Study on Dynamic Mechanical Properties and Constitutive Model of Deep Rock under Hydrothermal Coupling

Author:Zhang Rong Rong

Supervisor:jing lai wang


Degree Year:2019





Rocks are often in various complex hydrothermal environments,such as the surrounding rock of a mine,the surrounding rock of an underground engineering,and the rock foundation of a surface engineering,etc.It is of great significance to study the dynamic mechanical properties of deep rock under hydrothermal coupling.In this study,it is selected as the argillaceous sandstone sandstone from a depth of 725 m in the Xieqiao coalmine and the fine-grained sandstone from a depth of 906 m in the Zhujidong coalmine.The dynamic mechanical properties of deep rock under the coupling of water and temperature are studied by using the selected rock samples.In view of the influence of water,temperature and extermal force on the rock,the dynamic and static mechanical test of sandstone under hydrothermal coupling is designed by split-Hopkinson pressure bar(SHPB).The dynamic and static characteristics,each stage of energy dissipation characteristics,failure morphology and microcosmic characteristics of rock are studied in detail.Additionally,based on the results of the study?a dynamic constitutive relationship based on damage evolution of rock under hydrothermal coupling is established,and the experimental results are matched with the numerical calculation.The main contents and conclusions are as follows:(1)The characteristics of dynamic and static stress-strain curves of sandstone after different temperature under different condition are analyzed.The results show that the dynamic and static stress-strain curves can be divided into four stages:compaction stage,elastic stage,plastic deformation stage and failure stage.However,the djTiamic stress一strain curve increases with the strain growth rate in the eompaction stage,and the performance trend is different.The peak stress in the static dried state is almost twice as high as the peak stress in the wet state,and the peak stress in the dynamic wet state is affected by the free water bonding force and the Stefan effect.The dynamic peak stress in wet state is slightly larger than that in dried state.(2)The dynamic peak stress and the relative elastic modulus(^r)value decrease rate of Low-Temperature(L-T)group and Medium-Temperature(M-T)group after 20 cycles and the High-Temperature(H-T)group after 4 cycles were gradually slow.The degree of decrease in H-T group is much greater than that in L-T group and M-T group,indicating that the degree of sandstone damage in H-T group is much greater than that in other two groups.The mass fractal dimension of fragmentation in L-T group and M-T group was linearly related to the cycling number,while that in H_T group was exponential.The average fragmentation degree of sandstone after H-C cycles decreases with the increase of cycling number,and takes the form of exponential function.(3)During the whole process of loading and breaking,the samples all accord with U>Ue>Ud,and the total import strain energy in this stage is converted into elastic energy in the compaction stage.A few of them are dissipated during plastic deformation.The elastic energy and dissipation energy of each specimen exist simultaneously in linear elastic deformation stage,while the total import strain energy is converted into elastic energy and dissipated,meanwhile,the total import strain energy of sandstone begins to dissipate in this process and a small part is converted into elastic energy under different water-temperature coupled damage conditions in yield stage.At failure state,dissipation energy in this stage is larger than the total import strain energy,while the elastic energy is approximately zero,indicating the elastic energy stored in compaction stage is released.According to the principle of energy conversion,the damage of sandstone after H-C cycle is defined by the law of decreasing total input strain energy.(4)XRD test found that quartz expanded at high temperature,which improved the intermal structure of sandstone.It is found that kaolinite has strong hydrophilicity through XRD analysis when it is saturated with water,and hydration expansion leads to the weakening of cementation between mineral particles.When kaolinite reaches 480℃ and 600℃,it is transformed into metakaolinite.After 750℃,metakaolinite is transformed into Al-Si spinel,and amorphous SiO2 is decomposed at the same time.Calcite decomposes above 800℃ to produce lime(CaO).(5)The sandstone is compact and has some some cracks at 20℃(room temperature).The crystal of rock presents glass flake at-20℃9-10℃ and 0℃,and the BSE images show that with the decreasing of the temperature treatment,the internal cracks and pores will increase.The granular crystals with clastic structure gradually become larger from range 20℃ and 1000℃,and the initial cracks increase obviously.After 800℃,the tearing ridges gradually become the main form of fracture,and the local cracks appear and expand gradually.(6)In L-T group,the initial microcracks and pore propagation caused by the freezing expansion and thermal expansion of mineral particles will occur simultaneously after 10 H-C cycles,and,The thermal stress set is produced by the disharmony of thermal stress deformation among mineral particles after 40 H-C cycles,and repeated H-C cycle leads to fatigue firactrure along the boundary of mineral particles.Compared with EDS results of room temperature,L-T group,M-T group and H-T group,it was found that after 12 H-C cycles in H-T group,a large number of cracks and pores appeared in the sandstone specimen.Due to the presence of clay minerals,the clay was treated at 400℃.Hydroxyl loss of water will occur,resulting in irreversible damage inside the sandstone.(7)The damage evolution curve and the total damage rate curve of the rock under hydrothermal coupling are studied and analyzed,and the damage dynamic constitutive model of the sandstone is established based on the dynamic unpact test data of the sandstone under hydrothermal coupling.The model takes into account the influence of hydrothermal coupling damage and impact load on the dynamic mechanical behavior of rock damage.The fitting effect of theoretical and experimental stress-st:rain curves is appropriate,and the established dynamic constitutive model is feasible,and the error analysis of the experimental and constitutive results is also carried out.Figure[73]table[11]reference[253]