1. 1) N. J. Petch: Fracture, John Wiley and Sons, New York, (1959), p. 54.
2. 2) S. Matsuda, T. Inoue and M. Ogasawara: Trans. JIM, 9 (1968), 343.
3. 3) G. T. Hahn et al.: Fracture, John Wiley and Sons, New York, (1959), p. 91.
4. 4) A. M. Turkalo: Trans. Met. Soc. AIME, 218 (1960), 24.
5. 5) To be published.