Experimental and numerical investigations on dynamic mechanical responses and failure process of gas-bearing coal under impact load

Highlights

• Stress waveform, stress-strain and failure process of gas-bearing coal were researched.

• Stress distribution and plastic deformation ware discussed in numerical simulation.

• Dynamic disaster of gas-bearing coal in coal mine was discussed.

Abstract

With mining depth increasing, the mining structure becomes more and more complex. It is easy to cause dynamic disaster during coal mining. To reveal the failure process of gas-bearing coal at complex mining environment, the dynamics experiments were conducted through Split Hopkinson Pressure Bar of gas-bearing coal (SHPB-GAS) experimental system. The results showed that the filtered stress waveform presented to be sine-like. With the impact load increasing, the amplitude of incident wave, reflected wave and transmission wave also increased. After calculation of stress wave data, the stress-strain curves were obtained, which included linear elastic stage, step stage, yield and rupture stage. The dynamic mechanical strength increased with the increase of impact load, and the specimen developed from stratiform rupture with macro cracks to crush rupture in fragments. To verify the experimental results, COMSOL Multiphysics software was adopted to conduct numerical simulation. The simulation results indicated that the high stress area was formed at the end faces of specimen firstly and expanded to interior of specimen gradually under different impact load. High stress induced the plastic deformation in coal specimen, which also developed from end faces to specimen inside. These results were consistent with the phenomenon of experiments, which explained the causes of failure process of gas-bearing coal. Based on the experimental and simulation achievements, the formed mechanism of dynamic disaster induced by impact load were discussed.

Introduction

With the depletion of coal resources in shallow area, coal mining has entered the deep stage, and the mining depth of some mines has extended to 1000 m below. In such mining structure environment (Fig. 1), the stress sources of coal body are relatively wide, such as static load, confining pressure, gas pressure and dynamic load, etc (Fig. 1) [[1], [2], [3], [4], [5], [6], [7]]. Due to the complex mining structure, it's difficult to prevent and control the dynamic disaster [[8], [9], [10], [11]]. Therefore, it is urgent to accurately determine the dynamic mechanical responses and failure process of gas-bearing coal under impact load.

About dynamics properties test, Split Hopkinson Pressure Bar (SHPB) is commonly used to investigate the mechanical behavior of materials at high strain rates [[12], [13], [14]]. Since Kolsky developed SHPB technique, it has been always improved and developed further [15]. In order to gain the accurate mechanical characteristics, the interface friction between bars ends and specimen, lateral inertia of the specimen and stress equilibrium were researched in details [[16], [17], [18]]. To overcome friction effects, the dynamics experiments was performed with and without lubricant at the interfaces, and it was found that the lubricant is helpful for diminish friction [[19], [20], [21], [22]]. About inertia effects, it was concluded that the length and radius have the opposite responses by dynamics experiments, and the optimal ratio range (1.5≤L₀/D₀ ≤ 2) of longitudinal (L₀) and radial (D₀) was determined [20,23,24]. To solve the stress equilibrium problem and eliminate dispersion effect, the waveform of various stress waves was researched and the sine-line wave was generally recognized to be the best [25,26]. Therefore, the measured dynamics results will be more and more reliable on the bases of these researches.

Dynamic mechanical characteristics of materials is not only related with SHPB itself, they are also closely relative to the loading conditions [[27], [28], [29]]. About loading conditions, they mainly derived from the engineering application background of various materials. For example, concrete is commonly used in constructing protective structures to against high-rate loading such as impact and blast, so its' dynamic mechanical properties under one-dimension impact load is essential for reliable concrete structure design [19,30]. In addition, like glass material, the researchers have tried to develop improved glass material with higher strength and fracture toughness for avoiding structural failure, so the dynamic mechanical properties of glasses materials such as soda-lime, starphire, borosilicate, and fused silica under low-velocity impact have been studied [31]. Another material, like titanium alloy (Ti–6Al–4V), its’ mechanical behaviors are closely related with the temperature, which were studied under impact load and different temperature conditions [32]. In this paper, the dynamic mechanical characteristics of gas-bearing coal would be discussed. In terms of coal seam underground, it is in 3-dimensional stress equilibrium (axial load from overlying strata and confining pressure from confining rock). And coal as porous media [33], the mechanical properties also are influenced by the gas inside [[34], [35], [36], [37], [38], [39]]. So it is necessary to maintain axial static load, confining pressure and gas pressure as constant, then the dynamic mechanical properties of gas-bearing coal were researched under different impact load.

Dynamic mechanical properties test of coal or rock material have bee researched earlier, which experienced 3 stages, such as one dimensional impact loading, coupled static-dynamic load and impact loading under 3-dimension stress state [40,41]. At one dimensional impact loading condition, James et al. [42] investigated the dynamic fragmentation of granite, which offered insights into the catastrophic dynamic fragmentation process of rock. For coupled static-dynamic loading, Li et al. [43] studied the dynamic mechanical properties of siltstone specimens, and concluded that the compressive strength under coupling loads was higher than their corresponding individual static or dynamic strengths. For triaxial coal or rock loaded by impact load, Jin et al. [44] discussed the effects of confining pressure on energy dissipation of sandstone under cyclic impact load. However, gas pressure as a important factor to influence the coal mechanical properties, it has been short of studies in dynamics experiments due to backward equipment. To master dynamic mechanical and fracture properties of gas-bearing coal, Split Hopkinson Pressure Bar of gas-bearing coal (SHPB-GAS) has been built in this paper.

Because of heterogeneous and anisotropic properties about coal material, the mechanical characteristics and fracture evolution under different loading conditions are often non-continuous and non-linear [[45], [46], [47], [48], [49], [50]]. Numerical simulation have advantages to overcome these defects, so it was also adopted to verify experimental results. And with rapid development of commercial software, many numerical simulation software such as ABAQUS/Explicit [21], finite element analysis (FEA) [51,52], AUTODYN [19] and COMSOL Multiphysics [53] were used to conduct material dynamics simulation. By ABAQUS/Explicit, Zhong et al. (2015) simulated the effects of friction and specimen configuration on the material dynamic responses during SHPB experiments, and concluded that the transmitted signal decreases and reflected signal increases with friction coefficient increasing [21]. And Hao et al. (2013) also used AUTODYN to analyze the effects of friction confinement to dynamics testing results during SHPB experiments of concrete [19]. Ai et al. (2019) researched fracture process of rock under high strain rate impact loading by numerical simulation [45]. More importantly, the perfect state of material would be presented, which could eliminate the friction effects. So we also adopted numerical simulation to verify the experiments results further.

In this paper, the dynamic mechanical experiments of gas-bearing coal under constant axial static load, confining pressure and gas pressure were conducted. The dynamic mechanical characteristics of gas-bearing coal with different impact load were analyzed. And stress waves, stress - strain curves and failure mode of gas-bearing coal were obtained. Through numerical simulation, the experiments results were verified by illustrating the stress distribution and plastic deformation evolution. These results are of great significance to reveal the evolution mechanism of dynamic disaster such as coal and gas outburst during coal mining.