Theoretical And Laboratory Study On The Variation Of Cracks In Granite Under Ultrasonic Vibration

Hard rock has the characteristics of high strength,low drillability and weak abrasiveness,which is easy to cause short life of drill bit,low penetration rate and high economic cost.It has always been a problem in the field of geological drilling engineering,which has seriously affected the effectiveness and cost of resource exploration.Therefore,the search for new and efficient hard rock crushing technology has become the research goal of many scholars.Ultrasonic vibration technology has the characteristics of high vibration frequency,good propagation direction,strong penetrating power and fast energy propagation.It has been successfully applied in the fields of mechanical processing of hard and brittle materials,space planetary drilling,medical surgery,oil and gas formation infiltration and other fields.Ultrasonic vibration technology has become a potential new method for breaking hard rock.At present,the use of ultrasonic technology to break rock has quietly become a new research hotspot,but the problem is that the mechanism of ultrasonic vibration crushing rock has not been fully studied yet.According to the theory of material mechanics,the damage of materials is caused by the initiation,expansion and penetration of cracks.Correct understanding of the crack initiation and propagation laws of microcracks is essential for accurately understanding the damage evolution process and failure mechanism of rock under ultrasonic vibration.Therefore,in view of the problems in the research field of ultrasonic vibration broken rock in the international scope,this paper selects the common and representative hard rock-granite as the research object,and carries out the theory and experiment on the law of rock crack change under ultrasonic vibration from macroscopic and microscopic perspectives.The purpose is to clarify the internal crack cracking mechanism and rock failure mechanism of rock under ultrasonic vibration,and provide theoretical support and technical guidance for solving the hard rock problem by using ultrasonic technology,which has practical significance.In this paper,granite is used as the experimental research object.The combination of theoretical analysis,numerical simulation and laboratory test is used.The numerical simulation and test methods include PFC2D scatter element numerical simulation method,matlab digital picture processing technology,optical microscope observation method,scanning electron microscope observation method,nuclear magnetic resonance detection method,immersion flaw detection method,thermal infrared non-destructive testing method,surface micro-amplitude detection method,and uniaxial compression experiments.The crack initiation criterion of ultrasonic vibration,the time-space evolution process of crack dynamic expansion,the influence of rock-forming mineral on crack,the influence of vibration time on internal pores of rock,the spatial distribution characteristics of crack,the azimuth of crack preferential propagation,the influence of ultrasonic amplitude on crack initiation,the attenuation characteristics of stress wave inside rock,the effective depth of rock fracture,the crack cracking and rock fracture mechanism are studied in detail,and the main results obtained are as follows:(1)Based on the vibration theory,the motion equation and stress equation of rock particle element under ultrasonic vibration are established.The azimuth angle of rock crck preferential propagation under ultrasonic vibration load is calculated with Griffith strength theory.Based on the maximum tensile stress criterion,fatigue damage theory and Newton 's second law,the cracking criterion of rock crack under ultrasonic vibration is given.The theoretical results show that the stress motion of rock particles under ultrasonic vibration is consistent with the loading frequency.The amplitude of the stress movement and the stress of the particle unit are proportional to the amplitude of the applied ultrasonic wave.The amplitude of the stress motion,the stress of the particle unit is proportional to the amplitude of the applied ultrasonic load.In addition,the stress of the rock particle unit is proportional to the square of the ultrasonic vibration frequency.The internal crack of the rock preferentially expands in a direction parallel to the loading direction,and then expands in a direction that is "?"("?" satisfied cos 2?=?1-?2/2(?1+?2)with the loading direction.The cracking criterion for cracks in rocks under ultrasonic vibration is F???,??????,0<?<1.(2)Discrete element software PFC2D is used to model and calculate the dynamic evolution process of cracks in rock under ultrasonic vibration.The simulation results show that the internal stress of rock model is wavy downward under the ultrasonic vibration load,and the stress state of rock particle element is transformed between tension and compression at high frequency.The stress and amplitude curves of particle elements at different locations in rock model show that the stress wave generated by ultrasonic vibration attenuates during the transmission process inside the rock model,and the attenuation amplitude decreases with the increase of propagation distance.Under the ultrasonic vibration load,three kinds of cracks are mainly generated in the rock,the parallel bond tensile crack(crackpb_tension)is located inside the grain;the smooth joint tensile crack(cracksj_tension)is located between the grains;the smooth joint shear crack(crack_sj_shear)is located between the grains.Among them,the smooth joint tensile failure crack accounts for the largest proportion,reaching 88.6%,the number of smooth bond tensile failure cracks is the second,reaching 10.9%,and the number of smooth joint shear failure cracks is the smallest,0.5%.It can be seen that the crack preferentially propagates at the grain boundary under the ultrasonic vibration load,and then expands inside the grain.The proportion of tensile failure cracks reaches 99.5%.The rose diagram of the crack in the rock model shows that the crack preferentially propagates in a direction parallel to the ultrasonic vibration load under ultrasonic vibration,and then expands in a direction at an angle of 60°to the vibration load.(3)The propagation and development characteristics of cracks in rock-forming minerals of granite samples treated with different vibration time were observed by scanning electron microscopy(SEM).The results show that the crack propagation characteristics of different rock-forming minerals under ultrasonic vibration are obviously different.The types of cracks that cause quartz mineral failure are mainly intragranular cracks and grain boundary cracks,of which grain boundary cracks predominate.For feldspar and mica minerals,intragranular cracks play a major role in the types of cracks that cause them to fail because they contain different types of joints.In general,grain boundary cracks account for the majority of crack types that cause granite failure,and the test results are highly consistent with the simulation results.(4)Experimental study on microscopic pore evolution and spatial distribution of cracks in granite under ultrasonic vibration shows that the number of ultrasonic vibrations(20s per ultrasonic vibration)has obvious phase effect on the development of pores in rock.From the time scale,it can be divided into three stages,the rock pore volume ratio remains unchanged(1-3 times,0-60s),the rock pore volume ratio is stable growth stage(3-4 times,60-80s)and the rock pore volume ratio sharply increases(4-5times,80-100s).The rock after vibration is split along the axis to carry out the crack spatial distribution characteristics test.The results show that the rock interior can be divided into crack development zone(D:1.0e-1-7.0e-1),sub-crack development zone(D:1.0e-3-1.0e-1)and protolith zone(D:0-1.0e-3)according to the different values of damage variable D in different areas of rock surface.The rock crack preferentially expands in parallel with the load loading,and then expands in the direction of 60°with the load direction.The crack is mainly distributed in the top end of the rock.The experimental results are in good agreement with the theoretical results and numerical simulation results.(5)The experimental study on crack initiation threshold of granite under ultrasonic vibration shows that when granite is crushed by ultrasonic vibration,there exists an ultrasonic amplitude threshold,that is,only when the ultrasonic amplitude is greater than 0.8 Amax(32 um),crack initiation and propagation will occur in rock.The larger the amplitude of ultrasound is,the more obvious the effect of rock fragmentation is.The effective breaking depth of rock under ultrasonic vibration load is about 10 mm.Ultrasonic vibration will cause the temperature of the rock to rise(the highest temperature reaches 397.2 C).The thermal damage caused by ultrasonic vibration and the subcritical propagation of microcracks in rock caused by ultrasonic vibration are the main mechanisms of rock breaking by ultrasonic vibration.