The rate-sensitivity of the relatively homogeneous and isotropic Stanstead granite has been studied in terms of its unconfined compressive strength, Brazilian tensile strength, and mode-I fracture toughness. Variable load rates up to a maximum of 1.15 × 107MPa/s were applied to the test samples by means of a calibrated split Hopkinson pressure bar apparatus. The study showed that both tensile strength and fracture toughness increased by an order of magnitude compared to their static value, whereas, the compressive strength increased only by about 70%. All parameters however exhibited a linear dependence with load rate over this range. These findings, if confirmed in other rock types, would have a very significant effect in developing predictive numerical models for fragmentation of rock in drilling, crushing and blasting operations.
Except for some mono-mineralic rocks, most exhibit some degree of anisotropy. This non-uniqueness is however considered of secondary importance. The average values of such properties as various strength (e.g. compressive, tensile, shear, etc.) and fracture parameters (e.g. fracture toughness) are considered adequate for calculation of stress and strain fields, load-bearing capacity, and stability of various structures. A wide range of advanced numerical codes based on these parameters are currently available, and have been utilized very successfully to address geo-mechanical issues under static or quasi-static loads in both underground and above-ground operations. However, there is a more systematic and much more significant variation of these properties than regular anisotropy, when the rock is subjected to a rapidly varying load such as in high-velocity impact or explosive action. In the field of explosively driven fractures in rock and in areas of hazard from explosively driven projectiles, it is very important to estimate both fragment size distribution as well as well as their respective velocities. Accurate estimate of the former is a key element in all excavation and mining operations employing explosives. That the response of rocks and concrete to dynamic loads (i.e. varying stress-rate and strain-rate) would be different than under static or quasi-loads has been widely known for several decades (Green and Perkins, 1969; Goldsmith et al, 1976; Wang et al, 2006; Wang et al, 2009). This is in sharp contrast to the varying degree of anisotropy of properties found in most rock. In order to study strain-rate dependence of fracture and strength properties of rocks, Grady and Kipp (1987) employed the concepts of inherent flaw and energy dissipation in the fracture process involving both single and multiple cracks to predict dynamic fracture strength. However, the use of experimental data obtained under static loads in predicting response of rock to impact and explosive loads continues to be common practice, mainly because of lack of extensive experimental data on the response of such materials under dynamic loads. Often the same strength and fracture properties obtained under static loads are, or some arbitrarily modified values of these parameters are employed in these calculations to account for the effect of dynamic loads on the response of rock materials.