Slip Geometry Research Articles

Seismites in Quaternary sediments of southeastern Altai

We study earthquake-induced soft-sediment deformation (seismites) in reference Quaternary sections of southeastern Altai. Sediments in the sections bear signature of liquefaction and fluidization and deformation is localized in thin (few centimeters to 0.5–1.0 m) continuously striking and frequently repeated layers sandwiched between undeformed sediments. The soft-sediment deformation records coseismic motion of different slip geometries. Seismic origin is also inferred for layers and lenses of coarse colluvium slid into the lake bottom from the slopes, which intrude plane-bedded silt and sand and vary in thickness from a few centimeters to one meter. The occurrence of seismic soft-sediment deformation at different stratigraphic levels of the Quaternary and in the Upper Pliocene Beken Formation confirms the high seismicity of southeastern Altai in Quaternary time.

Grain orientation dependence of extended planar dislocation boundaries in rolled aluminium

The dislocation boundary alignment in 124 grains in cold-rolled aluminium has been investigated by a combination of backscattered electron channelling contrast and electron backscatter diffraction . The asymmetric slip geometry of the Copper and S orientations together with the existence of different symmetry variants of these orientations is exploited to analyse whether the grain orientation (and consequently the slip systems) or the symmetric rolling geometry determines the boundary alignment. The results unambiguously point at the grain orientation as the controlling factor.

Hierarchy of earthquakes in the Baikal rift system: implications for lithospheric stress

We investigated space-time lithospheric stress patterns of the Baikal rift system according to the hierarchy of earthquakes using mechanisms of 265 K P ≥ 10 events recorded from 1950 to 1998 and seismic moments of 802 K P ≥ 11 events from 1968 to 1994. The lithosphere of the region was confirmed to undergo rifting with mostly normal-slip events, while local areas of frequent strike-slip and reverse motions may record stress heterogeneity. The dominance of rifting, although being evident in the stress dynamics, is unstable, which is indicated by increase in strike-slip and reverse motions to as many as normal slip events in the latest 1980s–earliest 1990s. The lithospheric stress patterns inferred from seismic-moment data are generally consistent with those derived from the classical focal mechanism method. The suggested approach of seismic zoning according to earthquake slip geometry may provide a more reliable background for successful mitigation of seismic hazard in the region.

Heat transfer and entropy generation for forced convection through a microduct of rectangular cross-section: Effects of velocity slip, temperature jump, and duct geometry

This work presents numerical solutions for fully developed velocity, temperature, and entropy generation distribution due to forced convection in microelectromechanical systems (MEMS) in the Slip-flow regime, for which the Knudsen number lies within the range 0.001 < Kn < 0.1. Invoking the temperature jump equation, the H1 boundary condition is investigated. It was observed that some of the previous reports in the literature can be misleading and lead to erroneous results specially when it comes to Second Law (of Thermodynamics) aspects of the problem. The results can be generalized to the macroscale counterparts by letting Kn = 0.

Entropy generation for microscale forced convection: Effects of different thermal boundary conditions, velocity slip, temperature jump, viscous dissipation, and duct geometry

This work presents closed form solutions for fully developed temperature distribution and entropy generation due to forced convection in microelectromechanical systems (MEMS) in the Slip-flow regime, for which the Knudsen number lies within the range 0.001<Kn<0.1. Two different cross-sections are analyzed, being microducts (composed of two parallel plates) and micropipes, with the effects of viscous dissipation being included. Invoking the temperature jump equation, two different thermal boundary conditions are investigated, being isothermal and isoflux walls. Expressions are presented for the local and bulk temperature profiles, the Nusselt number, the Bejan number, and the entropy generation rate in terms of the key parameters. Though the results are obtained for the microscale problems, they can be generalized to the macroscale counterparts by letting Kn=0.

Seismic strain in the Altai-Sayan active seismic area and elements of collisional geodynamics

Seismic strain estimated from about 900 earthquake mechanisms in the Altai-Sayan area indicates that the area evolves mainly under N-S compression caused by the India-Eurasia collision. Plastic flow in the mountains of the western and central Altai-Sayan area is controlled by convergence of aseismic rigid blocks, including the Dzungarian microplate, the Minusa basin, the Tuva basin, Uvs Nuur and other basins in the region of Great Lakes in Mongolia. Earthquake rupture follows the existing fault pattern. The pattern of rigid blocks and mountain ranges correlates well with upper mantle thermal heterogeneity as imaged by seismic tomography. Earthquake mechanisms in the presence of plate convergence depend on different factors. Slip geometry depends on focal depth at a given geodynamic regime, and the latter, in terms of the seismic process, is controlled by lateral (W-E) constraint. Reverse-slip earthquakes most often originate at shallow depths in regions of constrained compression, such as the Tien Shan, and strike slip is the dominant mechanism under non-constrained and moderately constrained compression. The direction of slip is governed by the parallel component of convergence and by the fault pattern. In regions of strain shadow, slip occurs mostly on normal planes, and shallow earthquakes have strike-slip mechanisms. The crust in the collisional region is divided into systems of rigid and plastic blocks (rheological structure) and of fault blocks (fault-block structure). The two types of systems coexist and determine the generally similar but specifically different features of local mass transfer.

Crystal plasticity simulations of fretting of Ti-6Al-4V in partial slip regime considering effects of texture

Generalized plane strain finite element fretting simulations of a rigid cylinder in contact with a Ti-6Al-4V half-space represented via 3-D polycrystal viscoplasticity model are performed in the partial slip regime. The dual-phase nature of Ti-6Al-4V and realistic 3-D crystallographic textures are explicitly accounted for in the model. Earlier studies using idealized 2-D slip geometry in 2-D finite element calculations indicate that ratcheting is the dominant mechanism of cyclic plastic deformation for this material under fretting conditions. Herein, changes in the distribution and mode of cyclic microplasticity with respect to different representative 3-D textures are examined and compared to observations from simulations using the 2-D slip geometry. Results are discussed and quantified in terms of cumulative effective plastic strain distributions, plastic strain maps, and the effective ratchet strain increments.

Geometrical analysis of mesoscopic shear zones in the crystalline rocks of MCT zone of Garhwal Higher Himalaya

The crystalline rocks of the MCT Zone of Garhwal Higher Himalaya exhibit well-preserved mesoscopic shear zones. Majority of these shear zones are of ductile and brittle ductile type with both sinistral and dextral sense of movement. Detailed analysis of mesoscopic shear zones reveals that sinistral shear zones exhibit a strike variation from NNE to ENE and dextral shear zones exhibit variation from NNW to WNW directions thus forming a conjugate pair. The bisectors of statistically preferred orientations of the two sets of the shears indicate that they generated due to NNE–SSW horizontal compression. These dextral and sinistral shear zones exhibit strike–slip geometry developed during progressive ductile shearing.

A three-dimensional crystal plasticity model for duplex Ti–6Al–4V

A rate dependent crystal plasticity model for the α/β Ti–Al alloy Ti–6Al–4V with duplex microstructure is developed and presented herein. Duplex Ti–6Al–4V is a dual-phase alloy consisting of an hcp structured matrix primary α-phase and secondary lamellar α + β domains that are composed of alternating layers of secondary α laths and bcc structured residual β laths. The model accounts for distinct three-dimensional slip geometry for each phase, anisotropic and length scale dependent slip system strengths, the non-planar dislocation core structure of prismatic screw dislocations in the primary α-phase, and crystallographic texture. The model is implemented in the general purpose finite element code (ABAQUS, 2005. Ver 6.5, Hibbitt, Karlsson, and Sorensen, Inc., Pawtucket, RI) via a UMAT subroutine.

Strength, strain-rate sensitivity and ductility of copper with nanoscale twins

We present a comprehensive computational analysis of the deformation of ultrafine crystalline pure Cu with nanoscale growth twins. This physically motivated model benefits from our experimental studies of the effects of the density of coherent nanotwins on the plastic deformation characteristics of Cu, and from post-deformation transmission electron microscopy investigations of dislocation structures in the twinned metal. The analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating the twin boundary as an internal interface and allowing special slip geometry arrangements that involve soft and hard modes of deformation. This model correctly predicts the experimentally observed trends of the effects of twin density on flow strength, rate sensitivity of plastic flow and ductility, in addition to matching many of the quantitative details of plastic deformation reasonably well. The computational simulations also provide critical mechanistic insights into why the metal with nanoscale twins can provide the same level of yield strength, hardness and strain rate sensitivity as a nanostructured counterpart without twins (but of grain size comparable to the twin spacing of the twinned Cu). The analysis also offers some useful understanding of why the nanotwinned Cu with high strength does not lead to diminished ductility with structural refinement involving twins, whereas nanostructured Cu normally causes the ductility to be compromised at the expense of strength upon grain refinement.

Pyridylporphyrin Metallacycles with a Slipped Cofacial Geometry: Spectroscopic, X-ray, and Photophysical Characterization

Treatment of the octahedral Ru(II) complex [trans,cis,cis-RuCl(2)(DMSO-O)(2)(CO)(2)] with an equimolar amount of 5,10-bis(3'-pyridyl)-15,20-diphenylporphyrin (3'-cis-DPyP) yielded, upon selective replacement of the DMSO ligands, the neutral 2 + 2 metallacycle 2. NMR spectroscopy provided unambiguous evidence that only one highly symmetrical species, in which the two chromophores are held in a slipped cofacial arrangement by the external Ru(II) metal fragments, exists in solution. The unprecedented geometry of 2, and of the fully zincated analogue 2a, were confirmed in the solid state by X-ray structural investigations. The spatial arrangement of the two parallel chromophores in 2, with an interplanar distance of 4.18 A and a lateral offset (center-to-center distance) of 9.82 A, is reminiscent of those of the special pair of bacteriophylls in the reaction centers and of adjacent B850 units in the LH2 light-harvesting antenna systems of photosynthetic bacteria. For comparison, the X-ray structure of the corresponding metallacycle with 4'-cis-DPyP, 1a, is also reported. In 1a, the two porphyrins have an almost perfect coplanar arrangement. The semi-zincated metallacycles 1b and 2b, in which only one of the two chromophores bears an inner zinc atom, were prepared from 1 and 2, respectively, and isolated in pure form. Detailed photophysical investigations of the above porphyrin assemblies were performed. In particular, very fast photoinduced intercomponent energy transfer processes from the zinc porphyrin to the free-base unit were detected in the semi-metalated derivatives 1b and 2b (time constants: 14 and 12 ps, respectively).

Evidence for static stress changes triggering the 1999 eruption of Cerro Negro Volcano, Nicaragua and regional aftershock sequences

Remarkable evidence of coupling between tectonic and magmatic events emerges from investigation of three tectonic earthquakes, aftershock sequences and eruption of Cerro Negro volcano, Nicaragua in 1999. Here, we explain this coupling through static stress changes following three Mw 5.2 earthquakes. We use focal mechanism solutions to estimate fault system geometry and magnitude of slip from these events, which are then used to calculate the change in minimum horizontal principal stress (σ3) for the region and the change in Coulomb failure stress on optimally oriented fault planes. Results of these simulations indicate that σ3was reduced by ∼0.08 MPa and that Coulomb failure stress was raised by 0.001 to 0.2 MPa in the region. A Kolmogorov‐Smirnov test demonstrates spatial correlation of Coulomb failure stress changes and triggered seismicity and volcanism, and suggests that these small changes in static stress can trigger subsequent geophysical events under appropriate circumstances.

Effects of Local Grain Orientation on Fatigue Crack Growth in Multicrystalline fcc Metallic Materials

The anomalous behavior of microstructurally “short” cracks that can control fatigue life at very high cycles can be attributed to the local conditions around these cracks, since the length scale involved requires the consideration of anisotropic material behavior and the effect of changes in grain orientation as the crack grows. The effect of local crystallography was studied in multicrystalline Compact-Tension (CT) specimens of pure nickel and a cast Ni-based superalloy. Orientation Imaging Microscopy (OIM) was used to map the crystallography of the grains ahead of the notch. A standard fatigue crack growth test was then carried out to characterize the crack path in relation to the grain orientations. Two extreme cases were identified: at one end cracks grew with small deviations through all the grains ahead of it, whereas at the other end large deflections from a path perpendicular to the applied load were observed. Intergranular cracks were found to prefer high angle boundaries, whereas transgranular cracks had a tendency to nucleate and display stage I growth along slip traces of systems with high Schmid factors, as determined by the uniaxial conditions expected at the notch tips. In addition, crack path tortuosity was more pronounced in grains with loading axes close to 〈111〉. Finally, the influence of changes on slip geometry as cracks moved across grain boundaries is also discussed.

Investigation of the slip geometry of high-purity potassium by in situ X-ray diffraction

Abstract Plastic deformation of bcc metals at intermediate and low temperatures is dominated by [1 1 1] screw dislocations which macroscopically seem to glide on planes of high or irrational Miller indices. It is now well established that this non-crystallographic glide is composed of elementary steps of kink-pair formation on three symmetry-equivalent low-index planes. In the present investigation, the slip geometry of high-purity potassium single crystals was investigated by in situ X-ray measurements of the lattice rotation in uniaxial compression in the temperature range from 6 to 70 K. The different alternatives for the elementary glide planes, e.g., {1 1 0}, {2 1 1} or {3 2 1}, can be uniquely distinguished from changes of the primary slip direction within the unit triangle. The results showed that the elementary glide step in potassium occurs on {2 1 1} planes in the investigated temperature range. In addition, glide of [1 0 0] dislocations (obviously products from interactions between mobile and sessile [1 1 1] dislocations) becomes dominating in many samples at increasing plastic strain. Numerical simulations of the orientation change in 〈1 1 1〉{2 1 1} glide, based on the model of thermally activated kink-pair formation, were in good agreement with the experimental results.

An inverse method to derive fault slip and geometry from seismically observed vertical stratigraphic displacements using elastic dislocation theory

This contribution describes the development of an inverse method to extract fault slip and fault geometry from subsurface horizons derived from seismic reflection datasets. The inversion process uses non-linear minimisation together with a forward model using elastic dislocation (ED) theory. The forward model calculates subsurface vertical displacements from fault parameters such as fault slip, dip, length and width for planar, listric or seismically observed fault geometries. Vertical subsurface displacements are measured at selected stratigraphic horizons from seismic depth sections, and act as observations for the inversion process. The non-linear inversion uses a standard procedure (Powell's method) to minimise an objective function (misfit parameter). The method has been tested using synthetic data generated by theoretical forward models. The inverse method can successfully recover slip distributions and non-planar, including listric, fault geometries. The method has also been applied to stratigraphic horizons derived from seismic reflection data. Fault slip and fault geometry derived from the inverse method are consistent with direct seismic interpretations, and are reproducible and geologically realistic. This inverse method provides a useful complement to conventional subjective seismic interpretations of fault slip and geometry. ED theory together with the retrieved fault parameters from the application of inverse theory may be used to determine strains for fracture prediction.

Negative yield stress temperature anomaly and structural instability ofPt3Al

First-principles electronic structure and total energy calculations of the phase stability and dislocation prop­ erties of Pt3Al are employed to reveal the origins of its yield stress low temperature anomaly (LTA). An analysis of the dislocation structure and mobility, based on generalized stacking fault calculations, demon­ strates that the LTA is connected with the L12^ D 0 'c structural transformation rather than, as traditionally believed, with features of the dislocation structure in the L12 phase. We also explain why the yield stress and slip geometry have a strong orientation dependence and why small deviations from stoichiometry lead to dramatic changes in its mechanical behavior.

Dislocation creep in MgSiO3 perovskite at conditions of the Earth's uppermost lower mantle.

Seismic anisotropy provides an important observational constraint on flow in the Earth's deep interior. The quantitative interpretation of anisotropy, however, requires knowledge of the slip geometry of the constitutive minerals that are responsible for producing rock fabrics. The Earth's lower mantle is mostly composed of (Mg, Fe)SiO3 perovskite, but as MgSiO3 perovskite is not stable at high temperature under ambient pressure, it has not been possible to investigate its mechanical behaviour with conventional laboratory deformation experiments. To overcome this limitation, several attempts were made to infer the mechanical properties of MgSiO3 perovskite on the basis of analogue materials. But perovskites do not constitute an analogue series for plastic deformation, and therefore the direct investigation of MgSiO3 perovskite is necessary. Here we have taken advantage of recent advances in experimental high-pressure rheology to perform deformation experiments on coarse-grained MgSiO3 polycrystals under pressure and temperature conditions of the uppermost lower mantle. We show that X-ray peak broadening measurements developed in metallurgy can be adapted to low-symmetry minerals to identify the elementary deformation mechanisms activated under these conditions. We conclude that, under uppermost lower-mantle conditions, MgSiO3 perovskite deforms by dislocation creep and may therefore contribute to producing seismic anisotropy in rocks at such depths.

On the evolution of crystallographic dislocation density in non-homogeneously deforming crystals

A set of evolution equations for dislocation density is developed incorporating the combined evolution of statistically stored and geometrically necessary densities. The statistical density evolves through Burgers vector-conserving reactions based in dislocation mechanics. The geometric density evolves due to the divergence of dislocation fluxes associated with the inhomogeneous nature of plasticity in crystals. Integration of the density-based model requires additional dislocation density/density-flux boundary conditions to complement the standard traction/displacement boundary conditions. The dislocation density evolution equations and the coupling of the dislocation density flux to the slip deformation in a continuum crystal plasticity model are incorporated into a finite element model. Simulations of an idealized crystal with a simplified slip geometry are conducted to demonstrate the length scale-dependence of the mechanical behavior of the constitutive model. The model formulation and simulation results have direct implications on the ability to explicitly model the interaction of dislocation densities with grain boundaries and on the net effect of grain boundaries on the macroscopic mechanical response of polycrystals.

Monte Carlo Inversion of DInSAR Data for Dislocation Modeling: Application to the 1997 Umbria-Marche Seismic Sequence (Central Italy)

— The study of surface deformation due to seismic activity is often made using dislocations with uniform slip and simple geometries. A better modeling of coseismic and postseismic surface displacements can be obtained by using dislocations with variable slip and nonregular shapes. This is consistent with the asperity model of fault surfaces, assuming a friction distribution on faults made of locked zones with much higher friction than surrounding zones. In this paper we consider the 1997–1998 Colfiorito seismic sequence. The coseismic surface displacements in the Colfiorito zone are used in order to infer the slip distribution on the fault surface at different stages of the sequence. The displacement field has been modeled varying the slip distribution on the fault, and comparing the deformation observed by SAR and GPS techniques with model results. The slip distribution is calculated by Monte Carlo simulations on a normal fault with the dip angle equal to 40°. A good approximation is obtained by using square asperity units of 1.5×1.5 km2. In the first stage, we employed a simplified model with uniform slip, in which each asperity unit is allowed to slip a constant amount or not to slip at all, and in the second stage, we evaluate the slip distribution in the dislocation area determined by the Monte Carlo inversion: in this case we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. The results show that the 1997 seismic events of the sequence can be modeled by irregular dislocations, obtaining a good fit to the DInSAR and GPS observations. The model also confirms the results of previous studies by a different methodology, defining the distribution of asperities on the fault plane using the fault geometry, the geodetic data and the seismic moment of the 1997–1998 Colfiorito seismic sequence. Furthermore, the analysis of 1997 aftershocks in the seismogenic region shows a strong correlation between most events and the asperity distribution, which can be considered as an independent test of the validity of the model.

Influence of non-glide stresses on plastic flow: from atomistic to continuum modeling

The Schmid law, which is accurate for face-centered-cubic (fcc) metals, assumes that only the shear stress acting in the slip plane in the slip direction controls the plastic deformation. Hence, it is implicitly assumed that the critical resolved shear stress (CRSS) for the slip is not affected by any other components of the applied stress tensor. This rule is almost ubiquitously utilized in large-scale continuum computations of plastically deforming single and polycrystals. On the other hand, in materials with more complex structures and for some orientations of the dislocation line the cores can spread onto several non-parallel planes. The most widely-known example is the screw dislocation in bcc metals, though this phenomenon is quite universal in structures that are not close packed. Signatures of such core configurations commonly include unexpected deformation modes and slip geometries, strong and unusual dependence of flow stresses on temperature and strain rate, a high Peierls stress, and, in general, a breakdown of the Schmid law. In this paper, we first summarize results of atomistic computer simulations of the response of 1/2〈1 1 1〉 screw dislocations to the applied shear stresses. The calculations have been made using central-force many-body potentials and tight-binding based bond-order potentials for molybdenum. While the core structure found is not the same for the two descriptions of atomic interactions, both lead to a very similar orientation dependence of the critical resolved shear stress for the dislocation motion which takes place along the most highly stressed {1 1 0} plane. This dependence reveals the break down of the Schmid law invoked by the effect of shear stresses acting in another {1 1 0} plane, which are called non-glide stresses. These results are then transferred to macroscopic level by formulating single crystal yield criteria that include the effects of non-glide components of the stress tensor. These criteria form a basis for multislip yield criteria and flow relations for continuum analyses. Using this approach we demonstrate that the effects of non-glide stresses that have their origin at the level of individual dislocations also have significant effect on polycrystalline response, including a significant tension-compression asymmetry.