| With the construction in the central and western regions of country has been placed to the topper priority,the number and scale of projects covered with highway,railway,airport,water conservancy and other key fields is continuing to expand in the region.Because of the effect of low temperature environment caused by seasonal frigid climate,persistent extreme snow and ice weather and other factors,many rock and soil engineering under construction and planned here,such as tunnels,slopes,subgrade,will inevitably undergo a long or short period of freeze-thaw cycles.Rock is generally regarded as a heterogeneous body,and there are more or less micro-cracks and micro-pores as well as other defects inside.Considering it,mechanical properties and failure rule of rock materials are easily damaged by the effect caused by freeze-thaw cycles.As a result,it is threatening the long-term stability of engineering rock.So studying the mechanical properties and damage failure law of frozen-thawed rock can provide a theoretical reference for the analysis and prediction of strength and deformation and failure characteristics of rock materials in freeze-thaw environment.Moreover,it is of great research significance and academic value for the construction,maintenance and safety assessment of major projects in areas with hidden danger of freeze-thaw disasters around country.In the paper,the effect caused by freeze-thaw cycles on mechanical properties and damage characteristics of rock materials in practical engineering has been studied through tests and theoretical analysis.At first,sandstone samples undergone different freeze-thaw cycles have been tested by uniaxial compression test.Then,the failure section and its fractal characteristic of samples after tests have been observed and calculated on the basis of digital image technology and fractal theory.As a result,the paper have presented the mechanical properties,the change rules of the macro-failure feature and the fractal dimension of the compressive cross-section,the deterioration mechanism of damage,as well as a fractal damage statistical constitutive model for frozen thawed sandstone under uniaxial compression.Finally,based on the experimental results,to verify the feasibility of model and analyze the damage evolution of loaded sandstone under freeze-thaw.Several results and conclusions obtained follows as:(1)Through freeze-thaw cycles test firstly and uniaxial compression test secondly for sandstone samples,its deterioration rules of mechanical properties and change of failure characteristics caused by freeze-thaw cycles is presented.With the cycles of freeze-thaw increasing,the part curves of compaction and plastic yield stage is becoming more and more obvious but shorten at elastic deformation stage,and the part curves of post-failure stage more gentle as well as the increases of final deformation.The uniaxial compressive strength and elastic modulus of red sandstone tend to decrease rapidly at first and then decrease slowly,and all of them have well relationship with the number of freeze-thaw cycles in terms of exponential function,while the axial peak strain increase rapidly first and then decrease slowly during whole freeze-thaw cycles except individual cycle.In addition,there are three typical failure forms of sandstone after freeze-thaw cycles,as splitting tensile failure,conical shear failure and tensile shear mixed failure.The failure mode changes from the form of splitting tensile failure to that of shear failure with cycles increasing,and the integrity of broken rock block becomes worse.(2)Based on digital image technology and fractal theory,the change rules of the morphology characteristics for failure fracture and its fractal dimension have been presented.The difference on waviness of main fracture surface decreases with cycles of freeze-thaw increasing.As to the calculating results,the value of fractal dimension is also gradually decreasing(except for individual point),and the fractal dimension distribution of the three images of the same rock compression fracture becomes uniform.By the way,the confidence level of each one using Matlab is more than 0.985,and that means the results of fractal dimension calculation are more accurate.In addition,the fractal dimension value of cross-section morphology has good correlation with energy dissipation characteristics that the value of fractal dimension is smaller,that of energy dissipation is smaller,which means rock will be destroyed more easily.(3)Based on damage continuum damage mechanics and statistics theory,fractal damage statistical constitutive model for frozen-thawed sandstone under uniaxial compression is established.The model well defects the evolution characteristics of damage caused by freeze-thaw cycles and loading.And it combines macroscopic failure characteristics with damage failure mechanism in microscopic by taking advantage of fractal characteristic of failure fracture of rock under freeze-thaw cycles.It is found that unknown parameters of the model can be easily solved by the experimental data,with well applicability.(4)Comparing the calculating results of model with experimental results and to verify it,and the evolution rules of damage for loaded sandstone under freeze-thaw are presented.It is found that the stress-strain theoretical curves of sandstone are in good agreement with that of test,especially at peak point of curves,and well describes the deterioration rules of strength for frozen-thawed rock.In terms of damage evolving rules,total damage Dm caused by freeze-thaw and loading tends to increase rapidly at first and then increase slowly,with the cycles of freeze-thaw unchanged.As a result of the cycles of freeze-thaw increasing,total damage is increasing but the velocity of damage decreasing when the load is unchanged,and the damage evolution path of rock samples moving forward.It can be concluded that freeze-thaw cycles has an effect on aggravating the damage and strengthening plastic characteristics for sandstone. |
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