Study On Meso-Structure And Damage Mechanical Characteristics Of Frozen Rock Based On CT Image Processing

Frozen rock problems exist in many projects in different degree, including the rockengineering in cold regions, the reservoir pool at low temperature of liquefied natural gas andthe construction using frozen methods in mine building projects, etc. The research on frozenrock problems has extensive engineering background and practical significance. The damageon frozen rock at low temperature has a significant effect on the security and stability of therock engineering in cold region. Aiming at the engineering demands for practical problemsoccurred in frozen rock engineering, based on the computed tomography (CT) scanningexperiment of frozen rock at different minus temperature, and guided by the theories ofDamage Mechanics, Continuous Media of Thermodynamics, Meso-mechanics, Stereology,Ice mechanics, and taking the meso-structure of frozen rock as cut-in point, meso-structureand mechanical characteristics of frozen rock were investigated by the method combiningwith CT image processing technology and damage theoretical analysis and mesoscopicnumerical calculation. The following conclusions have been gotten:(1) CT scanning images of red and gray sandstones at different low temperature areobtained by carrying out the CT scanning experiment at room temperature20℃,0℃,-5℃,-10℃,-20℃and-30℃. CT scanning images can well show the change of themeso-structure and damage of the frozen rock, and the composition distribution whichincludes water, ice and mineral particles during process of freezing. Under low temperature,the new internal rock pore and crack are generated also accompany with the phenomenon ofcrack propagation and coalescence. Rock damage increases with the temperature dropping.When the temperature drops to-20℃, the rock meso-structure keep stable and almost changeno more.(2) Applying digital image processing theory to CT image, pseudo-color enhancement technique of CT image of frozen rock has been implemented. Histogram of frozen rock basedon CT value has been obtained. The double thresholds segmentation that based on geneticalgorithm is realized and the three-valued segmentation of frozen rock CT images iscompleted. By Canny operator’s edge detection, the binary images of frozen rockmeso-structure are gotten. Using CT image processing technology can improve the resolutionof frozen rock CT images, and reduce the error of visual judgment. According with thechanges of colors in pseudo-color enhancement image of frozen rock CT images andhistogram of frozen rock based on CT value, meso-structure, unfrozen water, ice, and damagedistribution can be analyzed quantitatively dependent on temperature during freezing process.he three-valued segmentation of frozen rock CT images is employed to distinguish water andice from rock in the internal of frozen rock. Digital expression of spatial location and thecontent of water, ice and rock are given definitely by the three-valued segmentation.(3) The stereology theory is introduced to the study of the characteristics of freeze rockmeso-structure. Combined with the CT images of frozen rock, the meso-structure compositionand their mutual relations are analyzed and the parameter calculation formulations ofmeso-structure of frozen rock are put forward. The meso-structure parameters are calculatedquantitatively, such as length, perimeter, area, width and circularity of pore and crack. Thecalculation results of unfrozen water and ice content shows that unfrozen water and icecontent in rock interior varied with the changes of temperature, and both are in a state ofdynamic balance.In frozen rock, the unfrozen water content is considered as an internal variable and thework conjugated with the internal variable is the latent heat of phase change of unfrozen rock.The continuum thermodynamic theory is employed to deduce the formula of the relationshipbetween the unfrozen water and ice content depends on temperature. The experimental resultsof CT scanning agree well with the result predicted by deduced formula. The changes ofunfrozen water and ice content in the process of freezing can be described by deducedformula, and the pure frozen point of frozen rock can be predicted.(4) Based on composition and characteristics of mesoscopic structure of frozen rock,from the idea of the rock mesoscopic mechanics, using mixed law methods to consider frozenrock with pore as isotropic medium, and frozen rock with crack as the transversely isotropicmedium, calculation formulas of equivalent elastic modulus of frozen rock under differentfrozen temperature were deduced.In view of the influence of frozen action on the properties of rock mechanics, the conceptof frozen negative damage was put forward. Damage variable was proposed with more new meaning to frozen and loaded rock, three new conceptions were put forward, which werefrozen damage, loaded damage and total damage. Proposed damage variable can describedifferent damage process of rock under the two different actions,one is low temperaturefrozen and another is load.Based on the continuum thermodynamics method, and takeing theinitial damage state as reference state, macro-mesoscopic damaged constitutive model offrozen rock was build under the load action according to the generalized principle of strainequality and theory of damage mechanics.(5) The CT image of frozen rock can be extended to be applicable to research frozenrock damage. The new method (DIP-FEM), Digital Image Processing—Finite ElementMethod was put forward. By the proposed method, the finite numerical analysis fomat of rockdamage process has been established under different low temperature environment and loadaction. The real mesoscopic structure digital model of frozen rock is obtained bythree-dimesion reconstruction technology of CT image. Vectorization algorithm ofmesoscopic structure digital model of frozen rock is presented, which can convert imageformat to graphics format. Then real meso-structure of frozen rock is imported into finiteelement software ANSYS to analysis damage characteristics of rock with pore or crack in lowtemperature environment. The proposed method can make up for the deficiencies of lowtemperature frozen rock physical experiment, and provide a new research avenues to realizedamage process research of frozen rock from the meso-scale to macro-scale.