1. Paris, P. C. and Sih, G. C., Fracture Toughness Testing and Its Applications, ASTM STP 381, p. 30 (1965).
2. Kantorovich, L. V. and Krylov, V. I., Approximate Methods of Higher Analysis, Inter-science, New York (1964).
3. Irwin, G. R., The Crack-Extension Force for a Crack at a Free Surface Boundary, U. S. Naval Research Laboratory Report 5120 (1958).
4. Lachenbruch, A. H., J. Geophys. Res., 66, p. 4273 (1961).
5. Broi, P., Acta Technical Csav, 15, p. 724 (1970).