Rock Indentation Research Articles

Peridynamic simulations of rock indentation

AbstractThis paper presents an ordinary state‐based peridynamic model to simulate failure in porous rocks and its application in the rock excavation processes. Rock indentation is a fundamental process for mechanical rock fracturing, which is frequently encountered in rock excavation. To model crushing occurring under high triaxial loads observed in the rock indentation experiments, peridynamic model has been extended to consider the pore‐collapse phenomenon. Rock heterogeneity is considered in the simulations by sampling the strength parameters from a probability distribution function. The model is qualitatively validated using the acoustic emission data from the indentation experiments. The developed model is able to form a crushed zone, which serves as the point of initiation for tensile cracks, which split the specimen. Quantitative validation relies on the comparison of the force‐penetration data as well as the indentation pressure‐penetration data obtained from the simulations and experiments performed on Bentheim Sandstone specimens.

The Modelling of Rock Fragmentation Mechanisms by Carbide Buttons Using the 3D Discrete Element Method

Button cutters are commonly used in hard rock drilling because the inserted carbide buttons provide exceptional wear resistance, impact resistance, and high strength in challenging geological formations. One of the most pressing issues in designing a button cutter is to study the rock breaking mechanisms of carbide buttons. In this study, the three-dimensional discrete element method (DEM) was employed to investigate the rock breaking mechanism and cutting performance of five widely used carbide buttons, i.e., spherical, saddle, wedge, conical, and parabolic buttons. The simulation results were compared with laboratory tests to reveal the rock indentation process. The crack propagation pattern, energy dissipation, and damage evolution associated with the force–penetration depth curve were investigated. Tensile damage was the primary determinant for crack propagation and coalescence. By systematically exploring the penetration index, specific energy, and crack propagation characteristics, the conical button had a high rock breaking efficiency when the penetration depth was low, and the saddle button had a high rock breaking efficiency when the penetration depth was high. The findings can provide references for the design of a button cutter.

Rock Indentation Characteristics Induced by Spherical Button Subjected to Dynamic and quasi-static Loads

Rock Indentation Characteristics Induced by Spherical Button Subjected to Dynamic and quasi-static Loads

Investigation on the Tool-Rock Interaction Using an Extended Grain-Based Model

The poor drill-ability of rock in deep hard formation leads to the problems of low rate of penetration (ROP) and high drilling cost. A detailed understanding of the rock-tool interaction is of great significance for improving the rock breaking efficiency and optimizing the cutting parameters. For this purpose, this paper establishes a numerical model of heterogeneous granite which considers the influence of mineral distribution and real grain structure. The fracture behaviors of granite under tool cutting and indentation are investigated subsequently. It is found that there are always four kinds of microcracks in granite, intragranular tensile crack, intragranular shear crack, intergranular tensile crack and intergranular shear crack, among those four crack types, the intergranular tensile crack and intragranular shear crack are dominated, almost no intergranular shear cracks occur. Lateral pressure has no great influence on the volume of cutting chips in rock cutting, but is the direct factor that affects the rock-breaking efficiency in rock indentation. The initiation and propagation of radial cracks will be obviously inhibited in large lateral pressure, and the number of radial cracks will decrease sharply, resulting in low rock-breaking efficiency. Lateral pressure seems to have little effect on the indentation force, the possible reason for this lateral pressure insensitivity is because the far-field lateral pressure has less influence on the indenter tip around field than the existence of free surface. The research results can provide the basis for improving granite rock-breaking efficiency and optimizing rock-breaking tools to a certain extent.

Cavity expansion analysis of normal indention of rocks with lateral confinement

Indentation of rocks with lateral confinement is strongly related to rock fragmentation in drilling, mechanized tunnelling, and mining. Yet, the interpretation of the results and its transfer to use in practice is complicated by both the limited specimen size and boundary conditions associated with the application of lateral confinement. In this study, a new semi-analytical solution for the problem of a finite-cavity expansion in rock specimens of finite radial extent is present for the analysis of rock indentation with lateral confinement. To account for possible strain hardening/softening effect, an extended linear strain hardening/softening, i.e. constant plastic tangential modulus, Drucker-Prager model with non-associated flow rule is used to describe the stress-strain behavior of rocks. The model predictions, valid up to crack initiation, agree well with experimental observations that peak indentation pressure increases with lateral confinement, indicating that for a correct interpretation of results, the ratio between specimen size and the tip width of a truncated indenter as well as the boundary conditions shall be considered. The results point to the need for further development of the cavity expansion model by incorporating more features to model the realistic rock behavior and for more experimental studies with lateral confinement using advanced testing technique to monitor the failure process.

Experimental investigation on the PDC cutter penetration efficiency in high-temperature granite

Hot dry rock (HDR) is a renewable and clean energy resource, its efficient exploration has attracted widespread attention from scholars. However, the rock encountered during drilling is characterized by poor drillability, high abrasiveness and great hardness, which aggravates the wear of bits and increases drilling costs. To address such an issue, much research has been conducted to investigate the failure mechanism of high-temperature rocks. But most of them focused on the rock cutting process by PDC cutters and the penetration of rocks was ignored. In the present study, a series of experiments were performed to determine the penetration efficiency of PDC cutters under different temperatures and back rake angles. The penetration force and 3D topography of penetration pits were measured to analyze rock failure characteristics. Besides, uniaxial compressive experiments were carried out to study the effects of temperature on rock properties. The results indicate that as the rock is heated from 25 to 240 ℃, its properties change slightly. Analogous rock failure characteristics can be found in rock indentation process, where chipping and crushing of rocks, on following another, are presented, respectively. Moreover, the occurrence of chipping is the major contributor to the increase in penetration efficiency. The further increase in rock temperature (greater than 360 ℃) results in a sharp decrease in the strength of rocks, which is accompanied by the enhancement of rock plasticity. At his moment, the plastic failure dominates rock indentation process under different back rake angles. The induced horizontal force cannot promote brittle failure and the penetration efficiency is hard to be improved, significantly affecting the performance of conventional PDC bits in high-temperature formations. Based on the results, the low penetration efficiency at high temperatures is a major obstacle that restricts the exploration of HDR. Increasing the cutting depth should be an important target in optimizing drilling parameters and the arrangement of PDC bits.

Influence of brittleness and confining stress on rock cuttability based on rock indentation tests

Influence of brittleness and confining stress on rock cuttability based on rock indentation tests

Quantification of dynamic damage and breakage in granite under confined indentation

Understanding of indentation rock damage and breakage under confining stress is necessary for underground engineering applications, as both loading rate and in-situ stress can affect the failure behaviour and mechanism. A triaxial apparatus capable of applying both static lateral confinement and dynamic loading is utilised to experimentally investigate the influence of both loading rate and confinement on rock indentation failure. The force-displacement relationships and energy evolution were determined for different dynamic loading and confinement conditions, while digital reconstruction of indented rock samples was achieved for crater depth and damage volume measurements. The failure mechanisms at microscale are studied using the optical microscopy and Synchrotron-based X-ray Computed Tomography (CT). The relationship between the incident energy and indentation force is correlated with the crater depth and volume of rock removed. The crater depth increases linearly with increasing force and incident energy, while the volume increases exponentially. The biaxial confinement controls the propagation of radial cracks, which propagate in the direction of major confining stress, while impact velocity dominates the crater depth and lateral crack formation beneath the crater. The formation of chipping during indentation is reflected on both the force-displacement relationships and X-ray CT measurements. Different minerals within the rock also affect the propagation and coalescence of cracks, harder mineral such as quartz has better resistance to crushing, whereas biotite deflects and terminates cracks.

Analyses and predictions of rock cuttabilities under different confining stresses and rock properties based on rock indentation tests by conical pick

The rock indentation tests by a conical pick were conducted to investigate the rock cuttability correlated to confining stress conditions and rock strength. Based on the test results, the regression analyses, support vector machine (SVM) and generalized regression neural network (GRNN) were used to find the relationship among rock cuttability, uniaxial confining stress applied to rock, uniaxial compressive strength (UCS) and tensile strength of rock material. It was found that the regression and SVM-based models can accurately reflect the variation law of rock cuttability, which presented decreases followed by increases with the increase in uniaxial confining stress and the negative correlation to UCS and tensile strength of rock material. Based on prediction models for revealing the optimal stress condition and determining the cutting parameters, the axial boom roadheader with many conical picks mounted was satisfactorily utilized to perform rock cutting in hard phosphate rock around pillar.

Failure characteristics of rocks subjected to PDC cutter indentation

Indentation of rocks by the Polycrystalline Diamond Compact (PDC) cutter has great impacts on the cutting depth. However, a vast majority of previous studies only focused on the rock cutting process. Further insight into the failure characteristics of rocks subjected to PDC cutter indentation is required to optimize the design of PDC bits. In this paper, a set of experiments were conducted to investigate the effects of back rake angles and mechanical properties of rocks on the PDC cutter indentation. Penetration forces and Acoustic Emission (AE) signals were captured to determine rock failure modes (brittle failure or plastic failure), and the three-dimensional profilometer was used to identify the morphology features of penetration pits. Besides, the penetration efficiency and brittle failure index were defined to evaluate the crushing characteristics of rocks during penetration. The results show that plastic failure dominates the indentation process as the back rake angle is smaller than 20°, which significantly limits the growth of penetration depth. Based on the penetration force and the number of AE hit counts, chipping occurs in the indentation process when using the PDC cutter with greater back rake angles (>20°). Such phenomenon can be further confirmed by topography features of penetration pits. Moreover, the position of brittle failure moves from the front to the back of the cutter as the back rake angle exceeds 40°. Compared with granite and sandstone, chipping occurs more easily in the marble, indicating a higher brittle failure index. In conclusion, back rake angle has great effects on the rock cutting process and the indentation of rocks. Our current study is expected to provide theoretical guidance for the cutter layout in PDC bits.

Experimental investigation of rock cuttability based on rock indentation tests by a conical pick

Stress conditions and rock properties significantly influence rock breakage. The rock indentation tests by a conical pick were conducted to investigate the rock cuttability corelated to confining stress conditions and rock strengths. Based on the test results, the regression analyses were used to find the relationship among rock cuttability, uniaxial confining stress applied to rock, uniaxial compressive strength (UCS) and tensile strength of rock material. It was found that the regression models can accurately reflect the variation law of rock cuttability, which presents decreases followed by increases with increases in uniaxial confining stress and the negative correlation to UCS and tensile strength of rock material. The established models are suitable for guiding the rock cutting by conical pick and will provide useful information for mechanized mining in deep hard rock.

On the Failure Mechanism of Brittle Granite in 2-D Rock Indentation

Abstract Rock indentation is one of the main mechanical rock breaking approaches, in which the indenter moves perpendicular to the rock surface. A detailed understanding of the failure mechanism of brittle granite is of great importance to achieve high efficiency in rock breaking and to optimize the cutting parameters during deep hard formation drilling. In the present study, a rock indentation simulation model of Ya’an granite is carried out, using the grain-based model (GBM), which is based on the discrete element method of Particle Flow Code (PFC2D), to reproduce the crack initiation, propagation, and coalescence processes in intact rock. This study also includes an experimental program to examine the crack initiation, propagation, and the acoustic emissions in rock specimens. The results show that the microcracks generate gradually in intact rock with respect to penetration depth, and some of the microcracks coalesce with the crack number increasing, eventually resulting in the macrofracture formation. Four types of microcracks will generate during the indentation: intragrain tensile crack, intragrain shear crack, intergrain tensile crack, and intergrain shear crack. Among these four types of cracks, the intergrain tensile crack and intragrain shear crack are the two main crack types generated in indentation, especially the intergrain tensile crack. The rock failure mechanisms are different with the variation of grain size, and the lateral fragmentation of rock is restricted because of the chip hold-down effect of hydrostatic pressure. This study leads to an enhanced understanding of rock breaking mechanisms of Ya’an granite and provides the basis for improving the rock excavation machine design.

Semi-analytical contact model for analyzing rock fragmentation induced by an indenter

Rock indentation is the basic process in drilling and excavation from the mechanical points of view. A better understanding of the fundamental mechanism of rock-indenter interaction is of great significance. This paper proposes a semi-analytical model (SAM) to analyze rock fragmentation processes induced by indentation loads. In SAM, the analytical expressions of the stress and strain relationship of each two elements are obtained based on the theory of continuum mechanics. The numerical approaches including the conjugate gradient methods, fast Fourier transform, and fix-point iteration are applied to solve the unknowns of the model. In the proposed model, rocks are considered as elasto-brittle, and the damage threshold is described by the Mohr-Coulomb failure criterion. Also, a fracture element is treated as an inhomogeneity and tackled by the equivalent inclusion method. The present model can precisely capture the coupling between rock fragmentation and contact status. Thus, the developments of the crushed zone and cracked zone can be duplicated well. To reveal the potential applications of the model, the effects of confining pressure, rock inhomogeneity, and indenter geometry on rock indentation are also carefully investigated.

Experimental study on rock indentation using infrared thermography and acoustic emission techniques

Experimental study on rock indentation using infrared thermography and acoustic emission techniques

The rock fragmentation mechanism and plastic energy dissipation analysis of rock indentation

Based on theories of rock mechanics, rock fragmentation, mechanics of elasto-plasticity, and energy dissipation etc., a method is presented for evaluating the rock fragmentation efficiency by using plastic energy dissipation ratio as an index. Using the presented method, the fragmentation efficiency of rocks with different strengths (corresponding to soft, intermediately hard and hard ones) under indentation is analyzed and compared. The theoretical and numerical simulation analyses are then combined with experimental results to systematically reveal the fragmentation mechanism of rocks under indentation of indenter. The results indicate that the fragmentation efficiency of rocks is higher when the plastic energy dissipation ratio is lower, and hence the drilling efficiency is higher. For the rocks with higher hardness and brittleness, the plastic energy dissipation ratio of the rocks at crush is lower. For rocks with lower hardness and brittleness (such as sandstone), most of the work done by the indenter to the rocks is transferred to the elastic and plastic energy of the rocks. However, most of such work is transferred to the elastic energy when the hardness and the brittleness of the rocks are higher. The plastic deformation is small and little energy is dissipated for brittle crush, and the elastic energy is mainly transferred to the kinetic energy of the rock fragment. The plastic energy ratio is proved to produce more accurate assessment on the fragmentation efficiency of rocks, and the presented method can provide a theoretical basis for the optimization of drill bit and selection of well drilling as well as for the selection of the rock fragmentation ways.

The investigation of rock indentation simulation based on discrete element method

Rock indentation is widely encountered in rock engineering, such as oil & gas drilling process. The rock indentation represents the fundamental process for mechanical rock breaking. Therefore, it is necessary to research the failure mechanism during the rock indentation process. For this purpose, the Uniaxial Compressive Strength (UCS) and Brazilian Tensile Strength (BTS) tests are performed to calibrate the relations between micro-properties and macro-properties of the rock specimens. The rock indentation process and crack propagation with the effects of lateral pressure, hydraulic pressure, ledge, wedge angle and joint are researched by PFC2D in this paper. The results show that: with the indenter penetrating into rock, the sub-vertical crack is formed from the damaged zone and it will extend to bottom edge of the rock at last; the initiation and propagation of the sub-vertical crack is mostly driven by the tensile contact force. The development of sub-vertical crack and damaged zone are restrained with increasing lateral pressure, the lateral pressure increases led to an increase in the critical penetration depth and the size of the damaged zone decreases and its shape flattens with the lateral pressure increasing. On the contrary, the development of sub-vertical crack and damaged zone are promoted with increasing hydraulic pressure. With the wedge angle increases the size of crushed zone underneath the indenter increases, it promotes the formation of sub-vertical crack; larger wedge angle causes a larger indentation force. The existence of a ledge leads to crack initiation and propagation towards the free surface and the presence of the joint also promotes crack initiation and propagation towards the joint; when the crack propagates to the joint, the crack will no longer propagate towards the intact rock mass but along the joint.

The effects of joints on rock fragmentation by TBM cutters using General Particle Dynamics

In this paper, a novel meshfree numerical method known as General Particle Dynamics (GPD) is proposed to reveal the mechanism of the rock fragmentation by TBM cutters. Rock fragmentation by two cutters in consecutive joints rock is investigated using GPD. The numerical results obtained from GPD are in good agreement with the field observed results. Moreover, the effects of the length of intermittent joints on rock fragmentation by two cutters are investigated using GPD. It is found from the numerical results that the length of intermittent joints can significantly influence the crack initiation and propagation and coalescence as well as the fragmentation pattern, and hence affect the penetration rate of the TBM. It indicates that GPD has the potential in simulating rock indentation and fragmentation by TBM cutters.

Numerical simulation of rock fragmentation mechanisms subject to wedge penetration for TBMs

The rock fragmentation mechanism and failure process induced by a wedge indenter for TBMs are numerically simulated utilizing the discrete element method (DEM) in the present study. A novel clustered assembly approach with irregular boundaries for grain-based brittle rock is incorporated into the particle flow code (PFC) to calibrate the macro-responses such as the low tensile to compressive strength ratio observed in laboratory tests. The simulation results are compared with laboratory tests which were conducted to reveal the rock indentation process and rock-tool interactions. The numerical analysis can be divided to two main parts. Part one is to investigate the crack propagation and damage evolution at meso-scale associated with the force–indentation curve. The stress field distributions with respect to indentation time are depicted. It is revealed that the tensile cracks are resulted from the chipping process and the shear cracks are induced by the crushing force with the orientation angle ranging from 45° to 55°, which is in good agreement with macro-observations. In Part two, the fragmentation efficiency and penetration rate under different cases are analyzed with several parameters such as the wedge angle, the confinement stress and the penetration velocity. Meanwhile, the blunt indenter is taken as a special case for analysis and comparison of the fragmentation mechanism.

Performance Assessment of Hard Rock TBM and Rock Boreability Using Punch Penetration Test

Rock indentation tests are often called punch penetration tests and are known to be related to penetration rates of drilling equipment and hard rock tunnel boring machines (TBMs). Various indices determined from analysis of the force-penetration plot generated from indentation tests have been used to represent the drillability, boreability, and brittleness of rocks. However, no standard for the punch penetration test procedure or method for calculating the related indices has been suggested or adopted in the rock mechanics community. This paper introduces new indices based on the punch test to predict the performance of hard rock TBMs. A series of punch tests was performed on rock specimens representing six rock formations in Korea with different dimensions, i.e., the core specimens had different lengths and diameters. Of the indices obtained from the punch tests, the peak load index and mean load index showed good correlations with the cutting forces measured in full-scale linear cutting machine tests on the same rock types. The indices also showed good linear correlations with the ratio of uniaxial strength to Brazilian tensile strength, which indicates the brittleness of rock. The scale effect of using core specimens was investigated, and a preferred dimension for the punch test specimens is proposed. This paper also discusses the results of the punch test and full-scale rock cutting tests using LCM. The results of this study confirm that the proposed indices from the punch tests can be used to provide a reliable prediction of the cutting forces that act on a disc cutter. The estimated cutting forces can then be used for optimization of cutter-head design and performance prediction of hard rock TBMs.

Brittle and Ductile Behavior in Deep-Seated Landslides: Learning from the Vajont Experience

This paper analyzes the mechanical behavior of the unstable Mt. Toc slope before the 1963 catastrophic collapse, considering both the measured data (surface displacements and microseismicity) and the updated geological model of the prehistoric rockslide. From February 1960 up to 9 October 1963, the unstable mass behaved as a brittle–ductile ‘mechanical system,’ characterized by remarkable microseismicity as well as by considerable surface displacements (up to 4–5 m). Recorded microshocks were the result of progressive rock fracturing of distinct resisting stiff parts made up of intact rock (indentations, undulations, and rock bridges). The main resisting stiff part was a large rock indentation located at the NE extremity of the unstable mass that acted as a mechanical constraint during the whole 1960–1963 period, inducing a progressive rototranslation toward the NE. This large constraint failed in autumn 1960, when an overall slope failure took place, as emphasized by the occurrence of the large perimetrical crack in the upper slope. In this circumstance, the collapse was inhibited by a reblocking phenomenon of the unstable mass that had been previously destabilized by the first reservoir filling. Progressive failure of localized intact rock parts progressively propagated westwards as a consequence of the two further filling–drawdown cycles of the reservoir (1962 and 1963). The characteristic brittle–ductile behavior of the Vajont landslide was made possible by the presence of a very thick (40–50 m) and highly deformable shear zone underlying the upper rigid rock mass (100–120 m thick).