Abstract
This paper describes a study of the influence of non-normal loading on the indentation behaviour of single bit-teeth. Indentation experiments have been performed on a soft, isotropic limestone using both sharp and blunt 60 degree wedge-shaped indentors constrained to penetrate the rock at 0, 15, and 30 degrees to the rock surface normal. For both sharp and blunt indentors, the normal force required to produce a given normal penetration decreased as the offset of the penetration direction from the rock surface normal increased. The crater volume produced by a given normal penetration with the sharp indentor was largely independent of the penetration direction. In contrast, the crater volume produced by the blunt indentor was increased markedly by non-normal penetration at small penetration depths, because the threshold penetration for chip formation decreased with non-normal loading. The crater volume produced by a given normal force was significantly increased by non-normal penetration, and this increase was particularly pronounced with the blunt indentor. The energy required to form unit crater volume was correspondingly reduced by non-normal indentor penetration. A simple method for predicting the reduction in the rock's resistance to chip formation is outlined, which is supported by the experimental data. The implications of these findings for rotary drilling are considered.
Introduction
The geometry of soft formation roller cone bits is designed to impose both normal and shearing loads on the bit teeth. In consequence the teeth do not penetrate the rock normal to its surface. Little attention has been paid to the influence of this non-normal loading on the indentation and crater formation behaviour of individual bit teeth, although it is generally recognised that it increases the rate of penetration achievable with the full bit. In consequence a study of indentation with non-normally loaded indentors has been conducted at SCR - this paper presents the results of the study.
Experimental Details
The indentors used in this study were all wedge-shaped with an included angle of 60 degrees and a length in the wedge symmetry plane of 10mm. Both sharp and blunt indentors were employed, the latter having a 2mm wide flat perpendicular to the wedge symmetry plane. The indentors were mounted on the crosshead of an Instron mechanical testing machine so that they were driven at a penetration rate of l mm per minute into a rock sample resting on its base. Non-normal penetration of the indentor was achieved by inclining the wedge symmetry plane and the rock surface by the same angle relative to the direction of penetration. This angle of offset was variously 0, 15, or 30 degrees. The instantaneous forces on the indentor parallel and perpendicular to the penetration direction were measured and resolved to give the loads normal and parallel to the rock surface. These were recorded during each indentation together with the indentor's normal penetration and its displacement parallel to the rock surface. The total energy of indentation was calculated from the measured loads and displacements. The volume of material removed during indentation was determined by collecting and weighing the displaced rock fragments.
