| The feasibility of ultrasonic vibration technology for rapid hard rock breaking has been fully verified,and the damage and breaking mechanism of granite under uniaxial ultrasonic vibration has been initially understood.The existing laboratory tests and numerical studies have shown that the axial splitting failure caused by the rapid accumulation of meso fatigue damage in a certain depth range of rock samples under ultrasonic vibration is the main mechanism of granite sample breaking.The meso damage of rock is mainly manifested as tensile fatigue damage and partial thermal damage,and there are vibration frequency threshold and static pressure threshold of rock damage development.Existing studies have explored and summarized the above rock damage and fracture mechanism from the macro and micro perspectives,but there are still research deficiencies.In the process of drilling,the rock is generally in the state of in-situ stress(confining pressure),and the boundary condition of lateral confining pressure will change the micromechanical state and macro crushing mode of rock samples.Therefore,whether the micro damage and macro fracture mechanism of rock under uniaxial ultrasonic vibration are suitable for confining pressure conditions is worth further exploration.Aiming at the lack of research,the theoretical analysis is carried out to explore the effects of confining pressure,vibration frequency and static pressure parameters on the stress wave propagation of ultrasonic vibration load.Then,the stress and deformation state and meso damage mechanism of rock model under confining pressure under ultrasonic vibration are solved by numerical simulation.Finally,the triaxial ultrasonic vibration rock crushing test-bed is constructed to carry out the vibration loading test of rock under confining pressure.The test includes multi parameter orthogonal test and ultrasonic vibration test with different confining pressure with equal time.The influence of confining pressure conditions and main ultrasonic vibration parameters(vibration frequency and static pressure)on the development of rock meso damage(meso porosity and longitudinal wave velocity)is quantitatively analyzed by multi parameter orthogonal test,and the coupling mechanism of three parameters on the development of rock damage is summarized.Based on the meso and macro damage evaluation indexes such as meso pore damage,internal crack propagation,macro fracture mode and surface infrared thermal radiation state of granite samples,the ultrasonic vibration test of rocks under equal time and different confining pressure conditions is to analyze the influence law of confining pressure conditions on granite damage evolution and fracture mode under ultrasonic vibration.The specific research results obtained in this paper are as follows:(1)Based on the theory of vibration mechanics,ultrasonic theory and one-dimensional stress wave propagation,the formula of stress vibration force borne by unit particles in a certain range of rock under ultrasonic vibration load is deduced.The effects of preload static pressure and confining pressure on the propagation of ultrasonic vibration stress wave and the development of rock damage are also analyzed.Theoretical analysis shows that the ultra-high vibration frequency gives enough vibration force between rock particles in a certain range,so as to drive the rapid meso damage accumulation of rock.When the excitation frequency is within the range of rock natural frequency,the amplitude of rock particles will reach a maximum,the rock sample will be in resonance state,and the rock damage efficiency will be greatly improved.Excessive preload static pressure will affect the propagation of vibration stress wave in rock,make the rock in static compression state,and inhibit the development of fatigue damage.Confining pressure conditions will increase the wave impedance of rock samples and enhance the vibration force required for effective damage of rock samples.Under a certain range of high confining pressure conditions,the static pressure threshold of rock damage and fragmentation under ultrasonic vibration will also be improved.Ultrasonic vibration frequency and static pressure are the main load parameters affecting rock damage behavior,and confining pressure condition is an important factor affecting rock damage efficiency under ultrasonic vibration.(2)The numerical simulation of rock fragmentation under ultrasonic vibration under subjected to confining pressure shows that the compaction effect of confining pressure on model particles inhibits the concentration of tensile stress at the tip of pores between particles and weakens the overall degree of rock fragmentation;During the loading process,the maximum tensile stress and axial tensile strain are concentrated in the upper part of the model,leading to local damage development;The micro-damage in the model is the coexistence of intergranular tensile crack and shear crack,and the shear connection between some tensile cracks causes the local breakage of the model,which proves that the ultrasonic vibration stress can effectively damage the granite model subjected to confining pressure.(3)Three factor mixing level orthogonal test(L18(6~1×3~3))shows that the vibration frequency is the primary factor affecting the overall damage degree(porosity increase)of granite samples under ultrasonic vibration,the static pressure is the primary factor affecting the local crushing degree(longitudinal wave velocity decrease)of samples under ultrasonic vibration,and the confining pressure controls the meso damage mode of rocks under ultrasonic vibration,In addition,the coupling effect of various factors on the development of rock damage cannot be ignored.Vibration frequency gives rock particles great alternating excitation stress and promotes the development of fatigue damage in rock.When the vibration frequency is close to the natural frequency range of rock,the stress amplitude of rock unit particles will reach the extreme value and the rock will break rapidly.Static pressure enhances the propagation energy of load stress wave in rock,intensifies the energy dissipation at rock microcracks,and promotes the propagation of fatigue cracks.With the increase of confining pressure,the meso damage mode of granite samples changes from tensile fatigue damage to shear damage.Static pressure and vibration frequency promote the development of fatigue damage.This is the basic mechanism of ultrasonic vibration load to effectively break hard rock.The limiting effect of vibration frequency and confining pressure on the effective damage range of the sample is the main reason for the effective crushing depth of the sample under ultrasonic vibration.(4)Within the confining pressure range of 2～6 MPa,the strengthening of axial deformation of granite sample leads to the concentration of tensile stress at the tip of internal defects of the sample.It causes a large number of fatigue tensile damage cracks in the sample,and gradually develops to the peak with the increase of confining pressure.The incremental porosity of the specimen is mainly contributed by the expansion of small pores of class I radius(0～1μm radius)to medium radius pores of class II(1～40μm).In the range of 8～14 MPa,as the specimen tensile stress concentration is weakened and shear stress is enhanced,shear damage within the specimen begins to coexist with fatigue tensile damage,resulting in a significant increase in the incremental porosity of Class III large pores(≥40μm)under the condition that the overall porosity of the specimen continues to decrease.When the enclosing pressure exceeds 14 MPa,the expansion of the shear crack gradually dominates until transverse shear through fracture occurs at higher enclosing pressures.With the enhancement of the enclosing pressure,the expansion angle of the damage crack(the angle between the crack and the axial load)changes from 30°to 60°,and the damage crack is gradually biased from the ultrasonic vibration loading direction to the direction of the enclosing pressure.(5)The evolution of the highest infrared thermal radiation temperature at the vibration loading surface of the rock specimens shows that the granite specimens form a ring-like dense zone of about 10 mm depth at the edge of the specimen under the joint compression of the central thermal expansion stress and the surrounding pressure in the range of 16-20 MPa surrounding pressure.The presence of the annular dense zone prevents the shear crack from extending through to the edge of the specimen,resulting in shear crack penetration in the depth range of 10-15 mm,thus producing transverse macroscopic fracture. |
