287 (a) d=2.5mm (b) d=4.0mm Figure 6 - Influence of double-inserted teeth penetration velocity and depth on load From Figure 6 (Variations in First Peak Load and Average Load during Dual Insert Penetration), the influences of penetration velocity, depth, and insert geometry on rockbreaking load can be observed. In the figure, the "First Peak Load" corresponds to the peak resistance at the first rock breakage during penetration, while the "Average Load" is the mean value of the peak loads over the entire process. The core trend is: as the penetration velocity increases from 0.6 mm/min to 1.8 mm/min, regardless of the insert geometry and depth, both the first peak load and the average load show an increasing trend. This is because, under higher penetration velocities, the dynamic mechanical strength of the rock increases, and the energy transfer time during breakage shortens, leading to a higher load threshold required for each breakage event. The insert geometry is a key factor determining load differences: under the same parameters, the first peak load and average load for the spherical-spherical dual inserts are consistently higher than those for the conical-conical dual inserts. For example, at a penetration depth of 2.5 mm and a velocity of 1.8 mm/min, the first peak load for sphericalspherical dual inserts is approximately 18 kN, while for conical-conical dual inserts, it is only about 15 kN. The difference in average load is even more pronounced (approximately 19 kN for spherical inserts versus 16 kN for conical inserts). The underlying reason lies in the contact characteristics of the insert geometries: the tip structure of conical inserts results in a much smaller "insert-rock contact area" compared to spherical inserts. Under the same total load, the concentrated load per unit area is greater for conical inserts, making it easier to exceed the rock's strength threshold. Therefore, the overall load required for rock breaking is lower for conical inserts. Changes in penetration depth modulate the magnitude of load increase: comparing the curves at depths of 2.5 mm and 4.0 mm, when the depth increases, the load growth slope becomes steeper for both types of dual inserts (e.g., for spherical-spherical dual inserts at a depth of 4.0 mm, the first peak load increases from 15 kN to 20 kN, a larger increase than at a depth of 2.5 mm). This occurs because, as the depth increases, the "residual stress" and "crack network complexity" within the rock rise. Subsequent breakage must overcome more complex structural resistance, and when combined with the effect of velocity, the magnitude of load increase is further amplified. 0.6 0.9 1.2 1.5 1.8 9 12 15 18 21 24 27 30 Peak load /kN Penetration velocity/mm·min-1 First Peak Load (Spherical teeth) Average load (Spherical teeth) First peak load (Conical teeth) Average load (Conical teeth) (a) 0.6 0.9 1.2 1.5 1.8 9 12 15 18 21 24 27 30 Peak load/kN Penetration velocity/mm·min-1 First Peak Load (Spherical teeth) Average load (Spherical teeth) First peak load (Conical teeth) Average load (Conical teeth) (b)
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