Evolution Of Grain Size Distribution Research Articles

Modeling for critical state line of granular soil with evolution of grain size distribution due to particle breakage

Determination of the critical state line (CSL) is important to characterize engineering properties of granular soils. Grain size distribution (GSD) has a significant influence on the location of CSL. The influence of particle breakage on the CSL is mainly attributed to the change in GSD due to particle breakage. However, GSD has not been properly considered in modeling the CSL with influence of particle breakage. This study aims to propose a quantitative model to determine the CSL considering the effect of GSD. We hypothesize that the change of critical state void ratio with respect to GSD is caused by the same mechanism that influences of the change of minimum void ratio with respect to GSD. Consequently, the particle packing model for minimum void ratio proposed by Chang et al. (2017) is extended to predict critical state void ratio. The developed model is validated by experimental results of CSLs for several types of granular materials. Then the evolution of GSD due to particle breakage is incorporated into the model. The model is further evaluated using the experimental results on rockfill material, which illustrates the applicability of the model in predicting CSL for granular material with particle breakage.

A SEM-based method to determine the mineralogical composition and the particle size distribution of suspended sediment

A robust method for characterizing the mineralogy of suspended sediment in continental rivers is introduced. It encompasses 3 steps: the filtration of a few milliliters of water, measurements of X-ray energy dispersive spectra using Scanning Electron Microscopy (SEM), and robust machine learning tools of classification. The method is applied to suspended particles collected from various Amazonian rivers. A total of more than 204,000 particles were analyzed by SEM-EDXS (Energy Dispersive X-ray Spectroscopy), i.e. about 15,700 particles per sampling station, which lead to the identification of 15 distinct groups of mineralogical phases. The size distribution of particles collected on the filters was derived from the SEM micrographs taken in the backscattered electron imaging mode and analyzed with ImageJ freeware. The determination of the main mineralogical groups composing the bulk sediment associated with physical parameters such as particle size distribution or aspect ratio allows a precise characterization of the load of the terrigenous particles in rivers or lakes. In the case of the Amazonian rivers investigated, the results show that the identified mineralogies are consistent with previous studies as well as between the different samples collected. The method enabled the evolution of grain size distribution from fine to coarse material to be described in the water column. Implications about hydrodynamic sorting of mineral particles in the water column are also briefly discussed. The proposed method appears well suited for intensive routine monitoring of suspended sediment in river systems.

Evolution of grain size distributions and bed mobility during hydrographs in gravel‐bed braided rivers

AbstractEvolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel‐bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally‐stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel‐bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high‐resolution (~1 mm precision) digital elevation models generated from close‐range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel‐forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near‐equal mobility at peak channel‐forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D90 and active widths were narrow (< 3% of wetted width). Above this discharge, both increased so that at channel‐forming discharge, the active depth had a local maximum of 9D90 while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley & Sons, Ltd.

On the modified grain-size-distribution method to evaluate the dynamic recrystallisation fraction in AISI 304 stainless steel

The purpose of the present work was to verify the grain size distribution (GSD) method, which was recently proposed by one of the present authors as a method for evaluating the fraction of dynamic recrystallisation (DRX) in a microalloyed medium carbon steel. To verify the GSD-method, we have selected a 304 stainless steel as a model system and have measured the evolution of the overall grain size distribution (including both the recrystallised and unrecrystallised grains) during hot compression at 1,000 °C in a Gleeble machine; the DRX fraction estimated using the GSD method is compared with the experimentally measured value via EBSD. The results show that the previous GSD method tends to overestimate the DRX fraction due to the utilisation of a plain lognormal distribution function (LDF). To overcome this shortcoming, we propose a modified GSD-method wherein an area-weighted LDF, in place of a plain LDF, is employed to model the evolution of GSD during hot deformation. Direct measurement of the DRX fraction using EBSD confirms that the modified GSD-method provides a reliable method for evaluating the DRX fraction from the experimentally measured GSDs. Reasonable agreement between the DRX fraction and softening fraction suggests that the Kocks-Mecking method utilising the Voce equation can be satisfactorily used to model the work hardening and dynamic recovery behaviour of steels during hot deformation.

A revisited Johnson–Mehl–Avrami–Kolmogorov model and the evolution of grain-size distributions in steel

Abstract The classical Johnson–Mehl–Avrami–Kolmogorov approach for nucleation and growth models of diffusive phase transitions is revisited and applied to model the growth of ferrite in multiphase steels. For the prediction of mechanical properties of such steels, a deeper knowledge of the grain structure is essential. To this end, a Fokker–Planck evolution law for the volume distribution of ferrite grains is developed and shown to exhibit a log-normally distributed solution. Numerical parameter studies are given and confirm expected properties qualitatively. As a preparation for future work on parameter identification, a strategy is presented for the comparison of volume distributions with area distributions experimentally gained from polished micrograph sections.

Evolution of dust extinction curves in galaxy simulation

To understand the evolution of extinction curve, we calculate the dust evolution in a galaxy using smoothed particle hydrodynamics simulations incorporating stellar dust production, dust destruction in supernova shocks, grain growth by accretion and coagulation, and grain disruption by shattering. The dust species are separated into carbonaceous dust and silicate. The evolution of grain size distribution is considered by dividing grain population into large and small gains, which allows us to estimate extinction curves. We examine the dependence of extinction curves on the position, gas density, and metallicity in the galaxy, and find that extinction curves are flat at $t \lesssim 0.3$ Gyr because stellar dust production dominates the total dust abundance. The 2175 \AA\ bump and far-ultraviolet (FUV) rise become prominent after dust growth by accretion. At $t \gtrsim 3$ Gyr, shattering works efficiently in the outer disc and low density regions, so extinction curves show a very strong 2175 \AA\ bump and steep FUV rise. The extinction curves at $t\gtrsim 3$ Gyr are consistent with the Milky Way extinction curve, which implies that we successfully included the necessary dust processes in the model. The outer disc component caused by stellar feedback has an extinction curves with a weaker 2175 \AA\ bump and flatter FUV slope. The strong contribution of carbonaceous dust tends to underproduce the FUV rise in the Small Magellanic Cloud extinction curve, which supports selective loss of small carbonaceous dust in the galaxy. The snapshot at young ages also explain the extinction curves in high-redshift quasars.

Grain Growth Characteristics of Hydrothermally Prepared Yttria Stabilized Zirconia Nanocrystals during Calcination

Yttria stabilized zirconia (YSZ) nanocrystals were prepared by hydrothermal synthesis and were calcinated at different temperatures and with different isothermal hold time. X-ray diffraction via whole powder pattern modeling approach was used to study the grain growth as well as the evolution of grain size distribution of YSZ nanocrystals. Results show that YSZ nanocrystals start to grow along with the grain size distribution broadening at about 300 °C. The grain growth rate and the grain size distribution of YSZ nanocrystals are dependent on the calcination temperature and the isothermal hold time. The grain growth exponent and the active energy of grain growth were calculated. Grain rotation induced grain coalescence is suggested as the predominant way of grain growth of YSZ nanocrystals.

Molecular dynamics simulations investigating consecutive nucleation, solidification and grain growth in a twelve-million-atom Fe-system

Continuous processes of homogeneous nucleation, solidification and grain growth are spontaneously achieved from an undercooled iron melt without any phenomenological parameter in the molecular dynamics (MD) simulation with 12 million atoms. The nucleation rate at the critical temperature is directly estimated from the atomistic configuration by cluster analysis to be of the order of 1034m−3s−1. Moreover, time evolution of grain size distribution during grain growth is obtained by the combination of Voronoi and cluster analyses. The grain growth exponent is estimated to be around 0.3 from the geometric average of the grain size distribution. Comprehensive understanding of kinetic properties during continuous processes is achieved in the large-scale MD simulation by utilizing the high parallel efficiency of a graphics processing unit (GPU), which is shedding light on the fundamental aspects of production processes of materials from the atomistic viewpoint.

Yielding of rockfill in relative humidity-controlled triaxial experiments

The paper reports the results of suction-controlled triaxial tests performed on compacted samples of two well-graded granular materials in the range of coarse sand–medium gravel particle sizes: a quartzitic slate and a hard limestone. The evolution of grain size distributions is discussed. Dilatancy rules were investigated. Dilatancy could be described in terms of stress ratio, plastic work input and average confining stress. The shape of the yield locus in a triaxial plane was established by different experimental techniques. Yielding loci in both types of lithology is well represented by approximate elliptic shapes whose major axis follows approximately the K0 line. Relative humidity was found to affect in a significant way the evolution of grain size distribution, the deviatoric stress–strain response and the dilatancy rules.

A particle model for rockfill behaviour

Particle crushing has been introduced into a discrete model for rockfill aggregates by a novel approach which combines a closed-form solution for stress distribution inside particles and a crack propagation criterion derived from fracture mechanics concepts. Two breakage modes are simulated: a local contact crushing and an equal volume splitting. The occurrence of both modes derives from experimental observations. Three main parameters control the model performance: contact stiffness, basic friction angle of the particle material and the particle toughness. Contact stiffness and friction were determined by experiments. A large diameter oedometer test on hard limestone gravel was successfully simulated. The simulation included the prediction of the compression response and the evolution of grain size distribution. Rock toughness and basic friction angle were determined independently from the oedometer results. Subsequently, large-scale triaxial tests on the same material and different grain size distributions were simulated. Model parameters fitting the oedometer test were maintained unchanged. The predicted triaxial response (deviatoric stress–strain response, volumetric behaviour and final grain size distribution) was close to actual measurements.

Dust processing in elliptical galaxies

We reconsider the origin and processing of dust in elliptical galaxies. We theoretically formulate the evolution of grain size distribution, taking into account dust supply from asymptotic giant branch (AGB) stars and dust destruction by sputtering in the hot interstellar medium (ISM), whose temperature evolution is treated by including two cooling paths: gas emission and dust emission (i.e. gas cooling and dust cooling). With our new full treatment of grain size distribution, we confirm that dust destruction by sputtering is too efficient to explain the observed dust abundance even if AGB stars continue to supply dust grains, and that, except for the case where the initial dust-to-gas ratio in the hot gas is as high as $\sim 0.01$, dust cooling is negligible compared with gas cooling. However, we show that, contrary to previous expectations, cooling does not help to protect the dust; rather, the sputtering efficiency is raised by the gas compression as a result of cooling. We additionally consider grain growth after the gas cools down. Dust growth by the accretion of gas-phase metals in the cold medium increase the dust-to-gas ratio up to $\sim 10^{-3}$ if this process lasts >10/(n_H/10^3 cm^{-3}) Myr, where $n_\mathrm{H}$ is the number density of hydrogen nuclei. We show that the accretion of gas-phase metals is a viable mechanism of increasing the dust abundance in elliptical galaxies to a level consistent with observations, and that the steepness of observed extinction curves is better explained with grain growth by accretion.

A study of the dynamic recrystallization kinetics of V-microalloyed medium carbon steel

The Avrami kinetics of the dynamic recrystallization (DRX) of V-microalloyed medium carbon steel is studied by measuring the flow curves and evolution of grain size distribution (GSD) due to DRX and by modeling the flow softening kinetics and flow curves under various deformation conditions. The e1/2 method is proposed as a new method of modeling the Avrami kinetics and is shown to be, unlike the conventional t 1/2 method, capable of modeling the flow softening kinetics and flow curves with a single variable Z (= $$ {\dot{{\varepsilon }}} $$ exp(Q def/RT)). The time exponent “n” of the Avrami kinetics decreases from 2.3 to 1.4 with increasing Z, which results from a shift in the mode of necklace recrystallization from boundary migration dominant mode to nucleation dominant mode. The GSD method is proposed as a new method for estimating the DRX fraction in deformation conditions where it is difficult to distinguish recrystallized grains from un-recrystallized grains. The softening fractions estimated from the flow curves tend to overestimate the DRX fractions at a later stage of deformation, where DRX kinetics appear to be retarded by dynamic precipitation.

Two-size approximation: a simple way of treating the evolution of grain size distribution in galaxies

Full calculations of the evolution of grain size distribution in galaxies are in general computationally heavy. In this paper, we propose a simple model of dust enrichment in a galaxy with a simplified treatment of grain size distribution by imposing a `two-size approximation'; that is, all the grain population is represented by small (grain radius a < 0.03 micron) and large (a > 0.03 micron) grains. We include in the model dust supply from stellar ejecta, destruction in supernova shocks, dust growth by accretion, grain growth by coagulation and grain disruption by shattering, considering how these processes work on the small and large grains. We show that this simple framework reproduces the main features found in full calculations of grain size distributions as follows. The dust enrichment starts with the supply of large grains from stars. At a metallicity level referred to as the critical metallicity of accretion, the abundance of the small grains formed by shattering becomes large enough to rapidly increase the grain abundance by accretion. Associated with this epoch, the mass ratio of the small grains to the large grains reaches the maximum. After that, this ratio converges to the value determined by the balance between shattering and coagulation, and the dust-to-metal ratio is determined by the balance between accretion and shock destruction. With a Monte Carlo simulation, we demonstrate that the simplicity of our model has an advantage in predicting statistical properties. We also show some applications for predicting observational dust properties such as extinction curves.

FEM-simulation of machining induced nanocrystalline surface layers in steel surfaces prepared for tribological applications

A formation of nanocrystalline grains due to dynamic recrystallization within the workpiece surface layer (AISI4140) resulting from machining has proven to be suitable for obtaining improved tribological and fatigue behaviour. In the work presented an optimization of the machining process is carried out with cutting simulations using a continuous remeshing method and describing surface layer generation. The simulations describe the influence of process parameters and tool geometry on evolution of grain size distributions within the subsurface and affected depth after this thermo-mechanical processing. The validation is performed by experimental analyses based on cutting technology and focused ion beam technique.

Evolution of grain size distribution in high-redshift dusty quasars: integrating large amounts of dust and unusual extinction curves

Abstract The discoveries of huge amounts of dust and unusual extinction curves in high-redshift quasars (z ≳ 4) cast challenging issues on the origin and properties of dust in the early Universe. In this Letter, we investigate the evolutions of dust content and extinction curve in a high-z quasar, based on the dust evolution model taking account of grain size distribution. First, we show that the Milky Way extinction curve is reproduced by introducing a moderate fraction (≃0.2) of dense molecular-cloud phases in the interstellar medium for a graphite–silicate dust model. Then we show that the peculiar extinction curves in high-z quasars can be explained by taking a much higher molecular-cloud fraction ( ≳ 0.5), which leads to more efficient grain growth and coagulation, and by assuming amorphous carbon instead of graphite. The large dust content in high-z quasar hosts is also found to be a natural consequence of the enhanced dust growth. These results indicate that grain growth and coagulation in molecular clouds are key processes that can increase the dust mass and change the size distribution of dust in galaxies, and that, along with a different dust composition, can contribute to shape the extinction curve.

Effects of grain growth on the interstellar polarization curve

We apply the time evolution of grain size distributions by accretion and coagulation found in our previous work to the modelling of the wavelength dependence of interstellar linear polarization. We especially focus on the parameters of the Serkowski curve $K$ and $\lambda_{\max}$ characterizing the width and the maximum wavelength of this curve, respectively. We use aligned silicate and non-aligned carbonaceous spheroidal particles with different aspect ratios $a/b$. The imperfect alignment of grains with sizes larger than a cut-off size $r_{V,\rm cut}$ is considered. We find that the evolutionary effects on the polarization curve are negligible in the original model with commonly used material parameters (hydrogen number density $n_\mathrm{H}=10^3$ cm$^{-3}$, gas temperature $T_\mathrm{gas}=10$~K, and the sticking probability for accretion $S_\mathrm{acc}=0.3$). Therefore, we apply the tuned model where the coagulation threshold of silicate is removed. In this model, $\lambda_{\max}$ displaces to the longer wavelengths and the polarization curve becomes wider ($K$ reduces) on time-scales $\sim (30 - 50) (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr. The tuned models at $T < 30 (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1} $ Myr and different values of the parameters $r_{V,\rm cut}$ can also explain the observed trend between $K$ and $\lambda_{\max}$. It is significant that the evolutionary effect appears in the perpendicular direction to the effect of $r_{V,\rm cut}$ on the $K$ - $\lambda_{\max}$ diagram. Very narrow polarization curves can be reproduced if we change the type of particles (prolate/oblate) and/or vary $a/b$.

Refinement of coarse grained austenite in Nb–V–Ti microalloyed steel during roughing rolling

The refinement of coarse grained austenite in a Nb–V–Ti microalloyed steel during roughing rolling of a medium thin slab casting and direct rolling process was investigated by considering the austenite grain size distribution. The evolution of grain size distribution was analysed by taking into account dynamic recrystallisation (DRX), static recrystallisation (SRX) and metadynamic recrystallisation (MDRX), strain induced precipitation and retained strain. The results indicated that the initial coarse austenite was significantly refined during roughing rolling. Static recrystallisation is the main post-dynamic softening mechanism during all interpass intervals, but SRX+MDRX may also take place at certain times. The microstructural homogeneity of microalloyed steel produced in this work is better than that in steel produced by thin slab and direct rolling (TSDR) process. This is associated with a higher reheating temperature, a higher entry temperature and a larger reduction due to increased slab thickness in this work compared with the TSDR process.

Theoretical breakage mechanics and experimental assessment of stresses surrounding piles penetrating into dense silica sand

This letter extends an earlier theoretical and numerical analysis that aimed to improve predictions for the large-displacement end-bearing capacity of piles in sand by considering the boundary conditions applying to steady penetration and the effects of grain crushing via breakage mechanics. The earlier work led to good agreement with the end-bearing capacities, soil displacements and evolution of grain size distributions observed in experiments of penetrating model piles. The purpose of the current paper is to complement the previous evaluation through comparisons with more recently published calibration chamber measurements of the stresses surrounding instrumented model piles penetrating into dense silica sand. Encouraging agreement is demonstrated for the radial, circumferential and vertical stress components. Scope for further improvements is also identified.

Austenite Grain Boundary Pinning during Reheating by Mixed AlN and Nb(C,N) Particles

The present study investigates the role of aluminium nitride (AlN) on grain boundary pinning during reheating in presence of niobium carbonitrides (Nb(C,N)) and subsequent evolution of grain size distribution. Three as continuously cast slabs of high strength low alloy steels containing different levels of Nb (between 0.045 – 0.019 wt%) and Al (between 0.057 – 0.02 wt%) have been characterized in terms of phase balance, ferrite grain size distribution and particle size distribution. Precipitate distributions have also been determined following simulated reheating schedules. The prior austenite grain size distributions after reheating have been correlated with precipitate distribution. Quantification of precipitate and prior austenite grain size distributions after reheating unveils the governing mechanisms for precipitate dissolution/ coarsening. It is observed that grain boundary pinning is controlled by AlN at temperatures below 1 125°C, but by Nb(C,N) at higher temperatures.

Scale effects in rockfill behaviour

The particle sizes in large rockfill structures such as dams prevent laboratory testing. The prediction of field behaviour requires the development of models that integrate size effects. A distinct element method model was developed in which grains are characterised by aggregations of a maximum of 14 elementary spherical particles (the resulting particle shape approaches real geometries and allows a reasonable breakage evolution) and the particle breakage criterion involves the subcritical propagation of fissures in the grain. Time effects are included through the velocity of crack propagation, a function of stress state and defect size, which is introduced as a random set of varying lengths. The model was used to simulate the stress–strain response, the evolution of grain size distribution and creep behaviour under oedometric conditions. The model has been used to simulate size effects in the range 0·28–56·0 cm of initial particle size (uniform distributions were tested). Compressibility and creep were partially validated by comparing calculations with test results covering a reduced range of particles. The paper presents the evolution of short-term compressibility and creep indices in terms of particle size. The model is a useful and novel tool with which to extrapolate laboratory results from scaled grain size distributions to prototype dimensions.