Pore Evolution Mechanism in Granite during Ultrasonic Vibration-Assisted Penetration by a Disc Cutter

The high strength and abrasiveness of hard rock formations are major factors contributing to the slow progress and high costs in tunnel boring machine (TBM) construction. Ultrasonic vibration, by generating stress concentration and fatigue damage through ultrahigh-frequency cyclic impacts, can effectively weaken rock strength and is considered a promising auxiliary technology for TBM applications. This study used nuclear magnetic resonance (NMR) to comparatively analyze the evolution of microscopic damage in granite under composite loading (ultrasonic vibration + static load) versus single loading. The results demonstrate that high-frequency ultrasonic vibration within the composite loading mode induces the microreciprocating motion of mineral particles, which intensifies stress concentration at grain boundaries and microdefects and leads to more pronounced fluctuations in nanopore volume. Furthermore, while the micro-nano pore volume ratio fluctuated under both modes, it exhibited an overall increasing trend with loading cycles. Compared to single loading, the composite mode significantly mitigated the initial decrease in this and amplified the maximum increase degree, indicating that ultrasonic assistance mitigates pore compression while promoting the initiation and accumulation of localized, irreversible microdamage. Under single loading, porosity decreased during the initial cycle due to the closure of primary pores, followed by fluctuating increases in subsequent cycles from new pore generation. Conversely, under a composite loading exceeding 2 kN, ultrasonic vibration prematurely induced damage, causing an immediate porosity increase from the first cycle and a significantly higher overall increase. This is attributed to the efficient promotion of microcrack propagation and coalescence by ultrasonic vibration at lower loads. This study reveals the differential impact of various loading modes on the microscopic damage mechanisms of granite, providing a microscale basis for a deeper understanding of the rock damage process.