Theoretical investigation of percussive drilling and drop test

This paper compares the energy transmission from the bit to the rock in percussion drilling (PD) and in the drop test (DT). The DT dynamics is essentially characterized by two time scales: T₁, the time taken by a perturbation to travel from one end of the assembly to the other end; and T₂, a measure of the duration of the bit penetration into the rock for a rigid assembly. Provided that ratio ? = T₁/T₂ « 1, the rigid body model of the DT leads to an accurate estimation of the penetration time and impact force/energy transmission. The laboratory PD test is controlled by two other time scales: T₃, the effective incident wave time length produced by the piston impact; and T₄, the time scale of the response, when the bit-rock interface (BRI) is subjected to an impulse impact load. Here it is assumed that the bit/rod assembly is long enough to ensure a separation of time scales between the wave travel time T₁ and T₃. Considering only one transmission cycle of percussive action, the energy transmission efficiency in PD first increases with the bit/rock interface pseudo-stiffness k =T₃/T₄ from zero to a peak, and then asymptotically reduces to zero.