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2. A. G. EVANS, in ?Fracture mechanics of ceramics?, Vol. 1, edited by R. C. Bradt, D. P. H. Hasselman and F. F. Lange (Plenum Press, New York, 1973) p. 17.
3. S. FREIMAN and R. W. RICE, unpublished work.
4. C. C. M. WU, K. R. MCKINNEY and R. W. RICE, unpublished work.
5. C. C. M. WU and R. W. RICE, unpublished work.