Experimental Investigation on the Effect of Microwave Heating on Rock Cracking and Their Mechanical Properties

Due to various advantages including high efficiency, energy-saving, and having no secondary pollution (no dust or noise), the technology of microwave-induced fracturing of hard rock has been considered as a potential method for rock fracturing and breaking. Realizing microwave-assisted mechanical rock cutting using the microwave-induced hard rock fracturing technique can prolong the mechanical life and improve the efficiency of rock-breaking operations. For example, to realize microwave-assisted TBM excavation for hard rock tunnel. At present, this technology is still in the laboratory research stage. By summarizing the research results of relevant scholars in this field, this paper generalizes the mechanism of microwave heating of rock, microwave heating system, heating characteristics, and the effect of microwave heating on rock cracking and mechanical properties. Microwave heating causes microscopic cracks on the surface of the rock and microscopic cracks inside the rock. The higher the microwave power, the longer the irradiation time, the more serious the cracks propagation. Uniaxial compressive, Brazilian tensile, and point load strengths all decreased with increasing microwave irradiation time at rates that were positively related to the power level. The conventional triaxial compressive strength of basalt samples decreased linearly with microwave irradiation time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. In addition, the elastic modulus and Poisson’s ratio of basalts decreased in a quasi-linear manner with the growth of microwave irradiation time under uniaxial compression. While microwave irradiation has a slight influence on elastic modulus and Poisson’s ratio under triaxial compression. The cohesion decreases with increasing microwave irradiation time and shows an approximately linear decrease over time.

Acoustic Emission Characteristics and Failure Prediction of the Granite with Orthogonal Cracks under Compressive Loading

Natural joints existing in rock significantly affect the stability of long-term served subsurface engineering. In this paper, granite specimens with two orthogonal cracks were made for uniaxial compressive tests. The acoustic emission monitoring (AE) and digital image correlation techniques were employed to record the acoustic events and cracking of rock. The stress, ring-down count, and cumulative ring-down count of AE during the tests were obtained. The b-value of AE was calculated based on the magnitude and number of AE events. The relationship between the b-value and rock cracking for the specimens with orthogonal cracks was discussed. Further, the effects of orthogonal cracks distribution on the b-value and rock cracking were investigated. Experimental results show that the specimens with orthogonal cracks would undergo multiple cycles of energy accumulation-release-reaccumulation-rerelease under the uniaxial compression. For the specimens with orthogonal cracks, the b-value of AE was volatile but generally decreased until complete failure. Every cracking event during the loading made the b-value drop and then the reaccumulation of energy made the b-value increase or stable. The specimen with orthogonal cracks was more prone to initial cracking than the intact rock. The orientation of cracks had effects on the b-value evolution and crack patterns. The b-value reaching about 1.5 can be used as the failure precursor of specimens with orthogonal cracks.