Experimental investigation on fracture behaviors and acoustic emission characteristics of sandstone under different strain rates

To further investigate differences of fracture behaviors and acoustic emission characteristics of rock under different strain rates, unconfined uniaxial compressive tests of thirty cylindrical specimens with 50 mm in the diameter and 100 mm in the height were performed within quasi-static strain rates of 10(-5)-10(-2) s(-1) by rock material testing system and multi-channel acoustic emission (AE) monitoring system. The uniaxial compressive strength, peak strain and elasticity modulus, failure mode, fractal dimension and equivalent average size of the rock fragments, and the three-dimensional spatial distribution of acoustic emission events were obtained and analyzed. The experimental results verify that the uniaxial compressive strength, elasticity modulus and peak strain are strain rate-dependent significantly. Particularly, with increasing strain rates, macroscopic failure modes present three types of single shear failure, multiple shear failure, and shear and tensile mixed failure, and the rock fragments show a smaller equivalent size and a larger fractal dimension, which resulted in a higher fragmentation degree. Besides, it is noted that the evolution process of acoustic emission ring-down count and spatial distribution of events under different strain rates are closely related to macroscopic failure modes. Furthermore, characteristic stresses based on the evolution process of acoustic emission ring-down count, such as the elastic stress, initiation stress and yield stress, were analyzed deeply at different strain rates. A simplified rock model consisting of discrete fracture network and equivalent elementary volume was proposed to discuss the effect of strain rate on energy accumulation and dissipation. Finally, to better reveal the micro mechanism of the effect of strain rates on rock mechanical behaviors, the microscopic fracture morphology was observed and characterized by means of scanning electron microscopy. The experimental results should be taken into account in underground mining, tunnel excavation and other rock engineering activities.