Study On Dynamic Mechanical Properties And Micro-Structure Mechanism Of Rock Under Freeze-Thaw Cycles

Owing to the practical problem that various kinds of geotechnical engineering were inevitably destroyed by freeze-thaw cycle, dynamic load and so on, study on rock dynamic mechanical properties under freeze-thaw condition has been a hot topic. In this paper, damage characteristics within rock after different freeze-thaw cycle experiments were detected by NMR. And contrast experiments on dynamic and static mechanical characteristics after different freeze-thaw cycles were studied through experiments. And one dimensional static-dynamic combination load experiments were carried out. Basing on the experiments results, the damage evolution law of sandstone under coupling effects of freeze-thaw cycle and dynamic load was discussed. The conclusive results are as follows:(1) After different freeze-thaw cycles, nuclear magnetic resonance (NMR) technology was used to analyze the sandstone porosity, nuclear magnetic resonance (NMR) imaging, T2spectrum distribution and area change characteristic. It revealed that the expansion of sandstone pore was a dynamic cycle process and the internal damage of sandstone was a fatigue damage process by different freeze-thaw cycles.(2) The change rules of sandstone specimens such as dynamic and static complete stress-strain curve, failure mode, peak strength and peak strain after different freeze-thaw cycles under dynamic and static loading tests were compared and analyzed. Sandstone samples after different freeze-thaw cycles under dynamic and static loading tests were presenting different failure modes:sandstone under static loading were given priority to splitting and shearing failure, and the failure mode of rock exhibit evolution from a single cleavage plane to "X "-type shearing plane with the increasing of freeze-thaw cycle; while sandstone under dynamic loading are given priority to tensile failure, and the sample fragments appear even and small distribution with the increasing of freeze-thaw cycles. In the process of0to20times and60to100times freeze-thaw cycles, internal porosity of sandstone extend acutely, which led to the weakening of peak strength and the fast growth rate of the peak strain under dynamic and static loading. The average decline of peak strength under dynamic loading was slightly less than static loading, while the increase of peak strain under static loading was greater than the dynamic loading because of compaction stage.(3) The dynamic strength relationship of axial static pressure and freeze-thaw cycles was obtained by one dimensional static-dynamic combination load experiments. To some extent, axial static pressure not only increases the dynamic peak strength of the rock, also causes the end effects that can change sandstone crushing modes:from axial tensile failure to shear-tension destroy. The influence of freeze-thaw and axial static pressure on energy dissipation rate of sandstone was analyzed, the results show that the rate of energy absorption changes with rise of axial static pressure, and exists an optimal value that matches the nature of the rock itself, freeze-thaw cycles influences energy absorption rate by changing internal porosity and pore distribution.(4) With the increases of NMR porosity and spectral area, dynamic peak strength and elastic modulus decreased exponentially. The relationship of NMR eigenvalues and macroscopic mechanical parameters was established and evolution of damage under the interaction of freeze-thaw cycles and dynamic load was investigated. The total damage model can be characterized by freeze-thaw cycles and two-parameter was presented...