| With the implementation of the "One Belt,One Road" strategy,more and more rock engineering problems in cold regions have been encountered in western my country.Affected by the alternating cycles of small day and night cycles and large seasonal cycles in cold regions,the initial fine-grained damage of rocks continues to intensify,and then the mechanical properties of rocks deteriorate,causing rock engineering disasters in cold regions.Therefore,it is of great significance to study the micro-damage evolution characteristics of sandstone under the combined action of freeze-thaw and load for the construction of a large number of cold regions.In this paper,taking the sandstone under the freezing and thawing environment as the research object,by carrying out the real-time CT scanning test under the action of uniaxial compression,using the digital image processing technology,based on the digital volume correlation method,the accurate pore(crack)structure data inside the sandstone is obtained,and the quantitative data is obtained.Analysis of structural damage characteristics of frozen-thawed sandstone during uniaxial compression.The main completed work and conclusions are:(1)The CT real-time scanning test of the structural evolution of the frozen-thawed sandstone under the action of uniaxial compressive load was completed,and the CT data and CT images of each scanning horizon of the rock sample at each failure stage under different freeze-thaw cycles were obtained.Therefore,it provides a feasible and effective method for the accurate identification of the mesostructure of the frozen-thawed sandstone during uniaxial compression.(2)Gaussian filtering,median filtering,anisotropic diffusion filtering and non-local mean filtering were used to denoise CT images respectively.The results show that non-local mean filtering can remove noise and protect frozen and thawed rock samples during loading.The detailed features of CT images are an effective method for noise reduction of frozen-thawed sandstone CT images.(3)Based on the porosity of the rock sample,using the inverse method,combined with the pore area-threshold curve inflection point interactive threshold segmentation method,the optimal segmentation threshold of the CT image of the rock sample before and during the loading process was obtained,and the frozen-thawed rock was completed.It provides a basis for quantitative analysis of microscopic damage of frozen-thawed rock samples under uniaxial compression.(4)On the basis of noise reduction,segmentation and 3D reconstruction of CT images of different scanning horizons during the loading process of the frozen-thawed sandstone obtained from the experiment,the two-dimensional CT images of the meso-structural damage of each scanning horizon during the loading process of the frozen-thawed sandstone were obtained.3D fissure image.The analysis shows that under the action of freeze-thaw cycles,the failure of the rock sample is from shear stress failure to produce a single through shear plane,which gradually becomes the combined failure of tensile stress and shear stress.The number of failure surfaces keeps increasing;in the process of uniaxial compression,the volume of pores(cracks)inside the rock sample firstly decreases slowly,then increases slowly,and finally increases sharply.The change process corresponds to the pore and fissure compaction stage,elastic deformation stage and peak damage of the rock sample under load;with the increase of the number of freeze-thaw cycles,the damage propagation direction of pores and cracks in the sandstone is from the top and bottom ends to the middle.expansion,the failure mode changes from brittle failure to ductile failure.During the uniaxial compression process,a large number of through-connection shear cracks are generated inside the rock sample,and the porosity increases significantly after the failure.Among them,the sandstone that has undergone low-order freeze-thaw cycles,the internal pore and fissure structure after failure is dominated by complex small pores,and the proportion of cracks is low,while the sandstone that has experienced high-order freeze-thaw cycles,the expansion of simple cracks after failure dominates,and the pore structure less proportion.(5)On the basis of the quantitative analysis of the three-dimensional reconstruction of the characterization unit,the three-dimensional pore-throat structure is used to characterize the quantitative relationship and connectivity between the pores and throats in the frozen-thawed sandstone.From the three-dimensional meso-scale,the evolution law of fine-tunnel damage structure during the loading process of frozen-thawed sandstone is discussed.The results show that:under the influence of freeze-thaw cycles,during the loading process of rock samples,the proportion of small pores decreases,while the proportion of mesopores and large pores increases,and the number of internal throats of the rock sample is closely related to the permeability,and the permeability is mainly Determined by the number of internal throats.Because the freeze-thaw cycle changes the initial meso-damage structure inside the sandstone,the uniaxial compressive load causes the rock mineral particles to further slip and dislocate,pores are initiated and expanded,the damaged area is gradually connected,and the throat merges and expands,resulting in The fracture volume shape changes,eventually leading to a change in the failure mode.(6)Based on the digital volume correlation method,the displacement field and strain field of the frozen-thawed sandstone loading process are analyzed.The analysis results show that the damage of the rock sample under the action of freeze-thaw cycle is a gradual process from the surface to the inside.Under the influence of the freeze-thaw cycle,the internal displacement and strain range and the size of the rock sample continue to expand during the loading process.Among them,the shape of the high displacement and strain distribution areas in the distribution graph of the displacement field and strain field before and after the failure is consistent with the crack shape when the rock sample is damaged.Therefore,the CT image-based digital volume correlation method proposed in this paper can predict the failure of frozen-thawed rocks under uniaxial compressive loading. |
