Study On The Effect Of The Pressure On The Ultrasonic Vibration In The Process Of Breaking Rock

In the drilling field, there exist problems such as low drilling efficiency, short drilling bit life and high cost during drilling in hard rock. Ultrasonic vibration can cause damage or crack in the rock. Rock strength will be greatly reduced, so that the difficulty of crushing will be reduced and the efficiency of rock crushing will improve. As an important drilling parameter, the drilling pressure directly affects the drilling efficiency. When ultrasonic vibration is applied to hard rock drilling, the drilling pressure can be reflected by the static pressure of the device. When ultrasonic vibration is applied to hard rock drilling, if the static pressure is too large, the maximum shear stress of rock will be reduced, and influence the formation of crack, leading to the decline of the drilling speed. If the static pressure is too small, the rock surface and debris will absorb part of the vibration energy, and the rock crushing efficiency will not increase. Therefore, ultrasonic vibration pressure is an important factor affecting the effect of rock breaking. The mechanism and condition of ultrasonic vibration for rock breaking is different from the conventional way of rock breaking. Through the combination of theory and experiment, under the ultrasonic vibration, exploring the effect of pressure on the rock breaking effect, can provide important theoretical support for the improvement of the efficiency of hard rock fragmentation、the study on the mechanism of ultrasonic vibration for breaking rock、extending the application of ultrasonic vibration in hard rock drilling field and overcoming the hard rock drilling technical problems.At present, the domestic and foreign research haven’t been involved in the study of pressure and other factors on the impact of ultrasonic vibration rock breaking effect,and there is no explanation of the mechanism of ultrasonic vibration for breaking rock. Based on theoretical analysis and experimental study, this paper studies the effect of pressure on rock fragmentation under ultrasonic vibration.In this paper, the effect of ultrasonic vibration on rock breaking is analyzed theoretically. Combining ultrasonic vibration with fatigue fracture theory, fracture mechanics theory and resonance theory, the effect of ultrasonic vibration on rock fragmentation is analyzed from three aspects of the fatigue damage, energy dissipation and resonance breaking of rock.In this paper, the effect of pressure on the effect of ultrasonic vibration for rock breaking is analyzed by theory and numerical simulation. A nonlinear dynamic model of rock system loaded with pressure and ultrasonic vibration is established. Combined with the energy dissipation theory and the fatigue damage theory, the influence of the pressure on the plastic strain energy of rock is analyzed from the point of energy. And the energy dissipation and breaking law of rock under ultrasonic vibration are obtained. Then, the paper derived the damage and fracture process of rock and the conditions of micro crack propagation under combined loading of pressure and ultrasonic vibration. By using WORKBENCH ANSYS software, the finite element analysis of the process of ultrasonic vibration on rock is carried out. Through the comparative analysis of rock plastic strain under different pressure, the paper summed up the effect of the pressure on the ultrasonic vibration in the process of breaking rock.The test of rock breaking under ultrasonic vibration is studied. Using the piezoelectric ceramic ultrasonic vibrator built the test platform, and the Ultrasonic vibration test is conducted under different pressure conditions. Using nuclear magnetic resonance instrument to detect the rock pore characteristics, and the uniaxial compressive strength of rock was tested by rock fracturing test machine. The variation law of rock porosity, T2 map distribution, nuclear magnetic resonance spectrum area, compressive strength and the amount of broken shapes are all compared and analyzed. The experimental results and theoretical analysis results are compared, and the results obtained are as follows:(1) Because ultrasonic vibration has the characteristics of high frequency and high vibration, the energy density of loading stress wave is high, so that the rock damage accumulation speed is fast, and the time of fatigue failure is short. The ultrasonic vibration can reach the resonance frequency of rock, causing the rock crack to expand and develop and reducing the strength,so the ultrasonic vibration has higher crushing rock efficiency.(2) When the combination of ultrasonic vibration and pressure acts on breaking rock, the energy density of the loading wave has a minimum threshold value, which is about 200N-300 N. Only when the pressure exceeds the threshold value of the energy of the loading wave, the micro crack begins to expand, and the damage accumulation process begins. In a certain range, with the increase of the pressure, energy density is bigger. The more plastic strain energy absorbed by rock, using for the production and development of rock cracks, the lower the fatigue strength of the rock. Therefore, the rock crushing efficiency is improved. When the pressure is in the range of less than 500 N, the pressure change has a great influence on the rock crushing efficiency, and the degree of influence is different with the different pressure value. In the range of 500 N to 2000 N, the increase of pressure can play a role in promoting the efficiency of rock breaking under ultrasonic vibration, but the promotion effect is relatively weak. This is because the plastic strain energy increases with the increase of the energy of the carrier, and the plastic strain energy increases with the increase of the pressure, and it tends to be gentle after reaching a certain value. The relative value of the elastic wave energy dissipation is slow down, the rock damage rate tends to be stable, and the internal crack of the rock keeps steady development at a fast speed.(3) Ultrasonic vibration can promote the breaking of rock, but there is a pressure threshold value, which is 200 N. Below this threshold, the influence of vibration on the rock fracture is weak, and there is little change in rock strength. Vibration does not cause damage to rock, and it does not promote the breaking of rock. When the pressure threshold is exceeded, the ultrasonic vibration can cause the formation and development of the rock crack, and the damage accumulation and strength decrease in the pressure range of 500 N. The promoting effect increases with the increase of pressure. When the pressure increases to a certain value(that is, 300N), with the pressure continue to increase, the promotion of the role has gradually tended to be gentle. The micro cracks of the rock are constantly produced and expanded under ultrasonic vibration. The experimental results are consistent with the theoretical analysis, which verifies the correctness of the theoretical analysis.(4) By simulation and experimental results: Under the condition of vibration frequency 20 KHz, and5 min, the effect of ultrasonic vibration on rock fragmentation increases with increasing pressure,When the pressure is 300N;When the pressure exceeds 500 N, the relative increase in the efficiency of the rock crushing efficiency is slowed down with the increase of the pressure. Therefore, under the same conditions, from the energy utilization and the rock breaking effect, there is an optimal pressure value of 300 N, which is the biggest change in the nuclear magnetic resonance characteristics and the compressive strength of the rock.