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 theory of percussive drill bit penetration
Abstract The author has developed a theory of percussive bit penetration. The theory is in substantial agreement with his experimental results. In developing the theory the author has assumed a mathematical model which is based on some of his experimental observations. The mathematical force-penetration relationships deduced by the author have been compared with experimental results of the author himself and also with the results obtained by Reichmuth, and Gnirk and Cheatham. It is concluded that (a) the force-penetration curve for each bit-rock combination reflects the characteristics of both the rock type and the bit. (b) During the crushing phase, rock in contact with the bit becomes pulverized to a certain depth and subsequently becomes compact under increasing force. The size of this zone is a function of bit geometry, penetration and angle of internal friction of the rock. (c) The friction between the pulverized wedge and the confining solid rock has a great influence on the chipping force. (d) The angle of fracture, profile and shape of the crater are in substantial conformity with the theoretical prediction and from the theory it is also possible to predict the forces for given depths of penetration.
Comparison of size effect for different types of strength tests
SummaryComparison of Size Effect for Different Types of Strength TestsDifferent theories have been proposed to explain and predict size effect. Notable is Weibull's “Weakest link theory”. In addition various theories have been founded on strain energy consumption at failure. The present paper suggests a theoretical approach, based on energy considerations and mode of failure, which holds for diverse types of mechanical testing. Maximum size effect is assumed to be associated with failure through development of a single fracture plane and a lack of size effect is associated with failure affecting a volume of material. The quantitative expression of the theoretical approach is based on the relationship between load at failure (P) and cross sectional area of the specimen (A):P=KAn wheren expresses size effect andK is a constant.The theoretically lowestn-value equals 0.75 expressing maximal size effect whereas the theoretically highestn-value is 1.00 expressing lack of size effect.n-values evaluated from published data for various mechanical tests indeed lie between these limits and appear to be related to type of test, material properties and specimen shape. Point load tests and Brazilian tests are generally associated with large size effects. This may be attributed to the test conditions which promote development of single fracture planes. On the other hand relatively small size effects are generally found for uniaxial compression tests. Failure in these tests characteristically occurs through multiple fracturing and crushing and therefore affects a volume of material rather than a single plane. Brittleness seems to be associated with large size effects and ductility with small size effects.