<p>The high strength and abrasiveness of hard rock formations lead to rapid wear of cutting tools, significantly increasing time and material costs, which restricts the progress and economic efficiency of tunnel excavation. As an effective auxiliary rock-breaking technology, ultrasonic vibration demonstrates advantages in promoting fatigue damage and reducing rock strength, showing potential to enhance TBM penetration rates and extend cutter lifespan in hard rock environments. Nevertheless, limited research has been conducted in this area to date. This study employs penetrant testing, digital image processing, and particle flow code (PFC) simulations to systematically investigate damage progression, crack evolution characteristics, and fracture distribution patterns in rock specimens. The results indicate that compared to single loading mode, the ultrasonic-assisted composite loading mode enables the proportion of damaged areas to increase from 45 to 58%, demonstrating that the auxiliary effect of ultrasound can significantly enhance the damage severity. Furthermore, single loading mode primarily induces intragranular cracks originating from inherent defects within feldspar minerals, whereas composite loading mode tends to generate grain-boundary cracks along feldspar grain boundaries and feldspar-quartz/mica interfaces, and shows enhanced cross-mineral propagation capabilities. The incorporation of ultrasonic vibration induces lateral outward shifting of crack initiation locations along the horizontal direction while significantly increasing the effective depth in the vertical direction. This helps to promote the extension and penetration of cracks towards the free face, thereby facilitating the formation of rock fragments and rapid detachment of the rock mass. This study contributes to a deeper understanding of damage characteristics and crack propagation mechanisms in ultrasonic-assisted TBM rock fragmentation technology, offering critical design guidelines for its practical implementation.</p>