Research On Dynamic Behavior And Failure Mechanism Of Water-Bearing Sandstone Under Microwave Irradiation

Microwave irradiation on rocks before excavation is an effective method to reduce equipment wear and energy consumption during mechanical cutting.However,current research on the mechanical properties of rocks under microwave irradiation is mainly focused on quasi-static conditions,while in the actual tunneling excavation and rock crushing operations,cutter head and rock are in strong and complex interaction,usually at medium to high strain rates.In addition,for most engineering practices,due to the seepage effect and aquifers in the mined-out area,rock mass excavation is usually carried out in a water-rich environment.Therefore,understanding the coupled effects of water content and loading rate on the mechanical behavior of rocks under microwave radiation is essential.In this paper,we have carried out microwave heating tests on different rock types based on a multi-mode microwave radiation system and systematically studied the dynamic mechanical behavior and fragmentation characteristics of water-bearing sandstone under microwave irradiation,using a combination of experimental research and theoretical analysis.The main research contents and conclusive results are as follows.(1)Four different rock types were selected for comparison tests under microwave and conventional heating conditions.The results show that,under the same microwave power conditions,the heating rate of the four rocks is: basalt > granite > sandstone > marble.Basalt specimens will burst under the effect of microwave radiation above 1 k W,and with the increase of microwave power,the bursting time of the specimens shortens;under the same heating rate,microwave radiation can cause severe damage to the rock than conventional heating.When the temperature reached 500 °C,the porosity of basalt,granite,sandstone and marble increased by 82.7%,59.5%,14.6% and 30%,respectively,under microwave radiation compared to conventional heating.The P-wave velocity of red sandstone shows a temperature-strengthening effect at100-200 °C,but this phenomenon disappears when the microwave power is increased above 1 k W.(2)Dynamic compressive tests on microwave-damaged sandstone were carried out using a split Hopkinson pressure bar(SHPB)system.With increasing microwave power and extension of radiation time,the dynamic failure mode of sandstone switches from the initial tensile failure to tensile-shear composite failure,and finally to the typical compression-shear failure,and the rock fragmentation gradually increases.The main reason for this phenomenon is the increase in crack density and the decrease of dynamic Poisson’s ratio.The SEM results indicate that microwave radiation causes dehydration,pore expansion and cracking.Extension of the time of microwave irradiation supports the extension and opening of cracks,while increasing the microwave power is more beneficial to the generation of transcrystalline cracks and the formation of fracture networks.Based on crack extension characteristics and microscopic thermodynamic mechanisms,five basic types of cracks generated within sandstone under microwave radiation are summarised.(3)Quasi-static and dynamic tensile behavior of water-bearing sandstone subjected to microwave irradiation were carried out.The results indicate that the presence of pore water can significantly improve the heating efficiency of microwave-insensitive rocks.With the increasing water content,the pore structure of sandstone shows a trend of increasing the volume of large pores and decreasing the volume of tiny pores after microwave radiation.When the initial water content of the specimen exceeds 3%,the damage caused by microwave radiation to the rock is significantly increased.For different initial water content,the dynamic tensile strength of microwave irradiated sandstone increases with increasing loading rate.The higher the initial water content of the sample,the stronger the dependence of the loading rate,which is related to the increasing crack density and the increase in the sample’s viscosity.Under the same loading rate,the energy consumption by fragmentation of the sample decreases with the increase of the moisture content,which indicates that the existence of pore water is beneficial to increase the damage caused by microwave radiation.(4)Water saturation effects on dynamic fracture behavior of sandstone subjected to microwave irradiation were studied.The dynamic fracture initiation,extensional toughness and fracture energy of the water-saturated sandstone specimens are less than those of the dry specimens after exposure to microwave radiation,indicating that the ability of the water-saturated sandstone to resist crack extension is reduced by microwave radiation and that the rock is more susceptible to low-stress fracture and rapid failure.The presence of pore water under microwave radiation exposes the sandstone to the dual weakening effects of high temperature and thermal shock,and the reduction in rock fracture toughness can be explained by the evolution of the microstructure around the precast crack tip and the change in the size of the fracture failure process zone(FPZ).