1. R. Kieffer and P. Schwarzkopf,Hartstoffe und Hartmetalle(Springer-Verlag, Vienna, 1953).
2. W. P. Mason,Physical Acoustics and the Properties of Solids(D. Van Nostrand Company, Inc., Princeton, New Jersey, 1958), p. 370.
3. J. J. Gilman inFracture, edited by Averbach, Felbeck, Halm, and Thomas (John Wiley & Sons, Inc., New York, 1959), p. 204.