Rock Indentation Research Articles

Rock Indentation Indices as Criteria in Rock Excavation Technology – A Critical Review

It has been known for thousands of years that materials differ in hardness, quantitative methods of measuring hardness by performing careful indentation experiments only began to be developed during the nineteenth century. At the beginning of the twentieth century, hardness testing machines began to be commercially available. The purpose of this review paper is to present the techniques and advances in Indentation of rocks. Such Rock Indentation is essential for studying the fundamental processes like drilling and cutting operations. The indentation testing is used for characterisation of hardness of rocks. The testing procedure was developed to be in line with other ISRM Suggested Methods. The results show that standardised indentation testing allows for characterisation of mechanical properties of rock and that there is a relationship between the value of the indentation hardens index and the uniaxial compressive strength. And also, rock indentation test is used in the development of models for rock fragmentation process. The review explains the correlation of indentation indices with mechanical properties of rock and the fracturing process of rock during indentation.

Numerical Crack Analysis of Blunt Rock Indenters by an Indirect Boundary Element Method

Linear elastic fracture mechanics principles are widely applied for the analysis of crack problems in rock fracture mechanics. Rock indentation is an important and complicated problem among rock engineering issues. In this paper, in addition to the fracture criterion of maximum tangential stress adjacent to crack tip, the higher order displacement discontinuity method (which is a version of the indirect boundary element method) has been used for modeling the crack propagation mechanism under blunt indenters. In order to achieve more accurate results, higher order boundary elements i.e. quadratic elements, has been used to calculate displacement discontinuities and also to reduce the singularities of stress and displacement fields near the crack tip, the special crack tip elements has been used to calculate the stress intensity factors (SIF) at the crack tips. In this modeling, the effect of crack angle on stress intensity factors has been investigated. The numerical results of stress intensity factors obtained from some example problems were compared to the theoretical and experimental results cited in the literature which always show a percentage error less than one percent. The simulated results may pave the way for increasing the efficiency of mining and drilling by improving the design of tools and indentation equipments.

Indentation of rock by wedge-shaped tools

This paper presents the experimental results obtained with a two-dimensional indentation device controlled by a servo-hydraulic loading system and monitored with the nondestructive techniques of acoustic emission and electronic speckle pattern interferometry. The goals of this research were to evaluate the indentation pressure as well as the size of a damage (plastic) zone, and to study the initiation of tensile fracture at the intact rock-damaged rock boundary, that is, the elasto-plastic interface. The key factors controlling the failure process are (1) the mechanical properties of the rock including the elasticity and strength parameters, (2) the geometric features of the tool such as the wedge angle, and (3) the lateral confinement simulating the far-field stress. A good agreement with regard to indentation pressure and damage-zone radius was found between the experimental and theoretical analyses. Furthermore, the intrinsic crack length, critical in establishing tensile fracture, was estimated and correlated to the grain size.

Numerical modelling of the effects of joint spacing on rock fragmentation by TBM cutters

The influence of joint spacing on tunnel boring machine (TBM) penetration performance has been extensively observed at TBM site. However, the mechanism of rock mass fragmentation as function of the joint spacing has been scarcely studied. In this study, the rock indentation by a single TBM cutter is simulated by using the discrete element method (DEM), and the rock fragmentation process is highlighted. A series of two-dimensional numerical modelling with different joint spacing in a rock mass have been performed to explore the effect of joint spacing on rock fragmentation by a TBM cutter. Results show that the joint spacing can significantly influence the crack initiation and propagation, as well as the fragmentation pattern, and can hence affect the penetration rate of the TBM. Two crack initiation and propagation modes are found to fragment the rock mass due to the variation of joint spacing. The simulation results are analyzed and compared with in situ measurements.

Numerical modeling of the effects of joint orientation on rock fragmentation by TBM cutters

The influence of joint spacing on tunnel boring machine (TBM) penetration performance has been extensively observed at TBM site. However, the mechanism of rock mass fragmentation as function of the joint spacing has been scarcely studied. In this study, the rock indentation by a single TBM cutter is simulated by using the discrete element method (DEM), and the rock fragmentation process is highlighted. A series of two-dimensional numerical modelling with different joint spacing in a rock mass have been performed to explore the effect of joint spacing on rock fragmentation by a TBM cutter. Results show that the joint spacing can significantly influence the crack initiation and propagation, as well as the fragmentation pattern, and can hence affect the penetration rate of the TBM. Two crack initiation and propagation modes are found to fragment the rock mass due to the variation of joint spacing. The simulation results are analyzed and compared with in situ measurements.

An Analytical Model for the Indentation of Rocks by Blunt Tools

A methodology based on the cavity expansion model is developed to analyze the indentation of rocks by a class of blunt indenters. The analysis covers the particular self-similar case of indentation by blunt wedges or cones. As an example, the main results for the indentation of rocks by a spherical tool are presented and the analytical solution is compared with experimental results obtained by indenting a sphere in Harcourt granite.

Identification of the governing parameters related to rock indentation depth by using similarity analysis

This paper focuses on the indentation depth in rocks caused by a hemispherical indenter. The problem is approached by a combination of similarity methods with an artificial neural network. The similarity methods offer a profound understanding of the physical problem and help to identify the most important governing parameters or factors that reflect the essence of the rock indentation events, thus simplifying the target problem. The artificial neural network provides an advanced computing model, which allows more factors to be involved. The predictions obtained using this combined approach are in better agreement with the experimental results than predictions using other methods.

The introduction of neural network system and its applications in rock engineering

The neural network system has been developing very fast recently. It has been widely used in many industries such as automation, nuclear power plant, chemical industry, etc. Neural network systems have a great advantage in dealing with problems in which many factors influence the process and result, and the understanding of this process is poor, and there are experimental data or field data. In rock engineering, many problems are of this nature. In this paper, a brief introduction to neural network systems is given. Problems such as what is a neural network, how it works and what kind of advantages it has are discussed. After this, several applications in rock engineering, made by us, are presented. Case 1 is ore boundary delineation. In this case, the rock are divided into three classes, i.e.: (1) waste rock; (2) semi-ore; and (3) ore for mining purposes. The neural network system built can judge whether it is ore, semi-ore or waste rock along the borehole according its corresponding geophysical logging data, such as gamma-ray, gamma-gamma, neutron and resistivity. Case 2 is aggregate quality prediction. In this case, the quality parameters: (1) impact value; (2) abrasion value I; and (3) abrasion value II are predicted by using a neural network system based on density, point load, content of quarts and content of brittle minerals. Case 3 is rock indentation depth prediction. In this case, the rock indentation depth under indentation load is predicted by the established neural network system based on the indentation load on rock, indenter type and rock mechanical properties, such as uniaxial compressive strength, Young's modulus. Poisson's ratio, critical energy release rate and density. In all these cases, the neural network systems have been applied successfully. The testing results are satisfactory and better than the existing techniques.

Fracturing at contact surfaces subjected to normal and tangential loads

Abstract A variety of rock engineering problems including drilling, cutting, abrasion, and milling involve rock tool contact and indentation. The pattern of indentation fractures and the role of slip conditions, surface roughness, tool radius and initial flaw size for an arbitrarily loaded contact are not fully known. The present paper aims to identify the elastic stress field for a contact subjected to both normal and tangential loads, and evaluate the condition for the fracture initiation and propagation. Stress fields within two spheres at contact are available when either only normal load is applied or when tangential load causes full-slip conditions. It is shown here that through appropriate superposition of the above two solutions, the stress field under partial-slip conditions, as well as during the unloading of tangential force may be determined. Maximum tensile stress increases significantly under partial-slip conditions as compared to the full-slip case even though the same magnitude of tangential force is applied. The location of maximum tensile stress moves inward from the trailing edge as the tangential force is unloaded. The stress-intensity factors for a penny-shaped crack which initiates at the contact periphery, and follows the minimum principal stress trajectory are obtained and utilized to study indentation fracturing. The dependency of critical loads on initial flaw size, indenter radius and slip conditions is quantified. The predictions of fracture density and spacing under a sliding indenter are achieved through a simple estimate of shielding interaction between adjacent fractures. Relation of these evenly spaced fractures with the formation of wear grooves on sliding surfaces is discussed.

Prediction of rock hardness and drillability using acoustic emission signatures during indentation

This paper reports rock indentation tests which indicated that there was a certain relation between the indentation hardness of rock and the AE parameters, such as accumulated number of events, peak RMS and integrated RMS. Among the above-mentioned AE parameters, the correlation of RMS of AE events with indentation hardness is pronounced and more than ever the integrated RMS is the best index to correlate with the indentation hardness of rock. It represents the sum of amplitudes of AE varied during all the indentation process and reflects the size of AE energy released in the time period of indentation. Indentation hardness corresponds well to peak RMS value for different rock types. The onset of significant AE activity appears at about 40% peak indentation load, and intense AE activity corresponding to fracture propagation and chipping initiates at about 80-90% of peak indentation load. The peak RMS and integrated RMS may be recommended as the indexes to express rock hardness and may be further employed to predict rock drillability using these indexes in association with penetration rate of drilling.

Role of wetting fluids during the indentation of porous rocks : Suarez-Rivera, F R; Cook, P J; Cook, N G W; Myer, L R US Symposium, Norman, 10–12 July 1991P683–692. Publ Rotterdam: A A Balkema, 1991.

Role of wetting fluids during the indentation of porous rocks : Suarez-Rivera, F R; Cook, P J; Cook, N G W; Myer, L R US Symposium, Norman, 10–12 July 1991P683–692. Publ Rotterdam: A A Balkema, 1991.

Failure mechanisms induced by indentation of porous rocks : Thiercelin, M; Cook, J Key Questions in Rock Mechanics: Proc 29th US Symposium, Minneapolis, 13–15 June 1988P135–142. Publ Rotterdam: A A Balkema, 1988

Failure mechanisms induced by indentation of porous rocks : Thiercelin, M; Cook, J Key Questions in Rock Mechanics: Proc 29th US Symposium, Minneapolis, 13–15 June 1988P135–142. Publ Rotterdam: A A Balkema, 1988

Limit analysis of quasi-static pyramidal indentation of rock

An approximate analysis of pyramidal indentation into a rock is presented in which the upper bound approach of limit analysis is utilized. The rock is assumed to obey the Mohr-Coulomb yield condition with a small tension cut-off. Two kinematically admissible mechanisms of possible deformation processes, based upon the non-associated flow rule, are adopted for predictions of geometrical variations. The phenomena experimentally observed in rocks, such as dilatancy and deterioration of strength while undergoing deformation, are accounted for approximately in the analysis. The kinematical approach of limit analysis to the problem of block-splitting under pyramidal indentation is also presented in the paper. A number of possible failure modes are examined and those which give the least upper splitting force are shown in detail. Although the particular soluions presented have limited engineering capacity, the method proposed in the paper may find application in solving many rock engineering problems related to drilling, tunnelling, excavating, etc.

Stress fields and subsurface crack propagation of single and multiple rock indentation and disc cutting

Stress fields and subsurface crack propagation of single and multiple rock indentation and disc cutting

Observations of crack growth in hard rock loaded by an indenter

A description is given of the processes of rock fragmentation induced by circular flat-bottomed punches from 5 to 20 mm dia loading orthogonal to the flat surface of cylindrical specimens of Sierra granite of 89 mm dia and confined by a steel belt, and 100 mm cube specimens confined in a biaxial frame. It is found that the rock deforms elastically until the applied punch load exceeds 45% of the maximum load that the rock cna sustain. At loads greater than 45% of the maximum, a crack is initiated around the perimeter of the punch and this crack propagates in the well-known conical Herizian manner. When the crack begins to propagate, the location of its front is reasonably well defined. However, as it extends with continued application of the load, an intense zone of microcracks surrounds and conceals the crack tip. Fractures initiating within this microcracked region propagate to the surface and form rock chips. The effects of punch size and confining stress, orthogonal to the punch, on the rock indentation strength are examined. Some decrease in punching strength with increasing punch size is noted; there is an increase in strength with increasing confining stress. At low confining stress, specimens split by cleavage through the axis of the punch. The theory of elasticity is used to show that this could occur with a punch loading against a semi-infinite rock surface and may therefore be an important phenomenon, not just an artifact of finite laboratory specimens.

Stress fields and subsurface crack propagation of single and multiple rock indentation and disc cutting

Indentation stress fields of one- and two-point loads applied on an ideal elastic half-space are presented. Laboratory observations, although few, are in surprisingly good agreement with long median and cone-type tensile cracks predicted by normal principal stresses. Results indicate that simultaneous loading by multiple indenters offers a possibility partly to control the direction and length of such cracks. This suggests the development of new cutter configurations with a possible increase in efficiency, as compared with present rock boring and rock cutting practice. A simple fracture mechanics calculation of the length of subsurface cracks is performed by applying indentation fracture studies of ceramics. Results demonstrate the influence of material parameters such as fracture surface energy, hardness and elastic constants.

Behaviour of the crushed zone in rock indentation. Technical note : Lindqvist, P A; Lai Hai-Hui Rock Mech Rock Engng, V16, N3, Aug 1983, p199–207

Behaviour of the crushed zone in rock indentation. Technical note : Lindqvist, P A; Lai Hai-Hui Rock Mech Rock Engng, V16, N3, Aug 1983, p199–207

Behaviour of the crushed zone in rock indentation

1. Micro hardness tests of compressed crushed rock indicate that the plastic behaviour of powder of ductile rocks, in this case a marble, resembles that of intact rock, irrespective of the deformation history of the crushed material. The inelastic deformation of such rocks under a bit can then be treated by plasticity theory. 2. The deformation properties of crushed material of granite and sandstone are of a brittle nature. Preserved grain interlocking, shape of grains (original or fractured) and degree of confinement are important factors in this respect. 3. Crushed rock powder can reconsolidate at pressures considerably lower than the contact pressure under a button of a drill or a disc cutter. Compacted crescents are then easily explained by the confined situation and the available pressure under a drill bit. 4. The inelastic zone of brittle rocks is partly kept in place by friction between tool-inelastic zone, inelastic zone-elastic matrix and by internal friction. If this friction can be reduced by some chemical agent, the inelastic zone could perhaps be squeezed out. Penetration will also increase after removal of the inelastic zone between cuts, e. g. by flushing or by some mechanical method.

Analysis of the indentation of a compacting material by a perpectly rough wedge

In previous analyses of wedge or cone indentation of rocks it has been assumed that the rocks were perfectly plastic materials obeying a Coulomb yield condition. The authors have recently shown that the Coulomb yield condition does not necessarily provide an accurate description of rock behavior and have proposed a new yield condition describing the plastic behavior of compacting and workhardening rocks. The present paper describes a method for the analysis of the indentation of rock that is assumed to obey the new yield condition. This analysis is compared with results assuming the rock obeys the Coulomb yield condition, and it is shown that the new yield condition provides improved agreement with the experimental data obtained during indentation of compacting rock.

Rock-Bit Tooth Friction Analysis

Abstract The influence of friction on the force required for an idealized bit tooth to penetrate a "plastic" rock is analyzed. The rock is assumed to obey the Coulomb-Mohr yield criterion and the tooth is represented by a two-dimensional sharp wedge. At the tooth-rock interface the stress field not only satisfies the yield condition by also is such that the ratio of the shearing stress to the normal stress is equal to the coefficient of friction. The solution relates the force on the tooth to the depth of penetration, the tooth angle, the coefficient of friction between the rock and bit tooth and the angle of internal friction and compressive strength of the rock. Introduction Most rocks are brittle at atmospheric pressure but exhibit ductility and increased strength as the confining pressure is increased. In a previous analysis the force required for a bit tooth to penetrate a rigid- plastic rock was computed for two limiting conditions:a perfectly lubricated tooth-rock interface which should correspond to a lower limit for the penetrating force, anda perfectly rough interface which should provide an upper limit. Experimental results for rock indentation under confining pressure have confirmed that the analysis generally predicts upper and lower limits for the penetrating force as a function of rock properties and tooth angle. The present analysis extends the previous work to account for the friction between the rock and the tooth. ANALYSIS The force required for incipient plastic flow of an idealized model of a rock under a bit tooth is computed subjected to the following assumptions. BIT TOOTH sharp two-dimensional wedge (end effects neglected) rigid tooth (elastic deformation of tooth neglected) displacement of tooth is normal to rock surface friction exists between tooth and rock. ROCK semi-infinite, isotropic and homogeneous rigid-plastic (elastic deformation of rock neglected) non-work hardening (horizontal stress-strain curve) Coulomb yield condition (linear Mohr envelope) smooth, flat surface According to the Mohr theory of failure it is assumed that failure occurs when the state of stress is such that the Mohr stress circle becomes tangent to an experimentally determined envelope. The Mohr failure curve is frequently approximated by a straight line and referred to as the Coulomb-Mohr failure criterion. If the rock is under sufficient pressure to be ductile we can use the linear envelope as a yield condition. The Coulomb-Mohr yield criterion is illustrated by Fig. 1 and is expressed by the following equation: where = shear stress on radial slip line = normal stress on radial slip line = cohesive strength= angle of internal friction The slip-line field for the plasticity solution of this problem is shown by Fig. 2. The angle depends on the coefficient of friction at the tooth rock interface. If the solution corresponds to the perfectly lubricated tooth without friction; and if, the solution becomes that for the perfectly rough tooth (see Fig. 1). The two families of slip lines intersect at angles of and each family intersects the direction of maximum compressive stress at angles of. In the triangular region adjacent to the free surface, both families of slip-lines are straight, which indicates a constant-state field in which the mean stress does not vary. Stresses change along the curved slip-lines of the "fan" but are constant along the straight radial lines. SPEJ P. 327^