attempted to show the limitation of the Evans and Charles approach for transformation- toughened ceramics, especially when studying residually stressed surfaces. Indeed, the use of their approach can substantially alter the form of the indentation data, and hence confuse its interpretation. Acknowledgements The author would lke to thank D. B. Marshall for pointing out the difficulties involved in [2] and [8]. References 1. Y. IKUMA and A. V. VIRKAR, J. Mater. Sci. 19 (I984) 2233. 2. A. G. EVANS and E. A. CHARLES, J. Amer. Ceram. Soc. 59 (1976) 371. 3. B. R. LAWN, A.G. EVANS and D.B. MARSHALL, ibid. 63 (1980) 574. 4. G. R. ANSTIS, P.R. CHANTIKUL, B.R. LAWN and D. B. MARSHALL, ibid. 64 (1981) 533. 5. D. B. BHAT, ibid. 64 (1981) C-165. 6. D. B. MARSHALL and A. G. EVANS, ibid. 64 (1981) (2182. 7. D. J. GREEN, F.F. LANGE and M.R. JAMES, ibid. 66 (1983) 623. 8. A. G. EVANS, "Fracture Mechanics Applied to Brittle Materials,"edited by S. W. Freiman (Specialist Technical PublicationsVol. 678, AmericanSociety for Testing and Materials, Philadelphia, 1978) p. 112. 9. D. B. MARSHALL and B. R. LAWN, J. Amer. Ceram. Soc. 60 (1977) 86. 10. D. J. GREEN, ibid. 66 (1983) C-178. 11. D. J. GREEN, F. F. LANGE and M. R. JAMES, "Advances in Ceramics", edited by N. Claussen et al., Vol. 12 (American Ceramic Society, 1984) p. 240. Received 9 October and accepted 9 November 1984 D. J. GREEN Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Reply to "Comments on "'Crack-size dependence of fracture toughness in transformation-toughened ceramics"" In recent years, several investigators have used an indentation technique for determining frac- ture toughness, Kc, of brittle materials. Two approaches, both based upon the indenter crack approximated as a penny-shaped crack under point loading at the centre, have been suggested. Evans and Charles [1] used data from various materials of known fracture toughness (Kc) values and generated a master curve by plotting (Kc~/Hal/2)(H/~E) ~ against c/a, where His the hardness, E is the Young's modulus of elasticity, c is the crack radius and a is half the indent diagonal. The other approach, proposed by Anstis et al. [2] sets Kc = )~P/c 3/2 with Z being a material dependent parameter. Green [3], in the preceding discussion of a recent paper by Ikuma and Virkar [4], suggests that the technique of Evans and Charles [1] leads to erroneous results and that one must use the method given by Anstis et al. [2]. The present authors have also noted that the two indentation techniques often give differing results and a discussion of this point is warranted. As will be shown in the following, however, the contention by Green [3] that the problem lies in the validity of the Evans and Charles [1] approach is without basis. The objective of our response is twofold. Firstly, we will examine Green's [3] data and assess the validity of the concept of the crack growing out of the zone of compression. Secondly, we will examine our data using both of the techniques and attempt to sort out the source of the dis- crepancy. Green [3] suggests that the increase in Ko (as determined using the method of Evans and Charles [1]) with increasing c ~/2 observed in our and his work is the result of nonlinearity in the (KcC~/Ham)(H/~E) ~ against c/a curve at low values of c/a. By contrast, the method of Anstis et al. [2] yielded K c independent of c [3]. In Fig. 1, the data of Ikuma and Virkar [4] on ZrO2 + 4.5mol%Y203 is replotted. Kc cal- culated using the method of Anstis et al. [2] is also plotted in the same figure. Note that Kc against c ~/2 using this technique actually decreases with increasing c J/2. For similarly prepared samples, Green [3] finds Kc indepen- dent of d/2. However, when K~ decreases with increasing c ~/2, Green [3] suggests that this 0022-2461/85 $03.00 + .12 9 1985 Chapman and Hall Ltd. 4241