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Overburden collapse and water inrush in mines are primarily caused by rock fractures. Mining safety can be enhanced by monitoring and identifying early signs of coal failure in the mines. This article collected acoustic emission data synchronously throughout a series of uniaxial compression (UC) experiments on natural and water-saturated coal. The influence mechanisms of water, mechanical properties, and acoustic emission signals on the stress–strain curve and the SEM results of water-saturated and dry samples are investigated. As a result, the mechanical properties of coal are not only weakened by water saturation, such as elastic modulus, strain, stress, and compressive strength but also reduced acoustic emissions. In comparison with saturated coal, natural coal has a uniaxial stress of 13.55 MPa and an elastic modulus of 1.245 GPa, while saturated coal has a stress of 8.21 MPa and an elastic modulus of 0.813 GPa. Intergranular fractures are more likely to occur in coal with a high water content, whereas transgranular fractures are less likely to occur in coal with a high water content. An innovative and unique statistical model of coal damage under uniaxial loading has been developed by analyzing the acoustic emission data. Since this technique takes into account the compaction stage, models based on this technique were found to be superior to those based on lognormal or Weibull distributions. A correlation coefficient of greater than 0.956 exists between the piecewise constitutive model and the experimental curve. Statistical damage constitutive models for coal are compatible with this model. Additionally, the model can precisely forecast the stress associated with both natural and saturated coal and can be useful in the prevention of rock-coal disasters in water conditions.

Various primary fissures and defects are widely present in a rock mass and have a significant impact on the stability of the rock mass. We studied the influence of the crack inclination angle on the energy dissipation and electromagnetic radiation (EMR) response of sandstone under an impact load. Impact tests were conducted on red sandstone samples with different inclination angles, in addition to test energy dissipation and EMR signals. The results showed that as the energy of the stress wave increased, the energy consumption density and damage variables of the sample gradually increased, and the electromagnetic radiation energy also increased. As the crack inclination increased, the energy consumption density first decreased and then increased, while the damage variable and electromagnetic radiation energy first increased and then decreased. In the process of impact damage, the main frequency of EMR was 0~5 kHz. As the energy of the stress wave increased, the dominant frequency band of the main frequency expanded from low frequency to high frequency, and the amplitude signal gradually increased; the α = 45° specimen frequency domain was the widest, and the amplitude was the largest. The crack inclination significantly changed the failure state of the sample, resulting in changes in the energy dissipation and the electromagnetic radiation response of the sample.

This study is aimed at investigating the effective precursory information of coal sample instability and failure under uniaxial compression. To achieve this aim, first, the infrared radiation temperature (IRT) variations of coal sample under the same loading mode were analyzed. Next, the variations of IRT counts on the coal sample surface, numbers of IRT points in high-temperature and low-temperature zones and IRT differences in the two zones were analyzed based on the infrared thermal image. Furthermore, the variations of variance and autocorrelation coefficient of IRT were explored with reference to the critical slowing down theory. The following results were obtained. The changes in IRT exhibit different characteristics in different stages of loading. Before the main rupture occurs, both the maximum infrared radiation temperature (MIRT) and the average infrared radiation temperature (AIRT) present a cooling precursor and an IRT rise buffer section. When the main rupture occurs, the MIRT surges, while the AIRT plunges. The high-temperature zone in the infrared thermal image keeps expanding with the increase of stress, and the low-temperature zone also expands considerably before the main rupture. The number of IRT points in the high-temperature zone decreases first and then increases with the increase of stress, while the number of IRT points in the low-temperature zone increases continuously with it. The IRT differences in the high-temperature and low-temperature zones vary in a basically consistent way with the number of IRT points in the two zones, respectively. The MIRT curve, and the curve of number of IRT points in the high-temperature zone and the variance curve of MIRT can all be taken as the precursor signal for identifying coal sample instability and failure. Among the three, the variance curve of MIRT boasts the most superior effect.