loading cell. The more the stiffness, the less crack path before arrest. It is evident that the stable crack growth leads to strengthening and to strength insensitiveness to the initial flaw size. In a layered material design process, it necessary to determine the range of stable crack growth and strengthening if the range of flaws in layers is known Usually, technological flaws are relatively small in laminates. Only rising dependence of the apparent fracture toughness on the crack length is not sufficient to obtain the desirable strengthening and toughening. The dependence is effective in imparting flaw tolerance only if the slope of the apparent fracture toughness curve is steep at short crack lengths. Obtaining high residual compressive stress is an effective way to provide high toughness at small crack lengths, thereby ensuring the improved flaw tolerance and surface damage resistance Conclusions. The toughening of ceramic-matrix asymmetric laminates with elastic inhomogeneity has been studied both analytically and experimentally. Applied and residual stress distributions are determined for an arbitrary alteration of different layers. Expression for the apparent fracture toughness of residually-stressed asymmetric layered material is obtained. The most appropriate coordinate system to analyze fracture conditions of laminar composite shown to be the system where the apparent fracture toughness is depicted depending on the crack length parameter a=r(o)a 4. The dependences of the apparent fracture toughness on the crack length parameter are calculated for the specimens under study. Conditions of crack arrest, stable and unstable crack growth in layered structure are analyzed. It is shown that the path passed by the crack before arrest can vary depending on the stiffness of a loading cell. The experimental values of the apparent fracture toughness are measured using the compliance technique. They are in good agreement with the calculated data REFERENCES M. Chan, "Layered ceramics: processing and mechanical behavior, " Ann. Rev. Mater. Sci., 27, 249-28 (1997) 2. W. J. Clegg, K Kendall, N. MCN Alford, et al., "A simple way to make tough ceramics, "Nature, 347 455-457(1990 3. R. Lakshminarayanan, D. K. Shetty, and R. A Cutler, "Toughening of layered ceramic composites with esidual surface compression, "J. Amer. Ceram Soc., 79, No. 1, 79-87(1996) 4. N. Lugovoi, N. Orlovskaya, V. Slyunyayev, et al., Crack bifurcation features in laminar specimens with fixed total thickness, "Comp. Sci. Tech, 62, 819-830(2002) 5. D. B. Marshall, JJ Ratto, and F. F. Lange, ""Enhanced fracture toughness in layered microcomposites of CeO-Zro, and Al,O3, J. Amer. Ceram. Soc., 74, No. 12, 2979-2987(1991) 6. N. Lugovoi, N. Orlovskaya, K. Berroth, and J. Kuebler, "Macrostructural engineering of ceramic-matrix layered composites, "Comp. Sci. Tech, 59, 1429-1437(1999) 7. A.J. Blattner, R. Lakshminarayanan, and D. K. Shetty, "Toughening of layered ceramic composites with residual surface compression: effects of layer thickness, " Eng. Fract. Mech., 68, 1-7(2001) 8. A G. Evans, "Perspective on the development of high-toughness ceramics, "J. Amer:. Ceram. Soc., 73, No. 2, 187-206(1990) 9. V. M. Sglavo, L. Larentis, and D. J. Green, " Flaw-insensitive ion-exchanged glass: I, theoretical aspects J.Amer. Ceram.Soce,84,No.8,1827-1831(2001) 10. R.J. Moon, M. Hoffman, J. Hilden, et al., "Weight function analysis on the R-curve behavior of multilayered alumina-zirconia composites, " J. Amer. Ceram. Soc., 85, No 6, 1505-1511(2002) 11. T. Fett and D Munz, "Influence of crack-surface interactions on stress intensity factor in ceramics, J Mater Sci.Let.,9,1403-1406(1990). 12. S P. Timoshenko and J. N. Goodier, Theory of Elasticity, 3rd edn., McGraw-Hill, New York(1970) A. E. Giannakopoulos, S. Suresh, M. Finot, and M. Olsson, ""Elastoplastic analysis of thermal cycling layered materials with compositional gradients, Acta Metall. Mater., 43, No. 4, 1335-1354(1995) 14. V. Sergo, D. M. Lipkin, G. de Portu, and D. R. Clarke, Edge stresses in alumina/zirconia laminate, ".Amer. Ceran.Soc.,80,No.7,1633-1638(1997) 302loading cell. The more the stiffness,the less crack path before arrest. It is evident that the stable crack growth leads to strengthening and to strength insensitiveness to the initial flaw size. In a layered material design process, it is necessary to determine the range of stable crack growth and strengthening if the range of flaws in layers is known. Usually, technological flaws are relatively small in laminates. Only rising dependence of the apparent fracture toughness on the crack length is not sufficient to obtain the desirable strengthening and toughening. The dependence is effective in imparting flaw tolerance only if the slope of the apparent fracture toughness curve is steep at short crack lengths. Obtaining high residual compressive stress is an effective way to provide high toughness at small crack lengths, thereby ensuring the improved flaw tolerance and surface damage resistance. Conclusions. The toughening of ceramic-matrix asymmetric laminates with elastic inhomogeneity has been studied both analytically and experimentally. Applied and residual stress distributions are determined for an arbitrary alteration of different layers. Expression for the apparent fracture toughness of residually-stressed asymmetric layered material is obtained. The most appropriate coordinate system to analyze fracture conditions of laminar composite is shown to be the system where the apparent fracture toughness is depicted depending on the crack length parameter ~aY a = α( ) 1 2 . The dependences of the apparent fracture toughness on the crack length parameter are calculated for the specimens under study. Conditions of crack arrest, stable and unstable crack growth in layered structure are analyzed. It is shown that the path passed by the crack before arrest can vary depending on the stiffness of a loading cell. The experimental values of the apparent fracture toughness are measured using the compliance technique. They are in good agreement with the calculated data. REFERENCES 1. M. Chan, “Layered ceramics: processing and mechanical behavior,” Ann. Rev. Mater. Sci., 27, 249–282 (1997). 2. W. J. Clegg, K. Kendall, N. McN Alford, et al., “A simple way to make tough ceramics,” Nature, 347, 455–457 (1990). 3. R. Lakshminarayanan, D. K. Shetty, and R. A. Cutler, “Toughening of layered ceramic composites with residual surface compression,” J. Amer. Ceram. Soc., 79, No. 1, 79–87 (1996). 4. N. Lugovoi, N. Orlovskaya, V. Slyunyayev, et al., “Crack bifurcation features in laminar specimens with fixed total thickness,” Comp. Sci. Tech., 62, 819–830 (2002). 5. D. B. Marshall, J. J. Ratto, and F. F. Lange, “Enhanced fracture toughness in layered microcomposites of CeO–ZrO2 and Al2O3,” J. Amer. Ceram. Soc., 74, No. 12, 2979–2987 (1991). 6. N. Lugovoi, N. Orlovskaya, K. Berroth, and J. Kuebler, “Macrostructural engineering of ceramic-matrix layered composites,” Comp. Sci. Tech., 59, 1429–1437 (1999). 7. A. J. Blattner, R. Lakshminarayanan, and D. K. Shetty, “Toughening of layered ceramic composites with residual surface compression: effects of layer thickness,” Eng. Fract. Mech., 68, 1–7 (2001). 8. A. G. Evans, “Perspective on the development of high-toughness ceramics,” J. Amer. Ceram. Soc., 73, No. 2, 187–206 (1990). 9. V. M. Sglavo, L. Larentis, and D. J. Green, “Flaw-insensitive ion-exchanged glass: I, theoretical aspects,” J. Amer. Ceram. Soc., 84, No. 8, 1827–1831 (2001). 10. R. J. Moon, M. Hoffman, J. Hilden, et al., “Weight function analysis on the R-curve behavior of multilayered alumina-zirconia composites,” J. Amer. Ceram. Soc., 85, No. 6, 1505–1511 (2002). 11. T. Fett and D. Munz, “Influence of crack-surface interactions on stress intensity factor in ceramics,” J. Mater. Sci. Lett., 9, 1403–1406 (1990). 12. S. P. Timoshenko and J. N. Goodier, Theory of Elasticity, 3rd edn., McGraw-Hill, New York (1970). 13. A. E. Giannakopoulos, S. Suresh, M. Finot, and M. Olsson, “Elastoplastic analysis of thermal cycling: layered materials with compositional gradients,” Acta Metall. Mater., 43, No. 4, 1335–1354 (1995). 14. V. Sergo, D. M. Lipkin, G. de Portu, and D. R. Clarke, “Edge stresses in alumina/zirconia laminate,” J. Amer. Ceram. Soc., 80, No. 7, 1633–1638 (1997). 302