Mechanical transitional behavior of binary mixtures via DEM: Effect of differences in contact-type friction coefficients

This report presents a numerical investigation of the shear behavior of binary mixtures via a two-dimensional discrete element method. The effect of the coarse particle content on the peak shear strength of mixtures is investigated for different contact-type friction coefficients. A detailed analysis on the anisotropies enables us to understand the microscopic mechanisms that result in the dependency of the peak shear strengths on the coarse particle content. The contributions of different contact types to the peak shear strength are quantified. Lastly, the inter-particles structures are examined when the binary mixtures become coarse-particle-supported structures.     

Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow

Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (∼ 400 μm and ∼ 200 μm, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture.     

Micromechanical viscoelasto‐plastic models and finite element implementation for rate‐independent and rate‐dependent permanent deformation of stone‐based materials

This paper presents parallel and serial viscoelasto‐plastic models to simulate the rate‐independent and the rate‐dependent permanent deformation of stone‐based materials, respectively. The generalized Maxwell viscoelastic and Chaboche's plastic models were employed to formulate the proposed parallel and serial viscoelasto‐plastic constitutive laws. The finite element (FE) implementation of the parallel model used a displacement‐based incremental formulation for the viscoelastic part and an elastic predictor—plastic corrector scheme for the elastoplastic component. The FE framework of the serial viscoelasto‐plastic model employed a viscoelastic predictor—plastic corrector algorithm. The stone‐based materials are consisted of irregular aggregates, matrix and air voids. This study used asphalt mixtures as an example. A digital sample was generated with imaging analysis from an optically scanned surface image of an asphalt mixture specimen. The modeling scheme employed continuum elements to mesh the effective matrix, and rigid bodies for aggregates. The ABAQUS user material subroutines defined with the proposed viscoelasto‐plastic matrix models were employed. The micromechanical FE simulations were conducted on the digital mixture sample with the viscoelasto‐plastic matrix models. The simulation results showed that the serial viscoelasto‐plastic matrix model generated more permanent deformation than the parallel one by using the identical material parameters and displacement loadings. The effect of loading rates on the material viscoelastic and viscoelasto‐plastic mixture behaviors was investigated. Permanent deformations under cyclic loadings were determined with FE simulations. The comparison studies showed that the simulation results correctly predicted the rate‐independent and rate‐dependent viscoelasto‐plastic constitutive properties of the proposed matrix models. Overall, these studies indicated that the developed micromechanical FE models have the abilities to predict the global viscoelasto‐plastic behaviors of the stone‐based materials. Copyright © 2009 John Wiley & Sons, Ltd.     

ON THE NUMBER OF FRACTIONS TO COMPUTE TRANSPORT OF SEDIMENT MIXTURES

1 INTRODUCTIONMany hydro-momhological mathematical models neglect the innuence of river bed maerialheterogeneity and its time and space changes during transport and related erosion/dePosition processes. Inthese models a rePresentative diameter of the river bed grain-size distribution (for examPle d5o) isspecified as initial data in each comPutational point of the modeled domain. Different d5o can be assignedto each ghd point but temPoral changes in bed material gradation cannot be simula…     

Computational microstructure modeling to estimate progressive moisture damage behavior of asphaltic paving mixtures

This paper presents a computational microstructure model to estimate the progressive moisture damage of multiphase asphaltic paving mixtures. Moisture damage because of water transport is incorporated with mechanical loading through a finite element method. To simulate nonlinear damage evolution in the mixtures, the model includes Fickian moisture diffusion, a cohesive zone model to simulate the gradual fracture process, and a degradation characteristic function to represent the reduction of material properties because of moisture infiltration. With the model developed, various parametric analyses are conducted to investigate how each model parameter affects the material‐specific moisture damage mechanism and damage resistance potential of the mixtures. Analysis results clearly demonstrate the significance of physical and mechanical properties of mixture components and geometric characteristics of microstructure for the better design of asphaltic paving mixtures and roadway structures. Copyright © 2012 John Wiley & Sons, Ltd.     

Spatial pattern recognition by decomposition

The decomposition problem—the assignment of sample observations to component populations—is studied in a spatial context. The observations are spatially located and the assignment to component populations takes into account the value of each observation as well as the values of neighboring observations. Both parameter estimation and assignment rules use a new method that integrates a standard multivariate decomposition algorithm with nonlinear regression. The method is illustrated and tested with artificial data. The distribution of the trace component Cr₂O₃ in recent Lake Michigan sediments is, then, analyzed by the method. It yields a pattern of component populations that is correlated with the Lake's bottom structure and depositional environments. Study carried out as visiting scholar at the Department of Geological Sciences, Northwestern University, Evanston, Illinois U.S.A.     

THE WAVELENGTH COMPONENT VECTORGRAM: A TOOL FOR RESOLVING TWO-COMPONENT FLUORESCENT MIXTURES

We consider the problem of resolving the total luminescence spectra of two-component mixtures.Non-negativity of the constituent spectra is commonly used as a criterion to accomplish this resolution,but its application seldom leads to unique answers. Sometimes, the condition must be relaxed because nosolution satisfying non-negativity exists. In these cases judgments must be made concerning which partsof the spectra will be allowed to go negative and by how much. The wavelength component vectorgramn (WCV) is presented here as a tool for assisting theexperimenter in making these judgments. Even when non-negative resolutions exist, it may happen thatresolutions with slight negative elements will more closely match the spectra of the pure constituents.The WCV can also be used as a tool for detecting these cases. The methodology is illustrated with realdata from a two-component mixture.     

Grinding of mineral mixtures in high-pressure grinding rolls

Attempts at improving comminution machines generally have been directed towards increasing the performance efficiency, particularly increasing throughput rate and decreasing energy consumption. The latest and most successful new comminution technology has been the high-pressure grinding rolls (HPGR), which have proved to be highly efficient in energy consumption and to have a relatively high throughput rate at low steel consumption. Already used extensively in cement plants worldwide, the first HPGRs in the mineral industry were installed in plants processing diamond ores. They are now finding their way into the large-scale base-metal mining industry. Since feed constituents in natural ores vary in their physical properties such as hardness, plasticity and brittleness, the present paper is concerned with an investigation of the behavior of a heterogeneous feed as it passes through the HPGR. The effect of feed composition on operational parameters, the energy efficiency of comminution, and energy distribution among the feed components is addressed. Mineral particles with high hardness act as energy transfer agents in the roll gap and enhance the grinding of softer mineral particles in a mixed feed.     

Evaluating legacy contaminants and emerging chemicals in marine environments using adverse outcome pathways and biological effects-directed analysis

Natural and synthetic chemicals are essential to our daily lives, food supplies, health care, industries and safe sanitation. At the same time protecting marine ecosystems and seafood resources from the adverse effects of chemical contaminants remains an important issue. Since the 1970s, monitoring of persistent, bioaccumulative and toxic (PBT) chemicals using analytical chemistry has provided important spatial and temporal trend data in three important contexts; relating to human health protection from seafood contamination, addressing threats to marine top predators and finally providing essential evidence to better protect the biodiversity of commercial and non-commercial marine species. A number of regional conventions have led to controls on certain PBT chemicals over several years (termed ‘legacy contaminants’; e.g. cadmium, lindane, polycyclic aromatic hydrocarbons [PAHs] and polychlorinated biphenyls [PCBs]). Analytical chemistry plays a key role in evaluating to what extent such regulatory steps have been effective in leading to reduced emissions of these legacy contaminants into marine environments. In parallel, the application of biomarkers (e.g. DNA adducts, CYP1A-EROD, vitellogenin) and bioassays integrated with analytical chemistry has strengthened the evidence base to support an ecosystem approach to manage marine pollution problems. In recent years, however, the increased sensitivity of analytical chemistry, toxicity alerts and wider environmental awareness has led to a focus on emerging chemical contaminants (defined as chemicals that have been detected in the environment, but which are currently not included in regulatory monitoring programmes and whose fate and biological impacts are poorly understood). It is also known that natural chemicals (e.g. algal biotoxins) may also pose a threat to marine species and seafood quality. Hence complex mixtures of legacy contaminants, emerging chemicals and natural biotoxins in marine ecosystems represent important scientific, economic and health challenges. In order to meet these challenges and pursue cost-effective scientific approaches that can provide evidence necessary to support policy needs (e.g. the European Marine Strategy Framework Directive), it is widely recognised that there is a need to (i) provide marine exposure assessments for priority contaminants using a range of validated models, passive samplers and biomarkers; (ii) integrate chemical monitoring data with biological effects data across spatial and temporal scales (including quality controls); and (iii) strengthen the evidence base to understand the relationship between exposure to complex chemical mixtures, biological and ecological impacts through integrated approaches and molecular data (e.g. genomics, proteomics and metabolomics). Additionally, we support the widely held view that (iv) that rather than increasing the analytical chemistry monitoring of large number of emerging contaminants, it will be important to target analytical chemistry towards key groups of chemicals of concern using effects-directed analysis. It is also important to evaluate to what extent existing biomarkers and bioassays can address various classes of emerging chemicals using the adverse outcome pathway (AOP) approach now being developed by the Organization for Economic Cooperation and Development (OECD) with respect to human toxicology and ecotoxicology.     

饱和膨润土及其与砂混合物的压缩变形特性

对用不同制样方法得到的饱和膨润土及其与砂混合物进行了压缩试验。试验结果表明,饱和膨润土的压缩曲线呈双线性,不同于普通黏土的压缩曲线。压缩试验中量测了侧向应力,由此得到的饱和膨润土的静止侧向压力系数值较一般黏土的数值要大。对膨润土与砂混合物的击实样进行了由非饱和到饱和状态的浸水试验,并得出试验过程中侧向应力的变化规律。由于浸水饱和的试样和抽真空饱和的试样在较高压力时压缩曲线趋于一致,可采用快速抽真空饱和的方法进行试验研究,以缩短非饱和混合物击实样浸水饱和所需时间。引入骨架孔隙比的概念,用来判断膨润土与砂混合物中砂骨架是否形成,得出影响混合物压缩特性的决定因素。