Finite element analysis of the impact process of concrete drilling
Percussive drilling, that uses rotary torque and axial impact simultaneously, is an established method of making holes in concrete. This paper deals with the development of a finite element model, based on three-dimensional wave propagation, which represents the impact process of percussive drilling in concrete. The solid model of the percussive drilling system consists of five separate components, viz. shank, fluted body, socket tip, carbide blade and concrete target. After integration and meshing, the finite element model is analysed using three-dimensional wave propagation techniques. The bias weight and the impact impulse were set up as inputs to the drilling system. A "surface to surface eroding" interface was selected for the drill tip-concrete contact surface while a "rigid wall" interface was chosen for the concrete-ground support contact surface. Simulation results showed the usefulness of the model in understanding how the three-dimensional wave, due to impact, propagates through the drilling system resulting in penetration of the drill bit into concrete. The accuracy of the model was successfully verified with prior-known one-dimensional analytical model and experimental penetration results. Finally, to illustrate the potential use of this model to design and analyse percussive drilling system, the effect of variation of rake angle, helix angle, point angle, and bias weight on the extent of penetration is presented. The results clearly showed the importance of simulating the impact process for designing new percussive drill bits.
A theoretical and experimental study of the percussive drilling of rock part I—theory of percussive drilling
Abstract The theoretical mechanics of the percussive drilling of rock are developed from analysis of stress-wave interactions in the drilling system. Particular attention is given to 1. (i) the efficiency of impact energy transfer from drill steel to rock, and 2. (ii) the minimum thrust force needed to ensure bit-rock contact at the beginning of each impact. A simple expression is derived from which the penetration rate of a percussion drill can be predicted, provided the amount of impact energy per unit volume of rock broken is known for the appropriate drill bit and rock type, together with certain and operational parameters of the drilling machine.
Penetration of rock by conical indenters
Abstract Experimental results for the penetration of conical bits into rock have been obtained and compared to theoretical results derived from an existing bit penetration model. The model is similar to the one established by Paul and Sikarskie for wedge penetration, and was developed independently by Miller and Sikarskie, and the author. The tests were made in Swedish Bohus Granite using seven indenters with apex angles in the range 60–150°. For the smaller apex angles chip failure was predominant whereas for the greater apex angles essentially crushing occurred in agreement with the predictions of the theory. Force-penetration curves and crater vol were measured, and a force-penetration parameter characterizing the chipping envelope and the ratio of volume to work were evaluated. The dependence of the force-penetration parameter on apex angle is similar to that predicted by the theory, but it is found that the best agreement between theory and experiment is obtained when lower values than the real values are assigned to the compressive strength, the internal angle of friction, and the angle of friction representing the interaction between the bit and the solid rock. This means that the predicted forces at chip failure are too high, which is believed to depend on the use of a global rather than a local failure criterion. Using those values of the mentioned parameters, which give the best fit for the chipping envelope, good consistency between theory and experiment is obtained for the crater volume to work ratio.