Abstract: To explore the rock fragmentation characteristics and crack propagation mechanism of tunnel boring machine (TBM) double-disc cutters, scaled penetration tests and numerical simulations were carried out on red sandstone, employing double-disc cutters with two distinct cutter-edge profiles at varying cutter spacings. The mechanical response, failure characteristics, and fragmentation efficiency during rock fragmentation induced by double-disc cutter penetration were subsequently analyzed. The penetration failure process and crack propagation mechanism were revealed using the continuous-discontinuous element method (CDEM). The results show that the rock fragmentation characteristics and mechanical response of the two blade-types of double-disc cutters exhibit significant differences. Compared with the double-disc wedge-shaped cutter, the double-disc cutters with constant cross-section generate a more regular crushing pit profile, characterized by reduced undulation at the crushing pit base and fewer macroscopic cracks on the rock profile. When double disc cutters work synergistically, their superimposed stress fields interact. Both the peak penetration force and the specific energy for rock fragmentation exhibit a trend of first decreasing and then increasing with increasing cutter spacing. Both parameters reach their minimum values at a cutter spacing of s=16 mm. At lower penetration depths, wedge-shaped cutters preferentially generate tensile cracks and cause localized failure, with a significantly higher proportion of tensile cracks compared to constant cross-section cutters. Tensile cracks between constant cross-section cutters can coalesce faster. As penetration depth increases, shear cracks in the compacted core zone continuously propagate, and the proportion of tensile cracks for two types of cutters converges. Double-disc constant cross-section cutters consistently produce larger damaged zones and severely damaged zones, indicating that their energy is concentratedly dissipated in damage zone expansion and shear crack propagation. This study can provide a reference for the design and selection of cutter spacing and cutter-edge profiles for TBM double-disc cutters.

Key words: TBM double-disc cutters, penetration test, rock breaking characteristics, crack propagation, continuum-discontinuum element method (CDEM)