Experimental study on the types and evolution of uniaxial compression cracks in defective sandstone

Abstract

[Objective] The mechanical strength of the fractured rock body will be deteriorated under loading, which is easy to further trigger rock rupture and affect the rock engineering, and analyze the acoustic emission energy characteristics, the main frequency change and the RA-AF value distribution of the defective sandstone under uniaxial compression. [Methods] Based on the acoustic emission parameters, we investigate the fracture mechanism, crack type and evolution of sandstone, and combined with scanning electron microscopy, we investigate the connection between the fracture micro-morphological characteristics and the macro-fracture. [Results] The presence of defects has obvious deterioration effect on the mechanical strength of sandstone, the change of acoustic emission energy exists in four phases, namely, “fluctuation period, smooth period, violent fluctuation period,and upward surge period”, and the acoustic emission parameter AF mainly distributes in the range of 0~500 kHz, and RA mainly distributes in the range of 0~ 2. 0 ms·V^-1, and the fracture surface of sandstone is divided into three types: tensile fracture,shear fracture, and composite fracture of tension-shear type. The sandstone fracture is divided into three types: tensile fracture,shear fracture, and tensile-shear composite fracture. [Conclusion] (1) The strength of sandstone shows an overall increasing trend with the increase of fracture angle;(2) The crack evolution process can be divided into: initial wing tension crackcomposite type crack(0°~45°) → initial anti-wing tension crack-anti-wing crack(60°~75°) → initial wing tension cracksecondary wing crack(90°);(3) The main frequency signal accounts for 71%~96% of the low-frequency, and the highfrequency accounts for 3%~23%, and the damage process mainly produces large-scale cracks;(4) the proportion of tension cracks is more than 70%, the proportion of shear cracks fluctuates around 20%, and the tension damage is dominant;(5) the fracture of the rupture surface of the sandstone is dominated by transgranular fracture(TG) and accompanied by along-granular fracture( IG), and the intact sandstone is mainly transgranular tensile-type fracture, and defective sandstone is mainly transgranular tension-shear composite-type fracture The macroscopic crack fracture forms of sandstones and the microscopic morphologic features have a corresponding relationship.

Keywords

uniaxial compression / acoustic emission / fracture mechanism / crack evolution / scanning electron microscope / influencing factors / mechanicalproperties / sand stone