Experimental Study on Continuous and Oscillatory Rotational Penetration

“Wiggling,” rotational and oscillatory motions are often adopted by many living organisms to penetrate into or burrow in the soil. For example, seeds of the Erodium cicutarium and Pelargonium can bury themselves into the ground for future germination with a continuous rotational motion. The angled worm lizards (Agamodon anguliceps) make tunnels by oscillating their head along the long axis of the trunk. Our previous numerical simulations indicate that both rotational modes lead to comparable reductions of penetration resistance of a rod with a conical tip. In this study, both modes of rotational penetration were further investigated through laboratory experiments. The penetrator consists of a cylindrical shaft and a conical tip with an apex angle of 60°. A gear motor is integrated into the penetrator for rotational motion control: both continuous rotation and oscillatory rotation can be achieved. The penetrator assembly is connected to a six-axis robotic arm, which controls the vertical penetration and collects the force/displacement data. A series of rotational penetration tests were conducted in Ottawa sand F65 with different combinations of vertical and rotational velocities while the resultant velocity was kept as a constant. Different oscillation amplitudes were used under the same rotational velocity for the oscillatory rotational penetration cases. The results confirmed the conclusions drawn from the numerical simulations: for both rotational penetration modes, the rotational penetration forces decrease with the relative slip velocity (ratio between the rotational and vertical velocity); comparing with the non-rotational case, the reduction of penetration force is about 80% for the whole-body-continuous-rotation (WCR) case and 60% for the cone-continuous-rotation (CCR) and cone-oscillatory-rotation (COR) when the relative slip velocity reaches 20. Under the tested conditions, the penetration forces were not affected by the oscillation amplitudes. Comparing with continuous rotational penetration, oscillatory penetration achieved less reduction of penetration force, but it is easier to be implemented in cone penetration tests or self-burrowing robots.