Showing papers on "Slip (materials science) published in 2009"

Mixed Convection Boundary-Layer Flow Near the Stagnation Point on a Vertical Surface in a Porous Medium: Brinkman Model with Slip

Abstract: The steady boundary-layer flow near the stagnation point on an impermeable vertical surface with slip that is embedded in a fluid-saturated porous medium is investigated. Using appropriate similarity variables, the governing system of partial differential equations is transformed into a system of ordinary differential equations. This system is then solved numerically. The features of the flow and the heat transfer characteristics for different values of the governing parameters, namely, the Darcy–Brinkman, Γ, mixed convection, λ, and slip, γ, parameters, are analysed and discussed in detail for the cases of assisting and opposing flows. It is found that dual solutions exist for assisting flows, as well as those usually reported in the literature for opposing flows. A stability analysis of the steady flow solutions encountered for different values of the mixed convection parameter λ is performed using a linear temporal stability analysis. This analysis reveals that for γ = 0 (slip absent) and Γ = 1 the lower solution branch is unstable while the upper solution branch is stable.

Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchuan earthquake

Abstract: The disastrous 12 May 2008 Wenchuan earthquake in China took the local population as well as scientists by surprise Although the Longmen Shan fault zone—which includes the fault segments along which this earthquake nucleated—was well known, geologic and geodetic data indicate relatively low (<3 mm yr−1) deformation rates Here we invert Global Positioning System and Interferometric Synthetic Aperture Radar data to infer fault geometry and slip distribution associated with the earthquake Our analysis shows that the geometry of the fault changes along its length: in the southwest, the fault plane dips moderately to the northwest but becomes nearly vertical in the northeast Associated with this is a change in the motion along the fault from predominantly thrusting to strike-slip Peak slip along the fault occurs at the intersections of fault segments located near the towns of Yingxiu, Beichuan and Nanba, where fatalities and damage were concentrated We suggest that these locations represent barriers that failed in a single event, enabling the rupture to cascade through several fault segments and cause a major moment magnitude (Mw) 79 earthquake Using coseismic slip distribution and geodetic and geological slip rates, we estimate that the failure of barriers and rupture along multiple segments takes place approximately once in 4,000 years The devastating Wenchuan earthquake in 2008 struck along a fault zone that showed low rates of deformation Analysis of GPS and InSAR data suggests that, as structural barriers failed during a single earthquake, the rupture cascaded across multiple fault segments, which may explain the high magnitude of the event

Fault zone fabric and fault weakness

Abstract: Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

Interfacial water at hydrophobic and hydrophilic surfaces: slip, viscosity, and diffusion.

Abstract: The dynamics and structure of water at hydrophobic and hydrophilic diamond surfaces is examined via non-equilibrium Molecular Dynamics simulations. For hydrophobic surfaces under shearing conditions, the general hydrodynamic boundary condition involves a finite surface slip. The value of the slip length depends sensitively on the surface water interaction strength and the surface roughness; heuristic scaling relations between slip length, contact angle, and depletion layer thickness are proposed. Inert gas in the aqueous phase exhibits pronounced surface activity but only mildly increases the slip length. On polar hydrophilic surfaces, in contrast, slip is absent, but the water viscosity is found to be increased within a thin surface layer. The viscosity and the thickness of this surface layer depend on the density of polar surface groups. The dynamics of single water molecules in the surface layer exhibits a similar distinction: on hydrophobic surfaces the dynamics is purely diffusive, while close to a h...

Micromechanical Model for Deformation in Solids with Universal Predictions for Stress-Strain Curves and Slip Avalanches

Abstract: A basic micromechanical model for deformation of solids with only one tuning parameter (weakening $\ensuremath{\epsilon}$) is introduced. The model can reproduce observed stress-strain curves, acoustic emissions and related power spectra, event statistics, and geometrical properties of slip, with a continuous phase transition from brittle to ductile behavior. Exact universal predictions are extracted using mean field theory and renormalization group tools. The results agree with recent experimental observations and simulations of related models for dislocation dynamics, material damage, and earthquake statistics.

Maximizing the giant liquid slip on superhydrophobic microstructures by nanostructuring their sidewalls.

Abstract: In an effort to maximize the liquid slip on superhydrophobic surfaces, we investigate the role of the nanoscale roughness on microscale structures by developing well-defined micro−nano hierarchical...

The Spring Loaded Inverted Pendulum as the Hybrid Zero Dynamics of an Asymmetric Hopper

Abstract: A hybrid controller that induces provably stable running gaits on an asymmetric spring loaded inverted pendulum (ASLIP) is developed. The controller acts on two levels. On the first level, continuous within-stride control asymptotically imposes a (virtual) holonomic constraint corresponding to a desired torso posture, and creates an invariant surface on which the two-degree-of-freedom restriction dynamics of the closed-loop system (i.e., the hybrid zero dynamics) is diffeomorphic to the center-of-mass dynamics of a spring loaded inverted pendulum (SLIP). On the second level, event-based control stabilizes the closed-loop hybrid system along a periodic orbit of the SLIP dynamics. The controller's performance is discussed through comparison with a second control law that creates a one-degree-of-freedom non-compliant hybrid zero dynamics. Both controllers induce identical steady-state behaviors (i.e., periodic solutions). Under transient conditions, however, the controller inducing a compliant hybrid zero dynamics based on the SLIP accommodates significantly larger disturbances, with less actuator effort, and without violation of the unilateral ground force constraints.

Analysis of viscous flow due to a stretching sheet with surface slip and suction

Abstract: The viscous flow due to a stretching sheet with slip and suction is studied. The Navier–Stokes equations admit exact similarity solutions. For two-dimensional stretching a closed-form solution is found and uniqueness is proved. For axisymmetric stretching both existence and uniqueness are shown. The boundary value problem is then integrated numerically.

Earthquake ruptures with thermal weakening and the operation of major faults at low overall stress levels

Abstract: We model ruptures on faults that weaken in response to flash heating of microscopic asperity contacts (within a rate-and-state framework) and thermal pressurization of pore fluid. These are arguably the primary weakening mechanisms on mature faults at coseismic slip rates, at least prior to large slip accumulation. Ruptures on strongly rate-weakening faults take the form of slip pulses or cracks, depending on the background stress. Self-sustaining slip pulses exist within a narrow range of stresses: below this range, artificially nucleated ruptures arrest; above this range, ruptures are crack-like. Natural earthquakes will occur as slip pulses if faults operate at the minimum stress required for propagation. Using laboratory-based flash heating parameters, propagation is permitted when the ratio of shear to effective normal stress on the fault is 0.2–0.3; this is mildly influenced by reasonable choices of hydrothermal properties. The San Andreas and other major faults are thought to operate at such stress levels. While the overall stress level is quite small, the peak stress at the rupture front is consistent with static friction coefficients of 0.6–0.9. Growing slip pulses have stress drops of ∼3 MPa; slip and the length of the slip pulse increase linearly with propagation distance at ∼0.14 and ∼30 m/km, respectively. These values are consistent with seismic and geologic observations. In contrast, cracks on faults of the same rheology have stress drops exceeding 20 MPa, and slip at the hypocenter increases with distance at ∼1 m/km.

Interplate seismogenic zones along the Kuril–Japan trench inferred from GPS data inversion

Abstract: Stress accumulation between earthquakes results from slip that is insufficient to fully accommodate plate movement. An inverse analysis of GPS data from the Kuril–Japan trench reveals a trench-parallel belt of stress accumulation with six peaks in the depth range of 10–40 km, suggesting potential source regions for future earthquakes. In the subduction zones around Japan, where four plates interact with one another, large earthquakes have occurred repeatedly1. These interplate earthquakes are part of the process of tectonic stress accumulation and release that is driven by relative plate motion2,3,4. Stress accumulation between earthquakes results from slip deficit (slip that is insufficient to fully accommodate plate movement). For the prediction of large earthquakes, it is therefore important to monitor the distribution of slip deficit on plate interfaces. Here we apply an inversion method based on Bayesian modelling (using direct and indirect prior information on the magnitude and distribution of fault slip5) to horizontal and vertical velocities from global positioning system data. For the seismically calm period between 1996 and 2000, we obtain a precise distribution of slip-deficit rates on the interface between the North American and Pacific plates around Japan, which reveals a trench-parallel belt of slip deficit with six peaks in the depth range of 10–40 km. These peaks agree with the source regions of past large interplate earthquakes along the Kuril–Japan trench. We conclude that the slip-deficit zones identified with our method are potential source regions of large earthquakes.

Seismic and geodetic evidence for extensive, long-lived fault damage zones

Abstract: During earthquakes, slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes. Seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic traveltimes, trapped waves, and interferometric synthetic aperture radar observations, we document seismic velocities reduced by 40%‐ 50% and shear moduli reduced by 65% compared to wall rock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures. No large earthquake has broken the Calico fault in historic time, implying that fault damage persists for hundreds or perhaps thousands of years. These fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.

Effect of particles on the formation of deformation twins in a magnesium-based alloy

Abstract: The deformation behaviour of the age hardenable alloy Mg–5%Zn after different precipitation treatments has been examined. It has been found that during compressive deformation, fine particles increase the number of twins that form, but reduce the size and total volume fraction of twins. Visco-plastic self-consistent modelling has been used to show that the presence of precipitates hardens the twin and prismatic slip systems more than the basal slip system. It is proposed that because the { 1 0 1 ¯ 2 } twin requires basal slip to accommodate the twinning shear, this twin type will always be hardened equal to, or more than, the basal slip system in response to precipitation.

Cyclic Slip Irreversibilities and the Evolution of Fatigue Damage

Abstract: In this article, the physical origin of fatigue crack initiation in ductile metals is discussed from a historical perspective. The main focus is to assess those cyclic slip irreversibilities in a microstructural sense that occur not only at the surface but also in the bulk at the dislocation scale and to show how they contribute to surface fatigue damage. The evolution of early fatigue damage, as evidenced experimentally in the last decades, is reviewed. The phenomenon of cyclic strain localization in persistent slip bands (PSBs) and models of the formation of extrusions, intrusions, and microcracks are discussed in detail. The predictions of these models are compared with experimental evidence obtained on mono- and polycrystalline face-centered-cubic (fcc) metals. In addition, examples of the evolution of fatigue damage in selected fcc solid solution alloys and precipitation-hardened alloys and in body-centered-cubic (bcc) metals are analyzed. Where possible, the cyclic slip irreversibilities p, defined as the fraction of plastic shear strain that is microstructurally irreversible, have been estimated quantitatively. Broadly speaking, p has been found to vary over orders of magnitude (0 < p < 1), being almost negligible at low loading amplitudes (high fatigue lives) and substantial at larger loading amplitudes (low fatigue lives).

Three‐dimensional boundary integral modeling of spontaneous earthquake sequences and aseismic slip

Abstract: [1] Fault processes involve complex patterns of seismic events and aseismic slip. This work develops a three-dimensional (3-D) methodology for simulating long-term history of spontaneous seismic and aseismic slip on a vertical planar strike-slip fault subjected to slow tectonic loading. Our approach reproduces all stages of earthquake cycles, from accelerating slip before dynamic instability, to rapid dynamic propagation of earthquake rupture, to postseismic slip, and to interseismic creep, including aseismic transients. We use the developed 3-D methodology to study interaction of fault slip with a small patch of higher normal stress over long-term slip history. For uniform initial prestress, dynamic rupture is significantly affected by the stronger patch in the first simulated event but not in subsequent ones. The change in behavior is due to redistribution of shear stress by prior slip, which demonstrates that distributions of fault strength and stress are related and illustrates the importance of simulating long slip histories even in studies of dynamic rupture. Despite no long-term effect on dynamic rupture, the small patch of higher normal stress influences nucleation processes and hence long-term slip patterns in the model. Comparison of the fully dynamic simulations and a widely used quasi-dynamic approach shows that the quasi-dynamic approach modifies long-term slip patterns in addition to resulting in much smaller slip velocities and rupture speeds during dynamic events. We show that the response of quasi-dynamic formulations with reduced radiation damping terms can be scaled to match the results of the standard quasi-dynamic formulation and hence cannot improve the comparison.

The SCEC/USGS Dynamic Earthquake Rupture Code Verification Exercise

Abstract: Numerical simulations of earthquake rupture dynamics are now common, yet it has been difficult to test the validity of thesesimulations because there have been few field observations and no analytic solutions with which to compare the results. This paper describes the Southern California Earthquake Center/U.S. Geological Surve(SCEC/USGS) Dynamic Earthquake Rupture Code Verification Exercise, where codes that simulate spontaneous rupture dynamics in three dimensions are evaluated and the results produced by these codes are compared using Web-based tools. This is the first time that a broad and rigorous examination of numerous spontaneous rupture codes has been performed—a significant advance in this science. The automated process developed to attain this achievement provides for a future where testing of codes is easily accomplished. Scientists who use computer simulations to understand earthquakes utilize a range of techniques. Most of these assume that earthquakes are caused by slip at depth on faults in the Earth, but hereafter the strategies vary. Among the methods used in earthquake mechanics studies are kinematic approaches and dynamic approaches. The kinematic approach uses a computer code that prescribes the spatial and temporal evolution of slip on the causative fault (or faults). These types of simulations are very helpful, especially since they can be used in seismic data inversions to relate the ground motions recorded in the field to slip on the fault(s) at depth. However, these kinematic solutions generally provide no insight into the physics driving the fault slip or information about why the involved fault(s) slipped that much (or that little). In other words, these kinematic solutions may lack information about the physical dynamics of earthquake rupture that will be most helpful in forecasting future events. To help address this issue, some researchers use computer codes to numerically simulate earthquakes and construct dynamic, spontaneous rupture (hereafter called “spontaneous rupture”) solutions. For these types of numerical simulations, rather than prescribing the slip function at each location on the fault(s), just the friction constitutive properties and initial stress conditions are prescribed. The subsequent stresses and fault slip spontaneously evolve over time as part of the elasto-dynamic solution. Therefore, spontaneous rupture computer simulations of earthquakes allow us to include everything that we know, or think that we know, about earthquake dynamics and to test these ideas against earthquake observations.

Cyclic Slip Irreversibilities and the Evolution of Fatigue Damage

Abstract: In this article, the physical origin of fatigue crack initiation in ductile metals is discussed from a historical perspective. The main focus is to assess those cyclic slip irreversibilities in a microstructural sense that occur not only at the surface but also in the bulk at the dislocation scale and to show how they contribute to surface fatigue damage. The evolution of early fatigue damage, as evidenced experimentally in the last decades, is reviewed. The phenomenon of cyclic strain localization in persistent slip bands (PSBs) and models of the formation of extrusions, intrusions, and microcracks are discussed in detail. The predictions of these models are compared with experimental evidence obtained on mono- and polycrystalline face-centered-cubic (fcc) metals. In addition, examples of the evolution of fatigue damage in selected fcc solid solution alloys and precipitation-hardened alloys and in body-centered-cubic (bcc) metals are analyzed. Where possible, the cyclic slip irreversibilities p, defined as the fraction of plastic shear strain that is microstructurally irreversible, have been estimated quantitatively. Broadly speaking, p has been found to vary over orders of magnitude (0 < p < 1), being almost negligible at low loading amplitudes (high fatigue lives) and substantial at larger loading amplitudes (low fatigue lives).

Direct numerical simulations of turbulent flows over superhydrophobic surfaces

Abstract: Direct numerical simulations (DNSs) are used to investigate the drag-reducing performance of superhydrophobic surfaces (SHSs) in turbulent channel flow. SHSs combine surface roughness with hydrophobicity and can, in some cases, support a shear-free air–water interface. Slip velocities, wall shear stresses and Reynolds stresses are considered for a variety of SHS microfeature geometry configurations at a friction Reynolds number of Reτ ≈ 180. For the largest microfeature spacing studied, an average slip velocity over 75% of the bulk velocity is obtained, and the wall shear stress reduction is found to be nearly 40%. The simulation results suggest that the mean velocity profile near the superhydrophobic wall continues to scale with the wall shear stress but is offset by a slip velocity that increases with increasing microfeature spacing.

Effective Stiffness of Reinforced Concrete Columns

Abstract: This study uses the experimental response of 329 concrete columns to assess existing and proposed models of the effective stiffness of reinforced concrete columns subjected to lateral loads. Existing models appropriate for design applications are shown to overestimate the measured effective stiffness and are unacceptably inaccurate, because they generally neglect the influence of anchorage slip on the effective stiffness of the column. Based on simplifications of a three-component model, a new procedure is proposed that explicitly accounts for deformations due to flexure, shear, and anchorage-slip. The new procedure is shown to provide a more accurate estimate of the measured effective stiffness for the database columns. For this model, the ratio of the measured stiffness to the calculated stiffness had a mean and coefficient of variation of 1.02 and 22% for circular columns and 0.95 and 25% for rectangular columns. The proposed procedure is more accurate and rational than existing models, and could easily be incorporated into design provisions for concrete structures.

Rupture history of the 2009 L'Aquila (Italy) earthquake from non‐linear joint inversion of strong motion and GPS data

Abstract: [1] We image the rupture history of the 2009 L'Aquila (central Italy) earthquake using a nonlinear joint inversion of strong motion and GPS data. This earthquake ruptured a normal fault striking along the Apennines axis and dipping to the SW. The inferred slip distribution is heterogeneous and characterized by a small, shallow slip patch located up-dip from the hypocenter (9.5 km depth) and a large, deeper patch located southeastward. The rupture velocity is larger in the up-dip than in the along-strike direction. This difference can be partially accounted by the crustal structure, which is characterized by a high velocity layer above the hypocenter and a lower velocity below. The latter velocity seems to have affected the along strike propagation since the largest slip patch is located at depths between 9 and 14 km. The imaged slip distribution correlates well with the on-fault aftershock pattern as well as with mapped surface breakages.