Numerical Modeling of S-Wave Generation by Fracture Damage in Underground Nuclear Explosions

The quasi-static micromechanical damage mechanics originally formulated by Ashby and Sammis (PAGEOPH, 1990) has been expanded in three important ways: (1) An energy density function has been derived that allows a self-consistent inclusion of the effects of dynamic damage evolution on the elastic and anelastic response. (2) whereas the Ashby/Sammis model was only applicable to the case where the initial cracks are all parallel and the same size, we can now include a specified distribution of initial crack sizes with random azimuthal orientation about the loading axis, and (3) we allow for yielding of the weaker minerals in granite at relatively low stress levels which produce strong nonlinearity in the failure envelope and stress-strain curve. This new damage mechanics is validated using published experimental data for Westerly granite. In collaboration with the mechanics group at U.C. Santa Barbara, we have built our new damage mechanics into the commercial finite element program ABAQUS and used it to simulate an explosion in damaged rock. We find that an explosion in a pre-stressed medium or one that contains anisotropic initial damage generates strong S-wave radiation in agreement with previous simpler calculations by Johnson and Sammis (PAGEOPH. 2001) and experimental observations in damaged photoelastic plates.
Presented at 2009 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies held in Tucson, AZ on 21-23 Sep 2009. Published in the Proceedings of 2009 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies, v3 p557-565, 2009. The original document contains color images. All DTIC reproductions will be in black and white.