Abstract
Subcritical crack growth has a strong dependence on the directions of crack opening and crack propagation. The maximum growth rate was higher by several orders of magnitude than the minimum one for Oshima granite. From the theoretical consideration on the torsion of two thin orthotropic plates, a way for analyzing the data of the double-torsion technique for anisotropic materials was established. Based on the Christoffel's equation, nine elastic stiffness constants of Oshima granite were determined from the sound velocities, Vp and Vs, propagating in various directions. The results of the anisotropic analysis showed that the observed difference in crack velocity of different directions was not an error due to the isotropic analysis but the intrinsic nature of the granite. A theory based on the microcrack growth model showed that the difference in crack velocity by several orders of magnitude brings in a difference of only several times in dilatancy and that of only several percents in the uniaxial compressive strength.
