Study on acoustic emission characteristics and damage evolution of shale under uniaxial compression

Abstract

Acoustic emission tests under uniaxial compression of shale were carried out by using MTS815 rock servo test system and PCI-2 acoustic emission system. The law of acoustic emission and its damage evolution characteristics are analyzed and a uniaxial compression damage constitutive model of shale based on acoustic emission characteristics is established. The research shows that: (1) The uniaxial compression process of shale can be divided into four stages: crack compaction stage - elastic stage to the stable development stage of micro-elastic fracture - unstable fracture development stage - post-rupture stage. The acoustic emission signals are less during crack compaction stage and elastic stage to the stable development stage of micro-elastic fracture. When the axial stress reaches the damage strength σcd, which is about 80% of the peak strength, a large number of acoustic emission signals begin to be generated. There will be a large number of acoustic emission phenomena in the unstable fracture development stage and post-fracture stage. (2) The acoustic emission signal can well reflect the internal damage and destruction process of shale. The damage factor D shows a trend of decreasing first and then increasing with the increase of acoustic emission cumulative ringing counts, rather than monotonically increasing. When the acoustic emission cumulative ringing counts and damage factor start to increase slowly, the corresponding stress can be regarded as the crack initiation strength σci. (3) The damage factor D is proportional to the cumulative acoustic emission ringing counts N. According to the acoustic emission characteristics of rock specimens at different stages, the relationship between the damage factor D and the acoustic emission cumulative ringing counts C at different stages is obtained by linear fitting using origin software, thus a more reasonable shale damage constitutive model is established. In this way, the theoretical stress-strain curve obtained by the damage constitutive model is more consistent with the actual stress-strain curve.