1531 average load, indicating that the pointed structure of conical inserts is inherently more conducive to penetrating rock, significantly reducing rock-breaking resistance. The influence of penetration depth is relatively moderate. Under the same insert geometry and velocity, when the penetration depth increases from 2.5 mm to 4.0 mm, the brittleness index shows only a slight decrease. This is because internal fractures progressively develop within the rock after initial penetration, leading to a modest decline in the resistance threshold for subsequent penetration. However, the overall decrease is limited, suggesting that depth has a weaker moderating effect on rock-breaking difficulty compared to insert geometry. Penetration velocity is a key variable affecting the brittleness index. For all insert combinations, as the penetration velocity increases from 0.6 mm/min to 1.8 mm/min, the brittleness index exhibits a clear upward trend. Furthermore, the growth slope for spherical-spherical dual inserts is far steeper than that for conical-conical dual inserts, with a distinct separation maintained between their brittleness index growth curves. This is due to the fact that high-velocity penetration increases the dynamic strength of the rock, and the blunt structure of spherical inserts is more sensitive to velocity changes, resulting in a much greater increase in rock-breaking difficulty with rising velocity compared to conical inserts. In summary, to enhance rock-breaking efficiency, it is advisable to prioritize the use of conicalconical dual inserts, paired with a relatively lower penetration velocity, while moderately increasing the penetration depth. This approach leverages the low-resistance characteristic of conical inserts, the lower brittleness index at lower velocities, and the slight reduction in resistance resulting from enhanced fracture development within the rock at greater depths, collectively working to reduce rock-breaking difficulty. 3.2 Analysis of Rock-Breaking Efficiency The differences in specific energy of rock breaking under the action of spherical and conical inserts at various penetration depths and velocities are shown in Figure 7. (a) d=2.5mm (b) d=4.0mm Figure 8 - Changes in SE under double-inserted teeth penetration velocity and depth As clearly shown in Figure 8, the energy consumed to break a unit volume of rock increases significantly with rising velocity, especially at shallower penetration depths (d=2.5 mm). In Figure 8(a), a distinct inflection point in Specific Energy (SE) is observed at 1.5 mm/min. When the velocity exceeds 1.5 mm/min, the SE for all experimental groups increases rapidly, whereas growth is slower before this point. It is also evident that the SE for the spherical-conical hybrid inserts is the lowest across all velocity stages. This indicates that the energy consumed for rock breaking by spherical-conical dual inserts is lower than that for spherical-spherical and conical-conical dual inserts under the same conditions, with conical-conical dual inserts showing particularly high consumption. At a velocity of 1.8 mm/min, the SE for conical-conical dual inserts increased by 214.43%, indicating a sharp rise in rock-breaking difficulty. At greater penetration depths (d=4.0 mm), the SE for all insert geometries decreased significantly, with reductions exceeding 33.16%. The decrease was most pronounced for conical-conical dual inserts, reaching 57.55%. Apart from a substantial increase observed for spherical-spherical dual inserts at 1.5 mm/min, the growth for other geometries remained relatively steady. Among them, the mean SE for conical0.6 0.9 1.2 1.5 1.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 SE/J·mm-3 Penetration velocity /mm·min-1 Spherical teeth Conical teeth (a) 0.6 0.9 1.2 1.5 1.8 0.00 0.08 0.16 0.24 0.32 SE/J·mm-3 Penetration depth/mm·min-1 Spherical teeth Conical teeth (b)
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