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| TOUGHNESS INCREASING OR DECREASING EFFECT OF HARD
ROCK FRACTURE WITH PRE-STATIC LOADING UNDER
DYNAMIC DISTURBANCE |
| GONG Fengqiang1，2，3，LU Daohui1，LI Xibing1，3，RAO Qiuhua4，FU Zhentao1 |
| (1. School of Resources and Safety Engineering，Central South University，Changsha，Hunan 410083，China；2. State Key Laboratory for Geomechanics and Deep Underground Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221008，China；3. Institute of Mechanics for Engineering Materials，Advanced Research Center，Central South University，Changsha，Hunan 410083，China；4. School of Civil Engineering，Central South University，Changsha，Hunan 410075，China) |
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Abstract In order to investigate the variation of fracture characteristics of hard rock under the condition of pre-static loading with dynamic disturbance，the semi-circular bend(SCB) marble specimens with central straight cracks were tested with the MTS Landmark electro-hydraulic servo testing machine under cyclic loading and unloading with different high disturbance frequencies. The effects of toughness increasing and decreasing of the hard rock fracturing were discovered. The value of 90% of the conventional static fracture load was set as the upper limit of the pre-static load. The cyclic loading and unloading tests were performed with the loading cycles of 5，10，20，40，80，100 and 167，respectively. The dynamic disturbance tests with different frequencies under pre-static load were conducted by using the semi-sine loading wave and the value of 10% of the conventional static fracture load was set as the amplitude of loading fluctuation and the loading frequency was 1，10，20 and 30 Hz，respectively. It was found that with the increase of the loading cycles，the fracture toughness of SCB specimen increased significantly firstly. The fracture toughness reached the highest value at the loading cycles of 40 with an increase of 19% compared with the conventional fracture toughness. Then，it gradually decreased and reached a stable value at the loading cycles of 80 with an increase of 11% compared with the conventional fracture toughness. Overall it exhibited a toughness-increasing effect. Under the condition of dynamic disturbance，rock fracture toughness values were reduced greatly compared to that of the normal condition. For example，when the loading frequency was 1 Hz，the rate of decrease was 9% compared with the conventional fracture toughness. The rock fracture toughness decreased linearly with the increase of the disturbance frequency. In conclusion，under the conditions with pre-static load(90% of conventional static fracture load)，cyclic loading and unloading can increase the fracture toughness of hard rock. While the fracture toughness of rock decreased under the combined loading of pre-static load and high frequency disturbance，which indicated that the high frequency disturbance was conducive to the damage of rock.
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Received: 28 April 2014
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2014, Vol. 33 
