Experimental Study On Dynamic Mechanical Properties Of Rock Under Low Temperature

Recently, with the implementation of the western development strategy in our country, a large number of geotechnical projects have been launched under frozen conditions, such as some coal mines in Xinjiang and Inner Mongolia, et al, and maintaining the resource channel to ensure the our country developed rapidly, such as the Kazakhstan-China and Russia-China oil, natural gas pipeline projects, a fully understanding to the mechanics of frozen rock in cold regions is needed to ensure these large projects safety in both construction and future maintenance. However, in terms of frozen rock, researchers most concentrated on the rock freeze-thaw cycle both at home and abroad, while, the physical and mechanical properties of rock under low temperatures are not well known, and the dynamic mechanic characters of frozen rock is more unknown.This paper focuses on the geotechnical engineering in western cold regions, especially the shaft construction of the coal mine in the Western China, which is funded by the national natural science fund project "rapid excavation and supporting base of large section roadway" and the project " the damage mechanism of frozen pipes and frozen wall under blasting", the change of mechanic properties of rock under the coupling of low temperature and impact loading is studied by considering the influence of temperature and strain rate using SHPB configuration, the energy properties and factual characteristics of rock fragmentation size is analysed, and fractal calculation model for rock fragmentation under low temperature is established, and the relation between fracture energy and fractal dimension is deprived. Based on the above tests, large number of research results are obtained as follows:(1) The impact rod of SHPB system is improved through experiments and numerical simulations, a bi-conical impact rod with cone section ratio of 31:13 is developed to realize the uniform distribution of stress/strain along sample length. It is found that the cone section ratio of bi-conical impact rod greatly affects the incident wave, the waveform of incident wave changed with different cone section ratio, the impact ends affects the rising stage, while the rear ends change the falling stage; the longer the cone section is, the more gentle the wave curve is, as well as less oscillation phenomenon, but correspondingly, the peak stress also declined.(2) Uniaxial compression tests and splitting tensile tests of red sandstone, marble, granite are carried out Using the TAW-3000 electro-hydraulic servo tri-axial test system. The rock failure under quasi static uniaxial experiments can divided into two failure modes(namely, the tensile fracturing and the single shear plane failure). The uniaxial compressive strength of saturated red sandstone reduced rapidly, while, it changed little in both saturated marble and granite, which indicates that the compressive strength decreased rapidly in highly porosity rocks when saturating, it is relatively smaller for low porosity rocks; the tensile strength of rock is more sensitive to water compared with compressive strength; the tensile strength of all three rocks are decreased significantly under saturated state.(3) Dynamic compression tests of three kinds of rock are carried out under normal temperature using SHPB configuration with the bars diameter of 75 mm. the size of fragmentation of all three rocks are reduced significantly with the strain rate increases, and the number of pieces are increased significantly, which shows greatly strain rate dependence; Both the dynamic compressive strength factor of rock and the energy absorption ratio increased linearly with the strain rate e. Where, the rate of dynamic compressive strength factor of red sandstone is highest, and are equal to 0.01668, the marble and granite are slower, and are equal to 0.01; the energy absorption ratio of red sandstone is highest in same strain rate, whereas, in terms of the increasing rate of energy absorption ratio, the marble is fastest, granite is medium, and the red sandstone is lowest.(4) Dynamic tensile tests of three kinds of rock are carried out under normal temperature using SHPB configuration with the bars diameter of 50 mm. the split failure path, which is perpendicular to the loading direction, occurs along the radial direction of the specimen under different strain rate, both the tensile sensitive coefficient and the dynamic tensile elastic modulus of all three rocks are increased with the strain rate increases, the dynamic tensile strength of rock is 3 ~ 4 times of the static tensile strength, it can be seen from the histories of tensile stress with radial strain,, the modulus of unloading in marble and granite are greater than that in the red sandstone, which implies that greater stress release rate or more abrupt destruction would occur during unloading for stiffer and tougher rock, such as marble and granite.(5) During the experimental tests coupling low temperature and high strain rate, the peak stress of saturated red sandstone presents a first increase, and then decreases with the temperature from 25℃ to-40℃, which is formed to be a "几" shape; the peak stress of marble shows a decrease at first, then increases, and finally decreases with the temperature decreases, which is more likely a "N" shape; the peak stress of granite holds constant at first, and then decreases with the temperature decreases, which is like a "乁" shape. The peak strain of red sandstone presents a “decreasing-increasing” tendency from 25℃ to-40℃, and the transition point is-10℃. While, as for marble and granite, the peak strain presents a “increasing-decreasing-increasing” tendency with the temperature decreases, and correspondingly, the transition point is-5℃ and-20℃ for marble, and-5℃ and-15℃ for granite. In the experimental tests of the dynamic mechanical properties of rock under low temperature with different strain rate, the dynamic elastic modulus, peak stress and peak strain of all three rocks grow in a weak power manner with the strain rate at-15℃.(6) The waters-ice phase changing has little effect on the rock strength under static loading from 0℃ to-6℃, but it is strongly weakened the strength under dynamic loading. This is because during the static loading, both the micro-cracks and micro-defects, induced by water-ice phase changing, can be closed, thus, the internal damage due to the volume expansion, cannot make the strength change; While, under the strong impact loading, both the internal crack closure and new crack formation are happened simultaneously, but crack initiation rate are largely higher than crack closure. Moreover, the new fissure are commonly induced by the stress concentration at the original microcracks, micro defects and weak structures, et al, thus, the internal crack, defects and the micro-cracks, produced by the volume expansion with water-ice phase changing, could greatly weakened the rock mechanical properties;(7) The dynamic mechanical properties of rock weakened again after below-15℃(it changed with different rocks and the satisfied degree of water), this is because although both the volume of rock matrix and ice shrinks with temperature decreases, the shrinkage rate of rock matrix is higher than that in ice medium after the temperature below-15℃. Meanwhile, since the rock is composed by a variety of mineral, the shrinkage rate of different mineral grains are also different at low temperature, thus the cracks, defects and weak structural surface could initiates in rock structural surface, matrix-ice interface, which induced mechanical properties of rock weakened again.(8) The macroscopic failure of rock has close relationship with the energy absorption(namely, the total dissipation energy), the higher the dissipation energy is, the greater the rock fragmentation degree is; the energy absorption(namely, the total dissipation energy) of rocks under low temperature is rate dependence with the strain rate increasing, the total dissipation energy of both freezing red sandstone and freezing granite increases at a weak power form with the strain rate under low temperature, and the corresponding total dissipation energy of marble presents a linear increase.(9) The fractal dimension D is introduced into the study of the size distribution in rock fragmentation under low temperature, the fragment-size distribution model of frozen rock with fractal theory is established, and the fracture energy of rock fragmentation increases at a weak power form with the fractal dimension based on this model, that is, the more the fracture energy consumes during the frozen rock fragmentation, the more the rock breaks, and the higher the fractal dimension is.(10) Low temperature gradient affects the fractal characters of rock fragment-size, for example, the fractal dimension of red sandstone at 25℃,-5℃,-10℃,-20℃,-30℃ and-40℃ are 2.37, 2.12, 2.29, 2.35, 2.41, 2.44, respectively; Therefore, there is a positive correlation between the fracture energy of fragmentation FDW and the fractal dimension D, while, the increasing rate of D gradually decreases with FDW increases, which meets the equation:-5 2FD FDD =-5.18123 e W +0.01972 W +0.58682.(11) Through the analysis of the fracture pattern type of the rock, the crack nucleation of the cryogenic rock under impact loading mainly due to crystal deformation, and depending on the nucleation mechanism, it will be grouped into three kinds: the grain does not match each other, crystal solids plastic deformation and interface slippage.(12) Depending on the analysis of the fracture morphology of red sandstone under different negative temperature, it is discovered that the properties of the cement and the fracture mechanism of red sandstone under negative temperature has a more significant effect. Under impact loading, micro-cracks generally develop in the cement to form the final fracture, merely occurring on mineral particles; As the ice medium in the saturated red sandstone under negative temperature will gather cement substance together, the cement strength will be improved, and the degree of cementation between cement and mineral particles improved as well, in a result,the red sandstone strength properties are enhanced; The shrinkage rate of sandstone mineral particles, ice media and cement will be significantly differentially after-30℃, and because of differences in the rate and elastic modulus mismatch of three materials, the interface of them are prone to crack and generates separation in shear under the external force, and less cement composition and peel potholes phenomenon will appear on the fracture surface, since the appearance of the microscopic fracture phenomena, the macroscopic properties of the rock, carrying capacity and strength, decrease.