289 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 conicalconical 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 rockbreaking 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 conical-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 0.6 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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