Experimental and Numerical Study of Fracture Behavior of Rock-Like Material Specimens with Single Pre-Set Joint under Dynamic Loading

Abstract

The Split Hopkinson Pressure Bar (SHPB) is an apparatus for testing the dynamic stress-strain response of the cement mortar specimen with pre-set joints at different angles to explore the influence of joint attitudes of underground rock engineering on the failure characteristics of rock mass structure. The nuclear magnetic resonance (NMR) has also been used to measure the pore distribution and internal cracks of the specimen before and after the testing. In combination with numerical analysis, the paper systematically discusses the influence of joint angles on the failure mode of rock-like materials from three aspects of energy dissipation, microscopic damage, and stress field characteristics. The result indicates that the impact energy structure of the SHPB is greatly affected by the pre-set joint angle of the specimen. With the joint angle increasing, the proportion of reflected energy moves in fluctuation, while the ratio of transmitted energy to dissipated energy varies from one to the other. NMR analysis reveals the structural variation of the pores in those cement specimens before and after the impact. Crack propagation direction is correlated with pre-set joint angles of the specimens. With the increase of the pre-set joint angles, the crack initiation angle decreases gradually. When the joint angles are around 30°–75°, the specimens develop obvious cracks. The crushing process of the specimens is simulated by LS-DYNA software. It is concluded that the stresses at the crack initiation time are concentrated between 20 and 40 MPa. The instantaneous stress curve first increases and then decreases with crack propagation, peaking at different times under various joint angles; but most of them occur when the crack penetration ratio reaches 80–90%. With the increment of joint angles in specimens through the simulation software, the changing trend of peak stress is consistent with the test results.

Numerical Simulation of Failure Modes in Irregular Columnar Jointed Rock Masses under Dynamic Loading

“…The dynamic failure modes of the CJRMs in this paper are consistent with similar past studies about physical and numerical simulation tests on the dynamic loading of rock mass [42,46,[75][76][77], which concluded that the failure mode of rock mass under dynamic loading is mainly tensile failure and the compressive shear failure of the CJRMs may be determined by the special properties of basalt columns at low angles. Pan et al [76] studied the dynamic loading of jointed rock mass, and the results show that rock mass with a dip angle of 45-60 is more prone to failure, which is consistent with the thesis in this research that the CJRMs with a dip angle of 60 is the most prone to failure. In addition, cracks in jointed rock mass mainly develop along the joints and are dynamic [77], which is a good mirror of the crack propagation pattern studied in this paper.…”

A New Index of Energy Dissipation Considering Time Factor under the Impact Loads

“…Liu [21] considered that the energy time density and incident energy show an upward trend. Following a further study by Pan [22], it was demonstrated that energy absorption reaches its highest level when the joint angle is 45 • . Li [23] found that the penetration rate of a joint can lead to the increase of energy time density.…”

Pressure-Shear Crack Initiation and Expansion Mechanism of Complex Cracked Rock Mass under the Seepage Stress

“…The place with a large opening becomes the dominant seepage channel, while for some gas-sealed areas; the seepage diameter is more complicated. Some exploration shows that although some cracks are below the underground water level, they are still dry or have only a small amount of crack water seepage, which shows the unsaturated state of the rock mass [12,13]. The inhomogeneity of seepage is mainly reflected in the different permeability coefficients at different positions in the rock mass space system.…”