Contribution to experimental study of mechanical and thermal damage in crystalline hard rocks

The main objective of this work is to investigate crack propagation and damage evolution using acoustic emission (AE) to predict failure of rock during loading. In this research we recorded AE parameters in granite and gabbro rocks under stress, to establish a pattern of AE variation before failure that can be used in earthquake prediction. In order to investigate the effect of thermal and stress induced damages on AE parameters, a few tests were also conducted using pre-damaged specimens. AE monitoring, mechanical tests, elastic wave velocity measurements and microscopic investigations were used for these purposes. In order to compare the AE records in pure uniaxial compression (UC) and indirect tensile tests, a few Brazilian tests were carried out using AE monitoring. Plotting of the accumulated hits and the accumulated energy AE parameters versus time shows that the AE energy parameter is a more effective way to monitor crack propagation and to predict the failure of specimens than the conventional AE hits. According to AE records from granite and gabbro under UC test, three sharp steps of dramatic AE activity were distinguishable. The first one occurred at about 25 % and the second one at 60% of stress. The last step occurred when the stress level reached 90 to 95% of the ultimate strength of the rock. These steps are comparable to crack initiation (σci), crack damage (σcd) thresholds and uniaxial compression strength (σucs) in the stress-strain diagram presented by Martin (1993). The same steps were detectable during the Brazilian tests. Comparing the acoustic emissions data for intact and thermally treated samples showed a considerable difference in the number and steps of acceleration of AE activity. It appears that microcrack evolution due to thermal damage plays a key role in the reduction of the number of AE hits during uniaxial tests on pre-heated specimens. The AE records of intact and thermally treated specimens of gabbro below 600°C are comparable. However, there is a significant difference between the records of specimens heated to more than 600°C and unheated gabbro. A drastic decrease in the uniaxial strength and elastic wave velocity was also evident in specimens that had been heated to above 600°C. Based on all the investigated parameters, we found that 600°C is a critical temperature that causes physical and mechanical (strength and elasticity modulus) degradation to the gabbro rock subjected to thermal treatment. On the other hand, it was observed that the damage due to ultra high stresses was less than that due to thermal effects, especially in the mechanical parameters. However, AE parameters were more affected by stress damage than thermal heating.