Abstract
The deformation and failure process of coal rock under different strain rates is a significant challenge which must be solved in dynamic excavation. It is important to study the influence of strain rate on the evolution of coal rock crack. The triaxial compression tests and acoustic emission tests under the strain rate of 10−5 s−1 to 10−3 s−1 were conducted on coal rock using MTS 815 hydraulic servo‐control testing machine. During the loading process, acoustic emission energy and spatial distribution have obvious stage characteristics. The damage variable is defined by acoustic emission energy, and the rate of damage evolution is obviously affected by the strain rate. Based on stage characteristics of acoustic emission energy, spatial distribution, and damage evolution, the use of damage evolution curve to determine stress threshold is proposed. In order to verify the rationality of the damage evolution method, the stress threshold values determined by damage evolution method and existing method are compared and analyzed. In order to study the effect of strain rate and confining pressure on the stress threshold, the stress thresholds under uniaxial and triaxial stress states at different strain rates were analyzed.
Highlights
With the development of social economy, the demand for energy has increased in recent years, and the exploitation of mineral resources has gradually emerged as a large-depth feature [1]
E Acoustic emission (AE) energy curve at different strain rates can be divided into four stages according to its evolution characteristics: (i) quiet stage, (ii) stable growth stage, (iii) fast growth stage, and (iv) postpeak stage. (i) In quiet stage, the rock is under small load, and defects such as primary cracks and holes in the coal rock are compacted under the action of axial pressure
Since the energy of AE signal produced by crack closure is low [37], it is not easy to be captured by the AE sensor, so no AE signal is produced at this stage, and the AE energy is 0. (ii) In stable growth stage, the material is in the elastic stage, and the stress of coal rock and AE energy keep linear relationship with time
Summary
With the development of social economy, the demand for energy has increased in recent years, and the exploitation of mineral resources has gradually emerged as a large-depth feature [1]. As mentioned above, confining pressure and strain rate have important effects on AE and mechanical properties of rock, but there are few reports on AE of coal rock considering strain rate under high confining pressure, so it is significant for the engineering safety to study the mechanical characteristics of high stress coal rock by using AE. Erefore, no matter the determination of the stress threshold of coal rock under quasi-static strain rate, or to have a reliable method to define the stress threshold in laboratory tests, it has important theoretical significance for the safety of coal mines engineering. In order to investigate the change of stress threshold of deep buried coal rock under different strain rates, this paper firstly analyzes the results of AE tests under quasi-static strain rate and uses AE to analyze the damage evolution characteristics of coal rock. The damage evolution curve is used to determine the stress threshold, and the effects of strain rate and confining pressure on the stress threshold are discussed
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