Abstract
A SHPB experiment of porous sandstone was conducted to discuss the influencing principle of porosity on the energy dissipation of stress wave across dry and saturated porous sandstones. Changes in porosity before and after the SHPB experiment were analyzed by nuclear magnetic resonance. Results show that the number of wave peaks in the T2 spectrum before and after the impact test of sandstone is unchanged, but the wave peak value corresponding to small pores increases. This result indicates that the sandstone structure remains in the microcrack formation stage because the stress wave energy is adequate only for forming new microcracks but not for tearing pores and increasing pore diameter. Results further show that, at the same porosity, saturated sandstone consumes less energy than dry sandstone. With increased porosity, the energy dissipation of saturated sandstone decreases, whereas that of dry sandstone increases. This phenomenon can be explained based on three aspects. First, according to the fracture mechanics theory, the surface tension of water and Stefan effect significantly hinder crack expansion. Therefore, generating a new surface to dissipate stress wave energy of saturated sandstone is more difficult than that of dry sandstone under the same porosity. Second, during impact loading, saturated sandstone is in a nondrainage state, and its deformation can be viewed approximately as the sum of compressive deformation of sandstone and water. Water has smaller comprehensive deformation than sandstone. The deformation of saturated sandstone decreases with increased porosity, i.e., water content. Plastic deformation and the probability of new cracks decrease, so energy dissipation of saturated sandstone decreases. Third, with increased porosity, the dynamic strength of dry sandstone declines accordingly. Developing new cracks during loading to dissipate stress wave energy as surface energy is easy. Therefore, high porosity causes great energy dissipation.
Highlights
Blasting is still a usual method of mineral exploration and excavation of underground chamber
The SHPB device was applied in the material impact dynamics laboratory. is device is mainly composed of a nitrogen cylinder that stores high-pressure gas, bullet for impact loading, laser speedometer to measure the bullet impact velocity, incident and transmission bars as the stress wave propagation media, energy absorption bar, and device protective damper
Comparison of Pore Characteristics before and after Impacts. e Nuclear magnetic resonance (NMR) relaxation spectra of sandstones (Figure 5), for 92# dry sandstone and 64# saturated sandstone show that the relaxation spectra of the same sample after the impact experiment still have three peaks, but the value of small pore peaks increased to some extent
Summary
Blasting is still a usual method of mineral exploration and excavation of underground chamber. In order to sufficiently utilize the explosion energy and enhance the efficiency of rock breaking, except for selecting appropriate explosive amount and location to install the explosive, one key scientific problem that needs to be solved is to deeply study the propagation principle and energy dissipation mechanism of the stress wave in the rocks with discontinuous structural surfaces, such as cracks, pores, and joints. Deducing the influencing principle of microscopic and macroscopic discontinuous structural surfaces in rocks (e.g., pores, fissures, and joints) on the propagation of stress wave and explaining the internal. Us, rocks with the same porosity but different diameters show different influencing principles on stress wave propagation tested by using SHPB. E influencing principle of changing rock porosity on stress wave propagation, and energy dissipation under dry and saturated conditions was discussed. E energy dissipation principle and the mechanism of porous sandstone were disclosed
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