Study On The Law Of Damage Evolution,Fracture Failure And Energy Dissipation For Sandstone Under Loading And Unloading

The excavation of rock mass causes different degrees of loading or reciprocating loading and unloading,causing the rock mass to undergo a complex nonlinear mechanical process from damage accumulation to failure from excavation to instability,and accompanied by energy dissipation.This mechanical process experienced damage accumulation to failure,accompanied by energy dissipation.The energy dissipation of rock leads to its strength deterioration,and the energy release of rock leads to catastrophic failure related to instability.The elastic energy storage limit of the rock can be obtained,which can determine the judgment conditions of the overall stability of engineering rock mass.In addition,there are many discontinuities in the rock mass,and the discontinuous surfaces weaken its strength.Therefore,the damage failure characteristics and energy dissipation of sandstone under loading and unloading have been studied in this paper.Taking sandstone and sandstone with surface cracks as research objects,the test scheme of one-time loading and unloading-loading failure and cyclic loading-unloading-reloading failure were set.Taking the energy dissipation of samples in the damage and failure process as the main line,a variety of measurement methods were used to quantitatively describe and analyze the energy dissipation law in the damage and failure mechanical process of sandstone.In this way,the mechanism behind the change of sandstone energy dissipation under loading and unloading can be discussed from macro to micro.The aforementioned multiple measurement methods include a pressure loading system,nuclear magnetic resonance measurement device,digital speckle measurement equipment and the self-proposed active microwave resonance electromagnetic energy consumption detection method.The main work and achievements of this paper are as follows.(1)An active microwave resonance rock electromagnetic energy consumption detection method is proposed.The electromagnetic energy transmission model of coupling resonance between rock sample and microwave was established.Such model can correlate the equivalent micro element resistance,inductance,and capacitance of rock under external load with microwave resonance electromagnetic energy consumption,and quantitatively analyze rock damage afterwards.The advantage of using the active microwave resonance rock electromagnetic energy consumption detection method to quantify and analyze the damage lies in the uninterrupted real-time reflection of the evolutionary state,which is different from the passive source mode of rock electromagnetic radiation detection method and acoustic emission detection method.(2)The mechanism of electromagnetic energy consumption in microwave resonance of rock under pressure is discussed from the point of view of resetting the microscopic particle structure,and also is analyzed the correlation between microwave resonance electromagnetic energy consumption and energy dissipation of rocks.A sandstone damage model characterized by microwave resonance electromagnetic energy consumption was established.The feasibility of using microwave resonance electromagnetic energy consumption to describe damage is verified by measurement results of combine with the macro strain and micro NMR porosity.(3)The axial strain cycle number curve of rock under cyclic loading and unloading(strain development curve)has a three-stage development law.That is,the initial stage is the development stage with a larger rate,the middle stage is the near-linear development stage with a smaller rate,and the later stage is the rapid development stage with a larger rate.The number of cycles corresponding to the endpoint of the first stage is recorded as N_A.The strain corresponding to the endpoint of the second stage is called the failure strain and is recorded asε_B.Combined with the energy parameter change rate corresponding to the second stage of the strain development curve as approximately a constant,a method of solving the elastic energy storage corresponding to the position ofε_B point in the strain development curve is proposed.This method is suitable for loading and unloading tests that only experience a limited number of cycles.The number of so-called finite cycles is expressed as N.In addition,N is greater than N_A and less than the total number of cycles N_F.(4)Based on the proposed method of solving the elastic energy storage corresponding to the position ofε_B point in the strain development curve,the dissipated energy of sandstone with surface cracks atε_B point is obtained.It is found that there is a linear relationship between the energy dissipated atε_B point and the proportion of NMR large porosity.Therefore,the microscopic pore changes inside the sample can be used to quantify the energy dissipation during the failure of the sample.(5)The digital image correlation method is used to analyze the crack initiation stress of sandstone with surface cracks under external load.It is found that under the combined action of external load and surface crack inclination,when the surface crack inclination decreases,the crack initiation stress of the sample decreases.