The aims of this paper are to comprehensively explore both the dynamic mechanical properties and crack evolution characteristics of coal and rock during impact failure. First, experimental specimens are prepared from coal seam, direct roof rock strata and direct floor rock strata in the same area to highlight the correlations between test pieces. Second, a dynamic strain gauge and high-speed (HS) camera are adopted to reflect the stress wave signal and crack evolution. Then, based on digital image correlation (DIC) technology and the mass screening method, the evolution laws of surface cracks during crushing and the distribution characteristics of sample fragments after crushing are studied from the perspective of fractal, and finally compared with those of the simulation analysis. The results are as follows. (1) The coal and rock samples from the same area have both consistency and differences. The dynamic mechanical properties of coal and rock are affected by the impact velocity and the physical properties of the specimen. Higher impact speeds and densities lead to the more obvious brittleness of the specimen when destroyed. Conversely, the sample shows more plasticity and ductile yield. (2) The self-similarity is significantly manifested in the evolution of surface cracks during impact and the distribution characteristics of fragments after impact. The box dimension and quality screening dimension are applicable to quantitatively characterize the evolution process and results of coal and rock fractures. (3) The simulation results based on the Holmquist–Johnson–Cook (HJC) and Riedel–Hiermaier–Thoma (RHT) constitutive models agree well with the experimental results, and the RHT constitutive model is more consistent. This study may contribute to a more comprehensive understanding of the dynamic characteristics and crack evolution laws of coal and rock under impact loading and provide references for further research and discussion.
Abundant engineering practice shows that coal and rock dynamic disasters often occur in fault zones; hence, the detection of faults in coal mines is important. As an essential branch of seismic tomographic application, research shows that ultrasonic P-wave velocity can detect mine faults and provide technical support for dynamic disaster prewarning systems. Traditional P-wave testing mostly takes rock or raw coal as the research object, which has some shortcomings in controllability, homogeneity and comparability. This paper compares the difference in ultrasonic P-wave propagation velocity in jointless and jointed briquettes through laboratory research, focuses on the effect of macro-joints on P-wave velocity and makes a preliminary theoretical analysis. The results show that: (i) the three-dimensional P-wave velocity throughout a jointless briquette specimen is similar, which reflects the high homogeneity of this medium and avoids the influence of the random distribution of primary bedding, joints and structural planes; (ii) the P-wave velocity in jointless and jointed briquettes is positively correlated with density and forming pressure, and is negatively correlated with the angle between the ultrasonic wave and the joint surface in a sample and (iii) when the P-wave encounters the macroscopic joint surface, it may reflect and refract, changing the propagation direction and inducing wave mode conversion. This study provides the necessary technical support and a theoretical guide to optimise acoustic property analysis of coal and rock as well as a field application for seismic tomography technology.
With the decline of shallow coal reserves and increase in demand of coal consumption, the hazard of coal and rock dynamic disasters is expanding. In this paper, we conducted some laboratory scale tests on coal, cement, and glass materials to figure out the microseismic (MS) characteristic differences among materials. A new method, denoted as WPT-LMD, is proposed to conduct signal denoising and analysis work. A series of basic analyses regarding MS are conducted, including the relationships between MS characteristics and loading rate, coal powder particle size, loading stage, and MS event statistics. Research results show that the damage of all these three materials is accompanied with MS events, abundant with low frequency component . But cement and coal specimens produce with relative wide frequency distribution in sample frequency domain, while glass specimens were found to only produce low frequency event. The powder particle sizes have obvious influence on the strength of coal specimen, but the loading rate seems to have no influence on the MS characteristics. The outcome of the paper will further provide a theoretical basis for understanding the mechanism of MS activities and has great significance to improve the coal mining safety.
It is very important from the point of view of gas control and production safety to efficiently extract gas from stress-relief fractures in the initial phase of the longwall working face. In the initial phase of longwall mining, the overburden deformation significantly affects the instability of gas emission and the mining speed positively correlates to the volume of gas emission, so appropriate mining speed can help to increase the efficiency of gas extraction. The monitoring data on surrounding rock deformation show that the horizontal separation fractures first appear from the outer to inner layers and continue to deform along with the mining working face progressing, then the vertical fractures gradually evolve from the lower to the upper layers and finally a stable fracture zone comes into being, which provides space and pathways for both gas ‘stress-relief and retention’ and its abnormal emission during the mining initial phase. Moreover, this paper proposes a new spatiotemporal division model called overburden ‘three belts and five zones’ in the mining initial phase, i.e. the vertical ‘three belts’ including the caving belt, the dynamic fracture evolution belt and the curved subsidence belt; and the horizontal ‘five zones’ consisting of the key gas drainage zone for fracture development, ‘stress-relief and gas-retention’, overburden pressure-relief zone, gas seepage zone in goaf, coal seam pressure-relief zone in front of the working face and the difficult gas drainage zone in coal wall stress concentration area. The research results of this paper can provide theoretical and technical support for optimizing establishment of gas drainage parameters.
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