Showing papers on "Volume fraction published in 1994"

Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites

Abstract: The effects of microstructure (namely, particulate volume fraction and particulate size) and the counterface materials on the dry-sliding wear resistance of the aluminum matrix composites 2014A1-SiC and 6061Al-Al2O3 were studied. Experiments were performed within a load range of 0.9 to 350 N at a constant sliding velocity of 0.2 ms-1. Two types of counterface materials, SAE 52100 bearing steel and mullite, were used. At low loads, where particles act as loadbearing constituents, the wear resistance of the 2014A1 reinforced with 15.8 µm diameter SiC was superior to that of the alloy with the same volume fraction of SiC but with 2.4 µm diameter. The wear rates of the composites worn against a steel slider were lower compared with those worn against a mullite slider because of the formation of iron-rich layers that act asin situ solid lubricants in the former case. With increasing the applied load, SiC and A12O3 particles fractured and the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. The transition to this regime was delayed to higher loads in the composites with a higher volume percentage of particles. Concurrent with particle fracture, large strains and strain gradients were generated within the aluminum layers adjacent to contact surfaces. This led to the subsurface crack growth and delamination. Because the particles and interfaces provided preferential sites for subsurface crack initiation and growth and because of the propensity of the broken particles to act as third-body abrasive elements at the contact surfaces, no improvement of the wear resistance was observed in the composites in this regime relative to unreinforced aluminum alloys. A second transition, to severe wear, occurred at higher loads when the contact surface temperature exceeded a critical value. The transition loads (and temperatures) were higher in the composites. The alloys with higher volume fraction of reinforcement provided better resistance to severe wear. Wearing the materials against a mullite counterface, which has a smaller thermal conductivity than a counterface made of steel, led to the occurrence of severe wear at lower loads.

Brownian dynamics of polydisperse colloidal hard spheres: Equilibrium structures and random close packings

Abstract: Recently we presented a new technique for numerical simulations of colloidal hard-sphere systems and showed its high efficiency. Here, we extend our calculations to the treatment of both 2- and 3-dimensional monodisperse and 3-dimensional polydisperse systems (with sampled finite Gaussian size distribution of particle radii), focusing on equilibrium pair distribution functions and structure factors as well as volume fractions of random close packing (RCP). The latter were determined using in principle the same technique as Woodcock or Stillinger had used. Results for the monodisperse 3-dimensional system show very good agreement compared to both pair distribution and structure factor predicted by the Percus-Yevick approximation for the fluid state (volume fractions up to 0.50). We were not able to find crystalline 3d systems at volume fractions 0.50–0.58 as shown by former simulations of Reeet al. or experiments of Pusey and van Megen, due to the fact that we used random start configurations and no constraints of particle positions as in the cell model of Hoover and Ree, and effects of the overall entropy of the system, responsible for the melting and freezing phase transitions, are neglected in our calculations. Nevertheless, we obtained reasonable results concerning concentration-dependent long-time selfdiffusion coefficients (as shown before) and equilibrium structure of samples in the fluid state, and the determination of the volume fraction of random close packing (RCP, glassy state). As expected, polydispersity increases the respective volume fraction of RCP due to the decrease in free volume by the fraction of the smaller spheres which fill gaps between the larger particles.

Heat transfer in foams

Abstract: One of the primary applications of polymeric foams is for thermal insulations Because of the modest proportion of solid in a foam and the consequent large volume fraction of gas which has a much lower thermal conductivity the resultant conductivity of the foam is much less than that of a solid body made of the same material In this chapter the foam conductivity refers to the effective value exhibited by the foam It is the ratio of the rate of heat transfer per unit cross-sectional area to the applied temperature difference

Particle size, volume fraction and matrix strength effects on fatigue behavior and particle fracture in 2124 aluminum-SiCp composites

Abstract: The effects of particle size, volume fraction and matrix strength on the stress-controlled axial fatigue behavior and the probability of particle fracture were evaluated for 2124 aluminum alloy reinforced with SiC particles. Average particle sizes of 2, 5, 9 and 20/~m and volume fractions of 0.10, 0.20 and 0.35 were examined for four different microstructural conditions. Tensile and yield strengths and fatigue life were substantially higher in the reinforced alloys. Strength and fatigue life increased as reinforcement particle size decreased and volume fraction loading increased. The frequency of particle fracture during crack propagation was found to be dependent on matrix strength, particle size and volume fraction and on maximum crack tip stress intensity. Particle fracture can be rationalized, phenomenologically, by the application of modified process zone models, originally derived for static fracture processes, and weakest link statistics which account for the dependence of matrix yield strength and flow behavior and particle strength on the probability of particle fracture during monotonic fracture and fatigue crack propagation.

Nanophase Separation in Monodisperse Rodcoil Diblock Polymers

Abstract: The authors have observed nanophase-separated structures in rodcoil polymers synthesized in their laboratory. Each polymer consists of a perfectly monodisperse, rodlike segment covalently bonded to a coil like segment of polyisoprene such that both rod and coil share the same molecular backbone. The polyisoprene was prepared by living anionic polymerization, and thus the rodcoil diblock molecules in the systems studied are of fairly uniform molar mass. Annealing of solution-cast thin films approximately 5--10 nm thick leads to ordered nanoscale morphologies that depend upon the volume fraction of rod segments in these diblock molecules. The authors find by transmission electron microscopy that the morphology varies from alternating strips of rod- and coil-rich domains to discrete aggregates of rods arranged in a hexagonal superlattice as rod volume fraction is decreased. At an intermediate volume fraction the authors see a coexistence of strips and aggregates. This breakup of rod domains may be the result of entropic coil stretching penalties analogous to those postulated recently by Williams and Fredrickson in lamellar assemblies of rodcoils.

Elastic Properties of Two‐Phase Composites

Abstract: Simple expressions were derived to predict the elastic properties of two-phase systems containing discontinuous reinforcements, on the basis of approximation of composite microstructure to a unit cell incorporating isostrain and isostress type elements, arranged in two different ways. The bounds on elastic modulus obtained in this manner have been shown to accurately describe the variation of elastic modulus as a function of volume fraction of one of the phases, for a wide variety of two-phase composites. The present expressions offer predictions of elastic modulus that are much closer to experimental data than the commonly used Hashin and Shtrikman bounds, particularly for composites with constituents having large differences in elastic moduli. Similarly, it has been shown that the shear moduli and the Poisson's ratios of composites as a function of second-phase volume fraction can also be predicted. the present unit-cell-based method of calculation presents a promising approach for the prediction of properties of multiphase materials. especially for those consisting of more than two dissimilar phases.

Finite element simulations of shear localization in plate impact

Abstract: S hear band development in a tungsten heavy alloy (WHA) during pressure-shear plate impact is analysed numerically. The alloy has a microstructure of hard tungsten grains embedded in a soft alloy matrix. A two-dimensional, plane strain model of the alloy microstructure is used in the computations. For this model microstructure a fully coupled thermo-mechanical initial boundary value problem is formulated and solved, accounting for finite deformations, inertia, heat conduction, thermal softening, strain hardening and strain-rate hardening. Calculations are carried out for distributions of uniform grains and for micro-structures obtained from digitized micrographs of the actual alloy. The effects of variations in grain volume fraction and grain size are considered. Experiments and the numerical calculations show that the two phase alloy is more susceptible to shear banding than either of the constituent phases. While the onset of shear localization depends on the grain distribution and volume fraction, the shear band width is found to be set by heat conduction and is insensitive to the grain volume fraction and the grain morphology, The shear band width obtained from the calculations is in good agreement with what is observed in the experiments. Furthermore, the computed shapes of the deformed tungsten grains inside the band resemble closely the observed shapes of the deformed grains in the experiments.

Micromechanical deformations in particulate filled thermoplastics: volume strain measurements

Abstract: Volume strain measurements were carried out on PP composites containing different CaCO3 fillers. During deformation, a volume increase was detected which could be divided into two linear sections as a function of elongation. Comparison of data with existing theories has shown that in the first part, mostly elastic deformation takes place and the slope can be related to the Poisson's ratio of the composite. Scanning electron microscopy revealed that in the second stage, the dominating micromechanical deformation process is debonding. Void formation is initiated at a certain stress which approximately corresponds to the yield stress of the composites, but data in the literature and model calculations indicate that separation of the matrix/filler interface may start at lower stresses. Initiation stress depends on the particle size of the filler and on interfacial interactions. The rate of volume increase has non-linear dependence on the volume fraction of the filler. Volume strain measurements reflect micromechanical deformations well, but further study is needed to explain contradictions between experimental results and theoretical predictions.

Measurement of Young's modulus and internal friction of an in situ Al-Al3Ni functionally gradient material

Abstract: An in situ Al-Al3Ni functionally gradient material (FGM) was produced by centrifugally casting an Al-20 mass% Ni alloy into a thick-walled tube. Four specimens, 90 mm long, with rectangular cross-sections (width × thickness) of 6 × 6, 6 × 5, 6 × 4 and 6 × 3 mm2 were machined from the tube such that the thickness direction of the specimens was in the radial direction of the tube. The microstructure of the FGM tube consisted of granular morphology Al3Ni as a second phase distributed within the aluminium matrix with an increasing volume fraction gradient from the inside to the outside of the tube. Thus, the thicker the specimen, the greater was the composition gradient and the thinner the specimen, the greater was the volume fraction of Al3Ni. The dependence of the Young's modulus and internal friction on the composition gradient of the FGM was determined by a flexural forced-resonance technique from the resonant frequency and the resonance peak width, respectively, as a function of nominal specimen thickness. The Young's modulus of the Al3Ni second phase was determined from a correlation plot of assumed Al3Ni Young's modulus values against the calculated resonant frequency values corresponding to the associated FGM Young's modulus values. The latter were calculated using a rule of mixtures with a fixed matrix Young's modulus and a gradient volume fraction of Al3Ni for each specimen thickness. By plotting the experimental FGM specimen resonant frequencies on this plot, the average Al3Ni Young's modulus was found to be 140 GPa. The Young's modulus of the FGM was found to vary between 81.5 and 100.8 GPa across the 6 mm tube-wall thickness from the inner to outer surface, reflecting the 15.2 and 43.2 vol % Al3Ni second phase, respectively. The measured internal friction increased with the volume fraction of Al3Ni, and owing to the relatively large Al3Ni particle size, was thereby dependent on the resultant increase in the second phase-matrix interface number density rather than the dislocation density.

Molecular weight-dependence of free volume in polystyrene studied by positron annihilation measurements

Abstract: Positron annihilation lifetime measurements are reported for four monodisperse polystyrenes with molar mass M = 4,000, 9,200, 25,000, and 400,000. The temperature dependences of orthopositronium (o-Ps) lifetime (τ3) and intensity (I3) were measured from 5°C to Tg + 30°C for each sample. From these data, the free volume hole size, 〈vf(τ3)〉, and fractional free volume hps=CI3〈vf(τ3)〉 were calculated. The temperature dependences of τ3, 〈vf(τ3)〉 and hps show a discrete change in slope at an effective glass transition temperature, Tg,ps, which is measurably below the conventional bulk Tg. This suggests that τ3 is sensitive to large holes which retain their liquid-like mobility in the glassy state. Good agreement was found for T > hg,ps between hps and the theoretical free volume fraction hth deduced from experimental P-V-T data for polystyrene using the statistical mechanical theory of Simha and Somcynsky. Below Tg,ps, deviations between hps and hth are observed, hps falling increasingly below hth as temperature decreases. Whereas hps and hth depend strongly on M in the melt, each essentially independent of M in the glass. A free volume quantity, computed from the bulk volume, which is in good numerical agreement with the Simha-Somcynsky h-function in the melt, gives improved agreement with hps in the glassy state. © 1994 John Wiley & Sons, Inc.

Use of rule of mixtures and metal volume fraction for mechanical property predictions of fibre-reinforced aluminium laminates

Abstract: This paper presents the results of a statistically designed programme conducted to validate feasibility of using the relationship of mechanical properties and metal volume fraction in fibre-metal laminates to make property predictions. Experimental and analytical practices employed to obtain these mechanical properties for tension, compression, in-plane shear, and bearing are described. Results from this pilot study show that use of the metal volume fraction may be useful for the prediction of strength mechanical properties in fibre-metal laminates. However, this needs further study to validate the concept. If the hypothesis is valid, the number of laminate configurations to be tested to qualify a fibre-metal laminate family can be minimized. The findings imply that the metal volume fraction approach using a rule of mixtures can be exploited to estimate design properties for a multitude of fibre-metal laminate variants, which is economically beneficial to the preliminary stages of aircraft design.

Electrical resistivity of bulk nanocrystalline nickel

Abstract: The effect of grain boundaries on the electrical resistivity of bulk nickel with a nanocrystalline structure has been investigated. For this study, nanocrystalline nickel was produced using a pulse plating technique that yields equiaxed nanostructures with negligible porosity. Using the four probe technique, resistivity measurements were performed at various temperatures ranging from 77 K to room temperature. The values of the resistivity were found to increase with decreasing grain size. This is mainly due to the increased volume fraction of interfaces at smaller grain sizes and the associated electron scattering events at the grain boundaries. The temperature coefficient of resistivity was found to decrease with decreasing grain size.

Residual stresses in Al2O3ZrO2 composites: A test of stochastic stress models

Abstract: In cooling sintered composites of Al2O3ZrO2 from their fabrication temperature residual stresses are created as a result of both the difference in thermal expansion between the two phases and the crystallographically anisotropic thermal expansion of the Al2O3 phase. In this work the first and second moments of the residual stress distribution have been measured as a function of volume fraction of zirconia from 0.01 to 0.90. The measurement technique used is piezo-spectroscopy based on the optical fluorescence from Cr3+ dopants in the alumina phase. For zirconia volume fractions up to 0.35 the average stress accurately fits the predictions given by the upper Hashin bound and this fit provides a value of the average thermal strain in the composites. Using this value, the effective medium approximation produces an excellent description of the average stress over the entire volume fraction. It is also shown that the fluorescence broadening due to stress fluctuations lies close to the predicted upper and lower Hashin bounds modified by the restrictions imposed by the principle of maximum entropy. The measured moments and those predicted by stochastic stress analysis compare well suggesting that the stochastic analysis provides a reliable method of calculating residual stress in composites.

The compositional dependence of thermodynamic interactions in blends of model polyolefins

Abstract: We have investigated the thermodynamic interactions as functions of component volume fraction φ and temperature T in six binary polymer blend systems. The components in all cases were model polyolefins, made by saturating the double bonds of nearly monodisperse polydienes. Small‐angle neutron scattering measurements for single‐phase melts, analyzed with the incompressible random phase approximation, were expressed in terms of the Flory–Huggins interaction parameter χ for 27 °C≤T≤167 °C and 0.1≤φ≤0.9. In most systems we found a characteristic upturn in χ at the composition extremes that diminished with increasing temperature. We also found that the component chain dimensions were unaffected by the strength of the interactions. Several theoretical attempts to explain the dependence of χ on component volume fraction were examined. Most were qualitatively inconsistent with the results, and none were fully satisfactory.

Residual stresses in alumina-SiC nanocomposites

Abstract: Residual stresses in an alumina-SiC particulate composite were studied as a function of SiC content by X-ray diffraction. The average microstresses in each phase and the stress fluctuations in the matrix were evaluated from a combination of X-ray reflection shift and line broadening analysis. The measured average microstresses show good agreement with those calculated theoretically from a simple model. The average microstresses in the matrix increases with the SiC content while the fluctuations of the stress field decrease. In effect, the mean dislocation density in the alumina matrix increases with the SiC volume fraction with more dislocations forming networks and subboundaries, as was confirmed by transmission electron microscopy.

Factors affecting the damping capacity of cast aluminium-matrix composites

Abstract: The damping capacity of stir-cast aluminium-matrix composites containing graphite and silicon carbide particles, were studied using a cantilever beam specimen and an HP 5423A Structural Dynamics Analyser. Damping data were determined in the first mode of vibration. Aluminium-matrix composites containing 5–10 vol % graphite particles and 10 vol % silicon carbide particles were prepared by the stir-casting technique and die cast to obtain standard samples (6 mm×25 mm100 mm). Graphite particles were found to be more effective in enhancing the damping capacity of composites compared to silicon carbide particles. The damping capacity of composites increased with the volume percentage of graphite within the range studied. However, no notable improvements in damping capacity were observed by dispersion of silicon carbide in aluminium alloy. The results have been analysed in terms of the effect of size, shape, nature and volume fraction of particles on the damping capacity of the aluminium matrix particulate composites and compared with the damping capacity data available in the literature. The effects of frequency, strain amplitude, temperature and processing on damping capacity of the aluminium matrix composites are reviewed.

Fracture at elevated temperatures in a particle reinforced composite

Abstract: The tensile fracture behavior of a cast and extruded 2014 aluminum alloy metal matrix composite (MMC) reinforced with 10, 15, and 20 vol.% aluminum oxide particles was investigated as a function of temperature between 100 and 300°C and hold time, and compared with the unreinforced alloy. In addition, the effect of aging condition was investigated in a 15 vol.% composite tested at 200°C. At lower temperature the composites have higher yield strength and UTS than the unreinforced material, and both decrease with increasing temperature. At higher temperatures all the materials have similar strength levels. The elongation is lower in the composites, decreasing with increasing level of reinforcement and increasing with increasing temperature, except at the highest temperature where all the composites are about the same. The microstructural damage in the composites also varies with temperature: particle fracture dominates at lower temperatures and interparticle voiding is the main damage feature at elevated temperatures. The time at temperature, and hence the degree of overaging, has little effect on the observed trends in the composite, in contrast with the unreinforced material where the density of voids decreases with increasing hold times. The transition temperature where the major damage changes from particle cracking to interparticle voiding increases with volume fraction and particle size, and decreases with overaging. The cracked particle density and void density both increase with strain, and the highest rate of increase occurs in the overaged material. In general, the tendency for particle cracking is reduced and for interparticle voiding is increased by any factor which permits accomodation of strain by the matrix, such as lower volume fraction of particles, small particle size, nonclustered particle distribution, and matrix softening from underaging or overaging.

Transverse strength of metal matrix composites reinforced with strongly bonded continuous fibers in regular arrangements

Abstract: The composite limit flow stress for transverse loading of metal matrix composites reinforced with a regular array of uniform continuous fibers is calculated using the finite element method. The effects of volume fraction and matrix work hardening are investigated for fibers of circular cross section distributed in both sqyare and hexagonal arrangements. The hexagonal arrangement is seen to behave isotropically with respect to the limit stress, whereas the square arrangement of fibers results in a composite which is much stronger when loaded in the direction of nearest neighbors and weak when loaded at 45° to this direction. The interference of fibers with flow planes is seen to play an important role in the strengthening mechanism. The influence of matrix hardening as a strengthening mechanism in these composites increases with volume fraction due to increasing fiber interaction. The results for a power law hardening matrix are also applicable to the steady state creep for these composites. The influence of volume fraction on failure parameters in these composites is addressed. Large increases in the maximum values of hydrostatic tension, equivalent plastic stain, and tensile stress normal to the fiber-matrix interface are seen to accompany large increases in composite strength.

Tetragonal zirconia growth by nanolaminate formation

Abstract: Multilayer films of polycrystalline zirconia and amorphous alumina were grown by reactive sputter deposition and characterized using x‐ray diffraction and high resolution electron microscopy. We demonstrate that the layer spacing can be scaled to insure nanosize crystallites in the zirconia layer. The result is that nanolaminates with a high volume fraction of retained tetragonal zirconia are produced, independent of deposition parameters and without the addition of a stabilizing dopant.

Thin film solar selective surface coating

Abstract: Solar energy absorptive coatings that comprise single cermet layers with a homogeneous metal volume fraction exhibit an absorptance of about 0.8, which is not high enough for practical solar applications. To achieve absorptance greater than 0.9, graded composite films have been developed, but they give rise to higher thermal emittance due to the absorption edge not being sharp enough. This leads to increased thermal emittance at high operating temperatures, in the range of 300° C. to 500° C. There is now disclosed a novel solar selective surface coating which is composed of two cermet layers, with different metal volume fractions in each layer. The two cermet layers have different thicknesses, and the layers have thicknesses and volume fractions such that solar radiation is absorbed by internal absorbing and phase cancellation interference, but the cermet layers are substantially transparent in the thermal infrared region.

Flow-Induced Anisotropic SALS in Silica-Filled PDMS Liquids

Abstract: Anisotropic butterfly-type patterns have been observed during shear flow in polydimethylsiloxane liquids (Mn = 8700 to 67,000) filled with colloidal silica (radius = 50 nm) using small-angle light scattering. These anisotropic patterns indicate the formation of particle or agglomerate fluctuations organized parallel to the shear direction. The shear-induced fluctuations appear to be due to deformation and alignment of large flocs or agglomerates formed in these systems by polymer bridging. Unsheared agglomerate structures were examined using static light scattering, transmission electron, and optical microscopy, and agglomerates subjected to shear flow were examined using light scattering and optical microscopy. The shear-induced ordering was found to increase with shear rate and the volume fraction of agglomerates.

Shear‐induced particle migration in suspensions of rods

Abstract: Shear‐induced migration of particles occurs in suspensions of neutrally buoyant spheres in Newtonian fluids undergoing shear in the annular space between two rotating, coaxial cylinders (a wide‐gap Couette), even when the suspension is in creeping flow. Previous studies have shown that the rate of migration of spherical particles from the high‐shear‐rate region near the inner (rotating) cylinder to the low‐shear‐rate region near the outer (stationary) cylinder increases rapidly with increasing sphere size. To determine the effect of particle shape, the migration of rods suspended in Newtonian fluids was recently measured. The behavior of several suspensions was studied. Each suspension contained well‐characterized, uniform rods with aspect ratios ranging from 2 to 18 at either 0.30 or 0.40 volume fraction. At the same volume fraction of solids, the steady‐state, radial concentration profiles for rods were independent of aspect ratio and were indistinguishable from those obtained from suspended spheres. Only minor differences near the walls (attributable to the finite size of the rods relative to the curvature of the walls) appeared to differentiate the profiles. Data taken during the transition from a well‐mixed suspension to the final steady state show that the rate of migration increased as the volume of the individual rods increased.

Analytical Characterization of the Anisotropy and Local Heterogeneity of Short Fiber Composites: Fiber Fraction as a Variable:

Abstract: A statistical analysis to characterize the system anisotropy and the local heterogeneity of short fiber composites is presented in this report. Firstly, using a fiber orientation probability density function (pdf), the statistical mean values and variations of the fiber cut ends and the parameters of the matrix material distribution on an arbitrary cross section of a composite are derived. The fiber area fraction Af, which is shown to be a function of the fiber orientation and the direction of the cross section, is used to calculate the composite elastic properties such as the tensile modulus associated with the cross section so that the direction-dependence or the anisotropic nature of the composite properties is predicted.However, to study other important system properties such as the strength, the fiber-matrix interaction, failure process and impact resistance, neither the overall fiber volume fraction Vf nor the fiber area fraction Af at cross sections is proved to be adequate. Local fiber volume frac...