Study On Mechanical Properties And Damage Mechanism Of Mudstone Under Freezing-thawing Cycle

Based on mudstone from Aba Prefecture,West Sichuan Plateau as the research object,Uniaxial compression tests were carried out on the processed standard rock samples under different freeze-thaw cycles to obtain the stress-strain curves of the mudstone under freeze-thaw cycles,and the deterioration of mechanical parameters with freeze-thaw cycles is analyzed.Damage variables were established based on wave impedance and the freeze-thaw damage equation of the rock was established;The SEM scanning electron microscopy test was carried out on the freeze-thaw samples to analyze the change rule of the pore characteristics of the rocks in the microscopic state with the freeze-thaw cycle.Image-Pro Plus was used to extract the microscopic pore parameters from the SEM images,and the fractal theory was combined to analyze the microscopic damage mechanism of the rocks in the freeze-thaw cycle;Finally,COMSOL Multiphysics was used to simulate the test process of rock freeze-thaw cycles,and the changes of thermal stress and thermal strain of rock under different freeze-thaw cycles were obtained.The main results are as follows:(1)Under the action of repeated frost heave load,the primary cracks of mudstone lead to fatigue evolution.The brittleness of mudstone decreases and the ductility increases,and the increase of secondary cracks caused by repeated freeze and thaw also decreases the uniaxial compressive strength,elastic modulus and longitudinal wave velocity of the rocks.The deterioration degree is used to characterize the deterioration law of mechanical parameter of mudstone under freeze-thaw cycles.The results show that the mechanical parameters of mudstone decrease in different ranges with the increase of the number of freeze-thaw cycles,but the rate of decrease gradually slows down.The mechanical parameters were fitted to the number of freeze-thaw cycles,the degradation model with exponential decay was obtained.Finally,based on the wave impedance as the damage variable,the damage equation of mudstone under freeze-thaw condition is established.The results show that the damage of mudstone increases as the number of freeze-thaw cycles increases,but the rate of increase gradually slows down.(2)According to the SEM images under different freeze-thaw cycles,with the increase of freeze-thaw cycles,the degree of cementation of kaolinite particles becomes worse,cracks develop,and pores gradually expand until they are connected due to frost heave.According to the plane porosity calculated by Image-Pro Plus,the plane porosity of rock increases as the increase of freeze-thaw cycles.Combined with the fractal dimension of pores under different freeze-thaw cycles calculated by MATLAB,it can be seen that the fractal dimension of pores increases during 0-4 freeze-thaw cycles,because the pore cracks in the rock gradually expand,and the pore distribution becomes more complex,However,during 4-16 freeze-thaw cycles,the fractal dimension of pores decreases.The reason is that the water in the pores of rocks is frozen,thawed and frozen in the process of freezing and pull-out force,and the expanded small rock pores penetrate into the large pores,resulting in uneven pores and irregular walls,which aggravate the deterioration of rocks.The reason is that the frost heave force generated during the continuous freezing and thawing process of the water in the rock pores expands the small pores in the rock into the large pores,and wears down the uneven and irregular inner walls in the pores as the pores in the rock become larger,thus aggravating the deterioration of the rock.(3)With the increase of freeze-thaw cycles,the maximum thermal strain of mudstone develops from the middle to the top of mudstone,and the maximum thermal strain increases as the increase of freeze-thaw cycles.With the increase of freeze-thaw cycles,the thermal stress of mudstone develops from the base of the rock upwards and towards the surrounding rock surface,the 16 times freeze-thaw cycles,the thermal stress concentration in the middle of the whole rock peripheral regions,stress concentration occurs at the top and the bottom center of mudstone and the maximum thermal stress increases as the increase of freeze-thaw cycles.The stress-strain curves of mudstone under freeze-thaw cycles by simulation are consistent with the experimental results,and the uniaxial compressive strength and elastic modulus of mudstone under freeze-thaw cycles are reduced.