Quantitative assessment of extrinsic damage in rock materials

Comminution (crushing and grinding) of rock materials is energy-intensive and expensive. Much effort has been directed to improve the efficiency of conventional milling practice, but relatively little attention has been given to the potential benefits of blast-induced (extrinsic) damage on comminution processes. The objective of this research is to investigate the effect of shock-induced "crack-like" damage on rock properties for three petrologically distinct rock materials under laboratory conditions. In order to evaluate the effect of shock-induced damage, a quantitative measure of rock damage is needed. Shock loading of rock material was accomplished with an explosively driven split Hopkinson pressure bar. Laboratory measurements show that the average uniaxial compressive strengths for damaged specimens are slightly lower than those for intact specimens. Based on damage mechanics, the scalar damage variable (omega) was experimentally determined as the relative reduction in ultrasonic wave velocity of damaged versus intact rock, omega increases as the shock energy dissipated in rock material increases. A crack density was determined by confocal image analysis. Measurements following shock loading indicate that ultrasonic wave velocity in rock partially recovers with time. Ultrasonic wave velocity and confocal image analysis may be useful in quantitatively assessing the internal crack-like damage in rock materials.