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 force-indentation model for brittle rocks
SummaryA force-deformation model is developed for localized loading of brittle rocks that depicts the action of a percussive drill bit. This model consists of a series of successive cycles each of which is composed of a crushing and a chipping phase. Crushing involves the fragmentation or comminution of individual grains or grain clusters and is manifested by an increase in the applied force, while chipping is associated with the ejection of crushed material accompanied by a reduction in the load. This work represents the foundation for the analysis of the response of a brittle rock to the action of a jackhammer.Based on the hypotheses of a constant resistive pressure exerted by the target and a constant friction coefficient between the bit and target, the reactive target force during crushing is shown to vary linearly for a wedge and quadratically for a conical bit tip as a function of indentation. During chipping, a linear force-identation relation with a negative slope is utilized and the shear force acting on the chip, but not the normal force, is reduced to zero. Thus, the force at the end of chipping is finite. Previous investigators have shown that, when using the Coulomb failure criterion, the curve connecting the points denoting the initiation of chipping in the force-indentation plane is also linear for a wedge indenter and quadratic for a conical bit tip. However, the corresponding force levels are known to have been overestimated.In the present study, the failure strength of brittle rock will be modified both by a Weibull parameter to account for a size effect and a tip shape factor. The Weibull parameters are also employed to define the initiation of chipping. Good correlation has been obtained between the predictions of the model and corresponding experimental results.
Mathematical Simulation of Combustion of Boron Powder Gas Suspension
Based on a two-phase two-velocity model of a reacting gas-dispersed medium, a mathematical model of combustion of a boron powder gas suspension has been formulated. The results of numerical solution of the problem are presented. The influence of the composition of the gas suspension, its size, size distribution of particles, mass concentration of particles, as well as of the presence of aluminum powder in the suspension on the visible and normal velocities of flame propagation in the gas suspension, is shown. A comparison of the results of calculation with the well-known experimental and theoretical data is carried out. Satisfactory agreement of the predicted results with experimental data is shown.