energies for each ASTM, 1997, p. 102 tested fiber type; (3)calculated strains in the multiply 20. Zhu, S, Mizuno, M, Kagawa, Y.Cao, J, Nagano, Y and cracked damage zone observed in sectioned and pol Kaya, H, Mater. Sci. Eng. 4, 1997, A225(1-2), 69 ished specimens compare favorably to predicted sin- 21. Mizuno, M, Zhu, S, Kagawa, Y and Kaya, H, Key Eng. gle-fiber creep strains, and (4) the increased creep 22. Cox, B. and Zok, F, in Britle Matrix Compos. 5.Proc. resistance of Hi-Nicalon fibers results in reduced Int. Symp., ed. A M. Brandt, V. C. Li and I. H. Marshall crack-growth rates of Hi-C composites compared to Sth ed. Woodhead, Cambridge, UK, 1997, p. 487 CG-C composites. Crack velocities, computed from 23. Wilshire, B Carreno,F and Percival,M.J.L,Scripta optical crack-length measurements, were in the range 24. Chermant, J.-L. and Boitier, G, Adv. Compos. Mater: 10-7 to 10-10 m/s and agree, albeit fortuitously with calculated effective elastic crack velocities. but 25. Zhu, s, Mizuno, M, Kagawa, Y, Cao, J, Nagano, still indicating the need for an improved crack Kaya, H, J. Am. Ceram Soc., 1999, 82(1), 117.and 26. Tressler R. E. Rugg K. L C. E. and Lamon. J Key Eng. Mater. 1999, 164-165 (High Temperature Cer- Acknowledgements-The authors thank L. Humason(ret 27. Begley, M.R. Cox, B.N. and mcMeekin, R. M, Acla P Keaveney, and G.K. Whiting for assisting with the mech- 28. Begley, MR, Cox, B N and McMeeking, R.M., Acta Walls, D. P Mech. Mater.,1997,26(2),81 1830 with Pacific northwest national laboratory. which is 30. Cox, B.N.,MarshalL, D. B, MCN R M. and operated for Doe by Battelle. Begley, M. R, Solid Mech. Its Appl., 1997, 49(IUTAM Symposium on Nonlinear Analysis of Fracture, 1995), 353. REFERENCES Materials. American Society of Mechanical Engineers, 1. Marshall, D. B. and Ritter, J. E, Am. Ceram. Soc. Bull, AMD, New York, NY, USA, 1995, p. 23 1987,66(2),309 32. Henager. C H. Jr Hoagland. R G. Acta mater. 2001 2. Henager, C H Jr and Jones, R. H, Mater. Sci. Eng, 1993. 49(18),3739-3753 A166(1-2),211. 33. Jones. R. H. Henager C. H. Jr and Windisch, C. F Jr 3. Henager. C. H. Jr. and Jones, R. H.J. Am. Ceram. Soc. later.Sci.Eng.A,1995,A198(1-2),103 1994,77(9),2381 4. DiCarlo, J A, Yun, H M, Morscher, G N and Goldsby, 4. Henager C. H. Jr. Jones. R. H. w C.Fr Stack J C, in High-Temperature Ceramic-Matrix Composites 1, oole, M. M. and bordia, R, Metall. Mater. Trans. A Ceram. Trans. 58. ed. A. G. Evans and R. Naslain. 1995 1996,27A(4).839 5. Nair. S. V. Jakus. K. and Lardner. T. J. Mech. Mater Lewinsohn, C. A, Henager, C. H. 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Soc., 1997, 80(3),569.HENAGER et al.: SUBCRITICAL CRACK GROWTH: PART I 3737 cracking offsets this and, fortuitously, brings the effective crack calculation into agreement with experiment (Fig. 9a). 5. SUMMARY AND CONCLUSIONS We have demonstrated conclusively that the growth of subcritical cracks in argon for Hi-Nicalon and Nicalon-CG fiber-reinforced CVI silicon-carbide composites (SiCf/SiC) is controlled by fiber creep processes. Four major observations support this assertion: (1) displacement–time data and crack length–time data during crack growth obey Sherby– Dorn (nonlinear) kinetics, similar to fiber creep kinetics, for each composite type; (2) activation energies for crack growth agree with independent measurements of fiber-creep activation energies for each tested fiber type; (3) calculated strains in the multiply cracked damage zone observed in sectioned and polished specimens compare favorably to predicted single-fiber creep strains; and (4) the increased creep resistance of Hi-Nicalon fibers results in reduced crack-growth rates of Hi-C composites compared to CG-C composites. Crack velocities, computed from optical crack-length measurements, were in the range of 107 to 1010 m/s and agree, albeit fortuitously, with calculated effective elastic crack velocities, but still indicating the need for an improved crackgrowth model. Acknowledgements—The authors thank J. L. Humason (ret.), J. P. 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