Study On Hysteretic Characteristics And Energy Dissipation Mechanism Of Rock Under Cyclic Loading

Rock mass in underground engineering is easily subjected to cyclic loading caused by geological and human activities,e.g.,earthquake,excavation or blasting.Understanding the deformation and failure mechanism of rock under cyclic loading is the key for evaluating long-term stability of rock engineering structures.Cyclic loading tests have always been the main research direction in rock mechanics.However,due to the complex composition and structure of rock,there are still many problems need to be solved in the deformation response of rock under cyclic loading,and there is still a lack of clear understanding of cyclic cumulative deformation and the controlled factors of hysteretic deformation.Therefore,based on the clear understanding of these deformation mechanisms,the corresponding constitutive relationship of rock subjected to cyclic loading can be established to better evaluate the long-term stability of rock structure,warn and prevent the occurrence of disasters reasonably,which is of great significance in researches.Firstly,twelve kinds of rock samples commonly used in engineering are selected as research objects,rock failure tests were carried out by rigid servo test system under monotonic loading and cyclic loading.Meanwhile,the acoustic emission monitoring system and scanning electron microscope were used to obtain the meso-structure of the damaged rock and acoustic emission signal parameters during loading and unloading deformation,the relationship between meso-structural characteristics and rock deformation stage evolution is analyzed comprehensively.Secondly,the typical samples are selected from the general samples,and the experimental tests after high temperature damage and triaxial cyclic tests under different confining pressures were carried out on the red sandstone with obvious pore characteristics,so as to explore the influence of the meso-structure changes on the cumulative deformation characteristics and hysteretic deformation characteristics.Finally,based on the progressive deformation characteristics of rock,combined with the viscoelastic characteristics of rock,energy distribution evolution and damage evolution law,the fractional order theory is introduced to establish the constitutive model of rock under monotonic loading and the cumulative deformation model under cyclic loading.Through experiments and theoretical analysis,some progress has been made in the following aspects:(1)The rock deformation under monotonic loading is characterized by stage variation,which is fundamentally caused by the continuous evolution of the internal structure of the rock.Expansion stress,as one of the important characteristic stresses in the classification of deformation stages,is the key to distinguish structural state transition.The rock with different lithology can be classified into pore structure rock and fracture structure rock by comparing the fracture micro-structure of rock.The nonlinear characteristics of the stress-strain curve show great difference under the influence of structural differences.Compared with the rock with fracture structure,the nonlinear deformation characteristics of the rock with pore structure are more obvious and the stage characteristics are clearer.(2)The hysteretic deformation mechanism under cyclic loading is mainly reflected in two aspects.One is the plastic deformation caused by structural rupture during the cyclic process,and the opening hysteretic curve features are caused by accumulated residual deformation after unloading.The other part is caused by differential damping effect caused by the different deformation responses of the meso-structure interface under the opposite direction loading,and thus a closed hysteresis loop appears in the mechanical curve.The closed hysteretic behavior is related to the complexity of the fracture surface characterized by fractal dimension.The larger the fractal dimension of section topography is(the more complex and rough it is),the fuller and more obvious the closed hysteresis is in the corresponding cycle curve.The smaller the fractal dimension is,the narrower the closed hysteretic loop of the corresponding cycle curve is,and the unloading and reloading curves tend to coincide.(3)The continuous cyclic loading results in the accumulation of residual strain.In the single-cycle multi-stage cycle test,the residual strain accumulation process in uniaxial state has three stages,which are attenuation accumulation,stable accumulation and accelerated accumulation respectively.With the increase of confining pressure in triaxial state,the three stage will change to two stage and the attenuation accumulation stage will disappear.The upper limit stress level is an important factor affecting the cumulative variation of residual strain in multi-cycle and multi-stage cycle tests.When the upper limit stress is lower than the expansion stress,the strain accumulation process presents two stages: attenuation accumulation and stable accumulation.When the expansion stress is exceeded,there are three stages of attenuation accumulation,stable accumulation and accelerated accumulation.(4)Based on the energy meaning given to the region divided by cyclic loading and unloading curve,the dissipated energy in the process was decomposed into damage and fracture energy and damping energy dissipation,and the evolution law of the two energy dissipation distributions in different deformation stages of rock was studied,and the corresponding stage deformation mechanism was described.By discussing the law of energy evolution,a method to predict the failure stress of specimens with increasing cyclic stress was established,and an average elastic energy index was put forward to evaluate the rock bursting liability under cyclic load.(5)Based on the discussion of deformation stage of rock,the characterized function of the competition mechanism of rock strengthening and deterioration in each deformation stage was summarized,the model of rock under monotonic loading was established by introducing the fractional theory.For the strain accumulation phenomenon in process of cyclic loading,the important effect of cyclic upper limit stress level on the change of deformation accumulation characteristics was analyzed.The difference of strain accumulation law before and after expansion stress was discussed,and the damage element considering evolution of cumulative damage energy and new variable-order fractional visco-elastic model were established.A constitutive model that can describe the cyclic cumulative deformation with constant amplitude was constructed.