Investigation on microstructure and damage of sandstone under cyclic dynamic impact

Abstract

In order to study the microscopic damage evolution of rock under the cyclic dynamic impact, red sandstone was selected as samples, and Split Hopkinson Pressure Bar (SHPB) loaded with impact pressure of 0.3 MPa was adopted to exert cyclic dynamic impact on samples. After each dynamic impact, nuclear magnetic resonance (NMR) system was then applied to test the microscopic structure of samples, and then, the parameters of red sandstone, including porosity, transverse relaxation time (T2) spectrum, permeability, and magnetic resonance images (MRI) after each dynamic impact, were obtained. The results show that both porosity and permeability increase with the number of dynamic impacts that red sandstone undertakes, and the increasing range of them also increases gradually. The T2cutoff value gradually decreases when the number of dynamic impacts increases and their decrease range increases gradually, which is consistent with the trend of microscopic damage represented by porosity and permeability. Also, it is found through analyzing the T2 spectrum that the dynamic impact produced by the impact pressure of 0.3 Mpa leads to the gradual increase of primary pore size, an increase of macro pores, and enhancement of samples' connectivity, rather than the generation of new microspores. The MRI images reveal that cyclic dynamic impact leads to the generation of microcracks in red sandstone, which coalesces and extends with the number of dynamic impact increase and forms larger micro crack zones finally.