Detecting freeze–thaw damage degradation of sandstone with initial damage using NMR technology

The nuclear magnetic resonance (NMR) technique was used to study freeze–thaw damage degradation of sandstone with initial damage. The initial damage was fabricated by preloading, and the pore structure was detected using the NMR technique. Porosity, T2 spectrum distribution, and T2 spectral area were obtained. The magnetic resonance imaging (MRI) technique was used to obtain two-dimensional pore distribution images. Combined with uniaxial compressive strength tests, the effects of initial damage and freeze–thaw cycles on the mechanical properties of rock were studied and the failure mode of rock was analyzed. The results show that in the first 20 cycles, the numbers of micropores (d ≤ 1 μm) and mesopores (1 μm < d ≤ 10 μm) both increased. As the number of cycles further increased, the porosity of the sample was mainly affected by micropores. When the initial damage level is 50% or lower, as the number of freeze–thaw cycles increases, the ability of sandstone to accommodate permanent strain before failure gradually weakens and then increases again after 40 cycles. When the initial damage level reaches 70% or higher, the ability to accommodate permanent strain continues to weaken. The strain produced by sandstone before elastic deformation is roughly positively correlated with the number of mesopores and macropores in the sandstone. Under loading, the sandstone presents four failure modes: splitting failure, single shear failure, cone failure, and multi-crack failure.