| One of the main reasons for rock fragmentation under ultrasonic vibration is the fatigue damage during this process,and when the damage accumulates to a critical value,the structure will be destroyed.Thus,in-depth investigation on the fatigue characteristics of rocks is necessary to understand the mechanism of rock fragmentation under ultrasonic vibration,which would further accelerate the usage of ultrasonic vibration-assisted rock crushing technology applied to the hard rock drilling field.However,the research on this issue is still in the initial stage,and the understanding of the mechanism of fatigue damage under high amplitude and low frequency ultrasonic vibration and how the key load parameters affect the fatigue damage of rocks is still unclear.Based on the fatigue damage theory of rocks and metal materials and the characteristics of ultrasonic vibration load,two fatigue damage models are established for rocks under high-frequency and low amplitude loading conditions of ultrasonic vibration,and the local and full-field deformation processes are analyzed in depth from the indoor experiments,in addition,the effect of vibration amplitude on the fatigue damage of rocks is studied.Finally,crack initiation and propagation and the mechanism of crack generation are investigated using meso-scale numerical simulation method,and the fatigue damage model is preferred according to the comparation between numerical and experimental results.The main research work and results are as follows.(1)The mechanical response characteristics of the rock under ultrasonic vibration is divided into two points,one is the nonlinear property of the stress-strain curve.This nonlinear property is mainly manifested by the non-overlap of the stress-strain curve during loading and unloading stage.The second feature is the localized damage,which is due to the high frequency of ultrasonic vibration loading conditions and thus energy dissipate fast during the transfer process,ultimately resulting in a significant damage inhomogeneity at different locations in the rock.Any kind of damage model is based on the concept of damage,when the material undergoes cyclic loading,the material produces a certain amount of damage in each cycle.The damage leads to the phenomenon of plastic deformation,and this kind of deformation can be accumulated.As the plastic deformation accumulated in each cycle of ultrasonic vibration fatigue is small,it is called as microplastic strain.When the damage accumulates to a critical value,the structure will be destroyed.Based on the above two characteristics,combined with the existing intrinsic model,two fatigue damage models called "macroscopic phenomena based " and " intrinsic mechanism based " are established to describe the mechanical response process of rocks during the ultrasonic vibration process,and analyze the fatigue damage mechanism.(2)The strain test is used to analyze the local deformation of rocks and investigate the fatigue characteristics under ultrasonic vibration,which mainly includes the local deformation process and damage evolution of rocks.The local strain test results show that the deformation process of rocks under ultrasonic vibration can be divided into three stages: the first stage is compression stage,followed by the deformation stabilization stage and finally is the deformation reversely increasing stage.In the compression stage,crack closure and slippage occur,the axial compression deformation increases,and the rock becomes more dense,which lead to the increase of the rigidity and natural frequency of the rock.This kind of behavior would further cause the increase of the internal stress;then enters the stabilization stage,in which new cracks sprout and existing cracks expand,and initial macro-fragment generates;finally,the stress concentration becomes more pronounced due to the initial fragmentation,and the cracks further interconnect with each other,resulting in the production of large amounts of rock fragments.(3)The radial and axial strain fields of the rock during ultrasonic vibration with different amplitude coefficients are monitored by the full-field DIC strain test.Based on the DIC test,the full-field fatigue deformation law of the rock under ultrasonic vibration is analyzed.In addition,the influence of ultrasonic amplitude on the fatigue characteristics of the rock is also investigated.The full-field DIC test results show that the process before the generation of the macro-crack can be divided into two stages.In the first state,the strain increases at a low rate and then begins to increase rapidly in the second phase,until macro-cracks are produced The radial strain concentration region of the rock corresponds to the region where the micro-crack generates,and the long axis direction of the strain concentration region coincides with the direction of the cracks and is nearly parallel to the loading direction.At the same time,the radial strain threshold that makes the rock macroscopically damaged is determined,and the larger the amplitude,the shorter the time required to reach this threshold.(4)The two-dimensional particle flow code analysis program(PFC2D)is used to carry out numerical simulation to investigate the law and mechanism of fatigue damage in rocks under ultrasonic vibration.The granular flow model is built by the cluster expansion method according to the geometric properties of granite minerals,and the flat-joint contact model is assigned between the particles to simulate the hard and brittle granite,while the mechanical parameters of different mineral components of rock materials are set separately to highly-reproduce the basic mechanics of real rocks.In order to clarify the fatigue damage characteristics of rocks during ultrasonic vibration,an intrinsic model based on the macroscopic phenomenon is implemented in the PFC software to further simulate the ultrasonic vibration rock fragmentation.In order to ensure the reliability of the simulation,the loading method,ultrasonic vibration characteristics,and the simulation of the earth semi-infinite body are also considered.According to the simulation results of crack direction angle and crack type,the fatigue damage and crack evolution process under ultrasonic vibration are divided into five stages,including undamaged stage,initial initiation of crack,accelerated development of crack,stable development of crack and final rock failure,and further revealed the generation mechanism of tensile and shear cracks under ultrasonic vibration.(5)PFC2D is further used to carry out numerical simulation,and the Voronoi grain granite model based on the concept of cluster with initial crack is established.Compared with the model constructed by cluster expansion method,the Voronoi rock model can more accurately reproduce the grain shape characteristics.Meanwhile,the model based on the intrinsic mechanism(using unbonded smooth-joint contact model to simulate the initial crack to reflect the slip characteristics of the crack)is incorporated into the meso-scale numerical simulation.The simulation results show that both the macroscopic phenomenon-based and intrinsic mechanism-based rock intrinsic models attribute the fatigue damage to the increasing irreversible deformation of the rocks.However,the intrinsic mechanism-based fatigue model is more accurate in describing the final failure characteristics.Based on the simulation results,the crack propagation mechanism under ultrasonic vibration with high frequency and low loading stress is revealed. |
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