Showing papers on "Slab published in 1993"

Mantle and Slab Contributions in ARC Magmas

Abstract: The current consensus is that most arc rocks crystallized from parental magmas generated in the mantle wedge, and that melting took place in the presence of water so that the temperature at the solidus was less than that beneath mid-ocean ridges and oceanic islands. The inference is that the water was released from the subducted oceanic crust, and that other more mobile elements were transported in the hydrous fluids, but the size and nature of the contribution from the subducted crust, and how it may be recognized, remain contentious. This review considers ways in which contributions from the subducted slab and the mantle wedge may be recognized, and examines estimated fluxes from subducted material. The emphasis is on the isotope and trace element geochemistry of the rocks themselves, which forms the basis for assessing the size and nature of the mantle and slab-derived components. -from Authors

A geodynamic model of mantle density heterogeneity

Abstract: Using Cenozoic and Mesozoic plate motion reconstructions, we derive a model of present-day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle. The thermal buoyancy of these slabs is estimated from the observed thermal subsidence (cooling) of oceanic lithosphere. Slab velocities in the upper mantle are computed from the local convergence rate. We assume that slabs cross the upper/lower mantle interface and continue sinking into the lower mantle with a reduced velocity. For a velocity reduction factor between 2 and 5, our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other. We have also computed a synthetic geoid from our density model. For a viscosity increase of about a factor of 40 from the upper to lower mantle, our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95% and explains 84% of the observed geoid assuming that the model C21 and S21 terms are absent due to a long relaxation time for Earth's rotational bulge. Otherwise, 73% of the geoid variance is explained. The viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle, since downwelling velocities are expected to scale roughly as the logarithm of viscosity (loge 40 = 3.7). These results show that the history of plate tectonics can explain the main features of the present-day structure of the mantle. The dynamic topography induced by this heterogeneity structure consists mainly of about 1-km amplitude lows concentrated along the active continental margins of the Pacific basin. Our model can also be used to predict the time variation of mantle heterogeneity and the gravity field. We find that the “age” of the geoid, defined as the time in the past before which the geoid becomes uncorrelated with the present geoid, is about 50 m.y. Our model for the history of the degree 2 geoid, which is equivalent to the history of the inertia tensor, should give us a tool to study the variations in Earth's rotation pole indicated in paleomagiietic studies.

Nonlinear Instabilities in Shock-Bounded Slabs

Abstract: (substantial changes to section 3.2, otherwise minor) We present an analysis of the hydrodynamic stability of a cold slab bounded by two accretion shocks. Previous numerical work has shown that when the Mach number of the shock is large the slab is unstable. Here we show that to linear order both the bending and breathing modes of such a slab are stable. However, nonlinear effects will tend to soften the restoring forces for bending modes, and when the slab displacement is comparable to its thickness this gives rise to a nonlinear instability. The growth rate of the instability, above this threshold but for small bending angles, is $\sim c_sk (k\eta)^{1/2}$, where $\eta$ is the slab displacement. When the bending angle is large the slab will contain a local vorticity comparable to $c_s/L$, where $L$ is the slab thickness. We discuss the implications of this work for gravitational instabilities of slabs. Finally, we examine the cases of a decelerating slab bounded by a single shock and a stationary slab bounded on one side by thermal pressure. The latter case is stable, but appears to be a special case. The former case is subject to a nonlinear overstability driven by deceleration effects. We conclude that shock bounded slabs with a high density compression ratio generically produce substructure with a strong local shear, a bulk velocity dispersion like the sound speed in the cold layer and a characteristic scale comparable to the slab thickness.

Numerical calculation of the reflection, absorption, and transmission of microwaves by a nonuniform plasma slab

Abstract: The reflection, absorption, and transmission of microwaves by a magnetized, steady-state, two-dimensional, nonuniform plasma slab is studied. A discussion on the effect of various plasma parameters on the reflected power, absorbed power, and transmitted power is presented. The nonuniform plasma slab is modeled by a series of subslabs. Even though it is assumed that the number density is constant in each subslab, the overall number density profile across the whole slab follows a parabolic function. The partial reflection coefficient at each subslab boundary is computed along with the absorption at each subslab. The total reflected, absorbed, and transmitted powers are then deduced and their functional dependence on the number density, collision frequency, and angle of propagation is studied. >

Pressure, temperature and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garhwal Himalaya

Abstract: Abstract Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a c. 10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south. Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted P-T gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab. Cooling rates for the upper MCT zone determined from 40Ar/39Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende 40Ar/39Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above c. 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.

Magmatic processes under arcs and formation of the volcanic front

Abstract: Temperature structure and stress under arcs are simulated in a two-dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge. Results of the calculations show three important features with respect to magmatic processes under arcs: (1) Temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction, the temperature structure of the slab, and that of the back-arc region. This indicates that physical conditions such as temperature and pressure are similar under various arcs. It is thus inferred that primary magmas generated under various arcs should have similar chemical compositions, if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same. (2) Calculated deviatoric stress magnitude is relatively large (more than a few tens of megapascals) in the partially molten mantle. Cracks may open under high differential stress, and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds. (3) The deviatoric stress values in the region over the partially molten mantle are relatively large, and the direction of the principal stress changes horizontally; the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore-arc region. It is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism. The propagation direction of magma-filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress. The calculated stress is highly compressive horizontally on the trench side, while it becomes tensile on the back-arc side. The location of this stress transition coincides with that of the volcanic front. The location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma-filled cracks under relatively high differential stress field.

Subduction of the lithosphere and back arc dynamics: Insights from physical modeling

Abstract: Physical modeling of the subduction of an elastico-plastic lithosphere underlained by a low-viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by Δρ (Δρ = ρ1 - ρa where ρa and ρ1 are the asthenosphere and lithosphere densities). At Δρ = 0, a steady state process develops which requires a horizontal lithosphere compression of σh = (0.3–0.4)σs ≈ 108 Pa, where σs is the lithosphere yield limit. The central part of the leading wedge of the overriding plate is nonisostatically elevated in this case, especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200–400 mGal. A decrease in Δρ (Δρ 0, compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction. At some point, compression changes into tension. Tensional nonhydrostatic stresses grow during the process to a value near σh = (0.1–0.2)σs ≈ 0.4 × 108 Pa. Both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the Benioif dip angle accompany this process. The gravity anomaly maximum is displaced into the volcanic arc in this subduction regime, where it is normally less than 150 mGal. If some weaknesses are present in the overriding plate, tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment (the arc plate) starts to move counter to the subduction plate. The arc plate slides along the underthrusting plate's surface without loss of contact and causes back arc opening. Hydrostatic suction prevents the plates from being separated. This effect is caused by the peculiarity of a mechanical system whereby a low-viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate. If the overriding lithosphere is not weakened enough to fail under the existing tension, then the hanging slab breaks down under its own weight when it reaches a specific length. Both interacting plates undergo isostatic recovery in this case, and the whole cycle is repeated again. The hydrostatic suction effect also provides back arc tension under other conditions, such as when asthenospheric flow forces the subducted plate to move away from the island arc.

Mechanics of oblique convergence

Abstract: The distribution and magnitude of the strike-parallel component of velocity in an obliquely converging thrust wedge or accretionary prism are determined by the geometry and mechanical properties of the wedge A mechanical analysis based on the assumption of a critical or stable geometry of the wedge, for which the rate of cross-strike deformation is zero, leads to the following conclusions for different bulk rheologies (1) In a linear viscous wedge, the strike-parallel motion relative to the underthrust slab decreases exponentially away from the rear and is effectively concentrated in a shear zone with a width comparable to the thickness of the wedge at the rear The wedge also deforms by corner flow, producing a circulation in the cross-strike plane The strike-parallel and corner flow velocities depend on the thickness and viscosity of the wedge and on the shear stresses applied to its lower and rear boundaries Convergence at the wedge front is normal to strike (2) A critically tapered perfect plastic wedge moves coherently without internal deformation For low and moderate obliquities of the convergence vector, the wedge moves at the same velocity as the backstop (upper plate) For high angles of obliquity, the wedge moves laterally relative to the underthrust slab at a maximum velocity dependent on its dimensions and the stress conditions on its boundaries, so that it is separated from the upper plate by a strike-slip fault, defining a forearc sliver No geometrical configuration exists that allows the strike-parallel motion to be distributed through the wedge (3) A noncohesive Coulomb wedge behaves in much the same way as a plastic wedge, but the geometry and velocity depend only on its mechanical properties and the shear stresses on its boundaries, and they are independent of scale

Polar wandering of a dynamic earth

Abstract: The rotational behaviour of a stratified visco-elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi-fluid approximation. This approximation allows for large displacements of the Earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years. Such a motion, called true polar wander (TPW), is detected by palaeomagneticiens assuming that the Earth's magnetic field remains on average aligned with the spin axis. Our model shows that a downgoing cold slab induces a TPW which quickly brings this slab to the pole for a mantle of uniform viscosity. The same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle. Our model is also suitable to investigate the effects of a non-steady-state convection on the Earth's rotation. We discuss these effects using a simple mass redistribution model inspired by the pioneering paper of Goldreich & Toomre (1969). It consists of studying the TPW induced by a random distribution of slabs sinking into the mantle. For such a mass redistribution, only a strongly stratified mantle can reduce the Earth's pole velocity below 1d Ma-1, which is the upper bound value observed by palaeomagnetic investigations for the last 200 Ma. Our model also shows that when corrected for the hydrostatic flattening, the Earth's polar inertia generally corresponds to the maximum inertia, as it is presently observed. However, this may not be the case during some short time periods. We also discuss the amount of excess polar flattening that can be related to tidal deceleration. This frozen component is found to be negligible. Copyright © 1993, Wiley Blackwell. All rights reserved

A detailed map of the 660-kilometer discontinuity beneath the izu-bonin subduction zone.

Abstract: Dynamical processes in the Earth's mantle, such as cold downwelling at subduction zones, cause deformations of the solid-state phase change that produces a seismic discontinuity near a depth of 660 kilometers. Observations of short-period, shear-to-compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the Izu-Bonin subduction zone. The discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab, with a deformation profile consistent with the expected thermal signature of the slab, the experimentally determined Clapeyron slope of the phase transition, and the regional tectonic history.

Real-time simulation of heat transfer in continuous casting

Abstract: A real-time heat-transfer model for continuous slab casting is presented. The model calculates the strand temperatures and the solid shell thickness profile along the machine as a function of the actual casting variables, strand geometry, and steel grade. The special requirements con-cerning the real-time use of the model and, in general, the accuracy of the model are also studied and discussed. The model has been tested by carrying out industrial trials. Some examples of the differences between the calculated and measured surface temperatures are presented. A spe-cial procedure to determine the boundary conditions for the secondary cooling zones from tem-perature measurements is also described.

Open-ended rectangular waveguide for nondestructive thickness measurement and variation detection of lossy dielectric slabs backed by a conducting plate

Abstract: Solutions for fields inside a slab of a generally lossy dielectric medium backed by a conducting plate, placed outside a waveguide-fed rectangular aperture, are used for the microwave nondestructive thickness measurement of such dielectric slabs. Upon construction of the waveguide terminating admittance expression from its variational form, an inverse problem is solved to extract the slab thickness form the conductance and susceptance in a recursive manner. A comparison between the experimental and theoretical results showed that the significance of higher order modes is minimal; hence, the dominant mode assumption is, in general, valid for describing the aperture field distribution. The validity of this assumption has led to the construction of a simple integral solution which is fast converging for generally lossy dielectric slabs, and may easily be implemented for real-time applications. Good agreement was obtained between the theoretical and experimental results. Multiple thicknesses of two different dielectric samples were estimated in this way. >

Tomographic inversion of P and pP data for aspherical mantle structure below the northwest Pacific region

Abstract: Summary To investigate the morphology of subducted slab in the mantle below northwest Pacific island arcs we inverted traveltime residuals for aspherical variations in P-wave propagation velocity relative to the radially symmetric iasp91 reference model. The tomographic method used is based on a step-wise linearization of the inversion problem. First, we relocated ISC (International Seismological Centre) hypocentres with re-identified P and pP phase data using the iasp91 traveltime tables. The variance of P residuals relative to iasp91 traveltimes was 17 per cent less than the variance of P data reported by the ISC relative to the Jeffreys-Bullen (J-B) traveltime tables. Second, we performed a linearized (LSQR) inversion for Earth structure and source relocation with the P and pP residuals obtained from the first step, using iasp91 as the reference model for seismic velocities. The incorporation of the depth phase pP in the tomographic inversions has two major advantages: (1) the pP data provide constraints on focal depth and thus reduce the trade-off between source relocation and structure; and (2) the pP ray paths improve the sampling of Earth structure in the shallow mantle and transition zone. We used more than 2 times 106 and about 1 times 105P- and pP-wave traveltime residuals, respectively, from about 40 000 earthquakes with epicentres in the study region that were recorded at one or more of the 2300 globally distributed seismological stations considered in this study. We assessed the spatial resolution in the tomographic images with checker board-type sensitivity tests. These tests reveal high resolution of upper mantle and transition-zone structure, particularly below the central part of our study region. Structure with wavelengths of the order of 100 km is resolved below Japan, whereas structure with wavelengths of the order of 300 km is well resolved below the Kuril, Izu Bonin and Ryukyu arcs. Small-scale structure is poorly resolved in depth below the northern part of the Kuril-Kamchatka arc and below the Izu Bonin and Mariana arcs. This limits the interpretation of slab structure and mantle flow from tomographic images alone. With this limitation in mind, we conclude from the tomographic images that subducted slab deflects in the mantle transition zone below the geographical area encompassed by the Kuril basin, the Japan Sea, and the northern part of the Philippine Sea. This is in good agreement with the results of other recently published tomographic studies, the occurrence of earthquakes several hundred kilometres off the inclined Wadati-Benioff seismic zones, and inferences about ‘660 km’ discontinuity topography. In contrast, slab-like structures of high P-wave velocity are imaged in the lower mantle below the deepest earthquakes of the northern Kuril-Kamchatka and Mariana seismic zones. This is indicative of local slab penetration of the lower mantle. From tomographic images we cannot discern between compositionally or thermally induced variations in seismic velocity. However, with regard to the nature of the boundary between upper and lower mantle, our observations argue against either compositional mantle layering with large contrasts in intrinsic density or phase changes with steep Clapeyron slopes.

Composite modular building panel

Abstract: A modular building panel comprising a single, monolithic, planar slab having a thickness and parallel inner and outer surfaces, the slab being formed primarily of cellular concrete and a pair of parallel linear members. Each of the linear members has outer and inner opposed parallel edges, a web connecting the edges, and an outer flange at the outer edge extending substantially perpendicular to the web. The slab extends between the webs of the outer side linear members, the outer edges and the outer flanges being embedded in the slab inwardly of its inner surface, and the outer surface of the slab being uninterrupted. The panel can additionally include at least one inner linear member intermediate and parallel to the outer side linear members. The outer edge, the outer flange, and at least part of the web of the at least one inner linear member are also embedded in the slab inwardly of its inner surface, and the inner surface of the slab is interrupted by the at least one linear member extending outwardly thereof. An insulation panel is positioned against the inner surface of the slab between pairs of adjacent linear members and is dimensioned to cover the inner surface of the slab between the pair of linear members.

Separating mantle from slab signatures in arc lavas using B/Be and radiogenic isotope systematics

Abstract: AT convergent margins, tectonic processes juxtapose subducted slab, mantle wedge and the crust of the upper plate in a column beneath the overlying arc volcano. As each of these components is expected to be chemically heterogeneous, and as all may contribute to magma chemistry, identifying the different sources of arc magmas has been difficult. A working hypothesis has emerged, in which tholeiitic and calc-alkaline lavas in island arcs are partial melts of the mantle produced by fluxing of the wedge by hydrous fluids from the subducted slab1,2. Trace-element and radiogenic isotope ratios have been used to define the chemical characteristics of these sources but cannot be unequivocally identified with one source; by contrast, high B/Be and 10Be/9Be ratios in arc lavas uniquely identify the subduction component3,4, and thus separate chemical variability owing to recent subduction from that reflecting other causes. Here we combine B/Be with Sr, Nd and Pb isotope systematics of alkaline, calc-alkaline and tholeiitic lavas from Java and Flores, Indonesia, to constrain the isotopic composition of their mantle and subduction sources. The alkaline lavas always have low B/Be, from which we conclude that they are derived from mantle that has not been modified by recent subduction (in agreement with refs 5 and 6). We also show that the isotopic composition of the subducted component is relatively homogeneous along the length of the arc, suggesting that the subduction of Australian continental lithosphere in the east started too recently to have changed the nature of the subducted material at present beneath Flores.

Modelling the plate-driving mechanism

Abstract: This paper demonstrates how sub-lithospheric loading gives rise to plate interior stresses and plate boundary forces. The stressing of the lithosphere resulting from plume heads in the upper mantle (hot spots), from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis. It is first demonstrated how a sub-lithospheric load, exemplified by a sub-continental plume head, produces local deviatoric stress in the strong upper lithosphere within a plate interior region. It is shown that a 500 km wide region of anomalous density averaging −8 kg m −3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 MPa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load. The large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load. When such a stress system is cut across by a zone or plane of weakness, then plate boundary forces and distant plate interior stresses are produced. A weak zone beneath the crest is incorporated in a model of a normal ocean ridge, and this yields a ridge push force of 2.5 × 10 12 Nm −1 referenced to old ocean floor and deviatoric compression of about 40 MPa in old ocean floor. A model of a ridge underlain by an anomalously low density upper mantle (plume head) yields a much larger ridge push force, and large deviatoric compressions of nearly 100 MPa extend into the bordering continents. Subduction pull (slab pull and trench suction) of about −4.5 × 10 12 Nm −1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault, and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges.

Behavior of Composite Timber‐Concrete Floors

Abstract: Timber‐concrete floors are widely used in the Persian Gulf region because of their resistance to the hot and aggressive environment of the area Because no shear connector is provided, the timber joists and concrete slab work independently In this study, it is suggested that relatively inexpensive high‐strength nails be used as shear connectors so that the timber joists and concrete slab resist the loads as a composite section Experimental investigations have shown that it is possible to attain full composite action through the use of these high‐strength nails The proposed composite timber‐concrete floor system was subjected to static short‐term loading, repeated loading, and long‐term sustained loading It is observed that in terms of strength and serviceability the behavior of the proposed system is within the limits set by standard building code This behavior is also verified by a finite‐element analysis The experimental and numerical results clearly reveal that it is possible to achieve a stiffer

Bend loss of slab and fiber modes computed with diffraction theory

Abstract: Radiation losses from optical waveguides that are bent into a circle are calculated with the help of Fraunhofer's diffraction theory. The validity of the result is tested by showing that it is in agreement with the known bend loss formulas of the slab waveguide of the LP/sub 01/-mode of the round step-index fiber. The accuracy of the bend loss calculations is limited by the use of a scalar theory to approximate a vector problem and by additional mathematical approximations that are required to solve an integral. >

Deformation of a weak subducted slab and variation of seismicity with depth

Abstract: Numerical simulations of slab evolution are used here to show that a weak slab model is consistent with seismic observations. Assuming that earthquakes occur at a rate proportional to deformation rate, the observed variation with depth of seismicity rate and focal mechanisms is reproduced along with the cessation of seismicity at 670 km depth. Provided that sinking material encounters resistance at depth, the pattern of seismicity can be explained by any mechanism for deep earthquakes in which the rate of seismicity is proportional to strain rate in the slab.

The influence of the layered character of snow cover on the triggering of slab avalanches

Abstract: One of the principal aims of avalanche warning is to prevent slab avalanches triggered by skiers. Other than explosives, the best practical tool for stability evaluation is the Rutschblock test. Whether the slab may be triggered by a skier depends on various slab characteristics. Important factors seem to be depth of the weak layer, slab hardness and sublayering of the slab. The stress distribution induced by a skier is calculated by the finite element method for typical snow-cover configurations. The additional shear stress is of the same order of magnitude as the shear strength of weak layers. Besides the critical weak layer - prerequisite for a slab avalanche - hard layers seem to be important. The analysis suggests that a shear failure is most probable at the transition from a hard to a soft layer. This corresponds well to observations of slab-avalanche profiles. The results may help to improve and quantify the analysis of snow profiles.

Transient wave propagation in reciprocal bi‐isotropic media at oblique incidence

Abstract: In this paper a new wave splitting is suggested that simplifies the analysis of wave propagation in reciprocal bi‐isotropic media. Two different methods to solve the scattering problem are analyzed: the invariant imbedding and the Green function approach. The medium is modeled by constitutive relations in the time domain (time convolutions) and the slab is assumed to be inhomogeneous with respect to the depth. It is shown that the cross‐polarized contribution to the reflected field at normal incidence is zero for the general reciprocal inhomogeneous slab. Moreover, the rotation and the attenuation of the wave front are calculated explicitly in the general inhomogeneous slab case. Special attention is paid to normal incidence and to the homogeneous semi‐infinite medium.

Control of molten steel flow in continuous casting mold by two static magnetic fields imposed on whole width

Abstract: An electromagnetic flow control device for molten steel in the mold of a continuous slab caster has been developed to improve the quality of steel sheet products. The newly designed device was characterized by two static magnetic fields imposed on the whole width of the slab. Numerical simulations and mercury model experiments confirmed the decrease of the liquid flow in the mold. Practical application to the No. 3 caster in Chiba Works resulted in the improvement of the surface and internal quality of the final products.

Fundamental phenomena governing heat transfer during rolling

Abstract: To quantify the effect of roll chilling on the thermal history of a slab during hot rolling, tests were conducted at the Canada Center for Mineral and Energy Technology (CANMET) and at the University of British Columbia (UBC). In these tests, the surface and the interior temperatures of specimens were recorded during rolling using a data acquisition system. The corresponding heat-transfer coefficients in the roll bite were back-calculated by a trial-and-error method using a heat-transfer model. The heat-transfer coefficient was found to increase along the arc of contact and reach a maximum, followed by a decrease, until the exit of the roll bite. Its value was influenced by rolling parameters, such as percent reduction, rolling speed, rolling temperature, material type,etc. It was shown that the heat-transfer coefficient in the roll gap was strongly dependent on the roll pressure, and the effect of different variables on the interfacial heat-transfer coefficient can be related to their influence on pressure. At low mean roll pressure, such as in the case of rolling plain carbon steels at elevated temperature, the maximum heat-transfer coefficient in the roll bite was in the 25 to 35 kW/m2 °C range. As the roll pressure increased with lower rolling temperature and higher deformation resistance of stainless steel and microalloyed grades, the maximum heat-transfer coefficient reached a value of 620 kW/m2 °C. Obviously, the high pressure improved the contact between the roll and the slab surface, thereby reducing the resistance to heat flow. The mean roll-gap heat-transfer coefficient at the interface was shown to be linearly related to mean roll pressure. This finding is important because it permitted a determination of heat-transfer coefficients applicable to industrial rolling from pilot mill data. Thus, the thermal history of a slab during rough rolling was computed using a model in which the mean heat-transfer coefficient between the roll and the slab was determined from an estimate of the rolling load. It was found that the heat loss of a slab to the roll was 33 pet of the total, which emphasizes the importance of accurately characterizing the heat-transfer coefficient in the roll bite during hot rolling.

Modeling of steel grade transition in continuous slab casting processes

Abstract: Mathematical models have been developed and applied to investigate the composition distributions that arise during steel grade changes in the continuous slab casting processes. Three-dimensional (3-D) turbulent flow and transient mixing phenomena in the mold and the strand were calculated under conditions corresponding to a sudden change in grade. The composition distribution in the final slab was then predicted. Reasonable agreement was obtained between predicted and experimental concentration profiles in the slab centerlines. Intermixing in the center extends many meters below the transition point, while intermixing at the surface extends above. Higher casting speed increases the extent of intermixing. Mold width, ramping of casting speed, and nozzle design have only small effects. Slab thickness, however, significantly influences the intermixing length of the slab. The axial transport of solute due to turbulent eddy motion was found to be many orders of magnitude greater than molecular diffusion and thus dominates the resulting composition distribution. Different elements, therefore, exhibited the same mixing behavior under the same casting conditions, despite having different molecular properties. Numerical diffusion caused by the finite difference schemes was investigated and confirmed to be much less important than turbulent diffusion. In the lower portion of the strand (lower than 3 m below the meniscus), the convection and diffusion can be reasonably approximated as one-dimensional (1-D) axial flow.

The Gravitational Stability of a Compressed Slab of Gas

Abstract: We consider the linear stability of an isothermal, pressure-bounded, self-gravitating gas slab. Such a configuration is unstable at sufficiently long wavelengths and grows at rate ∼ √Gρ, where ρ is the density of the gas slab, but at high external pressure the nature of this instability is quite different from that of the usual Jeans instability. We develop an analytical model that reproduces the behaviour of the instability found numerically by Elmegreen & Elmegreen. They found, surprisingly, that the critical wavenumber for the onset of the instability is always of the order of the layer thickness, independently of the level of self- gravity and the external pressure

The effects of phase transition kinetics on subducting slabs

Abstract: The effects of kinetics on the non-equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically. A one-dimensional model consisting of the kinetic equations and the heat-diffusion equation with latent-heat release has been constructed. Numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume. For slow slab velocities, less than 6 cm/yr, near equilibrium conditions are found. Finger-like structures emanating from the phase boundaries are obtained in this regime. These phase boundary prolusions may cause earthquakes. For slab velocities of around 10 cm/yr the metastable olivine region may be pushed down to a depth of about 600 km, with a sharp phase boundary. A direct transition from olivine to perovskite may be feasible.