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Raw coal was used as the study object in this paper to identify the evolution characteristics of acoustic emission (AE) during the dilation process of gas-containing coal. The coal specimens were stored in gas seal devices filled with gas at different pressures (0, 0.5, 1.0, and 1.5 MPa) for 24 h prior to testing. Then, the specimens were tested in a rock-testing machine, and the deformation and crack fracture patterns were recorded by using strain gauges and an AE system. The axial and volumetric strains-stress curves were analyzed in relation to the AE and the failure mode. Results show that as gas pressure increases, the uniaxial compression strength and elasticity modulus of gas-containing coal decreases, whereas the Poisson's ratio increases. In all the coal specimens, the dilation initiation stress decreases, and the dilation degree increases. During the dilation process, before the loaded coal specimens reach peak stress, and as the load increases, the changes in the specimens and in the AE energy parameter of specimens can be divided into four phases: crack closure deformation, elastic deformation, stable crack propagation, and unstable crack propagation (dilation process). Across the four phases, the AE energy increases evidently during crack closure and elastic deformation but decreases during stable crack propagation. As the gas pressure increases, the AE signal frequency increases from 4.5 KHz to 8.1 KHz during the dilation process. Thus, the gas presence in coal specimens exerts a significant influence on the closure of sample cracks and dilation damage. [ABSTRACT FROM AUTHOR]

In order to predict the rock rupture and instability, acoustic emission signals of natural sandstones and hydrous sandstones are analyzed through the Fast Fourier Transform and the Welch method. It is found that the acoustic emission signals of the hydrous sandstones have narrower range of frequency spectrum and lower average dominant frequencies than the natural sandstones. The dominant frequencies of the acoustic emission signals of the hydrous sandstones reduce more dramatically. In addition, two types of Welch power spectrum of the acoustic emission signals in terms of the shape are presented in this paper, which are named Type A and Type B. The Type B Welch power spectrum emerges when the acoustic emission occurs frequently, in which a disaster tends to appear. Time baselines are selected when relative stronger cracks and crack coalescence happens. We find that the greater signals energy at the time baseline, the higher probability of occurrence of Type B spectrum appears before the time baseline. Also, the probabilities of Type B before the corresponding the baseline time of rock last rupture in hydrous sandstones are higher than that of natural sandstones. [ABSTRACT FROM AUTHOR]

To reveal the effect of bedding plane and fracture distribution on crack propagation process in the layered rock mass, the prefabricated fractured layered rock-like specimens with different bedding plane strengths were prepared, and three-point bending tests were carried out on the specimens in combination with an acoustic emission monitoring system. Based on cohesive element aimming to analyze the path selection of crack propagation at the bedding plane, the influence of prefabricated fracture spacing on the crack propagation in the layered rock mass was investigated by using the numerical simulation method. Results show that the crack propagation path is closely related to the bedding plane strength. There are three fracture patterns during the crack propagation in the layered rock mass, such as crack deflection, crack penetration and coupling of crack deflection and penetration. The specimens with weak to moderate bedding plane have multiple peaks in the load-displacement curves, and the acoustic emission events are more abundant after the peaks accompanied by crack propagation process. The specimens with stronger bedding plane have a single peak in the loaddisplacement curve, showing the brittle fracture characteristics, and the acoustic emission events do not emerge until that close to the peak load. With the increasing of the prefabricated fracture spacing, the fractures generate first through the prefabricated fractures upwards, and the main fracture penetrates the intermediate rock layer within the fracture spacing when the spacing exceeds a certain critical value. The conclusions obtained in this study can provide a basis for predicting the fracture evolution of the layered rock mass. [ABSTRACT FROM AUTHOR]

The paper presents the results of the application of a newly developed method to study deformation and fracture processes in specimens produced by linear friction welding. This method combines mathematical modeling and experimental mechanical testing. By application of this method it has been demonstrated that during uniaxial testing of welded joint specimens of Ti-6Al-4V and Ti-6Al-3.5Mo shear bands occur at an angle of 45°. Fracture of the specimen took place near the weld line, in the heat-affected zone, of the Ti-6Al-4V alloy. Microcracks started in the centre of the tested specimen. Recording of acoustic emission signals during tensile testing identifies the appearance of the initial microcracks and can predict the precise moment of fracture. [ABSTRACT FROM AUTHOR]

The cemented filling body is taken as the object of this study, whose nonlinear dynamic characteristics of damage and instability are researched by virtue of acoustic emission (AE) under uniaxial cyclic loading and unloading test. The results show that there exists similarity in variation trend between correlation dimension and maximum Lyapunov exponent during each loading stage, which also verifies the disorder and chaos often reflect the complexity of fractal. Before the filling body gets to the instability, the correlation dimension declines continuously. So the filling body's unstable failure is a kind of ordered dimension reduction process. Such kind of characteristic can be as a criterion condition for filling body's failure. The maximum Lyapunov exponent varies little in magnitude, therefore, it can't be apt to be used as the criterion condition, but it can be used to predict the AE time series that is of chaos feature, and through it higher forecast precision can be got. [ABSTRACT FROM AUTHOR]