| With continuous increase of mineral resource exploration depth and high-density development of underground space,and in view of problems such as low efficiency,high cost and large energy consumption in conventional hard rock crushing manners,higher requirements have been proposed for rock crushing techniques for the purpose of prolonging the service life of drilling tools and reducing the cost.At present,the impact rotary drilling method commonly applied to hard rocks is deficient by low vibration frequency(14Hz)and non-obvious reduction of rock strength.In drilling projects,rocks are crushed by vibration in ways of single-time impact,low-frequency impact(12-16Hz)and sound wave impact(70-150Hz).Through modal analysis and tests,inherent frequencies of hard rocks are obtained(>20KHz).Rock resonance is generated due to the excitation force of ultrasonic wave(>20kHz)within this frequency scope,so resonance-fatigue crushed rocks are formed,the bit life is prolonged,and the efficiency is higher than conventional rock crushing manners.Ultrasonic detritus can be used for coring drilling in small-diameter geological exploration,comprehensive drilling in anchorage hole and drilling in large-diameter hard rock in foundation engineering,etc.Parameters of ultrasonic vibration mainly include vibration frequency,pre-pressure applied on rocks,vibration time and vibration amplitude.At present,a lot of tests and numerical simulation studies have been conducted on influences brought by vibration frequency and vibration amplitude of ultrasonic vibration as well as pre-pressure applied on rocks to rock crushing effects.On this basis,influential rules of each parameter on rock crushing effects are obtained.Nevertheless,rocks are crushed under joint effects of many parameters in practical application,and different degrees of influences are exerted by different parameters.Damage on rocks brought by pre-pressure applied on rocks,vibration amplitudes and vibration time is in positive correlation with parameter growth,but growth rates are distributed in parabolic curves.During drilling,it is necessary to know how to efficiently crush rocks on the bottom of drill bits through optimization coupling of different parameters and under the minimum vibration energy.Through research methods such as theoretical research,experiments and numerical simulation,the optimal multi-parameter combination of ultrasonic vibration is obtained.On this basis,hard rocks can be rapidly damaged in a certain longitudinal depth,and interactions between particles lead to tensile and shear damage of rock samples and formation of ultrasonic wave frequency resonance fatigue crushed rocks.In addition,the crucial scientific problem to be solved is to study influence degrees of different parameters on rock crushing effects.Meanwhile,due to heterogeneity of particles in rocks and inhomogeneity of the microstructures,dynamic behaviors conducted by rock particle units under vibration are quite complicated.Through systematic analysis of damage rules and failure mechanisms of cracks inside rocks under ultrasonic wave multi-parameter coupling vibration,a real rock crushing process is further approached.In this way,urgent demands for minimum economic cost and efficient crushing of hard rocks are satisfied.The research results will enrich the theory of rock microscopic damage,provide a theoretical basis for technologies of rock crushing with ultrasonic vibration,and exert theoretical significance and a good prospect for practical application.The research results are shown as follows:(1)Based on theory of rock crushing science,theoretical research was conducted on rock crushing mechanisms and resonance responses of rocks under multi-parameter coupling vibration of ultrasonic waves.Based onthe single freedom degree mechanical model,mechanical characteristics of rocks under ultrasonic vibration and main influential factors on parameter resonance rock crushing were analyzed.In addition,an effective resonance scope of rock damage of ultrasonic vibration under multi-parameter coupling was deduced.Vibration amplitude responses of rock particles under multi-parameter coupling vibration of ultrasonic waves were analyzed.Particles had very obvious dynamic characteristics because of rock particle energy,so vibration increase made collision between rock particles become a key factor for crack development.(2)With the ANSYS finite element analysis software,numerical simulation analysis was conducted on damage rules of granite under ultrasonic wave multi-parameter coupling vibration.A three-dimensional heterogeneous rock model in line with experimental phenomena was established to analyze rules of rock damage under ultrasonic wave multi-parameter coupling vibration.As found in the study,rock damage stages under ultrasonic wave multi-parameter coupling vibration were divided into initial damage,crack emergence,stable growth and rapid expansion.Rock damage was subject to threshold value time.When the acting time of ultrasonic wave multi-parameter coupling vibration exceeded the threshold value time,resonance damage of rocks could lead to rapid expansion of micro cracks.(3)Through testing with CT imaging and SEM,damage rules of granite under coupling of ultrasonic vibration parameters were studied.Based on theories of digital image processing and crack stereoscopy,visual change image information of rock micro structural holes and crack sizes in ultrasonic vibration was analyzed.Parameters of micro structure characteristics reflected changes in sizes and shapes of damaged zones in rocks.Large characteristic parameters such as cracks and holes corresponded to more irregular cracks and holes.Through assistance ofSEM,decision factors of mineral fracture modes and crack expansion were analyzed.As found,when rocks were subject to dynamic loading of ultrasonic wave parameter coupling,granite micro cracks of a tensile-pressure type were mainly generated,and the content of quartz played a decisive role in damage of granite.(4)Through a MRI test and a uniaxial compressive strength orthogonal optimization test,optimal vibration coupling parameters of ultrasonic vibration were discussed,and influences of multi-parameter coupling on evolution rules of the micro hole structure of granite were analyzed.As found,under multi-parameter coupling vibration of ultrasonic waves,medium apertures inside rocks were transformed to large ones.Dynamic changing rules in full courses of generation and development of rock micro apertures were obtained.A numerical simulation three-dimensional heterogeneous rock model was verified,in which rock damage was divided into three stages under multi-parameter coupling vibration of ultrasonic waves.In addition,matrix particles were influenced by longitudinal wave alternating stress along the diffusion direction.Specifically,due to shear waves,collision was generated between adjacent particles for high-energy release and expansion of a lot of particle cracks.Through test range analysis,the optimal combination under ultrasonic wave multi-parameter vibration was obtained,namely the optimal rock crushing effect was corresponding to ultrasonic vibration pressure of 400 N,vibration amplitude of 100% and frequency of 35 KHz.As analyzed,the prioritizing sequence of parameters was: vibration amplitude > pressure >frequency. |
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