288 3.1 Analysis of Brittleness Index (a) First peak brittleness index (b) Average brittleness index Figure 7 - Changes in rock brittleness index under double-inserted teeth penetration velocity and depth As shown in Figure 7, regarding the influence of insert geometry, under the same penetration parameters, the brittleness index for spherical-spherical dual inserts is consistently and significantly higher than that for conical-conical dual inserts. This difference remains stable for both the first peak load and the average load, indicating that the pointed structure of conical inserts is inherently more conducive to penetrating rock, significantly reducing rockbreaking 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 sphericalspherical 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 conical-conical 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 0.6 0.9 1.2 1.5 1.8 4 6 8 10 12 14 16 18 20 22 BIm1/kN·mm-1 Penetration velocity /mm·min-1 Spherical teeth (d =2.5mm) Conical teeth (d =2.5mm) Spherical teeth (d =4.0mm) Conical teeth (d =4.0mm) (a) 0.6 0.9 1.2 1.5 1.8 4 6 8 10 12 14 BIm-ave/kN·mm-1 Penetration velocity /mm·min-1 Spherical teeth (d =2.5mm) Conical teeth (d =2.5mm) Spherical teeth (d =4.0mm) Conical teeth (d =4.0mm) (b)
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