1532 conical dual inserts across all velocities was the lowest at 0.094 J/mm³, while spherical-spherical dual inserts had the highest at 0.120 J/mm³. The results above demonstrate that as the penetration depth increases, the SE for all insert groups decreases significantly. Furthermore, conical-conical dual inserts exhibit superior rock-breaking performance at penetration depths of 2.5 mm and 4.0 mm. During penetration, crack propagation tends to be radial for conical inserts and axial for spherical inserts. Consequently, conical-conical dual inserts can create a larger tensile stress zone between them, leading to the spalling of more surface rock samples and thereby reducing their specific energy consumption. However, they offer limited assistance in promoting the propagation of axial cracks. 3.3 Crater Morphology Figure 9 shows the top view of the rock-breaking surface after penetration and fragmentation. Figure 9 - Changes in surface crater morphology under double inserted teeth penetration velocity and depth As shown in Figure 9 (Crater Morphology of Dual Spherical and Conical Inserts), it can be observed that the craters formed by dual insert penetration encompass both the "area surrounding the penetration points" and the "region between the two inserts." Their morphology and extent exhibit clear patterns with changes in penetration parameters: Influence of Penetration Velocity: Regardless of the insert geometry or penetration depth, as the penetration velocity increases from 0.6 mm/min to 1.8 mm/min, the crater area shows a linear decreasing trend. This is because under high-velocity penetration, the rock failure mode shifts more towards "local impact fracturing," where energy concentrates near the penetration points and is less likely to diffuse into the inter-tooth region. Concurrently, the elliptical shape of the crater changes with velocity: the semi-minor axis gradually contracts, indicating that the "lateral extent" of the fractured zone decreases as velocity rises, making the crater more elongated and narrow. Influence of Penetration Depth: Under the same insert geometry and velocity, when the penetration depth increases from 2.5 mm to 4.0 mm, the crater area progressively expands. This occurs because greater depth enhances the compressive action of the dual inserts on the rock, making internal fractures more prone to propagate into the inter-tooth region. Furthermore, the position of the "lowest point" within the crater shifts. At d=2.5 mm, the lowest points are concentrated near the penetration points (indicating energy is primarily applied to the insert-rock contact zones). In contrast, at d=4.0 mm, the lowest points extend progressively into the region between the inserts (suggesting fractures have interconnected across the two inserts, forming a depression spanning the inter-tooth area). This demonstrates that increased depth enhances the "synergistic fracturing effect" between the dual inserts.
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