Influence of the particle size and volume fraction on micro-damage of the composites

The bulk and shear modulus of metal matrix composites with various volume fractions of particles are modified based on the Eshelby’s equivalent inclusion method combined with self-consistent scheme. By introducing the modified modulus, a new model, which can predict the particle size effects on the stress–strain relation under interfacial debonding damage between matrix and particles, is established. The results obtained from the present investigation show a better agreement with the experimental data.     

SiCp尺寸对SiCp／Cu基复合材料抗磨性能的影响

以纳米级(30nm)、亚微米级(130nm)、微米级(14μm)3种粒径的SiCp和微米级(10μm)铜粉为原料，用冷压烧结和热挤压方法制备了纳米、亚微米和微米级SiCp／Cu复合材料，考察了SiCp尺寸对SiCp／Cu复合材料抗磨性能的影响．结果表明，不同尺寸的SiCp颗粒作为增强相均能显著改善铜基复合材料的抗磨性能；随着SiCp尺寸的增大，SiCp／Cu复合材料的抗磨性能提高幅度显著增大，但偶件磨损表面的犁削加剧；以微米级SiCp为原料制备的SiCp／Cu复合材料的抗磨性能最佳，但其导致偶件40Cr钢犁削作用显著加剧．从摩擦副整体的摩擦磨损性能角度而言，宜采用SiCp尺寸为130nm的SiCp／Cu复合材料同40Cr钢组成摩擦副．     

WC/Al2O3颗粒增强Cu基复合材料爆炸粉末烧结实验研究

利用爆炸粉末烧结工艺,探索WC /Al2O3同时作为增强基颗粒制取多种颗粒增强Cu基复合材料的可行性,同时分析了工艺参数对压实坯致密度的影响。研究了复合材料的微观组织和致密度、韧性和硬度等性能,爆炸粉末烧结法可以成功制出WC/ Al2O3/Cu多种颗粒增强金属基复合材料。     

Anomalous rheology of polypyrrole nanoparticle/alginate suspensions: effect of solids volume fraction, particle size, and electronic state

The rheology of dispersions of polypyrrole (PPY) nanoparticles (nPPY) is compared to that of micron-sized PPY particles (CPPY), each suspended in aqueous sodium alginate. With increasing PPY volume fraction, the Newtonian viscosity of the CPPY/alginate suspensions exhibits a ??normal?? increase, whereas that of the nPPY/alginate suspensions decreases to a minimum and then increases again. Enhanced elasticity, indicative of agglomerate formation via bridging interactions with the alginate, is observed only in the CPPY rheology. By comparing doped versus dedoped nPPY particles, and investigating the effect of nPPY particle size, we conclude that the negative viscosity change of the nPPY dispersions is due to adsorption of a dense layer of alginate, resulting in a decrease in bulk alginate concentration. The viscosity upturn at higher nPPY volume fractions indicates the onset of particle agglomeration via bridging interactions with alginate. The results demonstrate improved dispersability of both doped and dedoped nPPY over CPPY particles.     

Cu改性C/C-SiC摩擦材料组织结构及摩擦磨损性能研究

以全网胎针刺整体毡为预制体,采用Cu粉与Si粉共同熔渗的工艺制备了Cu改性C/C-SiC摩擦材料,并且在MM-1000型惯性摩擦试验机上分别进行了不同速度下的制动试验.研究了Cu改性C/C-SiC摩擦材料的组织结构及摩擦磨损性能,研究结果表明:当初始转速为1 500 r/min时,Cu改性C/C-SiC摩擦材料摩擦系数为0.306,自身与对偶件的线性磨损率分别为0.036μm/(side.cycle),0.048μm/(side.cycle);当初始转速增加到6 500 r/min时,材料的摩擦系数减小到0.24,其自身与对偶件的线磨损率分别增加到1.87μm/(side.cycle),1.68μm/(side.cycle),其值均低于典型C/C-SiC摩擦材料,表明Cu改性C/C-SiC摩擦材料耐磨减摩效果优异,且在低速工况下更为显著.转速为1 500 r/min时磨粒磨损起主导作用;转速为3 000～4 500 r/min时,磨粒磨损以及黏着磨损共同主导着摩擦行为;转速为4 500～6 500 r/min制动时主要表现为氧化磨损与剥层磨损.     

The influence of particle shape,volume fraction and distribution on post-necking deformation and fracture in uniaxial tension of AA5754 sheet materials

Finite element analysis was performed over a small particle field, edge constraint plane strain post-necking model. The aim is to understand the roles of particle shape, volume fraction and distribution over the post-necking deformation and fracture of AA5754-O sheet materials. For models containing one single particle, the post-necking deformation decreases when the particle varies from circular to elliptical. The inter-particle spacing, the major parameter of distribution to determine whether a pair of particles belongs to a stringer or not, was varied for models with two particles of circular or elliptical shape. The general trend is that the post-necking deformation and fracture strains decrease with decreasing spacing between particles. There is considerable difference in terms of both fracture topographies and strains for models containing 16 particles when distributions varied from random/uniform to stringer distributions. The post-necking deformation and fracture strains monotonically decrease with particle volume fractions for models with 4–64 particles of random or stringer distribution. This indicates that the post-necking behavior for AA5754-O alloys where the matrix material is rather ductile is not solely controlled by a single or pair of particles although they may become initiation places of damage. Multiple damaging sources such as stringers or large particles can act cooperatively and speed up the damaging propagation of the material, and therefore produce small post-necking deformation and early fracture. The center clustering of particles can be beneficial for post-necking behavior and bendability of sheet materials.     

Cu/Ni-P多层膜的制备及其摩擦学性能研究

用电沉积方法制备了软硬层交替的Cu／Ni-P多层膜，用原子力显微镜观察分析了多层膜的表面形貌，用X射线衍射仪测定了其相结构；采用动一静摩擦系数精密测量装置考察了多层膜的摩擦学性能和摩擦磨损机制；采用扫描电子显微镜观察分析了其磨损表面形貌．结果表明：Cu／Ni-P多层膜与钢球对摩时具有较好的抗磨减摩性能；在低载荷和低滑动速度下，多层膜的磨痕表面存在擦伤痕迹和较浅的犁沟；在高载荷下多层膜发生严重粘着磨损和剥落。     

On the effect of small fluctuations in the volume fraction of constituents on the effective properties of composites

This study deals with three-scale composite materials comprised of nonlinear constituents. At the meso scale the composite can be considered as locally homogeneous with a macroscopic spatial variation of the constituents volume fraction. When these variations about a mean value are small, a Taylor expansion to second-order of the effective properties of the composite with respect to the fluctuations is given. This expansion can be used to discuss the beneficial or deleterious effects of clusters of inhomogeneities. It can also be used to derive new upper and lower bounds for the effective properties of nonlinear composites from dilute results. To cite this article: P. Suquet, C. R. Mecanique 333 (2005).     

Effects of particle size and concentration on bubble coalescence and froth formation in a slurry bubble column

A new approach for simulating the formation of a froth layer in a slurry bubble column is proposed. Froth is considered a separate phase, comprised of a mixture of gas, liquid, and solid. The simulation was carried out using commercial flow simulation software (FIRE v2014) for particle sizes of 60–150 μm at solid concentrations of 0–40 vol%, and superficial gas velocities of 0.02–0.034 m/s in a slurry bubble column with a hydraulic diameter of 0.2 m and height of 1.2 m. Modelling calculations were conducted using a Eulerian–Eulerian multiphase approach with k–ε turbulence. The population balance equations for bubble breakup, bubble coalescence rate, and the interfacial exchange of mass and momentum were included in the computational fluid dynamics code by writing subroutines in Fortran to track the number density of different bubble sizes. Flow structure, radial gas holdup, and Sauter mean bubble diameter distributions at different column heights were predicted in the pulp zone, while froth volume fraction and density were predicted in the froth zone. The model was validated using available experimental data, and the predicted and experimental results showed reasonable agreement. To demonstrate the effect of increasing solid concentration on the coalescence rate, a solid-effect multiplier in the coalescence efficiency equation was used. The solid-effect multiplier decreased with increasing slurry concentration, causing an increase in bubble coalescence efficiency. A slight decrease in the coalescence efficiency was also observed owing to increasing particle size, which led to a decrease in Sauter mean bubble diameter. The froth volume fraction increased with solid concentration. These results provide an improved understanding of the dynamics of slurry bubble reactors in the presence of hydrophilic particles.     

Effects of electric fields and volume fraction on the rheology of hematite/silicone oil suspensions

Electrorheological (ER) fluids composed of iron(III) oxide (hematite) particles suspended in silicone oil are studied in this work. The rheological response has been characterized as a function of field strength, shear rate and volume fraction. The dielectric properties of the suspensions were first studied in order to get information about the conductivity of the solid. Rheological tests under a.c. electric fields elucidated the influence of the electric field strength and volume fraction on the field-dependent yield stress. It was found that this quantity scales as a square power law in both cases. The viscosities of electrified suspensions were found to increase by several orders of magnitude as compared to the unelectrified suspension at low shear rates, although at high shear rates hydrodynamic effects become dominant and no effects of the electric field on the viscosity are observed. The ER behaviour of the suspensions was analysed by considering the fundamental forces (of hydrodynamic and electrostatic origin) acting on the particles and it is found that, at a given volume fraction, all the dependencies of relative viscosity on shear rate and field strength can be described by a single function of the Mason number, Mn. Finally, two different chain models were used to explain the shear-thinning behaviour observed: rheological measurements showed a power-law dependence of relative viscosity decrease on the Mason number, _F~Mn, with –0.95.     

Continuum modeling of size effects on the composition and stresses in nanoparticles of ionic solids with application to ceria

Owing to its broad potential applications, nanostructured ceria has been subject of intense investigation in the past few decades. Experiments have demonstrated that various material properties of the nanostructured ionic solids including ceria vary with the feature size. Here, we present a theoretical study of the size effects on the composition, defect concentrations and stresses in free-standing nanoparticles of nonstoichiometric ionic solids. To this end, a continuum model is developed which accounts for the highly nonlinear coupling between mechanical, chemical and electrical driving forces, and their effects on the thermodynamic equilibrium of the defect species. It is demonstrated that the model, once applied to the case of ceria, predicts size-dependent defect concentrations and surface stresses. It is further shown that the theoretical predictions of the size effects on the composition and lattice parameter are in good agreement with the experimental observations.     

Effects of the mass and volume shrinkage of ground chip and pellet particles on drying rates

The effects of varying the mass and volume of ground chip and pellet particles on the particle drying rate were analyzed. Samples of whole pellets and chips were hammer milled using a 3.2 mm screen and the ground chip and pellet particles were found to have similar size distributions, although the pellet particles were denser and more spherical than the chip particles. Prior to drying, water was added to the particles to obtain 0.10, 0.30, 0.50, 0.70, and 0.90 moisture contents (on a dry mass basis). The moistened particles were subsequently dried in a constant temperature thin layer dryer set at 50, 100, 150, or 200 °C under dry pure nitrogen, dry compressed air, or atmospheric air. The chip and pellet particles exhibited similar degrees of shrinkage, but the pellet particles underwent a greater reduction in their bulk volume during drying. It appears that the more spherical pellet particles are prone to shrinkage in more than one direction, whereas the needle-like chip particle shrink only in one direction. A variable radius first order drying model was found to fit the experimental data better than a fixed radius model.     

放电等离子快速烧结C/Cu复合材料的组织和摩擦磨损特性研究

利用机械合金化和放电等离子快速烧结法制备C/Cu复合材料,采用X射线衍射仪、显微硬度计、销-盘式摩擦磨损试验机和扫描电子显微镜对复合粉末和烧结体的组织结构、硬度、干摩擦条件下的摩擦磨损性能及其磨损机制进行分析.结果表明:C/Cu复合粉末尺寸随球磨时间的延长明显细化,C在Cu中形成过饱和固溶体;放电等离子烧结体组织致密、细小且均匀,随着碳含量增加,烧结体的硬度与密度减小;相对电解粗铜烧结样品而言,C/Cu复合材料表现出较低的摩擦系数和良好耐磨性,其磨损机制主要为粘着磨损和剥层磨损.     

Effect of surface/interface stress on the plastic deformation of nanoporous materials and nanocomposites

Surface and interface play an important role on the overall mechanical behaviors of nanostructured materials. We investigate the effect of surface/interface stress on the macroscopic plastic behaviors of nanoporous materials and nanocomposites, where both the surface/interface residual stress and surface/interface elasticity are taken into account. A new second-order moment nonlinear micromechanics theory is developed and then reduced to macroscopically isotropic materials. It is found that the effect of surface/interface residual stress is much more prominent than that of the surface/interface elasticity, causing strong size effect as well as asymmetric plastic deformation for tension and compression. The variation of yield strength is more prominent with smaller pore/inclusion size or higher pore/inclusion volume fraction. For a representative nanoporous aluminum, the surface effect becomes significant when the pore radius is smaller than about 50 nm. When hard inclusions are embedded in a ductile metal matrix, the interface effect and resulting size effect are much smaller than that of nanoporous materials. The results may be useful for evaluating the mechanical integrity of nanostructured materials.     

Effects of size on the strength and deformation mechanism in Zr-based metallic glasses

We report results of uniaxial compression tests on Zr₃₅Ti₃₀Co₆Be₂₉ metallic glass nano-pillars with diameters ranging from ∼1.6 μm to ∼100 nm. The tested pillars have nearly vertical sidewalls, with the tapering angle lower than ∼1° (diameter >200 nm) or ∼2° (diameter ∼100 nm), and with a flat pillar top to minimize the artifacts due to imperfect geometry. We report that highly-localized-to-homogeneous deformation mode change occurs at 100 nm diameter, without any change in the yield strength. We also find that yield strength depends on size only down to 800 nm, below which it remains at its maximum value of 2.6 GPa. Quantitative Weibull analysis suggests that the increase in strength cannot be solely attributed to the lower probability of having weak flaws in small samples - most likely there is an additional influence of the sample size on the plastic deformation mechanism.     

A new crystal plasticity scheme for explicit time integration codes to simulate deformation in 3D microstructures: Effects of strain path,strain rate and thermal softening on localized deformation in the aluminum alloy 5754 during simple shear

The predominant deformation mode during material failure is shear. In this paper, a crystal plasticity scheme for explicit time integration codes is developed based on a forward Euler algorithm. The numerical model is incorporated in the UMAT subroutine for implementing rate-dependent crystal plasticity model in LS-DYNA/Explicit. The sheet is modeled as a face centered cubic (FCC) polycrystalline aggregate, and a finite element analysis based on rate-dependent crystal plasticity is implemented to analyze the effects of three different strain paths consisting predominantly of shear. Finite element meshes containing texture data are created with solid elements. The material model can incorporate information obtained from electron backscatter diffraction (EBSD) and apply crystal orientation to each element as well as account for texture evolution. Single elements or multiple elements are used to represent each grain within a microstructure. The three dimensional (3D) polycrystalline microstructure of the aluminum alloy AA5754 is modeled and subjected to three different strain rates for each strain path. The effects of strain paths, strain rates and thermal softening on the formation of localized deformation are investigated. Simulations show that strain path is the most dominant factor in localized deformation and texture evolution.     

Effects of the gas phase molecular weight and bubble size on effervescent atomization

The effervescent atomization from an industrial Coker feed nozzle is compared for two different gas densities (air and mixed gas of 81.4 vol.% helium/18.6 vol.% nitrogen) at equivalent operating temperatures. The application is to observe the similarity of lab tests using air at 20 °C to the industrial process using steam at 300-400 °C. The effects of operating conditions, such as gas to liquid mass ratio, mixing pressure and void fraction on the flow regime, bubble size, and droplet size distribution were also examined in this study. The experiments were performed using mixtures of water with air or mixed gas, which resulted in gas to liquid mass ratios ranging from 1% to 4%.Stroboscopic back scattered imagery (SBSI) indicates that the average bubble size inside the nozzle conduit is similar when air and water are used as the process fluids, when compared to the case when mixed gas and water are used as the process fluids. Under similar conditions, the Phase Doppler Particle Anemometer (PDPA) data indicate that the droplet size in the spray is similar when using either mixed gas or air as the atomization gas.Experimental results obtained by high-speed video shadowgraphy (HSVS) indicate that the flow pattern inside the nozzle feeding conduit was slug flow with a tendency to attain annular flow with increased air to liquid mass ratios. Thus, from the experimental results it is evident that the smaller molecular weight of the mixed gas versus air (8.4 versus 29) does not significantly reduce the bubble (<±10% difference) and droplet size (<±1.5% difference), indicating a weak dependence of the gas phase density on two-phase atomization. This confirms that laboratory experiments on effervescent nozzles using air have reliable similarity to systems that use high temperature steam for the gas phase.     

Shape and size effects of ceria nanoparticles on the impact strength of ceria/epoxy resin composites

Ceria nanoparticles with various shapes (rods, cubes, and plates) and sizes were controllably synthesized and then introduced into epoxy resin. Subsequently, we investigated correlations between the shape and size of ceria nanostructures and the mechanical performance of composites. The samples were characterized by transmission electron microscopy, scanning electron microscopy, and X-ray diffraction. Compared with commercial ceria filled composites, the composites made with morphology-controlled ceria nano...     

C/C-SiC复合材料的制备及其湿式摩擦磨损性能研究

采用T700炭纤维针刺毡,经超声振动渗硅、化学气相沉积(CVD)、硅化处理及液相浸渍/炭化新工艺制备SiC呈"岛状"分布的C/C-SiC复合材料,利用偏光显微镜和扫描电子显微镜观察其微观组织结构并分析其形成机制,在MM-1000型湿式摩擦磨损试验机上研究C/C-SiC复合材料的摩擦磨损性能.结果表明:在初始转速恒定的条件下,动摩擦系数随制动比压的增加而逐渐减小;当制动比压恒定时,摩擦系数随初始转速的增加呈现出先增大而后降低的趋势;在本文试验条件下,摩擦系数稳定在0.088～0.126之间;在300次磨损试验后,其磨损量检测值为0.