Abstract
Ultrasonic waves, which constitute an active testing method, and acoustic emissions (AE), which can be applied as passive testing technology, can reveal rock damage processes in different ways. However, few studies so far have simultaneously adopted both, owing to the limitations of the experimental apparatus. However, the simultaneous use of both methods can improve the experimental efficiency and help to understand the rock damage evolution more comprehensively. In this study, concurrent experiments of ultrasonic waves and AE activities were carried out on rock salt under uniaxial compression, and the deformation characteristics were measured. The fracture process was divided into four stages with individual characteristics: the elastic compression stage, brittle-ductile transition with crack initiation, brittle-ductile transition with damage initiation, and plastic deformation and strain hardening stage. The ultrasonic wave velocity, crack density, ultrasonic wave amplitude, and attenuation coefficient were obtained to evaluate the damage process. The ultrasonic wave amplitude and the attenuation coefficient were recommended as forecast indicators, owing to their sensitivity and operability of measurement. The confining pressure had an inhibitory effect on crack expansion and on the AE activity, and the damage ultimate stress was defined and determined according to the AE activity and energy release characteristics. Four critical strengths of the crack initiation threshold stress, dilatancy boundary stress, short-term strength, and damage ultimate stress of rock salt were determined and then discussed. These results are valuable in evaluating rock damage and guiding the operation of underground salt caverns.
Highlights
Salt formations are almost perfectly impermeable and they have good creep properties; many countries have used them to store energy and nuclear waste [1,2,3,4]
Based on the axial stress–strain curve and volume strain information, the deformation process of rock salt can be divided into four curve and volume strain information, the deformation process of rock salt can be divided into four stages (Figure 2b): the elastic compression stage, the brittle-ductile transition with crack initiation, stages (Figure 2b): the elastic compression stage, the brittle-ductile transition with crack initiation, the the brittle-ductile transition with damage initiation, and the plastic deformation and strain brittle-ductile transition with damage initiation, and the plastic deformation and strain hardening hardening stage [29,34]
Rock deformation is accompanied by energy release, which can be recorded by acoustic emissions (AE) sensors
Summary
Salt formations are almost perfectly impermeable and they have good creep properties; many countries have used them to store energy and nuclear waste [1,2,3,4]. Applied ultrasonic wave attenuation to detect and study damage development in granite and demonstrated that the ultrasonic wave velocity was a good indicator for the physical and mechanical capability of rocks, as well as the gestation environment. Some of the limitations of these methods are the following: (1) ultrasonic wave and AE techniques are important means in studying the physical and mechanical properties of rocks, both methods were used simultaneously in the same loading test in only a few experiments; (2) due to the limitation of the test equipment, the AE activities and the locations of rock under the triaxial loading condition are studied less. In order to address these challenges and to answer open questions in the field, in this paper, an integrated testing device was used to study the ultrasonic wave and AE activity of rock salt under triaxial compression conditions (Section 2). The obtained results provide a new insight into the damage evaluation in salt rock and they lay an important foundation for further studies
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