AN EXPERIMENTAL AND ANALYTICAL INVESTIGATION OF THE RESPONSE OF ROCK TO IMPACT LOADING

An experimental and analytical investigation of the interaction between metallic strikers and the rock associated with many mechanical rock breaking methods was conducted. Cylindrical strikers of 6.35 mm and 12.7 mm diameter with flat, conical and hemispherical tips and masses ranging from 2 to 30 g were fired by a gas gun at blocks of hard granular rock, diorite and spessartite, and soft porous rock, green shale, in an initial kinetic energy range of 5 to 30 J. While the impact on harder rock involved considerable fracturing in the target, large permanent deformation and compaction were observed in softer rock.

Information has been collected on the damage pattern and extent in both diorite and spessartite, including an examination of a diorite specimen using a scanning electron microscope. A synthetic model which consisted of long cement paste square bars bonded with an adhesive was constructed to further ascertain the mechanism of the failure process in diorite.

It was found that the geometry of the striker tip greatly affects the resultant damage pattern and extent in diorite. A crucial difference was observed between the damage patterns in diorite, a coarse-grained rock, and in spessartite, a finer-grained rock. While the crack network in diorite consisted of numerous kinked cracks extending a distance not exceeding 20 times its grain size, only 5 to 7 nearly straight cracks longer than 20 times the grain size were found in spessartite. Furthermore, these cracks in spessartite seemed to have propagated ignoring both the grain packing structure and the defects in the material unlike those found in diorite. The synthetic rock was successful in reproducing the crack pattern observed in diorite.

An analytical model to predict the region of grain and grain boundary failure in coarser-grained rock due to impact loading was constructed. The model incorporates two failure criteria: one for grains and the other for grain boundaries. The first consisted of the creation of failure surfaces using an empirical criterion having similar characteristics as the modified Griffith criterion. The advent of grain boundary failure combined a critical tensile stress and a Coulomb type shear criterion. The model was successful in delineating the major features of damage in the synthetic rock and diorite and in establishing an upper bound prediction in the extent of damage.

Depth of penetration, degree of deformation in the material, and the velocity history of the projectiles were measured for the impact of 60(DEGREES) conically- and hemispherically-tipped cylindrical projectiles with a diameter of 6.35 mm on green shale. For both types of projectile, the depth related quadratically to the initial kinetic energy but the depth for the 60(DEGREES) cone tip was about 1.5 times more than that of hemispherically-tipped striker.

An analytical model to predict the contour of the deformed region and the degree of penetration in green shale for a given initial kinetic energy and geometry of the projectile was constructed. The model determines the energies associated with compaction and distortion without any volume change. Final penetration depth predictions within (+OR-)10% of the experimental results were obtained over an initial striker energy range of 1 to 500 J including data from other investigations.