Experimental investigation on the evolution of rock compression-shear fracture using the natural frequency
Brittle failure under compression-shear stress is a frequent and important form of rock failure.Laser Doppler vibration technology was applied to investigate the development of the natural frequency in a direct shear test of specimens with different brittleness degrees.The specimen sequentially passed through a crack closure stage,elastic deformation stage,stable crack growth stage,unstable crack growth stage,and failure stage,with each stage showing a clear signal of a change in the natural frequency.The average value of the natural frequency in the unstable crack growth stage was positively correlated with the axial stress during loading and the brittleness of the specimen.The response of the natural frequency revealed two key mechanical characteristic points.These characteristic points were the crack damage stress point,which was the turning point of the natural frequency from a steady rise to a steady jit-ter,and the unlock point,which was the turning point of the natural frequency from a stable jitter to a decline.The degree of brittleness of the specimen affected the location of the unlock point.During the transformation from brittle failure to ductile failure,the unlock point moved from near the peak stress to after the peak stress.The coefficient of variation(Cv)of the natu-ral frequency was introduced to characterize the co-evolution law of the specimen before unloc-king.Cv showed a decreasing trend in the unstable crack growth stage,indicating that the de-gree of synergy of the natural frequency was enhanced before the specimen failed.The strain ratio of the crack damage stress point to the unlock point increased with the brittleness degree,which potentially provides an early warning indicator for rock failure.
brittleness degreenatural frequencysynergistic characteristicsprecursor infor-mation of instability
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