| The electromagnetic emissions of granites under the impact loads are the external manifestations of atomic motions inside the granites,and also the real-time dynamic responses of rock mineral composition,crack and damage.The electromagnetic emissions involve from radio waves,infrared light to visible light,and also spread in the vacuum with the light velocity,and this paper focus on the radio waves.We investigate the electromagnetic emissions of granites under the impact loads,which is conducive to present the generation mechanism of electronmagnetic emissions induced by atomic motions inside the granites,and also have a great scientific value and application prospect in the important fields of astrophysics,deep-space exploration,aerospace engineering,geophysics,earthquake monitoring,blasting engineering,geotechnical engineering,mining engineering and other fields.Based on the quasi-static compression experiments,we conducted the compression experiments from different strain（loading）rates,determined and investigated the mechanical and electromagnetic responses under the different intensity impact loads,by using rock testing machine,split Hokinson pressure bar and one-stage light gas gun.Compression strength determined by the compression experiments in the strain rate range 10-4104s-1 is0.21.8GPa,and the corresponding frequency of electromagnetic emissions is in the range of 105108Hz,and we indicate the frequency is proportional to the strain rate,inversely proportional to the microcrack sizes,and consequently depend on the vibration mode of the atoms around microcracks.Based on the laboratory experiment results,we investigate two dynamic mechanisms on piezoelectric effect and cracks.From the piezoelectric constitutive equations of quartz crystal,based on Maxwell equations and elastic-dynamics equations,we deduced the wave equation of electric field on electromagnetic emissions.The laboratory compression experiment results indicate the transitions from the intergranular microcracks to transgranular microcracks is the mechanism of the strain（loading）rate sensitivity,and also physical mechanism that the amplitude and frequency of electromagnetic emissions increase with the strain rate.We conducted Brazilian disk and flatten Brazilian disk experiments to investigate the electromagnetic emissions under the tensile condition,by using rock testing machine and split Hokinson pressure bar.Tensile experiments indicates electric dipole moment of the origins is perpendicular to crack plane.Experimental results indicate the amplitude of electromagnetic emissions is directly proportional to crack velocity,and has a positive correlation with crack propagation toughness.Based on the signal generated by the single crack,we propose the theoretical model of damped oscillations of electric dipoles induced by transgranular microcracks and thus indicate that transgranular microcracks enable the atomic bond breakage,which generates oscillated electric dipoles,thereby resulting in the electromagnetic emissions.We conducted semi-cicular bend and notched semi-cicular bend experiments to investigate the electromagnetic emissions under the flexible-tensile condition,by using rock testing machine and split Hokinson pressure bar.Experimental results indicate the amplitude of electromagnetic emissions is directly proportional to crack initial toughness.Based on Barenblatt cohesion theory and theoretical model of damped oscillations of electric dipoles induced by transgranular microcracks,we indicate the amplitude of electromagnetic emissions is directly proportional to the cohesive strength of atomic bonds,and thus depend on the proportions of transgranular microcrack.We present the engineering application of rock electromagnetic emissions in monitoring the dynamic disaster,through the engineering practice of the earthquake electromagnetic precursor,blasting engineering and rockburst in the underground engineering.Based on IPE（Institute of Physics on the Earth）seismogenic model,we develop the electromagnetic emission model induced by transgranular microcracks of dry rock.For seismo-electromagnetic precursor in the atmosphere and ionosphere,we proposed the electromagnetic emission model induced by dynamic tensile failure when seismic waves reach the ground and turn to reflected tensile waves.The two models are verified by the electromagnetic response of the lithosphere and ionosphere of Wenchuan earthquake.The theoretical model of damped oscillations of electric dipoles induced by transgranular microcracks establishes the foundation of electromagnetic emissions as a method to determine dynamic crack propagation velocity,and also have a wide application in the blasting engineering and rockburst in the underground engineering. |
