Experimental investigation on the effect of preexisting orthogonal cross flaws on cracking behaviors and acoustic emission characteristics of red sandstone

Abstract

Uniaxial compression tests with combined acoustic emission (AE) technique and camera monitoring were conducted to investigate the influence of orthogonal cross flaws on the mechanical properties, cracking behaviors and AE characteristics of red sandstone. The effect of the primary flaw inclination angle α on the main mechanical parameters, crack initiation and propagation characteristics and failure modes were analyzed. The experimental results demonstrate that (1) The peak strength and elastic modulus of the specimens rise as α increases; the peak strain (the axial strain corresponding to the peak strength) εp increases initially and subsequently declines with increasing α, and reaches its maximum value at α = 45°. (2) The location of crack initiation is closely related to α. The new crack first initiates from the tip of the secondary flaw at the condition of α = 0°, 30° and 90°; However, when α is 45° and 60°, the new cracks first initiate from the primary flaw tip. (3) The ultimate failure mode of specimens is affected by α. When α is less than 45°, the primary flaw controls the specimen's failure mode and the specimen mainly shows shear failure; When α is greater than 45°, the secondary flaw plays a major role in controlling specimen's failure mode and the specimen shows tensile-shear mixed failure characteristics. (4) Propagation of anti-wing crack initiated from the primary flaw leads to coalescence behavior between the primary and secondary flaws under the condition of α = 30°, 45° and 60°. (5) When α is 30°, the cumulated AE counts and cumulated AE energy are the lowest, and the AE overall b-value is the highest. (6) Based on the analysis of AF value (average frequency) and RA value (the ratio of rise time to amplitude), tensile and shear cracks are classified during the loading process. The number of tensile cracks is substantially higher than that of shear cracks and α has no obvious effect on the proportion of the two types of cracks.