EFFECT OF MECHANICAL ANISOTROPY ON TRANSMISSION OF LOW-AMPLITUDE STRESS WAVES IN BRITTLE ROCK

Results are presented of a study of the transmission of low-amplitude stress waves in dry brittle rock materials which are characterized by a mechanical anisotropy resulting from a nonrandom distribution of open cracks within the rock structure. This form of anisotropy can result from an intrinsic mechanical anisotropy arising from a nonrandom open crack distribution and/or a stress-induced anisotropy which may result from a selective crack closure in response to a nonhydrostatic state of stress. It is assumed that the dynamic response of brittle rock is dependent only on the intrinsic (crack-free) rock properties and the density and distribution of open cracks within the rock structure. It is shown that, irrespective of the direction of wave transmission, the dynamic stress-strain relationships display a symmetry characteristic of materials exhibiting orthotropic anisotropy. This behavior is a result of a distinctive coordinate transformation property of the dynamic stress-strain relationships of isotropic materials containing a large number of open cracks distributed throughout the structure. Equations relating bar and free medium velocities to the orthotropic 'elastic' constants are discussed and it is shown that a combination of the two methods can be used to completely specify the dynamic elastic constants for a single oriented specimen of rock. © 1969.