Current Issue : January-March Volume : 2022 Issue Number : 1 Articles : 5 Articles
Previous studies have shown that water can reduce the acoustic emission (AE) energy and other parameters during rock failure. However, the fracture mechanism of rock can be better reflected by analyzing the AE waveform. Therefore, this paper conducted experiments of uniaxial compression on sandstone samples of various water contents and collected AE signals simultaneously. Analyses of fast Fourier transform (FFT) and Hilbert-Huang transform (HHT) were performed on the AE waveform when the sample failed. The results show that as the water content increases, the frequency and intensity of the AE signal will decrease. The influence of water on the intensity of the AE signal is greater than that on the frequency. Through the analysis of the energy mechanism of rock failure, it is pointed out that the frequency and intensity of AE signal are closely related to elastic energy index WET and burst energy index KE. The research results have guiding significance for the monitoring of rockburst....
The study of the damage process of rock under external loads is good guidance for geotechnical construction design. The differences in rock damage processes and damage modes under different stress paths are rarely reported. To explore the effects of stress paths on rock damage processes, uniaxial compression experiments under three stress paths were conducted. Numerical simulation is also used to simulate the rock acoustic emission (AE) and fracture process. The results of the study indicate that the maximum acoustic emission events are at the peak of stress, and fractures are mainly formed at this stage. The peak of AE energy occurs before the peak of AE events. The damage pattern and fragmentation size of the rock are related to the way the stresses are loaded. It is noticed that there is appearance of a quiet period of AE events prior to the production of significant cracks. Minor damage to the rock is accompanied by the generation of bright white spots in the specimen, which is due to the high tensile or shear stress in the units. When the stress in these units exceeds their strength, the units break down and tiny cracks appear. As the external load increased, the cracks developed and penetrated, and the specimen was damaged. Under cyclic loading and unloading, the number of AE events increased significantly compared with the controlled displacement and controlled stress loading methods, and the radius of the AE circle became larger and the energy also increased, which indicates a greater degree of destruction of the rock under cyclic loading and unloading. The results of the study are of reference significance for rock crack propagation and fracture mode influenced by stress conditions and provide some guidance for construction design under different working conditions....
The objective of this article was to investigate the acoustic emission (AE) precursor information and crack propagation mechanism of brittle granite specimens containing a hole under uniaxial compression using AE and digital image correlation (DIC) techniques. The results show that the existence of the opening in the specimen leads to the degradation of the mechanical properties, and the stress-strain curve exhibits several stress drops. Each deformation stage of rock had different AE characteristics, so AE signals can be used to characterize the microscopic damage evolution of rock and predict the macroscopic failure process of the surrounding rock of the opening. The deformation field evolution process of the specimen containing an opening gradually develops from a uniform distribution to prominent local deformation, and the deformation of the local area near the opening is greater than that of other areas. The initiation of cracks begins around the opening, and the propagation of tensile cracks around the opening is primarily influenced by the nucleation and propagation in the strain localization area. The crack propagation evolution derived from DIC images can be combined with AE monitoring to accurately reveal the deformation and failure mechanism of the intact specimen containing an opening....
In UWAC (underwater acoustic communication), UWA (underwater acoustic) channels change rapidly due to varying environment conditions. AMC (adaptive modulation and coding) is an efficient technique to improve system efficiency by changing transmission parameters according to channel conditions in UWA channels. In this paper, we propose an environmental cognition orthogonal frequency division multiplexing (OFDM) UWAC system and compare it with AMC algorithm with six transmission modes together with three threshold detection algorithms. Simulation and experimental results show the effectiveness of the proposed system. The system will play an important role in future communication networks which can significantly improve the efficiency of the system....
In terms of coal’s stability and failure, soaking time and water content play a significant role in geotechnical engineering practice. To determine the soaking time effect on the mechanical behavior of coal samples and the response of AE (acoustic emission) signal throughout loading, the samples with different soaking times (0–120 hours (h)) were prepared and tested under uniaxial compression. AE signals were continuously monitored during loading to examine the AE characteristic response via the AEwin Express-8.0 system. The results revealed that the mechanical characteristics of the coal samples decreased with an increase in soaking time. When coal samples were subjected to uniaxial compression, AE events occurred due to the formation of the cracks, which further propagated to cause coal fracture. AE counts and the accumulative counts fluctuated with time and corresponded very well to the load. Therefore, AE counts and the trend of the accumulative counts of AE qualitatively explained the rupture of the coal under stress. In addition, the variation in trends of AE counts, AE accumulative counts, and load with time at various phases of all samples were obtained. It is concluded that AE counts increase suddenly during a slow increase phase and peak at the active increase phase. During the attenuation phase, the AE counts first decrease significantly with stress drop, but also a slight increase was observed due to the initiation of secondary cracks. These research results are of great significance as a precursor in coal and rock failure....
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