Ostanin I.,University of Minnesota |
Ballarini R.,University of Minnesota |
Potyondy D.,Itasca Consulting Group Inc. |
Dumitrica T.,University of Minnesota
Journal of the Mechanics and Physics of Solids | Year: 2013
Mesoscale simulation techniques are becoming increasingly important due to the interest in complex mechanical problems involving nanomaterials. We propose applying the established macroscopic modeling concept of distinct spherical elements down to the mesoscale to simulate mechanical behavior of carbon nanotube systems. Starting from a microscopic description, the important interactions are encapsulated into two types of contact models that act simultaneously. Each individual nanotube is coarse-grained into a chain of spherical elements interacting by parallel-bonded contacts, representing the short-ranged covalent bonding. An anisotropic van der Waals model with aligning moments acts at the contact between elements located in different tubes to represent the long-ranged interactions. The promising potential of the proposed methodology to model large scale carbon nanotube assemblies is illustrated with several examples, including self-folding of individual nanotubes, mechanical testing of nanotube ropes, self-assembly of a high-porosity nanotube paper, and mechanical testing of a low-porosity nanotube paper. © 2012 Elsevier Ltd.
Onederra I.A.,University of Queensland |
Furtney J.K.,Itasca Consulting Group Inc. |
Sellers E.,African Explosives Ltd |
Iverson S.,U.S. National Institute for Occupational Safety and Health
International Journal of Rock Mechanics and Mining Sciences | Year: 2013
This paper presents one of the latest developments in the blasting engineering modelling field-the Hybrid Stress Blasting Model (HSBM). HSBM includes a rock breakage engine to model detonation, wave propagation, rock fragmentation, and muck pile formation. Results from two controlled blasting experiments were used to evaluate the code's ability to predict the extent of damage. Results indicate that the code is capable of adequately predicting both the extent and shape of the damage zone associated with the influence of point-of-initiation and free-face boundary conditions. Radial fractures extending towards a free face are apparent in the modelling output and matched those mapped after the experiment. In the stage 2 validation experiment, the maximum extent of visible damage was of the order of 1.45. m for the fully coupled 38-mm emulsion charge. Peak radial velocities were predicted within a relative difference of only 1.59% at the nearest history point at 0.3. m from the explosive charge. Discrepancies were larger further away from the charge, with relative differences of -22.4% and -42.9% at distances of 0.46. m and 0.61. m, respectively, meaning that the model overestimated particle velocities at these distances. This attenuation deficiency in the modelling produced an overestimation of the damage zone at the corner of the block due to excessive stress reflections. The extent of visible damage in the immediate vicinity of the blasthole adequately matched the measurements. © 2012 Elsevier Ltd.
Damjanac B.,Itasca Consulting Group Inc. |
Fairhurst C.,Itasca Consulting Group Inc.
Rock Mechanics and Rock Engineering | Year: 2010
The mechanical response of brittle rock to long-duration compression loading is of particular concern in underground disposal of nuclear waste, where radio-nuclides must be isolated from the biosphere for periods of the order of a million years. Does the strength decrease without limit over such time, or is there, for some rock types, a lower "threshold" strength below which the rock will cease to deform? This paper examines the possibility of such a threshold in silicate crystalline rocks from several perspectives, including: (1) interpretation of the results of short-term creep tests on rock; (2) numerical analysis of the effect of decrease in fracture toughness due to stress corrosion on the strength of a crystalline rock; and (3) evidence from plate tectonics, and observations of in situ rock stress in granite quarries. The study concludes that there is clear evidence of threshold strength. The threshold is of the order of 40% of the unconfined compressive strength or higher for laboratory specimens under unconfined compressive loading, and increases rapidly in absolute value with confinement. Field evidence also leads to the conclusion that the long-term strength of crystalline rock in situ is of comparable magnitude to the laboratory value. © 2010 Springer-Verlag.
Potyondy D.O.,Itasca Consulting Group Inc.
46th US Rock Mechanics / Geomechanics Symposium 2012 | Year: 2012
The bonded-particle model (BPM) consisting of parallel-bonded disks or spheres suffers from the limitation that if one matches the unconfined- compressive strength (qu) of a typical hard rock, then the direct-tension strength (σt) of the model will be too large. This limitation can be overcome in two dimensions by introducing a polygonal grain structure to provide rotational restraint arising from inter-granular interlock. The flat-jointed BPM (in which each disk-disk contact simulates the behavior of a finite-length interface between two disks with locally flat notional surfaces such that even a fully broken interface continues to resist relative rotation) provides such a structure and supersedes the parallel-bonded BPM by mimicking more of the micro- and macro-mechanisms associated with rock damage. Copyright 2012 ARMA, American Rock Mechanics Association.
Kwok C.-Y.,Itasca Consulting Group Inc. |
Kwok C.-Y.,University of Cambridge |
Bolton M.D.,University of Cambridge
Geotechnique | Year: 2010
Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each intergranular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range 20.8 to 21; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared quantitatively with the test data of soils from the literature. The effects of activation energy are considered in relation to the different magnitudes of creep encountered in sands and clays.
Zhang F.,Georgia Institute of Technology |
Damjanac B.,Itasca Consulting Group Inc. |
Huang H.,Georgia Institute of Technology
Journal of Geophysical Research: Solid Earth | Year: 2013
The coupled displacement process of fluid injection into a dense granular medium is investigated numerically using a discrete element method (DEM) code PFC2D® coupled with a pore network fluid flow scheme. How a dense granular medium behaves in response to fluid injection is a subject of fundamental and applied research interests to better understand subsurface processes such as fluid or gas migration and formation of intrusive features as well as engineering applications such as hydraulic fracturing and geological storage in unconsolidated formations. The numerical analysis is performed with DEM executing the mechanical calculation and the network model solving the Hagen-Poiseuille equation between the pore spaces enclosed by chains of particles and contacts. Hydromechanical coupling is realized by data exchanging at predetermined time steps. The numerical results show that increase in the injection rate and the invading fluid viscosity and decrease in the modulus and permeability of the medium result in fluid flow behaviors displaying a transition from infiltration-governed to infiltration-limited and the granular medium responses evolving from that of a rigid porous medium to localized failure leading to the development of preferential paths. The transition in the fluid flow and granular medium behaviors is governed by the ratio between the characteristic times associated with fluid injection and hydromechanical coupling. The peak pressures at large injection rates when fluid leakoff is limited compare well with those from the injection experiments in triaxial cells in the literature. The numerical analysis also reveals intriguing tip kinematics field for the growth of a fluid channel, which may shed light on the occurrence of the apical inverted-conical features in sandstone and magma intrusion in unconsolidated formations. © 2013. American Geophysical Union. All Rights Reserved.
Damjanac B.,Itasca Consulting Group Inc. |
Cundall P.,Itasca Consulting Group Inc.
Computers and Geotechnics | Year: 2016
The Distinct Element Method (DEM) represents a rock mass as an assembly of blocks (polygonal or polyhedral). Contacts between blocks correspond to discontinuities (i.e., fractures or joints) that can exhibit non-linear mechanical behavior, including slip and opening. If flow in rock fracture is approximated using the lubrication equation, coupled hydro-mechanical DEM models can be used for simulation of rock mass treatment by fluid injection. However, this approach has a limited capability for simulating fracture propagation. The synthetic rock mass (SRM) concept overcomes this limitation. In SRM, the bonded particle model (BPM), which is an assembly of circular or spherical particles bonded to each other, represents deformation and damage of intact rock. If pre-existing discontinuities are represented in the BPM, the resulting model, referred to as SRM, has the capability of simulating hydraulic fracturing in naturally fractured reservoirs. The model delivers a pattern of hydraulic fractures that evolves in response to both intact rock fracturing and sliding and opening of pre-existing joints. © 2015 Elsevier Ltd.
King M.S.,Imperial College London |
Pettitt W.S.,Itasca Consulting Group Inc. |
Haycox J.R.,Applied Seismology Consultants Ltd |
Young R.P.,University of Toronto
Geophysical Prospecting | Year: 2012
A polyaxial (true-triaxial) stress-loading system, developed originally for determining all nine components of P- and S-wave velocities and attenuation and fluid permeability for 50.8 mm-side cubic rock specimens tested to failure, has been modified to permit the measurement of acoustic emission events associated with the failure process. Results are reported for Crosland Hill sandstone tested to failure under loading conditions leading to the formation of sets of aligned microcracks, achieved by maintaining the minor principal stress at a low value while increasing the two other principal stresses until failure of the rock. An ultrasonic survey associated with the test has been employed to map the transversely-isotropic velocity structure created by through-going parallel fractures resulting from the sets of aligned microcracks. This velocity structure has then been employed to locate acoustic emission events recorded during the test by four acoustic emission sensors located in each of the six specimen loading platens. A selection of acoustic emission events associated with one of the fractures has been processed for moment tensor analysis information, in order to determine the source type and orientation of microcracking as the fracture grows. The mechanisms indicate tensile behaviour during initial fracture propagation. Shear failure, however, appears to dominate as the fracture finally approaches the opposite face of the cubic specimen. The work presented here has, in part, led to the development of new rock testing systems and geophysical monitoring and processing technologies that will enable laboratory study of rock behaviour under conditions better resembling those experienced in situ. © 2011 European Association of Geoscientists & Engineers.
Potyondy D.O.,Itasca Consulting Group Inc.
Geosystem Engineering | Year: 2015
We generalize our view of a bonded-particle model (BPM) to consist of a base material (that is a packed assembly of rigid grains joined by deformable and breakable cement at grain–grain contacts) to which larger-scale joints can be added and whose mechanical behavior is simulated by the distinct-element method using the two- and three-dimensional discontinuum programs PFC2D and PFC3D. The micromechanical processes that control brittle fracture, and thus, should inform any micromechanical theory or model, are summarized. The rich variety of microstructural models that can be produced by the bonded-particle modeling methodology are described and classified with respect to their microstructural and larger-scale features. These models provide a wide range of rock behaviors that encompass both compact and porous rock at both an intact and rock-mass scale, and examples are provided of how BPMs are being used to model rock at these scales. The examples include an intact anisotropic material that may swell and contract in response to changes in saturation, the behavior of two alternative BPMs that can match both the uniaxial and tensile strengths of compact rock and the embedding of an intact BPM within a larger continuum model to study fracturing around a gold-mine stope in quartzite. © 2014 Taylor & Francis.
Varun,Itasca Consulting Group Inc. |
Assimaki D.,Georgia Institute of Technology |
Shafieezadeh A.,Georgia Institute of Technology
Soil Dynamics and Earthquake Engineering | Year: 2013
We present a macroelement for soil-structure interaction analyses of piles in liquefiable soils, which captures efficiently the fundamental mechanisms of saturated granular soil behavior. The mechanical model comprises a nonlinear Winkler-type model that accounts for soil resistance acting along the circumference of the pile, and a coupled viscous damper that simulates changes in radiation damping with increasing material nonlinearity. The formulation for a gap element is also proposed to account for formation of gap at pile-soil interface. Validation of the macroelement is conducted using full-scale forced vibration test data from a blast-induced liquefaction test bed, and centrifuge data for seismic loading of piles with superstructure. The macroelement parameters are estimated as a function of the measured soil properties and the level of effective stress. Predictions of bending moments and acceleration time histories at the top of pile and the superstructure are found to be in good agreement with both the full and the model scale data. A comparison with predictions from alternative established methodologies is also presented. © 2012 Elsevier Ltd.