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Abstract The mortar is used to simulated the rock material to study its microcracks evolution laws. The distribution properties and statistical characteristics of density in rock materials are investigated by means of CT scanning during the failure process under uniaxial compression；and the activities of microcracks are reflected by the density varieties of same location in mortar. The evolution behavior of microcracks is studied through the statistical characteristics of density changes and the fractal index Rd. The research results show that：(1) The changes of amplitude and shape of grey frequency statistical curve reflect the different microcracks evolution effects. When load level is at 0%–50%，the curve is a sine curve，its wave peak rises and the amplitude increases evidently. When load level is at 50%–70%，the wave peak declines，the frequency amplitude increases，and the curve has a deviation from sinusoid. When load level is at 70%–90%，the curve shape，wave peak location and frequency amplitude do not change basically，and only a few gray interval?s frequencies change. When load level is greater than 90%，the wave peak rises largely and the curve becomes sinusoid again. Meanwhile，the movement of wave trough is opposite to the wave peak. The change laws of the amplitude and curve shape indicate that the dominant evolution action of microcracks is initiation in prophase，metaphase closing and propagation in anaphase during the failure process. (2) Through the gray transformation of CT images and its fractal index description，the relationship between Rd and load presents three-stage increment，which is related to the interaction of generating effect and closing effect of microcracks in compression process. In prophase，Rd increases with microcracks initiation remarkably. At middle stage，because the microcracks initiation becomes slow and its propagation develops slowly as well，the microcracks closing effect is dominant. So，Rd decreases tardily. In anaphase，the microcracks propagation and influx speed up and Rd is increasing rapidly. (3) The proposed fractal index Rd of microcracks volume fraction can characterize the evolution behavior of the microcracks in the compression fracture process.
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Received: 14 February 2011
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2011, Vol. 30 
