Effect of microgravity on grain coarsening during liquid phase sintering in the Fe–Cu system

Samples within the Fe–Cu system with three different volume fractions of solid (50, 60 and 70 vol% Fe) and four different sintering times (2.5, 5, 17 and 66 min) were liquid phase sintered (LPS) in microgravity. Particle coarsening during LPS is generally known to increase with increasing volume fraction of solid. Contrary to expectations, there was an enhancement in particle coarsening with a decrease in the volume fraction of solid. The agglomerated microstructures observed in these samples (especially those with a lower volume fraction of solid) also exhibited a higher grain growth constant consistent with their higher 3D coordination number. The relevant analysis discussed in this paper strongly suggests that agglomeration is promoted by Brownian motion that dominates any density-driven force in the absence of gravity. The observed particle growth characteristics were in excellent agreement with the Lifshitz–Slyozov encounter modified theory, which incorporates the effect of higher solid volume fraction and particle coalescence into the LSW theory. The particle distributions appear to remain unchanged with processing time beyond 2.5 min, suggesting thereby, that agglomeration promotes an equilibrium particle configuration early on in the process and enables scaled grain growth with time.     

Jamming in granular shear flows of frictional,polydisperse cylindrical particles

In this work, frictional, cylindrical particle shear flows with different size distributions (monodisperse, binary, Gaussian, uniform) are simulated using the Discrete Element Method (DEM). The influences of particle size distribution and interparticle friction coefficient on the solid phase stresses, bulk friction coefficient, and jamming transition are investigated. In frictional dense flows, shear stresses rise rapidly with the increasing solid volume fraction when jamming occurs. The results suggest that at the jamming volume fraction, stress fluctuation and granular temperature achieve the maximum values, and the rate of the stress increase with increasing solid volume fraction approaches the peak value. Meanwhile, the degree of cylindrical particle alignment approaches a valley value. In the polydisperse flows, the jamming volume fraction exhibits significant dependences on the fraction of the longer particles and the particle size distribution. Two models considering the effect of particle size distribution are discussed for predicting the jamming volume fractions of polydisperse flows with frictional, cylindrical particles.     

Effects of coarser fine aggregate on tensile properties of ultra high performance concrete

This experimental research investigates the mechanical properties and shrinkage of ultra high performance concrete (UHPC) incorporating coarser fine aggregates with maximum particle size of 5 mm. To adequately design UHPC mixtures using various sizes of solid constituents, particle packing theory was adopted. UHPC mixtures containing either dolomite or basalt, and four fiber volume fractions up to two volume percent were investigated. Uniaxial tension test was performed to evaluate the first cracking tensile strength, ultimate tensile strength, tensile strain capacity and cracking pattern. The UHPC mixtures with dolomite and steel fibers with more than one volume percent achieved more than 150 MPa of compressive strength at the age of 56 days, and showed strain hardening behavior and limited decrease in tensile strength compared to typical UHPC without coarser fine aggregates. The experimental results highlight the potential of dolomite used as coarser fine aggregate in UHPC.     

增强颗粒含量和尺寸对SiCp/Fe复合材料性能的影响

利用电流直加热动态热压烧结工艺,分别制备了增强颗粒体积含量从5%到15%,尺寸从3 μm到45 μm的SiCp/Fe复合材料,研究了粒子含量与尺寸对复合材料硬度、强度、延伸率和耐磨性能的影响.研究表明:增强颗粒的体积含量从5%提高到10%,可以明显提高材料的性能;随着增强颗粒含量进一步提高,颗粒团聚将导致材料性能降低;...     

注凝成型制备ZTA复相陶瓷

讨论了pH值、分散剂添加量、 ZrO₂(3Y)含量和固相体积分数对氧化锆增韧氧化铝(Zirconia toughened alumina, ZTA) 陶瓷注凝成型浆料粘度的影响,研究了注凝成型的烧结样品的性能和显微结构。结果表明,当pH值为8.5、分散剂添加量为0.9%时,可以制备固相体积分数达55%的低粘度ZTA (20%ZrO₂) 悬浮体。高固相悬浮体制备的烧结试样具有结构致密、ZrO₂分布较均匀和t-ZrO₂含量高等特征,其强度和断裂韧性分别达631.5 MPa和7.64 MPa · m1/2。      

Failure of cemented granular materials under simple compression: experiments and numerical simulations

We investigate the strength and failure properties of a model cemented granular material under simple compressive deformation. The particles are lightweight expanded clay aggregate beads coated by a controlled volume fraction of silicone. The beads are mixed with a joint seal paste (the matrix) and molded to obtain dense cemented granular samples of cylindrical shape. Several samples are prepared for different volume fractions of the matrix, controlling the porosity, and silicone coating upon which depends the effective particle–matrix adhesion. Interestingly, the compressive strength is found to be an affine function of the product of the matrix volume fraction and effective particle–matrix adhesion. On the other hand, it is shown that particle damage occurs beyond a critical value of the contact debonding energy. The experiments suggest three regimes of crack propagation corresponding to no particle damage, particle abrasion and particle fragmentation, respectively, depending on the matrix volume fraction and effective particle–matrix adhesion. We also use a sub-particle lattice discretization method to simulate cemented granular materials in two dimensions. The numerical results for crack regimes and the compressive strength are in excellent agreement with the experiments.     

Rubber toughening of plastics

Measurements of particle size distribution, particle volume fraction?/, Young's modulus, tensile and compressive yield stress and Charpy impact strength were made on a series of 14 high-impact polystyrene (HIPS) polymers of widely varying structure. In materials throughout the series containing 8.5 wt % polybutadiene, it was found that?/ varied between 0.17 and 0.44 as the mean particle size increased from 0.2 to 1.8 μm. Modulus and yield stresses depended principally upon particle volume fraction but the ratio of polybutadiene to polystyrene within the particles also appeared to have some influence upon properties. By contrast, variations in ? provided only a partial explanation for the observed differences in Charpy impact strength. It is concluded that impact strength is affected by rubber particle size to a much greater extent than properties measured at low strain rates.     

Effects of random particle dispersion and size on the indentation behavior of SiC particle reinforced metal matrix composites

This study investigates the effects of particle size, volume fraction, random dispersion and local concentration underneath a spherical indenter on the indentation response of particle reinforced metal matrix Al 1080/SiC composites. The ceramic particles in certain sizes and volume fractions were randomly distributed through the composite structure in order to achieve a similar structure to an actual microstructure as possible. The particle size and volume fraction affected considerably indentation depths and deformed indentation surface profiles. The indentation depth increases with increasing particle size, but decreases with increasing particle volume fraction. The experimental indentation depths were in agreement with numerical indentation depths in case the local particle concentration effect is considered. The local particle concentration plays an important role on the peak indentation depth. For small particle sizes and large volume fractions the random particle distribution affects the deformed surface profiles as well as the indentation depths. However, its effect is minor on residual stress and strain distributions rather than levels in the indentation region.     

Role of residual stress field interaction in strengthening of particulate-reinforced composites

A quantitative analysis was conducted on the effect of residual thermoelastic stress concentrations on the strength of particle-reinforced brittle matrix systems. The analysis is derived from the stress intensity factor for a periodic array of coplanar cracks emanating from the matrix-particle interface. It is shown that the major drop in strength occurs at smaller volume fractions of second phase where the residual stress field interaction effects are minimal. The effect of volume fraction on strength becomes important at larger volume fractions (normally above 10–15%). The theory is compared with experimental measurements of strength for glass and alumina matrix composites as a function of the particle volume fraction, its size, and thermal mismatch .     

粗骨料与钢纤维对超高性能混凝土单轴拉伸性能的影响

为了提高含粗骨料超高性能混凝土(Ultra-high performance concrete,UHPC)的单轴拉伸性能,采用单轴拉伸试验和图像分析技术分别研究了粗骨料掺量、颗粒粒径对含粗骨料UHPC单轴拉伸性能和钢纤维在UHPC体系中分散性能的影响规律。结果表明,随着粗骨料掺量及颗粒粒径的增大,钢纤维在UHPC体系中的分散系数和取向系数显著降低,含粗骨料UHPC的单轴拉伸初裂强度、裂后强度和耗能也随之减小。根据粗骨料颗粒最大粒径与钢纤维体积分数、直径间的匹配关系式(Dmax=3df/(Vf)0.5),采用纤维混杂可以充分发挥多尺度纤维与具有不同粒径分布的骨料间的分级匹配关系;粗骨料体积分数和颗粒最大粒径分别为10%和10mm时,采用平直钢纤维(直径0.12mm、长度10mm、体积掺量1.2%)和端钩钢纤维(直径0.35 mm、长度20mm、体积掺量1.8%)混杂实现了含粗骨料UHPC的单轴拉伸性能的提升,其裂后强度和耗能分别为8.69 MPa和11.10J。     

Mechanical behavior of cast SiCp-reinforced Al-4.5%Cu-1.5%Mg alloy

SiC_p-reinforced Al-4.5%Cu-1.5%Mg composite specimens were processed by vigorous stirring of the carbide in a semi-solid alloy slurry, followed by remelting and casting (stir-casting). The tensile and fatigue properties were evaluated in the as-cast and in the heat-treated conditions. In monotonic tensile testing, reinforcement with SiC_p produced a substantial increase in the work hardening of the material. This increase became more significant with increasing volume fraction of carbide. The yield and ultimate tensile strength, and the elastic modulus of the material, increased with heat-treatment and volume fraction of carbide at the expense of ductility. These properties are inferior to those of other reinforced, more complex aluminum alloys processed by other methods. In stress-controlled fatigue tests under fully reversed (R = −1) bending conditions, the fatigue life of the composite was longer than that of the unreinforced specimen at intermediate and lower stress levels. At higher stress levels the improvement was negligible. In heat-treated reinforced alloy specimens the fatigue strength at 1 × 10⁷ cycles decreased with increasing carbide particle size. With solid solution and precipitation strengthening, as well as carbide dispersion strengthening of the alloy, the crack growth threshold stress intensity factor K_th, increased, as did the crack initiation time and the crack growth rate.     

SiC粉体的微观差异性及其浆料的流变性质

选取国内外2种不同的SiC粉体,通过X射线衍射、能谱、透射电镜和粒度分布等测试方法对其组成、晶型、粒度及表面性质进行研究。结果表明,2种粉体的化学组成、微观结构几乎一致,但浆料的流变性质却存在显著性的差异;用国外粉体可制备出固相体积分数高达55%的低黏度稳定浆料,而国产粉体可形成最高固相体积分数为40%的浆料,且出现剪切变稠现象;粉体表面性质的微观差异性是导致浆料流变性质不同的主要原因。     

Emergence of coherent structures and large-scale flows in motile suspensions

The emergence of coherent structures, large-scale flows and correlated dynamics in suspensions of motile particles such as swimming micro-organisms or artificial microswimmers is studied using direct particle simulations. A detailed model is proposed for a slender rod-like particle that propels itself in a viscous fluid by exerting a prescribed tangential stress on its surface, and a method is devised for the efficient calculation of hydrodynamic interactions in large-scale suspensions of such particles using slender-body theory and a smooth particle-mesh Ewald algorithm. Simulations are performed with periodic boundary conditions for various system sizes and suspension volume fractions, and demonstrate a transition to large-scale correlated motions in suspensions of rear-actuated swimmers, or Pushers, above a critical volume fraction or system size. This transition, which is not observed in suspensions of head-actuated swimmers, or Pullers, is seen most clearly in particle velocity and passive tracer statistics. These observations are consistent with predictions from our previous mean-field kinetic theory, one of which states that instabilities will arise in uniform isotropic suspensions of Pushers when the product of the linear system size with the suspension volume fraction exceeds a given threshold. We also find that the collective dynamics of Pushers result in giant number fluctuations, local alignment of swimmers and strongly mixing flows. Suspensions of Pullers, which evince no large-scale dynamics, nonetheless display interesting deviations from the random isotropic state.     

Non-Settling Super-Strong Magnetorheological Fluids

A magnetorheological (MR) fluid is generally called a suspension in which magnetic particles are dispersed in a non-magnetic medium. When an external magnetic field is applied, a pseudo-phase transition occurs within a short time to generate yield stress, and when the magnetic field is released, it returns to the suspended state. Due to these unique characteristics, it is classified as a smart material to be widely applied in various industries. High performance MR fluids require high yield stress and stability for long-term use. However, it is very difficult to improve performance and stability simultaneously due to the limited amount of magnetic particles in the suspension and particle sedimentation caused by the density mismatch between the suspending particles and the liquid phase. In this study, an MR slurry is developed that is completely different from the MR suspension, starting from the opposite concept. An innovative non-settling (i.e., permanently stable) magnetorheological slurry is successfully created that exhibits unprecedented ultra-high yield stress. This result is expected to be a turning point for applying MR fluids to more diverse industries. In addition, a simple fitting equation expressing the yield stress as a function of the particle volume fraction is proposed.     

Libre parcours moyen dans la phase solide des materiaux frittés

A simple mathematical model is used to predict the evolution of the mean free path in the solid phase in a sintered material. The knowledge of the volume fraction of the solid phase and of the particle coordination number are only required. Good agreement is observed between experimental data and the purposed relationship.     

Interlaminar shear fatigue of pultruded graphite fibre-polyester composites

The effect of cyclic loading on the interlaminar shear strength of pultruded graphite fibre-polyester was determined. Two fibre volume fractions, 0.5 and 0.33, were studied. The results indicate that the deterioration in the interlaminar shear strength with cycling is significantly greater than in flexural fatigue. The higher volume fraction material showed a greater drop in the interlaminar shear strength than the lower volume fraction material. Unlike the monotonic strengths, the effect of the fibre volume fraction on interlaminar shear fatigue strength at high cycles is small, indicating that there is little advantage in increasing the fibre volume fraction to improve the interlaminar shear strength in high cycle fatigue environments. A critical stress was determined above which interlaminar shear fatigue failure did not occur within 10⁷ cycles for the materials tested.     

The influence of particle size on the tensile strength of particulate — filled polymers

The tensile strengths of a particulate-filled rigid polyurethane resin are presented at varying volume fractions and a wide range of particle sizes. These results are compared with exisiting theories of the strength of particulate-filled composite systems. A linear relationship is proposed to exist between the mean particle diameter and the tensile strength at a given volume fraction. A method of normalizing data is presented which removes the stress-concentration effects of finite particle sizes and allows comparison of the data with a simple equation relating tensile strength and volume fraction. The effects of particle size and volume fraction in relation to crack propagation are discussed, and the proposed method of analysis is shown to give similar results when applied to published data.     

Automatic generation of 2D micromechanical finite element model of silicon–carbide/aluminum metal matrix composites: Effects of the boundary conditions

Two-dimensional finite element (FE) simulations of the deformation and damage evolution of Silicon–Carbide (SiC) particle reinforced aluminum alloy composite including interphase are carried out for different microstructures and particle volume fractions of the composites. A program is developed for the automatic generation of 2D micromechanical FE-models with randomly distributed SiC particles. In order to simulate the damage process in aluminum alloy matrix and SiC particles, a damage parameter based on the stress triaxial indicator and the maximum principal stress criterion based elastic brittle damage model are developed within Abaqus/Standard Subroutine USDFLD, respectively. An Abaqus/Standard Subroutine MPC, which allows defining multi-point constraints, is developed to realize the symmetric boundary condition (SBC) and periodic boundary condition (PBC). A series of computational experiments are performed to study the influence of boundary condition, particle number and volume fraction of the representative volume element (RVE) on composite stiffness and strength properties.     

Modeling filtration performance of elliptical fibers with random distributions

The fibrous media with elliptical cross sections may improve the filtration performance, however, current researches mainly focus on the capture mechanisms of a single elliptical fiber, and the fibrous media with randomly distributed fibers are rarely involved. In this work, a 2D numerical model was developed to predict the pressure drop and particle penetration for the fibrous filter composed of randomly distributed elliptical fibers. The results show that a big solid volume fraction of filter increases the effective collision area, and enhances the capture at a low face velocity. The particle penetrations through the fibers with the diameter of 5 μm are conspicuously weaker than those with the diameter of 10 μm, especially at big solid volume fractions and high face velocities. The blunt elliptical fibers restrain the penetration more effectively than the circle ones when the solid volume fraction is high. Though the blunt fibers lead to a large drag force, the increased pressure drop cannot improve the filtration performance at low solid volume fractions. In most cases, the slim elliptical fibers can enhance the filtration performance. A bigger aspect ratio of elliptical fibers leads to a low quality factor, showing the capture efficiency increases with the penalty of a high pressure drop.     

低毒单体DMAA用于ZrO2/Al2O3坯体注凝成型

采用低毒的单体N, N-二甲基丙烯酰胺(DMAA)制备了氧化锆增韧氧化铝(ZrO₂/Al₂O₃)坯体。讨论了分散剂的用量、 ZrO₂/Al₂O₃浆料的pH值、 粉体中ZrO₂含量、 粉体所占浆料的固相体积分数、 球磨时间、 预混液中DMAA的浓度(质量分数)对ZrO₂/Al₂O₃浆料黏度的影响。并研究了注凝成型ZrO₂/Al₂O₃坯体的性能和显微结构。结果表明, 当浆料pH值为9, 分散剂的添加量为ZrO₂/Al₂O₃粉体质量的0.6%, 球磨时间为6 h, ZrO₂/Al₂O₃浆料具有最小的黏度。固相体积分数的提高和DMAA加入量的增大都会提高ZrO₂/Al₂O₃浆料的黏度, ZrO₂的加入会降低浆料的黏度。用DMAA制备得到的ZrO₂/Al₂O₃坯体结构均匀, 抗弯强度达到25 MPa。