| With the "One Belt,One Road" strategy and the shift of coal energy base to the West,the construction of projects in cold areas and the development of mineral resources are increasing year by year,and inevitably suffer from the effects of freezethaw at different temperatures caused by seasonal fluctuations,day and night cycles.At the same time,combined with the effects of mechanical construction,blasting,seismic disturbance,and other dynamic stresses,a series of engineering geological disasters can easily occur,seriously hindering the sustainable development of construction projects in cold areas and the safe and efficient extraction of mineral resources.In this thesis,the sandstone mined on the slope of an open-pit coal mine in the cold region is used as the research object.The effect of freezing temperature on the mechanical properties,impact dynamics,and deformation damage of frozen-thawed sandstone is investigated by a combination of indoor tests and theoretical analysis.Freezing temperature is considered as an important influencing factor in the freezethaw process,and the number of freeze-thaw cycles and impact loads are considered.Meanwhile,the correlation between the mesoscopic parameters and the dynamic mechanical properties of the frozen thawed sandstone was analyzed using the CT technique,and the dynamic damage constitutive relationship was constructed considering the evolution of the mesoscopic structure and verified by the results of impact tests.The main research work is as follows:(1)According to the changes of unfrozen water content of sandstone during freezing and thawing,four kinds of freeze-thaw cycle tests with different lower temperature limits were carried out to obtain the development law of mechanical properties of sandstone under the influence of freezing temperature and reveal the macroscopic damage mechanism.The results show that: the lowering of the freezing temperature extends the range of influence of the freeze-thaw action and the object of deterioration passes from the pores and the cement to the mineral particles;the lower the freezing temperature,the more evident is the variability of the peak strength and the elastic modulus of the sandstone with the increase of the freeze-thaw cycles,and the macroscopic change from a linear to an exponential decrease;under the influence of cyclic loading,the decrease of freezing temperature increases the plastic deformation of the unloading section and accelerates the deterioration of the frozen-thawed sandstone.(2)Impact tests were performed on frozen-thawed sandstone under different loads.The lower temperature limit for the change in sensitivity of the effect of strain rate in frozen-thawed sandstone was determined,and the response mechanism between fullfield strain and dynamic fracture properties was elaborated.The results show that the dynamic strength of sandstone is logarithmically related to the loading strain rate and has an obvious rate effect,while the dynamic deformation modulus is not significantly related to the strain rate.As the freezing temperature decreases,the sensitivity of the dynamic mechanical properties of sandstone to the strain rate effect first increases and then decreases,and the inhibitory effect of rate effect on freeze-thaw damage is gradually weakened;the evolution of the strain field is closely related to the dynamic failure characteristics of sandstone.The lower the freezing temperature,the earlier the local concentration of the strain field occurs,the lower the dynamic stress threshold for the development of microfractures,and the more drastic the macroscopic fragmentation degree.(3)Based on CT and digital image processing techniques,the mechanism of freezing temperature variations on the mesoscopic characteristics of freeze-thaw damage was revealed,and the main mesoscopic parameters affecting the deterioration of dynamic bearing capacity of fronzen-thawed sandstones were identified.The results show that lowering the freezing temperature promotes the change of freeze-thaw damage in sandstone from the development of primary pores to the development of new pores and microfractures,and that the number of pores changes from decreasing to monotonically increasing with freeze-thaw cycles;combined with the theory of gray relational analysis,it is found that the connected porosity is the key parameter for determining the freeze-thaw damage in sandstone,while the average throat length determines the performance of late freeze-thaw damage.Furthermore,using the twomedia phase model,a parameter characterizing the degree of throat channel development was introduced to test the validity of this micro-scale parameter.(4)The cyclic impact tests on frozen-thawed sandstone under different loads were carried out to obtain the evolution of peak stress and deformation modulus with the number of impacts under the influence of freezing temperature.Using the CT technology,the damage variables were defined by the variation of the average throat length and the evolution of the cumulative damage during the cyclic impact was given.The results show that: the peak stress and deformation modulus decrease at an accelerated rate with the decrease in freezing temperature,and the overall change from upward convexity to linear decrease with the number of impacts,while the maximum deformation and deformation rate show the opposite trend;with the increase of cyclic impacts,the accumulated damage passes through the three phases of slowing down and increasing,stable development and rapid growth,and the decrease of freezing temperature shortens the stable development phase and accelerates the rapid growth phase.(5)Based on the principle of energy dissipation of SHPB test,the law of energy distribution with freezing temperature during impact was determined and the mechanism of dynamic deformation damage of frozen-thawed sandstone under impact load was revealed.The results show that: the decrease in freezing temperature increases the proportion of absorbed energy and reflected energy in the incident energy,and the higher the strain rate,the greater the proportion of absorbed energy and reflected energy;the greater the absorbed energy,the greater the proportion of development of defects of any size in the sandstone,and the greater the macroscopic fragmentation;with the number of cyclic impacts,the transmitted energy decreases significantly,while the absorbed energy and reflected energy increase gradually.The lower the freezing temperature,the more the absorbed energy increases.The decrease in freezing temperature weakens the ability of the frozen-thawed sandstone to resist dynamic impacts and reduces the number of cycles in which critical damage to the sandstone occurs.(6)A dynamic damage factor was introduced to reflect the behavior of throat and pore development during impact,and a mesoscopic constitutive dynamic damage model was constructed to describe the mechanical behavior of frozen-thawed sandstone during impact by combining the initial freeze-thaw damage defined by mesoscopic parameters.The results show that the present constitutive equations better describe the dynamic stress-strain behavior of sandstone under the influence of a variable freezethaw temperature and can accurately reflect the changing characteristics of the stressstrain curve under the influence of the freezing temperature.Based on the improved ZW-T model,the main nonlinear viscoelastic constitutive equation for the damage of frozen-thawed sandstone under cyclic impact loading is derived and established,and the theoretical results of the model agree well with the results of the cyclic impact tests.There are 111 figures,27 tables and 232 references. |
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