Thermal-mechanical Coupled Discontinuous Deformation Analysis And Its Application To Microwave Rock Breaking

As the effectiveness of microwave-assisted rock fragmentation has been verified,more and more scientific researchers have begun to try the engineering application of microwave rock fragmentation.The sample size is getting bigger and bigger and the equipment is getting more and more complicated for physical experiments on the engineering scale.As a result,the time and financial costs are getting higher.At this time,a numerical tool for microwave-assisted rock is desperately needed.This tool must be able to simulate the complex thermal-mechanical coupling and discontinuous deformation fo rock.In this paper,the refined modeling,computational efficiency(contact detection,parallel computing)and computational accuracy(improvement of thermal mechanical coupling algorithm,optimization of damage and failure criteria)under the framework of discontinuous deformation analysis(DDA)algorithm are comprehensively improved.The numerical method is calibrated and verified by the laboratory experiments of diabase samples and granite slices irradiated by single-mode microwave.Then,the mechanism of rock breaking by microwave and the process of crack propagation are discussed.Finally,the damage area of microwave rock breaking is defined and discussed to guide the engineering application of microwave rock breaking.This paper mainly includes four parts,which are summarized as follows:The first part is about the improvement of the missing part of the numerical framework itself.Since the original DDA is not accurate enough in the part of rock fine modeling and the aspect of result display,the pre-processing module of DDA which based on the adaptive mesh generation strategy of rock surface texture is improved.The fine modeling of heterogeneous rock mass is carried out.In order to improve the efficiency of DDA,the two most time-consuming parts of DDA,i.e.contact retrieval and solving nonlinear equations,are improved.A dynamic distributed neighborhood search contact algorithm is proposed,which reduces the complexity of the algorithm from O(N2)to O(N),and improves the computing speed greatly.The numerical simulation results of uniaxial compression show that the program has a good parallel acceleration effect,and the calculation speed is increased by 13 times.In the second part,the thermal mechanical coupling function of DDA is realized and the accuracy of cracking simulation is improved.The multi degree of freedom equations of the internal temperature field are derived,and the steady-state and transient heat conduction equations are solved.Besides,the thermal damage criterion is also improved and verified by numerical test.Finally,a set of numerical simulation tools suitable for microwave-assisted rock breaking simulation is formed,which improves the function of fracture process simulation under temperature field.The third part is about the microwave rock breaking experiment and numerical verification.The experiment and numerical test of granite slice showed the sensitive minerals control the distribution of cracks in the thin section,and the thermal stress mismatch between microscopic minerals at high temperature is the main reason of crack initiation.The fine model based on digital image is used to simulate the fracture process of granite flakes.and the shape,particle size and proportion of minerals have a great influence on the fracture effect.T he effects of microwave power and irradiation time on diabase rock breaking effect were studied by using P-wave attenuation velocity,temperature field distribution and fracture mode as damage indexes.It is found that increasing power is more likely to increase the number of cracks,while increasing time is more likely to increase the length of cracks.The high power and low time can weaken the rock better under the same energy consumption.The nucleation and propagation of microcracks depend on the thermal stress caused by the temperature rise,the geometry of the specimen and the primary crack.The crack propagates gradually under the cyclic action of thermal stress loading and cracking unloading.The fourth part is about the industrial application research of microwave rock breaking.The microwave crushing experiment of rock in engineering scale is designed.The experimental results show that there are many microcracks in the radiation center under the effect of high temperature.while radial macroscopic cracks are distributed around the radiation center.The microwave action area is divided into the main tensile stress zone(S zone),the main compressive stress zone(P zone)and the transition zone(T zone)according to the stress state of rock mass under single mode microwave irradiation.The radial macroscopic cracks are mainly distributed in the transition zone(T zone),and the range of the three zones changes continuously with the crack expansion.Then the effects of heat conduction,confining pressure and distance of dual radiation system on rock breaking effect are discussed in detail.Heat conduction hinders the destruction of rock mass under microwave radiation,rock mass has better breakage effect while heat conduction is not considered.The existence of confining pressure affects the crack distribution and promotes the crack propagation in the direction of confining pressure but inhibits the crack propagation in other directions.The optimal distance of double radiation system is twice the width of waveguide(86 mm).When the distance between two radiating systems exceeds four waveguide widths,the influence between two radiating systems is small.