Showing papers on "Volume fraction published in 2002"

Thermal Conductivity of Suspensions Containing Nanosized SiC Particles

Abstract: Nanosized SiC suspensions were prepared, and their thermal conductivities were measured using a transient hot-wire method. The experimental results showed that the thermal conductivities of the studied suspensions were increased as expected, and the enhancement was proportional to the volume fraction of the solid phase, but the increasing ratio of the thermal conductivity was not significantly related to the base fluid. The effects of the morphologies (size and shape) of the added solid phase on the enhancement of the thermal conductivity of the nanoparticle suspension are reported for the first time.

Rheology and microstructure in concentrated noncolloidal suspensions

Abstract: The rheological behavior of a monodisperse suspension of non-Brownian particles undergoing simple shear flow in the presence of a weak interparticle force is studied using accelerated Stokesian dynamics. The availability of a faster numerical algorithm permits the investigation of larger systems (typically of 512 particles), and accurate results for the suspension viscosity, first and second normal stress differences, and the particle pressure are determined as a function of the volume fraction. The system microstructure, expressed through the pair-distribution function, is also studied and it is demonstrated how the resulting anisotropy in the pair-distribution function is correlated with the suspension non-Newtonian behavior. The ratio of the normal to excess shear stress is found to be an increasing function of the volume fraction, suggesting different volume fraction scalings for different elements of the stress tensor. The relative strength and range of the interparticle force is varied and its effec...

Toughening of a brittle thermosetting polymer: Effects of reinforcement particle size and volume fraction

Abstract: Micron- and nanometer-sized aluminum particles were used as reinforcements to enhance the fracture toughness of a highly-crosslinked, nominally brittle, thermosetting unsaturated polyester resin. Both particle size and particle volume fraction were systematically varied to investigate their effects on the fracture behavior and the fracture toughness. It was observed that, in general, the overall fracture toughness increased monotonically with the volume fraction of aluminum particles, for a given particle size, provided particle dispersion and deagglomeration was maintained. The fracture toughness of the composite was also strongly influenced by the size of the reinforcement particles. Smaller particles led to a greater increase in fracture toughness for a given particle volume fraction. Scanning electron microscopy of the fracture surfaces was employed to establish crack front trapping as the primary extrinsic toughening mechanism. Finally, the effects of particle volume fraction and size on the tensile properties of the polyester-aluminum composite were also investigated. The measured elastic modulus was in accordance with the rule-of-mixtures. Meanwhile, the tensile strength was slightly reduced upon the inclusion of aluminum particles in the polyester matrix.

Thermal conductivity of polystyrene–aluminum nitride composite

Abstract: The thermal conductivity of polymer composites having a matrix of polystyrene (PS) containing aluminum nitride (AlN) reinforcement has been investigated under a special dispersion state of filler in the composites: aluminum nitride filler particles surrounding polystyrene matrix particles. Data for the thermal conductivity of the composites are discussed as a function of composition parameters (aluminum nitride concentration, polystyrene particle size) and temperature. It is found that the thermal conductivity of composites is higher for a polystyrene particle size of 2 mm than that for a particle size of 0.15 mm. The thermal conductivity of the composite is five times that of pure polystyrene at about 20% volume fraction of AlN for the composite containing 2 mm polystyrene particle size. The relationship between thermal conductivity of composites and AlN filler concentrations has been compared with the predictions of two theoretical models from the literature.

A molecular dynamics simulation study of the viscoelastic properties of polymer nanocomposites

Abstract: We have carried out molecular dynamics simulations of model polymer–nanoparticle composites (PNCs) consisting of coarse-grained bead-necklace polymer chains and roughly spherical nanoparticles comprised of like beads for the purpose of gaining understanding of the influence of the nanoparticle–polymer interface on the viscoelastic properties of PNCs. The dynamic shear modulus Gc(t) and viscosity ηc of the PNCs were determined as a function of nanoparticle volume fraction, specific nanoparticle–polymer interfacial area, and the nature of the nanoparticle–polymer interaction. The viscoelastic properties of the PNCs were well described as a product of a polymer matrix dynamic shear modulus or viscosity and a particle volume fraction dependent effect independent of particle size, analogous to treatments of conventional particle composites. In contrast to many conventional composites, however, the viscoelastic properties of the polymer matrix were strongly perturbed by the nanoparticles and depended upon the n...

Compaction dynamics of a granular medium under vertical tapping

Abstract: We report new experimental results on granular compaction under consecutive vertical taps. The evolution of the mean volume fraction and of the mean potential energy of a granular packing presents a slow densification until a final steady state, and is reminiscent of usual relaxation in glasses via a stretched exponential law. The intensity of the taps seems to rule the characteristic time of the relaxation according to an Arrhenius's type relation. Finally, the analysis of the vertical volume fraction profile reveals an almost homogeneous densification in the packing.

Oxidation bonding of porous silicon carbide ceramics

Abstract: A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al2O3 and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al2O3 content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al2O3, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.

Compaction dynamics of a granular media under vertical tapping

Abstract: We report new experimental results on granular compaction under consecutive vertical taps. The evolution of the mean volume fraction and of the mean potential energy of a granular packing presents a slow densification until a final steady-state, and is reminiscent to usual relaxation in glasses via a stretched exponential law. The intensity of the taps seems to rule the characteristic time of the relaxation according to an Arrhenius's type relation >. Finally, the analysis of the vertical volume fraction profile reveals an almost homogeneous densification in the packing.

Compositional and orientational ordering in rod-coil diblock copolymer melts

Abstract: The phase behavior of a melt of monodisperse rod−coil diblocks is studied. We derive a Landau free energy functional for both a compositional and a nematic order parameter. The excluded volume interaction between the rod blocks is modeled by an attractive Maier−Saupe interaction. The incompatibility between rod and coil blocks is modeled by the usual Flory−Huggins interaction. For a large volume fraction of the rods, a transition from isotropic to nematic to smectic C is observed upon decreasing the temperature, whereas for small rod volume fraction, spherical, hexagonal, and lamellar structures prevail. In the smectic C phase, the rod orientation angle with respect to the lamellar normal increases rapidly from 35 to 40° close to the nematic/smectic-C phase boundary to values between 45 and 55°.

Effect of heat treatment on corrosion and electrochemical behaviour of AZ91D magnesium alloy

Abstract: An AZ91D ingot in the as-cast condition was homogenized by solution treatment and then aged for various periods of time The microstructures produced were studied in detail and the β phase volume fraction was measured quantitatively The Corrosion resistance of all the different microstructures was studied in 35% NaCl solution through weight loss measurement in constant immersion conditions and potentiodynamic polarization experiments The corroded surfaces were analysed using SEM and XRD The volume fraction of the β phase was found to have a significant influence on the corrosion behaviour The T4 condition improved the corrosion resistance of AZ91D alloy compared to the T6 heat treatment The results support the idea of microgalvanic coupling between cathodic β phase and anodic α matrix

Volume fraction optimization for minimizing thermal stress in Ni–Al2O3 functionally graded materials

Abstract: Functionally graded heat-resisting material, in which the volume fraction of constituents varies continuously and functionally, is investigated for high-temperature engineering applications In this advanced material, the thermomechanical behavior of FGMs is strongly influenced by the spatial distribution of the volume fraction So, the determination of volume fraction distribution becomes a crucial part in the FGM design, for a given specification and loading condition This paper is concerned with the volume fraction optimization for minimizing steady-state thermal stresses in Ni–Al 2 O 3 heat-resisting FGM composites Interior penalty-function method and golden section method are employed as optimization techniques, together with finite difference method for the sensitivity analysis and an appropriate material-property estimate for calculating thermomechanical properties of the graded layer The introduced optimization method, through the numerical experiments, is found to provide optimal volume fraction distributions that minimize thermal stresses significantly, as well as the rapid and stable convergence

Prediction of stiffness and stresses in z-fibre reinforced composite laminates

Abstract: The mechanical properties of z-pinned composite laminates were examined numerically. Finite element calculations have been performed to understand how the through-thickness reinforcement modifies the engineering elastic constants and local stress distributions. Solutions were found for four basic laminate stacking sequences, all having two percent volume fraction of z-fibres. For the stiffness analysis, a micro-mechanical finite element model was employed that was based on the actual geometric configuration of a z-pinned composite unit cell. The numerical results agreed very well with some published solutions. It showed that by adding 2% volume fraction of z-fibres, the through-thickness Young's modulus was increased by 22–35%. The reductions in the in-plane moduli were contained within 7–10%. The stress analysis showed that interlaminar stress distributions near a laminate free edge were significantly affected when z-fibres were placed within a characteristic distance of one z-fibre diameter from the free edge. Local z-fibres carried significant amount of interlaminar normal and shear stresses.

Isolating Components of Processing Induced Warpage in Laminated Composites

Abstract: This study considers laminate warpage (or spring-in) associated with the curing of reinforced thermoset composites. This processing induced spring-in shows up in flat parts as well as those with curved geometry, and is most prevalent with parts that do not have a closed cross section. Three contributions to spring-in were considered, namely: thickness cure shrinkage, mold expansion, and fiber volume fraction gradients. These effects were combined into a predictive finite element model (FEM). Mold stretching and thickness shrinkage were described through a modified in-and out-of-plane material thermal expansion, respectively, while fiber volume fraction gradients were accounted for by scaling the thermal and elastic properties of the composite through the thickness with the fiber volume fraction. For thin parts ( 2 mm) spring-in was dominated by thickness cure shrinkage. The FEM was able to acco...

Intermetallic-reinforced light-metal matrix in-situ composites

Abstract: Transition-metal trialuminide intermetallics such as Al3Zr and Al3Ti, having low densities and high elastic moduli, are good candidates for the in-situ reinforcement of light-metal matrices based on Al and Mg alloys. In this work, in-situ composites based on Al and Al-Mg matrices reinforced with an Al3Zr intermetallic were successfully processed by conventional ingot metallurgy. The microstructural studies showed that “needle” or “feathery”-like particles of Al3Zr phase, whose volume fraction increased with increasing concentration of Zr, were formed in the Al matrix in the investigated range of Zr contents from 0.9 to 11.6 at. pct. Properties of Al-Zr alloys were investigated as a function of volume fraction of Al3Zr. It is shown that the density, hardness, and yield strength of the in-situ Al/Al3Zr composites can be quite adequately described by the composite rule-of-mixtures (ROM) behavior. Alloying of a binary Al-2.4 at. pct Zr alloy with Mg up to ∼25 at. pct reduces profoundly its density and, additionally, strengthens the matrix by a Mg solid-solution strengthening mechanism.

Effect of reinforcement volume fraction on the evolution of reinforcement size during the extrusion of Al-SiC composites

Abstract: The effect of reinforcement volume fraction on the evolution of the reinforcement particle size during extrusion was investigated for the Al-SiC model composite system Composites with 64, 100, and 168 volume percent of SiC particles were extruded at a temperature of 350 °C using an extrusion ratio of 12:1 Image analysis showed that the SiC particles were refined by the extrusion process and that the extent of the refinement increased with an increase in reinforcement volume fraction A mechanistic rationale for the experimentally observed trends is formulated with reference to the corresponding change in the stress fields in the composite microstructure with the change in the reinforcement volume fraction Possible implications of extrusion-induced particle fracture for the processing of fine particle-reinforced metal matrix composites are also explored in this paper

The microstructure and mechanical properties of Al-Si-B4C metal matrix composites

Abstract: B4C particles have been added to molten Al- 7wt% Si- 0.3 wt% Mg alloys, at levels of 5 and 10 wt%, using a propriatory K-Al-Ti-F flux. The resulting composites were examined metallographically and mechanically tested in the as-manufactured condition and after heat treatment for 48 hours at 500°C and 700°C. During incorporation into the melt, a complex Ti-B-C reaction layer was formed on the particle surfaces. The reaction layer remained intact during heat treatment and the stable, protective nature of this layer gave rise to a significantly reduced rate of particle degradation compared to other Al-B4C composites. Significant increases in stiffness were observed; modulus increases per volume percent of particles added were similar to those for the Al-TiC system where strong interfacial bonding occurs. Improved adhesion between the solidified matrix and the B4C reinforcement was encouraged by the enhanced metallic character of the reaction layer. Solid state reaction at 500°C produced little change in mechanical properties. Heat treatment and reaction at 700°C resulted in an increase in the volume fraction of stiff, brittle reinforcing phases, leading to an increase in stiffness and a decrease in ductility.

Evaluation of a.c. conductivity of rubber ferrite composites from dielectric measurements

Abstract: The effect of frequency, composition and temperature on the a.c. electrical conductivity were studied for the ceramic, Ni1−xZnxFe2O4, as well as the filler (Ni1−xZnxFe2O4) incorporated rubber ferrite composites (RFCs). Ni1−xZnxFe2O4 (where) (bix)varies from 0 to 1 in steps of 0.2 were prepared by usual ceramic techniques. They were then incorporated into a butyl rubber matrix according to a specific recipe. The a.c. electrical conductivity (σa.c) calculations were carried out by using the data available from dielectric measurements and by employing a simple relationship. The a.c. conductivity values were found to be of the order of 10−3 S/m. Analysis of the results shows that σa.c. increases with increase of frequency and the change is same for both ceramic Ni1−xZnxFe2O4 and RFCs. σa.c increases initially with the increase of zinc content and then decreases with increase of zinc. Same behaviour is observed for RFCs too. The dependence of σa.c on the volume fraction of the magnetic filler was also studied and it was found that the a.c. conductivity of RFCs increases with increase of volume fraction of the magnetic filler. Temperature dependence of conductivity was studied for both ceramic and rubber ferrite composites. Conductivity shows a linear dependence with temperature in the case of ceramic samples.