In-place Preparation Technique And Uniaxial Compression Properties Of Ice Samples Frozen Under High Pressure

With the increasing of the depth of artificial freezing engineering and natural ice exploration,the deformation behaviour of ice frozen under high pressure in deep environment changes significantly.In order to reveal the influence of freezing pressure on the mechanical properties of ice,a test device and an in-place ice sample preparation technique were developed according to the formation environment and test requirements of pressurized freezing ice.Based on these,the studies of fabric characteristics and the in-place uniaxial compression properties of ice frozen under high pressure were carried out.Firstly,a virtual infinite stiffness triaxial test device with control of temperature field was developed.Through the numerical simulation,the structure of circulating channel,the locations of upper,side and lower parts,as well as the layout of spiral line were determined for the freezing channels of the pressure cell,forming a freezing system that can effectively and diversely control the temperature field of the sample.A loading technology which compensates the deformation of the loading system according to the force data was used,and the high-precision measurement of the real strain and the high-accuracy control of the sample deformation were achieved.The axial temperature field of the sample was measured by a distributed optical fiber which was sealed with glue filling technique.Secondly,a preparation technique freezing pressured water to a standard cylindrical ice sample in a triaxial cell was developed.Through the theoretical analysis,the design of an assembly cylinder and the glue sealing,the heat shrinkable tube was selected as the thin film outside the sample,and a shaping and sealing technique of cylindrical water sample with high geometric accuracy and excellent sealing performance was obtained.A freezing pressure control method that limits the volume of confining medium and constants the axial pressure was proposed.Through theoretical analysis and ice sample preparation tests,the permissible changing range of confining medium temperature during the freezing process and the sample frozen mode were determined.Based on that,a freezing operation that firstly pre-cools the triaxial cell,then injects and pressures the oil,finally freezes the sample under stable pressure was worked out,the temperature control parameters of the operation should be determined by numerical simulation.After the test researches of the temperature field of ice sample,a high effective and non-destructive method that adjusts the stress and temperature fields of ice samples to the test initial state was gain.The whole technique realized the continuous control of the sample stress state from freezing to testing,and the geometric accuracy and temperature distribution of prepared ice samples meet the test standard.Thirdly,ice samples frozen under pressure of 0.5–30 MPa were prepared by the test device and preparation technique,and their fabric characteristics were analyzed by the Langway method and image processing technology.The crystal structure of ice samples is composed of fine columnar crystals growing rapidly from bottom to bottom and coarse crystals growing slowly from outside to inside,and the crystal size increases from bottom to top on the whole.There is no obvious relationship between the crystal characteristics and the freezing pressure.The analysis also revealed the distribution law,macroscopic morphology and microstructure of bubbles in ice samples.The phenomenon of local bubbles disappearance caused by the formation of air hydrate under high pressure was discovered.A quantitative influence of the freezing pressure on the content and size of bubbles was obtained by means of density conversion calculation and microscopic analysis.Finally,the in-place uniaxial compression tests for ice samples frozen under pressure of 0.5–30 MPa were carried out with a temperature of-20?and strain rates of 5×10^-5–1.5×10^-6/s.The effect laws of freezing pressure on peak stress,residual stress and failure strain were obtained,and the effect mechanism was explained based on the competitive relationship between the impacts of the decrease of defect degree and the increase of defect internal pressure on strength characteristics according to the impact of freezing pressure on the fabric characteristics of ice samples.The relationship between the peak stress and the strain rate of pressurized freezing ice was verified to be a power function.The relationship between the brittleness index and the freezing pressure of the ice samples was also obtained,whose mechanism was explained by the influence of the bubble characteristics on the deformation energy dissipation law.In addition,a more precise uniaxial compression constitutive model for pressurized freezing ice was derived.The research results can provide the parameter basis for the thickness calculation of deep frozen wall and the improvement of drilling technology for extremely thick ice sheet.In this thesis,there are 86 figures,15 tables and 232 references.