The strength properties of two granitic rocks, Laurentian and Stanstead (with average grain size of 0.62mm and 1.13mm respectively) have been studied as a function of sample diameter and loading rate. The static and dynamic tensile strength tests were conducted over a diameter range of 19mm to 75 mm. The static Uniaxial Compressive Strength (UCS) tests were conducted over the same range of the diameters whereas the dynamic UCS tests were conducted using 19mm and 25mm diameters. The compressive strengths under static load were found to be insensitive over these diameters, but for tensile tests under both static and dynamic loading, 25mm diameter would be considered the lower limit for both rocks. Both rocks exhibited significant rate sensitivity showing an approximately linear increase with stress rate within the range of load rate of 106–107 MPa/s employed. The dynamic amplification factor (DF) for the coarser grained Stanstead was higher than that of Laurentian under compression. The amplification factor with increasing load rate for tensile strength was five to eight times that of the static value; the same for the dynamic compressive strength was less than a factor of two even over a much higher load rate.
Rocks are characterized by discontinuities ranging from macro- to micro-scale. These discontinuities play a major role in controlling their response under different loading conditions at all scales. Split-Hopkinson pressure bar (SHPB) apparatus is the most commonly used method for the measurement of dynamic tensile and compressive strength. Many significant improvements have been made to the test since its inception, and an increase in the dynamic strength with varying loading rates have been reported by many researchers (Grady & Kipp, 1987, Frew et al. 2001, 2002 & Wang et al. 2006, 2009).