An analytical and experimental investigation of the effect of impact on coarse granular rocks

SummaryAn Analytical and Experimental Investigation of the Effect of Impact on Coarse Granular Rocks The interaction between metallic strikers and coarse, granular rock, associated with many mechanical rock breaking methods was investigated by theoretical and experimental methods. Cylindrical steel strikers of 12.7 mm diameter with flat, conical and hemispherical tips and masses of about 18.5 g were fired by means of a gas gun at blocks of diorite and spessartite with initial energies ranging from 4 to 34 J, generating substantial fracturing. The damage pattern in the rocks was ascertained, partly with the aid of a scanning electron microscope. A synthetic model consisting of long square bars of cement paste bonded with an adhesive was constructed as a simulator of diorite and tested to further delineate the failure mechanisms in this material.As previously, it was also found here that the geometry of the striker tip significantly affects the damage pattern and extent in diorite. A crucial difference in this pattern was observed between that found in diorite, a coarsegrained rock, and in spessartite, a more finely-grained substance. The crack network in diorite consisted of numerous kinked fractures extending a distance not in excess of 20 grain lengths, whereas only 5 to 7 nearly straight cracks with a length in excess of 20 times the grain size were found in spessartite that appeared to have propagated without regard to the grain packing structure or material defects. The synthetic rock model successfully reproduced the crack pattern found in diorite under impact.An analytical model to predict the region of grain and grain boundary failure incorporating one failure criterion for grains and another for grain boundaries was constructed. The first involved the development of failure surfaces based on an empirical limiting strength analogous to the modified Griffith criterion. Grain boundary failure was stipulated upon attainment of a combination of critical tensile and Coulomb type of shear stress. The model successfully delineated the major features of damage in the synthetic rock and in diorite and established upper bound predictions for the extent of damage.With 19 Figures