Experimental study of dynamic bending failure of Laurentian granite: loading rate and pre-load effects

In deep underground rock engineering projects, rocks are under static pre-load and they may further experience dynamic load due to earthquakes or production blasts. It is thus desirable to consider dynamic failure of rocks subjected to static pre-load. Besides, bending load is commonly encountered near underground openings. Therefore, this study considers the effect of the pre-load on the dynamic bending strength of Laurentian granite (LG). Using a modified split Hopkinson pressure bar system, the semi-circular bend (SCB) method is applied to carry out the bending tests. Five groups of SCB specimens are tested under different pre-loads and loading rates. The results show that under a given pre-load, the flexural tensile strength of LG increases with the loading rate, and decreases with the static pre-load at a given loading rate. The total flexural tensile strength is roughly independent of the pre-load. An empirical equation is used to represent the effects of the loading rate and the pre-load force on the dynamic flexural tensile strength. Furthermore, the flexural tensile strengths measured from SCB tests have higher values than the tensile strengths measured using the Brazilian disc method for the same rock. A nonlocal failure theory is utilized to quantitatively interpret this discrepancy.