Dynamical Mechanical Properties And Fragmentation Of Microwave And Heat Treated Granite

Rock fragmentation is an important issue in rock mechanics and rock engineering.Low efficiency in fragmentation and rapid wear of cutting tools are the main problems for the hard rock breakage using the conventional crushing machine.Microwave and heating assisted rock breakage are thus proposed to overcome these two problems.These two rock breakage methods are fundamentally dynamic.It is therefore desirable to understand the dynamical responses and fragmentation behaviors of microwave and heat treated rocks.The comparative experimental studies were systematically carried out using split Hopkinson pressure bar(SHPB),coupled with momentum trap,digital image correlation(DIC)and other key measuring techniques.Meanwhile,considering the heterogeneity of damage,a new modeling method based on the statistical damage theory was employed and introduced to discrete element method(DEM)to conduct the numerical simulations.The main points of this work are as follows:(1)Dynamic compressive and tensile strengths are both loading rate dependent.The strengths increase with the increase of loading rate and are negatively correlated with irradiation duration and temperature.The strengths of microwave irradiated granite are larger than those of heat treated granite with the same damage level.The impacts of microwave irradiation and heating treatment on DIF,dynamic modulus,and stress-strain curves are quite different.These can be attributed to the differences in the type and distribution of micro cracks induced by these two preconditioning methods.The empirical models of dynamic strengths for granite were established.After that,the variations of strengths with damage and strength contours were also fully discussed.(2)The overload effect can be significantly observed in the dynamic Brazilian disc(BD)tests.The overload ratio is strongly rate dependent and increases with the increase of irradiation duration and temperature.It is thus essential to recorrect the apparent tensile strength.The crack propagation velocity was also determined using the virtual extensometers.The results shows that both loading rate and damage have positive effect on the increasing of the velocity.(3)The fragmentation type and degree are different between irradiated and heated granite when they are dynamically loaded at the same range of loading rate.The fragment size distributions(FSDs)with different degrees of fragmentation can be fitted by different functions.The variations of the fitting parameters indicate that the changing trend of FSD with the loading rate is preconditioning method-dependent.The new generated surface area and the kinetic energy can be determined using the proposed methods,then the evolution law of fragmentation is also revealed.For a given condition,the heated granites are fragmented more severe than the irradiated ones,but the efficiency of pretreatment is relatively low for heating.(4)According to the theoretical analysis,assumption and experimental data,the relation between fragment size and energy,referred to as fragmentation-energy fan(FEF),is quantitively given.The FEF behavior of irradiated granite is not similar with that of heated granite.Furthermore,the FEF of the damaged granite in the whole range of dissipated energy is predicted.(5)Normal distribution and Weibull distribution were used to describe the distributions of microwave induced and heating induced damages in granite,respectively.The numerical studies on dynamic properties and fragmentation of damaged granite were then conducted using DEM.Simulation results agree well with the experimental results,verifying the validity of the proposed method.