Research On The Characteristics Of Granite Crack Variation Under Ultrasonic Vibration

With the rapid development of world economy,more mineral resources are required by the human being,as a result,the exploration depth of mineral resources is growing and more and more hard rock strata appear.Hard rocks characterized by high density,high strength,strong abrasiveness and large fracture energy are highly difficult to break.To solve this technical problem,new technical method of drilling and relevant mechanism of rock fragmentation should be investigated in order to reduce rock strength and produce mass broken.The natural frequency of complete,high-density hard rock is commonly in the range of 20 KHz to 40 KHz.When rock resonates reasonably well,the strength will drop sharply and quickly.In this situation,rock will be easily broken and drilling speed and bit life will also be improved substantially.Therefore,thustechnical method of using ultrasonic vibration in combination with cutting drilling is put forward to solve the problem,and it seems that this method is feasible.Rock is composed of all kinds of mesoscopic defects,the damage process of rock is controlled by the internal mesoscopic structure,the evolution of mesoscopic defects will be irreversible under loading,resulting in the decrease of rock strength.To investigate the characteristics of rock crack variation under ultrasonic vibration will reveal the inner mechanism of rock strength decline regulation under ultrasonic vibration fundamentally,provide theoretical guidance for the ultrasonic vibration assisted drilling method,and this research is significantly important to solve the technical problem of hard rock drilling.This paper investigate the characteristics of granite crack variation under ultrasonic vibration in three areas,which is the condition of rock cracking,the characteristics of rock crack extension,and the influence regulation of static load against rock damage,using the finite element method combined with the experiment method.Firstly,this paper briefly introduced the granite’s physical mechanics parameters.On this basis,a mathematical model of the condition of rock cracking under ultrasonic vibration was derived by using freterman’s theory on joint strength,and based on the joint strength criterion,a damage model of granite under ultrasonic vibration was also got by using rock damage mechanics.Secondly,this paper established a numerical simulation model which could reflect the heterogeneous level of granite by using the ANSYS software combined with the MATLAB software.On this basis,the process of granite crack variation under ultrasonic vibration was simulated,and then the following conclusions was reached: granite natural frequency increases with static loading,the static load-natural frequency curve is logarithmic,with the increasing of static load,the increasing amplitude of natural frequency slows down gradually;when the vibration frequency is close to granite natural frequency,the internal stress and strain of granite is much bigger,the influence of the vibration frequency against the change rate of internal stress and strain is very significant,when the vibration frequency is the same as granite natural frequency,the internal stress and strain of rock is the biggest;only when the micro units located in the crack tip of granite meet the strength criterion will the crack extends,the extension process can be divided into three stages,which is initiation,coalescence and interaction,in initiation stage,rock internal stress concentrates to some nodes,those nodes crack and form micro-pores,in coalescence stage,the micro-pores extend gradually,and then form the principal crack along a direction,there exists some randomly scattered sub-cracks in the principal crack,in interaction stage,the interaction of cracks occurs,and finally lead to the formation of new cracks.Thirdly,this paper experimented on the characteristics of granite crack variation under ultrasonic vibration using a special ultrasonic vibration device,and the experimental results are consistent with the results obtained by simulation calculation basically.Compared experimental results with numerical simulation results,it could be seen that the numerical result that using mathematical model of the condition of rock cracking based on freterman’s theory was much closer to the experimental results than it using the second or the third strength theory separately.The influence rule of static loading against rock damage can be concluded by sort out experimental data: there exists a threshold value of static loading in theprocess of ultrasonic vibration,when the static loading is less than the value,changes of the static loading have no influence on rock damage,otherwise changing the static loading will have a directly influence on rock damage;when the static loading is bigger than the threshold value,granite damage process can be divided into two stages,which is the buffer stage and the damage stage,in the buffer stage,the decrease of granite elastic modulus is not obvious,and in the damage stage,granite elastic modulus decrease sharply;if other conditions remain no changed,There is an optimal value of static loading which can make the biggest degree of rock damage,the value is 400 N in this test.Finally,a conventional turning scan test was carried in order to get CT images under different conditions of vibration,and the extension process of granite crack was analyzed through the CT images,The process could be divided into three stages which is initiation,coalescence and interaction,and this is the same as the numerical simulation result,it could be seen that cracks gradually extend to the hub of the granite with the increasing of time in the process of vibration.