Mechanical Properties and Damage in Lignite under Combined Cyclic Compression and Shear Loading

In this paper, uniaxial cyclic compression and shear test was carried out for lignite samples. The effects of inclination angle (θ) and upper limit of cyclic stress (σmax) on mechanical properties of coal samples were analyzed, and the damage variables of coal samples were studied based on energy dissipation theory. The results show that the uniaxial compressive strength (UCS) of coal samples after uniaxial cyclic compression and shear tests decreases with the increase of the upper limit of cyclic stress and inclination angle. The shear stress component generated by the increase of inclination angle can effectively reduce the UCS and increase the damage degree of coal samples. With the increase of inclination angle, the failure mode of coal samples is changed from tensile failure (θ = 0°), the combined tensile failure and shear failure (θ = 5°) to shear failure (θ = 10°). The peak axial and radial strain of coal samples first increases rapidly and then stagnates. The peak volume strain rapid increases and then stagnates (θ = 0° and θ = 5°). When the inclination angle is 10°, the peak volume strain first decreases rapidly and then stagnates. Even if the upper limit of cyclic stress is lower than its UCS, it will still promote the propagation of micro cracks and the generation of new cracks and increase the internal damage of coal samples. With the increase of the cycle number, damage variables of coal samples after uniaxial cyclic compression and shear tests nonlinearly increase, and the growth rate decreases gradually

Investigations of Damage Characteristics in Rock Material Subjected to the Joint Effect of Cyclic Loading and Impact

Investigation of the damage characteristics of rock material under the combined effect of cyclic load and impact load is extremely crucial for many mining engineering applications. Based on energy dissipation theory, we considered factors such as circulation times, the cyclic stress of a uniaxial cyclic load test, and the impact pressure (strain rate) of a split Hopkinson pressure bar (SHPB) test, studying the damage mechanism of red sandstone under the combined action of a uniaxial cyclic and impact loads. We found that when the load stress is 60%, 70%, and 80% of the uniaxial compressive strength (UCS) of red sandstone, the stress can still promote the development of microcracks and the generation of new cracks in the rock, increasing the inner damage of the rock so that it reduces the rock strength. As the cyclic time increases, the energy dissipation ratio presented a trend of decreasing dramatically and then maintaining basically no change, and the peak strain and the damage variable of the rock both increased first and then tended to remain relatively constant. The damage variable of the rock specimen under the combined action of the uniaxial cyclic loading and impact load had a significant corresponding relationship with the initial damage. As the rock specimen initial damage increases, the damage variable and the peak stress of the rock specimen both decreases almost linearly with initial damage. Meanwhile, the average particle diameter of the rock specimen after breakage gradually increased, showing a positively correlated tendency. The ratio between the fragment quality of the bigger particle diameter in the total rock specimen quality gradually increased. Under the conditions of the same initial damage, a higher impact pressure resulted in smaller rock fragment average particle diameters

Mechanical properties experiment of load capacity on a mechanical yielding steel prop (MYSP) and its application in roadway support

As the coal mining depth increases year by year, the deformation and failure of deep roadway become more serious. Therefore, new support equipment with high supporting force and yieldable character is quite necessary for mining safety. In this research, a new mechanical yielding steel prop (MYSP) with high stable load capacity was introduced, which features sustaining large deformation in the field. The test shows that the load capacity provided by doublelayer steel balls is greater than that of single-layer steel balls, and that provided by high-hardness steel balls is higher than that of low-hardness steel balls. When double-layer high-hardness steel balls are adopted, the load capacity firstly increases and then remains stable with the increase of displacement, while it firstly increases and then decreases and finally remains stable with the increase of displacement when double-layer low-hardness steel balls are adopted. The load capacity decreases with the increase of inclined angle of the outer tube, but the influence of the inclined angle of the outer tube on load capacity provided by high-hardness steel balls is small. The load capacity increases gradually with the yielding strength of the prop. Therefore, the MYSP with different load capacities can be designed by changing the yielding strength of the outer tube and inner tube. The field application shows that the MYSP has good characteristics of yielding and high constant resistance. It is very effective for controlling the deformation of surrounding rock mass using the MYSP for roadway pre-support, which also reduces the supporting cost significantly