Avalanche Dynamics Analysis Of Deformation And Failure And Study Of Non-monotonic Relaxation Of Salt Rock Polycrystals

The salt cavern after solution mining is widely used as energy storage chamber or waste disposal space.It is challenged for the long-term stability and tightness of salt cavern.Therefore,it is necessary to study the microstructure and macroscopic deformation behavior of salt rock under the action of in-situ stress.As a kind of sedimentary rock,salt rock possesses the characteristics of non-continuous,heterogeneous and non-linear.In addition,it is also a polycrystal composed of sodium chloride crystal,which has crystal nature.For polycrystalline salt rock,it presents progressive failure under uniaxial compression,and catastrophic failure under accelerated creep.Although its stress-strain curve seems smooth and continuous at the macro level,its plastic deformation fluctuates at the micro level,which can be captured by acoustic emission(AE).In this paper,avalanche dynamics is introduced to analyze the acoustic emission signals of uniaxial compression and uniaxial creep of polycrystalline salt rock.The damage accumulation is reflected by the dynamic change of critical exponent.Combination with the particle flow code(PFC)numerical simulation,the evolution process of heterogeneous microstructure defects and complex deformation behavior of polycrystalline salt rock are analyzed.Through two-step relaxation test,the relationship between mesoscopic structure and non-monotonic relaxation of salt rocks is investigated.This dissertation is structured as follows:(1)Single crystal of sodium chloride and polycrystalline salt rock under uniaxial compression tests were carried out.Correlation analysis and statistical distribution analysis were conducted on the absolute energy,amplitude and duration of acoustic emission events,and their distribution forms and critical exponents were obtained.The avalanche dynamics statistical analysis method is adopted to compare the single crystal of sodium chloride and the polycrystalline salt rock from the perspective of seismology statistical law(Gutenberg Richter's law,Omori's law and Bath's law)and the distribution of waiting time,so as to illustrate the influence of crystal structure on its distribution form and critical exponents.The AE avalanches of sodium chloride single crystal and polycrystalline salt rock have different compositions,and their macroscopic failure modes are also different.However,the statistical characteristics of AE avalanches are very similar,except that the critical exponent is slightly different,which reflects different avalanche source information,namely,different failure mechanism in the damage process.(2)Voronoi polygon is used to reflect the geometric topological structure of polycrystalline salt rock,and Voronoi structure is imported into PFC software.The grain and grain boundary of the polycrystalline salt rock model were described by parallel bond model and smooth joint model,respectively.The effects of micro parameters and grain size on the macroscopic mechanical properties of the polycrystalline salt rock model under uniaxial compression were investigated in detail.The comparion of PFC numerical simulation results by microscopic parameters calibration with laboratory test results shows good consistency.The crack size simulated by PFC is highly correlated with the AE signal obtained from the test,which can be used as the simulated acoustic emission signal.The critical exponent of crack size obtained by PFC is also very close to the critical exponent of absolute energy obtained by the test,which indicates that the Voronoi polycrystalline salt rock model can better reflect the real salt rock samples.(3)Polycrystalline salt rock under uniaxial compression creep tests were carried out.The creep constitutive model was established according to element constitutive model and experience model.Based on the statistical analysis of avalanche dynamics,the variation of AE energy critical exponent during the loading stage,creep stage and at the end of creep stage were studied.The critical exponent of loading stage is smaller than that of creep stage,which indicates that different mechanisms between the constant rate loading and zero rate loading.At the end of creep stage,the critical exponents of absolute energy and amplitude of the active interval that far away from the failure are larger than that of the critical interval near the failure time,which is consistent with the change of the critical exponent when uniaxial compression approaches the failure time.A large number of micro-cracks and subgrains and high density dislocation and slip line indicate that the creep failure mechanism of polycrystalline is dominated by dislocation creep,pressolution and cataclasis.(4)Two-step relaxation tests of single crystal of sodium chloride and polycrystalline salt rock were conducted.The relationship between the non-monotonic phenomenon with the compression,release and the hold time in relaxation process is studied.The occurrence of non-monotonic relaxation is closely related to the amount of compression.When the release is fixed,the peak time t_p is linearly correlated with the hold time t_w,which reflects the memory effect,it means the previous loading history will influence the relaxation behavior of the latter stage.Based on the microscopic structure changes in the non-monotic process,this paper explores the generation and disappearance of non-monotonic relaxation in the elastic-plastic transformation process,and analyzes the non-monotonic phenomenon from the phenomenological model.