Experimental Study on Mechanical Properties and Cracking Behavior of Pre-cracked Sandstone Specimens Under Uniaxial Compression

Abstract

This study analyzes the mechanical behavior, cracking modes, crack initiation stress, initiated crack types and crack coalescence process for the sandstone specimens containing artificial flaws with different fissure angle from 0° to 90° (in 15° intervals) and different ligament length from 3 to 23 mm (in 5 mm intervals) under uniaxial compression. The flaw geometry is a combination of a single hole and an inclined fissure underneath, which is different from that reported in the previous studies. Basically, mechanical parameters of the specimens containing combined flaws are lower than those for the intact specimens, and the effect of fissure angle and ligament length on mechanical parameters for the flawed specimens is analyzed based on the axial stress–strain curves. Peak strength, peak axial strain, elastic modulus and the secant Young’s modulus all display a similar variation trend with the increasing fissure angle, i.e. first decreases in the range 0°–45° and then keeps basically constant in the range 45°–75° before presents an increase trend in the range 75°–90°. Variation trends of mechanical parameters for the flawed specimens with increasing ligament length are simple, decreasing gradually in the range 3–18 mm before showing an increasing trend in the range 18–23 mm. Crack initiation stress for the flawed specimens, which also depends on the fissure angle and ligament length, is analyzed. Using a high speed camera, initiated crack types and crack coalescence modes for the flawed specimens are observed. With a small fissure angle, the cracking modes are generally characterized by cracks initiated from the hole-wall and evolved to the specimen boundary. For flawed specimens with a larger fissure angle or ligament length, cracks initiated from both the hole-wall and fissure tips generate the main failure surfaces. In addition, relation between the axial stress–strain curve and real-time crack coalescence process for the flawed specimens during the entire deformation process is also captured.