Abstract：Size effect of rock static strength is validated by carrying out the conventional static loading tests on granite，sandstone and limestone specimens with diameters of 22，36 and 75 mm respectively and a constant length-diameter ratio of 0.5. In order to investigate the rock dynamic strength size effect and the strain-rate sensitivity subjected to axial impact，half-sine stress waveform with wave length proportioned to the specimens¢ length is chose as the loading method to carry out a series of split hopkinson pressure bars(SHPB) tests under different strain rates on granite，sandstone and limestone specimens with diameters of 22，36 and 75 mm respectively and a constant length-diameter ratio of 0.5. The test results show that the rock dynamic strength increases in form of power with the increase of strain rate and presents strong strain rate dependence. The bigger the specimen size is，the more notable the sensitivity of rock dynamic strength dependence to strain rate is，and the smaller discreteness of the obtained rock dynamic strength is. It is shown that the rock dynamic strength increases with the increase of specimen size under the same strain rate condition which is opposite to the size effect under static loading condition. In addition，the size effect of rock dynamic strength become weaker with the decrease of strain rate. This implies that there has a critical strain rate where the size effect will disappear completely. The static size effect takes the dominant position under the condition that the strain rate is below the critical strain-rate，while the dynamic size effect takes the dominant position under when the strain rate is above the critical strain rate.
