Mechanisms of Surface and Subsurface Damage in Longitudinal–Torsional Ultrasonic Vibration‐Assisted Milling of <scp>CFRP</scp>

ABSTRACT Carbon fiber reinforced polymers (CFRP) are widely utilized in the aerospace industry owing to their exceptional specific strength, high modulus, and structural tailorability. Longitudinal‐torsional ultrasonic‐assisted machining (LTUAM) has emerged as an effective technique to enhance machining efficiency while mitigating surface and subsurface damage. To elucidate the underlying damage suppression mechanisms, this study investigates the transition of carbon fiber failure modes through a combination of theoretical modeling, mesoscale simulation, and comparative milling experiments. A shear fracture damage criterion was established to model the CFRP material removal process under LTUAM. Utilizing this criterion, the regulatory effect of longitudinal‐torsional vibration on fiber fracture modes across different fiber direction angles was analyzed. The theoretical analysis reveals a micro‐physical transition in the material removal mechanism from bending‐dominated to shear‐dominated fracture, which fundamentally suppresses the evolution of subsurface damage. Comparative milling experiments validated these findings, demonstrating that LTUAM significantly improves macroscopic surface quality and restricts the deep propagation of micro‐defects. The experimental data indicated a negative correlation between the proposed shear fracture criterion and subsurface damage depth. Compared with traditional milling, LTUAM can reduce surface roughness by 13.5%. Notably, at a severe fiber direction angle of 135°, LTUAM decreased fiber bending deformation from 22° to 13° and substantially reduced the subsurface damage depth from 72.8 to 42 μm. These findings offer a robust technical pathway for the high‐efficiency, low‐damage machining of advanced composite components in aerospace and automotive lightweighting applications.