Experimental studies on a granite under dynamic compression are conducted at strain rates from 10–4 to 100 s-1 and confining pressures from 0 to 170 MPa. It is reported that the rock strength increase with increasing strain rates. Under dynamic triaxial compression, the rising rates of rock strength with increment of strain rate have a tendency to decrease with increasingconfining pressures. As that of under static loads, the rock strength increase with increasing confining pressures at different strainrates. The rising rates are almost identical under 4 different strain rates studied. It is also observed that the results of measuredYoung's moduli and Poisson's ratio are scatter and have no clear tendency changing with increasing strain rates and confiningpressures.
Dynamic mechanical properties of rock materials are basic Information in assessing the stability of rock structures as well as blasting wave propagation in rock structure under dynamic loads induced from blasting excavation or earthquake. Studies on this topic have been conducted primarily by laboratory tests. These tests showed that the mechanical properties of rock materials are rate dependent. For example, the strength of rock material has a general trend to increase with increasing strain rate. In addition, it is observed that the rates of increase aredifferent for different rocks and confining pressures. For example, Donath and Fruth conducted dynamic triaxial compression tests on a marble at confining pressure of 100 and 200 MPa, and strain rate from 10–7 to 10–3 s-1. They reported that at confining pressure of 100 MPa, the strength increase by 30% when strain rate increase by 5 orders of magnitude, while under the confining pressure of 200 MPa, the strength increase by 40% for the same increment of strain rate. Similarly, Logan andHandin conducted dynamic triaxial compression tests on the Westerly granite at confining pressures up to 700 MPa, they found that the strength increases proportionally With increasing Strain rate and the rising rate increases with increasing confining pressure. The same results are also reported byMasuda et al on granite. However, some researchers found thatthe strain rate sensitivity seems to decrease with increasing Confining pressure. For example, Ju and Wu showed that the strength of a tuff increases by 40% and 20% under confining pressure of 0 MPa and 90 MPa respectively, while the strain rate changes from 10–5 to 10–1s-1. Similar experimental results are also reported by Sangha and Dhir on a sandstone, Yang andLi on a marble.This paper presented a series of dynamic compression testsfor a granite at strain rates from 10–4 to 10° s-1 and confining pressures from 0 to 170 MPa. The change of the rock strength, Young's moduli and Poisson's ratio with strain rate and confining pressures are reported. In addition, the mechanics involve the rate dependent properties of rock material are briefly introduced.
The dynamic testing system is shown in figure I.