Changes In Grain Size Distribution Research Articles

The Sr and Nd isotopic variations of the Chinese Loess Plateau during the past 7 Ma: Implications for the East Asian winter monsoon and source areas of loess

143Nd/ 144Nd and 87Sr/ 86Sr ratios of the acid-insoluble residues of the red clay and overlying loess–paleosols from the Lingtai profile of the Loess Plateau, China, were investigated. The results show that variations of the 87Sr/ 86Sr ratios of the Lingtai profile can be divided into two stages. From ∼ 7 Ma B.P. to 2.5 Ma B.P., the acid-insoluble residues in the Red Clay (RC) Formation, are characterized by higher 87Sr/ 86Sr ratios with an average of 0.7230. From 2.5 Ma B.P. to the present, the acid-insoluble residues in the Wuchen Loess (WL4-WS1), Lishi Loess (L15-S1), Malan Loess (L1) and Holocene Loess (S0) have relatively lower 87Sr/ 86Sr ratios and display an overall descending trend from 0.7223 at ∼ 2.5 Ma B.P. to 0.7182 at the present. Among three possible interpretations for the variations of 87Sr/ 86Sr ratios of acid-insoluble residues of the loess (provenance change, chemical weathering change, or grain-size distribution change), only grain-size distribution change provides a satisfactory interpretation. This implies that the East Asian winter monsoon strength was weak and relatively stable from ∼ 7 Ma B.P. to ∼ 2.5 Ma B.P., but became continuously enhanced from ∼ 2.5 Ma B.P. to the present. All the red clay and the overlying loess-paleosols in the Lingtai profile have generally identical ε Nd (0), implying that the source regions of these Eolian deposits of the Chinese Loess Plateau may be relatively stable and has not changed since 7 Ma B.P.

Changes in Grain Size Distribution of a Submicron Grained Al-Sc Alloy during High Temperature Annealing

Severe plastic straining is an established method for producing submicron grain (SMG) structures in alloys. However, the development of such a fine grained structure in single-phase alloys is usually futile if they are to be exposed or processed at elevated temperatures. This is a direct consequence of the natural tendency for rapid and substantial grain coarsening which completely removes the benefits obtained by grain refinement. This problem may be avoided by the introduction of nanosized, highly stable particles in the metal matrix. In this work, a SMG structure was generated in an Al-0.3 wt.% Sc alloy by Equal Channel Angular Pressing (ECAP). The alloy was prepared initially to produce a fine grained microstructure exhibiting a large fraction of high angle grain boundaries and a dispersion of nanosized Al3Sc particles. The evolution of microstructure during annealing at temperatures up to 550 °C was examined in detail and grain size distributions generated from the data. It was shown that grain coarsening is rapid at temperatures above 450 °C and the initial log-normal grain size distribution exhibiting low variance and skewness was altered considerably. The statistical information generated from the grain size distributions confirms that discontinuous grain coarsening occurs in this alloy only at temperatures greater than 500 °C.

Sediment trend analysis of the hylebos waterway: Implications for liability allocations

Sediment trend analysis of the hylebos waterway: Implications for liability allocations

Effects of grain size distribution on the mechanical response of nanocrystalline metals: Part II

The model of Zhu et al. [Zhu B, Asaro RJ, Krysl P, Bailey R. Transition of deformation mechanisms and its connection to grain size distribution in nanocrystalline metals. Acta Mater 2005;53(18):4825–38] is further developed and used to explore the effect of grain size and grain size distribution, along with the influence of material parameters, on the mechanical response of nanocrystalline face-centered cubic aggregates. This model accounts for the simultaneous contributions of deformation mechanisms including grain boundary emission of dislocations and/or stacking faults, as well as for mechanisms such as grain boundary sliding and for natural transitions between the relative dominance of each. The effect of grain growth during deformation is also quantitatively assessed via simulation of recently obtained data on indentation tests in which dynamic grain growth was documented through the measurement of changes in grain size distribution and concomitant changes in hardness. The simulations provide a plausible description of the observed phenomenology and further underscore the unstable nature of nanocrystalline grain size distributions. The possibility of incorporating additional potential deformation mechanisms such as Coble creep, as has been proposed in other models, is discussed and shown to be straightforward addition to the model. Recently obtained data on texture development is analyzed via texture predictions for aggregates subject to finite deformations via high pressure torsion (HPT). The phenomenology is assessed specifically with regard to the potential use of texture measurements for confirming the importance of crystallographic mechanisms vs. those such as grain boundary sliding.

Attrition and particle breakage under monotonic and cyclic loading

The morphological evolution of gravel grains under monotonic and cyclic loading, typically present in road structures, is presented. Two types of mineralogy are considered: a limestone and a sandstone. Loading in uniaxial strain conditions are applied, with a maximum mean pressure of 5 MPa and a maximum number of cycles of 250 000. Variations in grain size distribution and grain shape, measured with image analysis techniques, are discussed in detail. In the case of uniform grain size distribution, particle breakage does appear for stresses around 500 kPa for the limestone grains and 100 kPa for the sandstone grains. For well-graded materials under low cyclic pressures (<1 MPa), changes in grain size distribution can hardly be detected. To cite this article: F. Mayoraz et al., C. R. Mecanique 334 (2006).

Lena River discharge events in sediments of Laptev Sea, Russian Arctic

To reconstruct the variability of freshwater discharge to the Arctic via the Lena River, 22 gravity cores were collected along two transects that extend from the delta floodplain onto the Laptev Sea shelf. High magnetic susceptibility (MS) signals appear to be good indicators for river sediment deposition. Sedimentation of this terrestrial material forms fluvial units distinguished by coarse grain size and high MS signal produced by the magnetic properties from the river-derived material. The criteria to distinguish marine units are low magnetic susceptibility signals when compared with fluvial units, lithological descriptions and granulometric analyses. Sedigraph granulometric analyses in combination with wet sieving analyses (grain size > 4 Φ) reveal changes in grain-size distributions along the cores that are well correlated with magnetic susceptibility data. Fluvial and marine units were found to alternate sequentially along the cores. Data from cores comprising two transects in the Laptev Sea revealed 17 total fluvial events during the last 2000 years. The Lena River-derived sediment bedload could be traced 34.5–47.5 km far from the delta edge of Bykovskaya branch showing direct impact of Lena River discharge to the Laptev Sea.

A Constitutive Model for Crushed Granular Aggregates Which Includes Suction Effects

ABSTRACT The mechanical response of rockfill is largely dominated by particle breakage, which is controlled by stress level and the prevailing relative humidity (RH) in the large rockfill voids. The paper presents a model for rockfill behaviour under triaxial stress states, which is based on: (a) existing knowledge on rockfill compressibility, (b) experimental and theoretical developments on sand behaviour and (c) a series of suction-controlled triaxial tests performed in a new large-diameter triaxial cell, in which samples of gravel were tested under two relative humidities: 36% (“dry”) and 100% (saturated). Changes in grain size distribution were measured in all the tests performed. The model is formulated within the framework of work hardening plasticity and critical state concepts. Specimen stiffness and limiting conditions depend on the accumulated plastic work and on the prevailing RH. The model reproduces in a satisfactory way the set of triaxial tests performed.

Milankovitch-scale multi-proxy records from fluvial sediments of the last 2.6 Ma, Pannonian Basin, Hungary

The continuous cores of two nearly 500 m deep boreholes, cutting Pleistocene fluvial sequences in the eastern part of the Pannonian Basin, Hungary have been studied. Magnetostratigraphy indicates that the sediment sequences span the last 2.6 Ma without significant hiatuses. Comparison of systematic variations in magnetic susceptibility with changes in grain size distribution revealed Milankovitch-scale cyclicity in sedimentation of 40 and 100 kyr cyclicity, which also shows a correspondence with the δ 18O variations of the ODP 677 site. Comparison of pollen and gastropod paleoecological data with the magnetic susceptibility proxies suggests that climate was a major allogenic control on the cyclicity. Zones of high magnetic susceptibility correspond closely with sandy and coarse silty parts of the borehole successions which largely coincide with pollen and molluscs indicating a warm climate. On the contrary zones with low magnetic susceptibility correspond to silts and clays, and in most cases coincide with cold climate indicator pollen and molluscs. Heavy mineral and XRD studies revealed that magnetic susceptibility reflects the absolute amount of detrital Fe-oxide minerals, mostly magnetite. Variations in the amount of magnetite are related to changes in heavy mineral content, which arise primarily from changes in sediment supply to the basin by rivers. The depositional model for the Pleistocene fluvial sediments suggests that sediment flux increased towards the basin during interglacials, due to the increased discharge and transport capacity of the rivers, while a decreased sediment supply to the distal parts of the basin occurred during glacials. A delay between erosion and storage of weathering products near to the source area and subsequent transportation and re-deposition of sediments in the basin interior setting is an important cause of this effect.

A constitutive model for granular materials with grain crushing and its application to a pyroclastic soil

AbstractA constitutive model for granular materials is developed within the framework of strain–hardening elastoplasticity, aiming at describing some of the macroscopic effects of the degradation processes associated with grain crushing. The central assumption of the paper is that, upon loading, the frictional properties of the material are modified as a consequence of the changes in grain size distribution.The effects of these irreversible microscopic processes are described macroscopically as accumulated plastic strain. Plastic strain drives the evolution of internal variables which model phenomenologically the changes of mechanical properties induced by grain crushing by controlling the geometry of the yield locus and the direction of plastic flow.An application of the model to Pozzolana Nera is presented. The stress–dilatancy relationship observed for this material is used as a guidance for the formulation of hardening laws. One of the salient features of the proposed model is its capability of reproducing the stress–dilatancy behaviour observed in Pozzolana Nera, for which the minimum value of dilatancy always follows the maximum stress ratio experienced by the material. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Impact of offsite sediment transport and toxicity on remediation of a contaminated estuarine bay

Sediment of Ostrich Bay, an arm of Dyes Inlet on Puget Sound, was historically contaminated with ordnance compounds from an onshore US Navy facility. An initial recommendation for a sediment cover to mitigate benthic risks was followed by studies of sediment transport and deposition to determine whether contaminated sediment from Dyes Inlet or other offsite sources in Puget Sound may contribute to Ostrich Bay impacts. A Sediment Trend Analysis (STA ®) identified net sediment transport pathways throughout the bay and inlet by examining changes in grain size distributions in multiple adjacent samples. Results indicated that fine-grained sedimentary material transports into and deposits throughout the Dyes Inlet system, with no erosion or transport out of Ostrich Bay. Echinoderm larvae mortality bioassay results were elevated in fine-grained sediments of both Ostrich Bay and Dyes Inlet. Ordnance compounds were undetected, and although sediment mercury concentrations were elevated at 0.48–1.4 mg/kg in both waterbodies, the relationship with toxicity was weak. Results of the studies and sedimentation modeling indicate that impacted sedimentary material deposits throughout the Dyes Inlet/Ostrich Bay system from unknown sources and will prevent natural recovery of Ostrich Bay as well as negate long-term effectiveness of active remedial measures. Stakeholders have recognized that remediation of the bay can be achieved only after the toxicity of depositing sediment decreases.

Analytical and Experimental Investigations into Microstructure Evolution in Hot Bar Stretching.

Transition in austenite grain size distribution after hot forging, which is closely related to recrystallization, is investigated. Rapid change in grain size distribution just after hot bar stretching is experimentally obtained by hot compression test with controlled rapid quenching, and experimental results are examined by microstructure analysis using incremental approach with three-dimensional finite element analysis for plastic deformation. Through investigation, it has become clear that transient change in microstructure is very rapid with higher amount of plastic deformation. Microstructure analysis using incremental approach with three-dimensional finite element analysis for plastic deformation is helpful to evaluate rapid change in cross-sectional grain size distribution after hot bar stretching.

Burrow architecture and turbative activity of the thalassinid shrimp Callianassa subterranea from the central North Sea

The architecture and development of the burrows of the endobenthic shrimp Callianassa subterranea from the central North Sea were studied in sediment-filled containers and thin cuvettes in the laboratory. Three-dimensional burrows of 81 shrimps were used to describe the 3-dimensional burrow architecture. In total, 41 shrimps made 2-dimensional burrows in cuvettes tailored to their body widths. Development of 8 burrows over time was registered by regularly mapping burrow outlines and sediment surface levels. Excavation Velocities and sediment expulsion rates were derived from changes in the burrow outlines and sediment surface levels on the maps. The total tunnel length increased at a rate of 23.4 +/- 6.0 mm h(-1) during the initial stage of burrow development. The length increase levelled off during the completion of the first and second U-tubes to 5.6 +/- 1.8 mm h(-1). Initial sediment expulsion rates up to 15.0 cm(3) d(-1) were established. The average sediment expulsion rate was 1.080 +/- 0.096 cm(3) d(-1). Extrapolation to a yearly dry weight (dry wt) sediment turnover, including population density and water temperature effects, resulted in an estimate of 15.5 +/- 2.7 kg dry wt m(-2) yr(-1), equivalent to a 1.2 cm layer. Samples of burrow lining, expelled sediment and unprocessed sediment did not show changes in grain size distributions or organic content due to manipulation or processing by C. subterranea. Burrow development experiments carried out in enriched seawater systems did not reveal consistent effects of particulate organic matter (POM) on sediment expulsion rates or on the composition of processed or unprocessed sediment.

Image analysis of structural changes in laser irradiated thin films of photodeposited a-Se

Write and erase features by the photodeposition effect have been accomplished in recent investigations. Quantitative analysis of the surface grains size and structure in the laser affected areas of the deposited thin films is of importance to the understanding of the material surface redistribution phenomena. Fractal dimensionality and 2D-Fourier spectral analyses have been employed as the most straightforward quantitative approaches connecting the change of grain size distribution with the driving forces involved in the formation of the grain shapes. These approaches have been used since the irregular spatial shapes of the grains cannot be approximated by simple or regular geometrical shapes of definite size. Hence, we use these two topological analysis methods for classifying the surface morphology in the various zones of the laser beam written thin film.

Neutron-induced microstructural alteration of GlidCop TM alloys at ∼415°C and high neutron exposure

GlidCop TM internally oxidized copper alloys remain the leading candidates for high heat flux applications in fusion reactors. This paper presents the microstructural changes incurred in three GlidCop TM alloys exposed to long term, high temperature neutron irradiation. Irradiation at high temperature produced a microstructure containing a much lower dislocation density than the unirradiated specimens. Although 10–50 nm size triangular oxide particles were observed in areas with a very low number density of particles, spherical oxide particles on the order of 5–7 nm in diameter, though to be CuAl 2O 4, were the predominant morphology. The changes in grain size distribution, dislocation density, and precipitate type and distribution saturate in the range of 34 to 50 dpa, as reflected in the saturation of mechanical properties.

Change in Grain Size Distribution during Grain Growth

Change in Grain Size Distribution during Grain Growth

On the pteropod pavement of the eastern Rio Grande Rise

A pteropod-rich surface layer covers the seafloor on the Rio Grande Rise and its vicinity in less than 3400 m water depth. Sedimentologic analyses of seven box cores, including oxygen isotope stratigraphy and radiocarbon dating, indicate that the pteropod pavement is a lag deposit, and that agitation by bottom currents delays the burial of pteropod shells. Changes in grain size distribution within cores suggest that the strength of bottom circulation has varied considerably through time. Maximum winnowing of fines occurred near the end of full glaciation, decreased in intensity during mid-deglaciation and increased again within the Holocene, culminating in recent erosion. The recent increase in winnowing is probably tied to an increase in the production of North Atlantic Deep Water, which also would have produced a drop in the aragonite compensation depth (ACD). Changes in the abundances of pteropods downcore support this hypothesis. Fe-staining of pteropod shells in the surface layer occurs throughout the Holocene with Fe-accumulation rates on the order of 1 μ m 1000 years .

Grain Coarsening of Water-Saturated Snow

AbstractExperimental measurements were made of changes in grain-size distribution with time in snow saturated with solutions of various impurity contents. Qualitatively, the changes in grain-size distribution occur by shrinkage and eventual disappearance of the relatively small particles and growth of the relatively large particles by a solid mass-exchange process which conserves the total solid mass. The distribution of relative grain size is found to be essentially time independent except for transient effects lasting only several to several tens of hours after the time of initial saturation. Mean grain volume increases at a constant rate, which for solutions of impurity concentration less than about 0.01 mol 1–1 is (5 to 6) X 10–3 mm3 h–1. In pure solutions the smallest particles shrink at a characteristic rate of about 1 x 10–2 mm3 h–1. Once the steady relative-size distribution is established, the rate of volume change of typical grains varies linearly with grain volume from the characteristic negative rate for the smallest particles through zero for particles of mean volume to positive values for particles of larger volume. The basic features of the changes that take place are explained in terms of heat-flow controlled melting and freezing determined by temperature differences associated with the effect of particle surface curvature on melting temperature. The constant rate of increase of mean grain volume is a consequence of conservation of total ice volume. The expectation that particles of intermediate size would be neither shrinking or growing leads to the conclusion that the actual rate of increase in mean grain volume is about one-half the characteristic melting rate of the smallest particles, which fits the observations. The process of grain growth is slowed by impurities in a way which can be predicted from the melting-temperature depression caused by the impurity and its diffusion coefficient. The transport of heat between grain surfaces is largely through the liquid-filled gaps between them, but about 19% is conducted through the solid.

Grain Coarsening of Water-Saturated Snow

AbstractExperimental measurements were made of changes in grain-size distribution with time in snow saturated with solutions of various impurity contents. Qualitatively, the changes in grain-size distribution occur by shrinkage and eventual disappearance of the relatively small particles and growth of the relatively large particles by a solid mass-exchange process which conserves the total solid mass. The distribution of relative grain size is found to be essentially time independent except for transient effects lasting only several to several tens of hours after the time of initial saturation. Mean grain volume increases at a constant rate, which for solutions of impurity concentration less than about 0.01 mol 1–1is (5 to 6) X 10–3mm3h–1. In pure solutions the smallest particles shrink at a characteristic rate of about 1 x 10–2mm3h–1. Once the steady relative-size distribution is established, the rate of volume change of typical grains varies linearly with grain volume from the characteristic negative rate for the smallest particles through zero for particles of mean volume to positive values for particles of larger volume. The basic features of the changes that take place are explained in terms of heat-flow controlled melting and freezing determined by temperature differences associated with the effect of particle surface curvature on melting temperature. The constant rate of increase of mean grain volume is a consequence of conservation of total ice volume. The expectation that particles of intermediate size would be neither shrinking or growing leads to the conclusion that the actual rate of increase in mean grain volume is about one-half the characteristic melting rate of the smallest particles, which fits the observations. The process of grain growth is slowed by impurities in a way which can be predicted from the melting-temperature depression caused by the impurity and its diffusion coefficient. The transport of heat between grain surfaces is largely through the liquid-filled gaps between them, but about 19% is conducted through the solid.

Clay Mineral Diagenesis in the Rappahannock Estuary: An Explanation

Mineralogical changes observed in bottom sediments along the Rappahannock salinity gradient include: (1) decrease in kaolinite, (2) decrease in dioctahedral vermiculite, (3) increase in illite, (4) appearance of thermally stable chlorite, and (5) appearance of clay-sized feldspar. All of these changes are real. They correlate with increase in bottom water salinity from 0 to 20%, increase in bottom sediment pH from 5.5 to 7.8, change in grain size distribution, and change in adsorbed ion populations. In the critical area the bottom sediments lose clay by selective sorting and become more silty. The adsorbed ions are Ca and Al in fresh water, Ca and Na with minor K at low salinities, and Na and Ca with major fractions of exchangeable K and Mg at high salinites. Studies of the suspended sediments over a period of years reveal that illite, vermiculite, kaolinite, and clay-sized feldspar are the normal constituents in the load of the fresh water stream. No mineralogical segregation, by differential flocculation for example, occurs during transport of this suspension along the salinity gradient. The increase in illite and feldspar content of the bottom sediments is due to normal processes of sediment deposition. The high kaolinite and dioctahedral vermiculite contents of fresh water bottom samples is due to two possible causes. (1) Samples are obtained from drowned, Pleistocene-weathered, terrace materials that possess this basic mineralogy. (2) Bedload sediment with kaolinite-dioctahedral vermiculite mineralogy is deposited in the fresh water region. Thermally stable chlorite is generated in the saline environment, probably from vermiculite. Simple ion exchange is insufficient to generate the chlorite. It forms by diagenesis.