Experimental Study On Damage Characteristics And Energy Evolution Law Of Damaged Sandstone Under Pore Water Pressure

With the rapid development of national economic construction,many large scale highways,dams,etc have been built and the safety of these projects has gradually become the focus of attention.Rock damage refers to a process in which cracks generates and develops continuously under the action of external loads,resulting in the decline of various rock mechanical parameters.Almost all rocks in nature have different degrees of damage,and the mechanical properties of these damaged rocks are the key to the safety of the project.Based on the above problems,this paper combines the key project of the National Natural Science Foundation of China "Deformation and failure mechanism and protection of reservoir bank slope under complex conditions(51439003).The energy conversion relationship of the damaged rock during the destruction process is analyzed by introducing energy and the constitutive evolution model is established based on the principle of energy dissipation.After research and analysis,the following conclusions are reached:1)The peak strength of damaged sandstone under pore water pressure is lower than that of saturated sandstone in saturated state,which indicates that the existence of pore water pressure will weaken the damage strength.When other test conditions are the same,the peak strain of rock with hydraulic pressure is smaller than that of saturated rock.The larger the pore water pressure is,the smaller the peak strain is.The relationship between peak strain and loading rate as well as pore water pressure can be fitted by exponential function.Under the same test conditions,the larger the pore water pressure value is,the smaller the elastic modulus is;the larger the loading rate is,the larger the elastic modulus gradually become.Moreover,the relationship between elastic modulus and the pore water pressure as well as the loading rate conform to an exponential functional.Poisson's ratio has a positive correlation with pore water pressure and a negative correlation with loading rate.When it is in the same condition,the value of Poisson's ratio under the condition without pore water pressure is smaller than that of sandstone under the condition of pore water pressure.2)With or without pore water pressure,the total energy and elastic energy of saturated sandstone increase with the increase of loading rate,and the dissipation energy decreases with the increase of loading rate.Compared with the same condition without pore water pressure,the existence of pore water pressure will weaken the difference of total energy at different rates.The energy data without pore water pressure is greater than the data with pore water pressure during the entire test process,and the larger the pore water pressure is,the greater the energy difference is.It shows that pore water pressure has a certain weakening ability on sandstone samples.The energy generation is affected by external test conditions.The total energy absorbed by rock samples at different stress states and loading rates may be the same.After the loading-unloading process,the sample has different stress damage.The higher the damage is,the greater the total energy,elastic energy,and dissipation energy is.In the reloading phase,the total energy,elastic energy,and dissipation energy are smaller when the rock fails.It means that the change of energy is consistent with the structural damage of rock.3)Considering that the damage strength of micro element in rock obeys the Weibull statistical distribution theory and combined with the continuous damage theory and the HB strength criterion as well as the generalized Hooke's law,the constitutive model of sandstone loading under pore water pressure is derived by selecting the proportion of dissipative energy in the process of reloading as the damage variable and then determining the comprehensive damage variable according to the equivalent principle.After verification,the results show that the constitutive model is basically coincide with the curve composed of the test data and can well reflect the damage evolution relation of sandstone under pore water pressure,which shows that the model has certain rationality and feasibility.