Mechanical characteristics and pore evolution of red sandstone under ultrasonic high-frequency vibration excitation

Ultrasonic vibration rock breaking is a new type of rock breaking technology. By studying the mechanical properties of red sandstone under ultrasonic vibration, the mechanical behavior and damage mechanism of rock under the impact of high-frequency vibration can be revealed more comprehensively from macro- and microscopic standpoints. In this paper, the cylindrical red sandstone specimen is used as the study object subjected to vibration excitation via the ultrasonic vibration device. The change in the mechanical parameters of red sandstone specimens is analyzed via a single-axial compression test. The red sandstone specimens are vibrated to study the effects of high-frequency vibration on their natural frequency. The latter's natural frequency is measured using the knocking method, while the micro-disruption characteristics of the red sandstone are observed via electron microscopy. The T-2 spectrum, aperture distribution, porosity, and nuclear magnetic resonance image (MRI) evolution characteristics of red sandstone specimens are obtained via nuclear magnetic resonance technology. The results show that ultrasonic vibration deteriorates the red sandstone compressive strength and elastic modulus by 55.3% and 26.9%, respectively, after 120 s of excitation. Under ultrasonic vibration excitation, the rock specimen's natural frequency is reduced by 2.4% due to its mass and elastic modulus variation. Many transgranular cracks are generated in the sandstone, splitting the crystal nucleus into smaller blocks. The generation of new micropores is observed in the T-2 spectrum, and the maximum increase in the dimensions of micropores and mesopores at the two peaks is 58.7% and 4.67%, respectively. The variation trend of rock specimen porosity is completely consistent with the variation in micropores' content. MRI images indicate that the microcrack aggregation occurs in the edge area of the contact surface between the exciter and rock.