Abstract
The natural and water‐saturated states of coal samples under static and static‐dynamic loads were tested using the Split‐Hopkinson pressure bar (SHPB) method and RMT‐150 system, respectively. The differences in the strength reduction coefficient and elastic modulus reduction coefficient of water‐saturated coal samples under static and static‐dynamic loads were discussed. The experimental results for coal were compared with the corresponding characteristics of typical sandstone samples under static and static‐dynamic loads. Furthermore, a fracture model of a hydrous wing branch fracture under static‐dynamic loading was established based on the theory of fracture damage mechanics. The difference in dynamic strength between coal and sandstone samples for both the natural state and water‐saturated state was analyzed. On this basis, the effect of water on the fracture surface of coal and the tensile strength and shear strength of the branch fracture surface were fully considered. In addition, criteria of the branch fracture surface for crack initiation and crack arrest were also established. Finally, the phenomenon of increasing elastic modulus in saturated coal samples was explained with this criterion.
Highlights
Coal is a heterogeneous natural medium that contains a large number of pores and microcracks
Pu and Ma [4, 5] analyzed Split-Hopkinson pressure bar (SHPB) experimental results for sandstone in a coalmine with different moisture contents and concluded that the dynamic uniaxial compressive strength of sandstone increased as a power function with increasing moisture content of the specimens
Wang et al [6, 7] carried out static-dynamic load experiments on coal specimens with different moisture contents using the improved SHPB method and the RMT-150 test system. ey found that the dynamic peak strength of the coal specimens decreases with increasing water-saturation time under static-dynamic loads
Summary
Coal is a heterogeneous natural medium that contains a large number of pores and microcracks. Ey found that the dynamic peak strength of the coal specimens decreases with increasing water-saturation time under static-dynamic loads (intermediate strain rate). E results showed that the uniaxial compressive strength of the specimens gradually decreased with increasing moisture content and the water in the surface of unsaturated clay has a significant effect on its mechanical properties. It can be seen that, under the intermediate strain rates, the dynamic strength of different coal and rock materials varies from the natural to water-saturated state. This difference is difficult to explain from macroscopic mechanics experiments or theoretical analyses. The hydration corrosion of coal materials and stress distribution at the branch fracture surface, which have a significant impact on watersaturated low strength materials, are rarely taken into account. erefore, integrating the above factors for coal materials, analyses of the strength properties of watersaturated coal and rock under static-dynamic loads have theoretical and practical significance
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