91 research outputs found
The apparent friction of granular fault gouge in sheared layers
No full textData are presented from a series of experiments on layers of granular quartz gouge in the double-direct-shear geometry at a normal stress of 25 MPa. The apparent friction of a layer shows considerable variability depending on the thickness of the layer and the particle size distribution of the gouge. Measurements of layer thickness during the experiments also show that the layers thin as shearing proceeds. When densification is also admitted, a simple flow law with one adjustable parameter is required to relate the volumetric and shear strain rates. -from Author
Frictional behavior and constitutive modeling of simulated fault gouge
No full textPresents an investigation of the frictional properties and stability of frictional sliding for simulated fault gouge. In these experiments gouge layers (quartz sand) were sheared under saturated drained conditions and at constant normal stress (50-190 MPa) between either rough steel surfaces or Westerly granite surfaces in a triaxial apparatus. Porosity φ was monitored continuously during shear. Measurements indicate that granular gouge exhibits strain hardening and net compaction for shear strains γ >0.5-1.0. For γ sliding occurs at approximately constant shear stress and net compaction from one load/unload cycle to the next ceases. Dilatancy occurs at 1/3 to 1/2 the shear stress required for sliding and d2φ/dγ2 becomes negative at about the peak stress in a given loading cycle, indicating the onset of shear localization. Experiments with an initial gouge layer exhibit velocity strengthening, and initially bare granite surfaces exhibit velocity weakening. Data suggest that slip within unconsolidated granular material, such as some natural fault gouges, is inherently stable. -from Author
Flow-to-Friction Transition in Simulated Calcite Gouge: Experiments and Microphysical Modeling
Get PDFA (micro)physical understanding of the transition from frictional sliding to plastic or viscous flow has long been a challenge for earthquake cycle modeling. We have conducted ring-shear deformation experiments on layers of simulated calcite fault gouge under conditions close to the frictional-to-viscous transition previously established in this material. Constant velocity (v) and v-stepping tests were performed, at 550°C, employing slip rates covering almost 6 orders of magnitude (0.001–300 μm/s). Steady-state sliding transitioned from (strong) v-strengthening, flow-like behavior to v-weakening, frictional behavior, at an apparent “critical” velocity (vcr) of ~0.1 μm/s. Velocity-stepping tests using v < vcr showed “semi-brittle” flow behavior, characterized by high stress sensitivity (“n-value”) and a transient response resembling classical frictional deformation. For v ≥ vcr, gouge deformation is localized in a boundary shear band, while for v < vcr, the gouge is well-compacted, displaying a progressively homogeneous structure as the slip rate decreases. Using mechanical data and post-mortem microstructural observations as a basis, we deduced the controlling shear deformation mechanisms and quantitatively reproduced the steady-state shear strength-velocity profile using an existing micromechanical model. The same model also reproduces the observed transient responses to v-steps within both the flow-like and frictional deformation regimes. We suggest that the flow-to-friction transition strongly relies on fault (micro)structure and constitutes a net opening of transient microporosity with increasing shear strain rate at v < vcr, under normal stress-dependent or “semi-brittle” flow conditions. Our findings shed new insights into the microphysics of earthquake rupture nucleation and dynamic propagation in the brittle-to-ductile transition zone.Applied Geophysics and Petrophysic
Modelling the Dynamics of Iceberg-Soil Interaction during Seabed Gouging
No full textNot only in the Arctic, but also well outside the Arctic, icebergs can be a danger to buried offshore pipelines due to seabed gouging. As it is not economically feasible to bury these pipelines at depths where soil deformations are small, these pipelines are buried at sub-gouge depths where the soil experiences significant plastic deformations. Before considering the sub-gouge soil deformations that are vital to determine optimal pipeline burial depths, a better understanding of dynamic iceberg-soil interaction during seabed gouging is required. In the past, several models have been developed to describe the interaction forces between an iceberg and the soil in front of the iceberg keel based on static soil failure theory or from experimental observations. These models do not fully account for the dynamic behaviour of the iceberg, nor do they consider the corresponding response of the sub-gouge soil. In fact, how the extent of sub-gouge deformations depends on the geotechnical parameters of the seabed is not at all well understood. Therefore, a three-dimensional physics-based model is being developed that describes the transient iceberg-soil interaction (i) based on plasticity theory, (ii) accounting for the dynamic response of the iceberg during gouging and (iii) including the hydrodynamics of the iceberg. This paper presents a two-dimensional version of the iceberg-soil interaction model assuming the iceberg keel to be rigid and plane-strain conditions for the soil. Model predictions are presented and compared for different seabed characteristics
Fusion Engineering (Technical Summary of the 13th Symposium, Knoxville, TN, USA, 2-6 October 1989)
No full textDevelopment of a Tritium Extruder for ITER Pellet Injection
No full textAs part of the International Thermonuclear Experimental Reactor (ITER) plasma fueling development program, Oak Ridge National Laboratory (ORNL) has fabricated a pellet injection system to test the mechanical and thermal properties of extruded tritium. Hydrogenic pellets will be used in ITER to sustain the fusion power in the plasma core and may be crucial in reducing first-wall tritium inventories by a process of "isotopic fueling" in which tritium-rich pellets fuel the burning plasma core and deuterium gas fuels the edge. This repeating single-stage pneumatic pellet injector, called the Tritium-Proof-of-Principle Phase II (TPOP-II) Pellet Injector, has a piston-driven mechanical extruder and is designed to extrude and accelerate hydrogenic pellets sized for the ITER device. The TPOP-II program has the following development goals: evaluate the feasibility of extruding tritium and deuterium-tritium (D-T) mixtures for use in future pellet injection systems; determine the mechanical and thermal properties of tritium and D-T extrusions; integrate, test, and evaluate the extruder in a repeating, single-stage light gas gun that is sized for the ITER application (pellet diameter -7 to 8 mm); evaluate options for recycling propellant and extruder exhaust gas; and evaluate operability and reliability of ITER prototypical fueling systems in an environment of significant tritium inventory that requires secondary and room containment systems. In tests with deuterium feed at ORNL, up to 13 pellets per extrusion have been extruded at rates up to 1 Hz and accelerated to speeds of 1.0 to 1.1 km/s, using hydrogen propellant gas at a supply pressure of 65 bar. Initially, deuterium pellets 7.5 mm in diameter and 11 mm in length were produced-the largest cryogenic pellets produced by the fusion program to date. These pellets represent about a 10% density perturbation to ITER. Subsequently, the extruder nozzle was modified to produce pellets that are almost 7.5-mm right circular cylinders. Tritium and D-T pellets have been produced in experiments at the Los Alamos National Laboratory Tritium Systems Test Assembly. About 38 g of tritium have been utilized in the experiment. The tritium was received in eight batches, six from product containers and two from the Isotope Separation System. Two types of runs were made: those in which the material was only extruded and those in which pellets were produced and fired with deuterium propellant. A total of 36 TZ runs and 28 D-T runs have been made. A total of 36 pure tritium runs and 28 D-T mixture runs were made. Extrusion experiments indicate that both T2 and D-T will require higher extrusion forces than D2 by about a factor of two
Role of fault gouge dilatancy on aseismic deformation transients
Get PDFAuthor Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): B10414, doi:10.1029/2010JB007522.In the vicinity of episodic aseismic transients in several subduction zones, the presence of interstitial fluids and near-lithostatic pore pressure has been proposed to interpret seismic observations of high P to S wave speed ratio and high Poisson's ratio. Under such conditions, fault stabilization by dilatancy-induced suction during increased shear strain rates becomes very efficient. We analyze the frictional and hydraulic conditions for spontaneous transients on a fluid-infiltrated fault including dilatancy and pore compaction in the framework of rate and state friction with a “membrane diffusion” approximation. In both a simplified spectral model and a 2-D Cascadia-like subduction fault model, the fault response is mainly controlled by three nondimensional parameters: (1) W/h*, the along-dip width of the high pore pressure, velocity-weakening fault relative to a characteristic nucleation size, (2) a drainage parameter U, the relative time scales for fluid diffusion and friction evolution, and (3) a dilatancy parameter E, the relative contributions to stress drop from dilatancy and friction evolution. The incorporation of dilatancy enables aseismic transients at much larger values of W/h* than is possible under conditions of constant pore pressure. An analytic estimate of the maximum slip velocity as a function of W/h*, E, and U is derived and agrees reasonably well with the simulation results. The dependence of the properties of modeled transients on the drainage parameter U is similar to that on the dilatancy parameter E. For U (E) less than 1, maximum velocity decreases, while recurrence period remains relatively constant. For U (E) greater than 1, maximum velocity approaches the steady state velocity, and recurrence period approaches the period at neutral stability. In the subduction fault model using gabbro gouge friction properties, the slip per episode and the recurrence period increase with W/h*, generally following the trend defined without dilatancy. The maximum velocity with dilatancy can be several orders of magnitude smaller than that without, in particular for larger values of E and values of W/h* near the no-dilatancy stability limit.This study was supported by a Department of Geosciences Hess postdoctoral fellowship to Yajing Liu at Princeton, USGS-NEHRP award 08HQGR0047 to Allan Rubin that partly supported Y. Liu, and the J. Lamar Worzel Assistant Scientist Fund to Y. Liu at WHOI
Offshore pipelines and ice gouge geohazards: Comparative performance assessment of decoupled structural and coupled continuum models
Get PDFOffshore pipelines in ice environments may be subject to unique geohazards such as seabed ice gouging. These events involve nonlinear processes including large deformations and strains, contact mechanics, and failure mechanisms.
Current pipeline engineering design practice employs decoupled, structural finite element modelling procedures to assess system demand and capacity. The inherent error and uncertainty within this approach drives conservative engineered solutions. Physical modelling and continuum numerical simulation tools complement this engineering framework to improve confidence in predicted outcomes.
The relative performance of engineering models, used in current practice, and numerical simulation tools, including structural and continuum finite element modelling procedures, to predict the deformation and strain response of a buried pipeline subject to an ice gouge event is examined. Refinements to the numerical modeling procedures and establishing a consistent and compatible reference framework for the performance evaluation differentiate this study from others, which are subsets of the current investigation. For the parameter analysis conducted, within an equivalent reference framework, the outcomes demonstrate key factors, including superposition error and directional load decoupling, that influence model error may not be as significant as previously considered. The scope and extent of this outcome is not fully understood and requires further investigations to delineate the significance across a wider parameter range.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
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