Showing papers on "Young's modulus published in 2000"

Size-dependent elastic properties of nanosized structural elements

Abstract: Effective stiffness properties (D) of nanosized structural elements such as plates and beams differ from those predicted by standard continuum mechanics (Dc). These differences (D-Dc)/Dc depend on the size of the structural element. A simple model is constructed to predict this size dependence of the effective properties. The important length scale in the problem is identified to be the ratio of the surface elastic modulus to the elastic modulus of the bulk. In general, the non-dimensional difference in the elastic properties from continuum predictions (D-Dc)/Dc is found to scale as αS/Eh, where α is a constant which depends on the geometry of the structural element considered, S is a surface elastic constant, E is a bulk elastic modulus and h a length defining the size of the structural element. Thus, the quantity S/E is identified as a material length scale for elasticity of nanosized structures. The model is compared with direct atomistic simulations of nanoscale structures using the embedded atom method for FCC Al and the Stillinger-Weber model of Si. Excellent agreement between the simulations and the model is found.

Oil Palm Fibre Reinforced Phenol Formaldehyde Composites: Influence of Fibre Surface Modifications on the Mechanical Performance

Abstract: Oil palm fibres have been used as reinforcement in phenol formaldehyde resin. In order to improve the interfacial properties, the fibres were subjected to different chemical modifications such as mercerisation, acrylonitrile grafting, acrylation, latex coating, permanganate treatment, acetylation, and peroxide treatment. The effect of fibre coating on the interface properties has also been investigated. Morphological and structural changes of the fibres were investigated using scanning electron microscopy and IR spectroscopy. Mechanical properties of untreated and treated fibres were studied. Changes in stress–strain characteristics, tensile strength, tensile modulus and elongation at break of the fibres upon various modifications were studied and compared. The incorporation of the modified fibres resulted in composites having excellent impact resistance. Fibre coating enhanced the impact strength of untreated composite by a factor of four. Tensile and flexural performance of the composites were also investigated. Finally, inorder to have an insight into the failure behaviour, the tensile and impact fracture surfaces of the composites were analysed using scanning electron microscope.

Evaluation of size effect on mechanical properties of single crystal silicon by nanoscale bending test using AFM

Abstract: This paper describes a nanometer-scale bending test for a single crystal silicon (Si) fixed beam using an atomic force microscope (AFM). This research focuses on revealing the size effect on the mechanical property of Si beams ranging from a nano- to millimeter scale. Nanometer-scale Si beams, with widths from 200 to 800 nm and a thickness of 255 nm, were fabricated on an Si diaphragm by means of field-enhanced anodization using AFM and anisotropic wet etching. The efficient condition of the field-enhanced anodization could be obtained by changing the bias voltage and the scanning speed of the cantilever. Bending tests for micro- and millimeter-scale Si beams fabricated by a photolithography technique were also carried out using an ultraprecision hardness tester and scratch tester, respectively. Comparisons of Young's modulus and bending strength, of Si among the nano-, micro-, and millimeter scales showed that the specimen size did not have an influence on the Young's modulus in the [110] direction, whereas it produced a large effect on the bending strength. Observations of the fractured surface and calculations of the clack length from Griffith's theory made it clear that the maximum peak-to-valley distance of specimen surface caused the size effect on the bending strength.

A Conewise Linear Elasticity mixture model for the analysis of tension-compression nonlinearity in articular cartilage.

Abstract: A biphasic mixture model is developed which can account for the observed tension-compression nonlinearity of cartilage by employing the continuum-based Conewise Linear Elasticity (CLE) model of Curnier et al. (J Elasticity 37:1–38, 1995) to describe the solid phase of the mixture. In this first investigation, the orthotropic octantwise linear elasticity model was reduced to the more specialized case of cubic symmetry, to reduce the number of elastic constants from twelve to four. Confined and unconfined compression stress-relaxation, and torsional shear testing were performed on each of nine bovine humeral head articular cartilage cylindrical plugs from 6 month old calves. Using the CLE model with cubic symmetry, the aggregate modulus in compression and axial permeability were obtained from confined compression (H−A =0.64±0.22 MPa, kz = 3.62 ± .97 × 10−16 m4/N.s, r2 =0.95±0.03), the tensile modulus, compressive Poisson ratio and radial permeability were obtained from unconfined compression (E+Y = 12.75 ± 1.56 MPa, ν− =0.03±0.01, kr =6.06±2.10×10−16 m4/N.s, r2 =0.99±0.00), and the shear modulus was obtained from torsional shear (µ=0.17±0.06 MPa). The model was also employed to successfully predict the interstitial fluid pressure at the center of the cartilage plug in unconfined compression (r2 =0.98±0.01). The results of this study demonstrate that the integration of the CLE model with the biphasic mixture theory can provide a model of cartilage which can successfully curvefit three distinct testing configurations while producing material parameters consistent with previous reports in the literature.

Strain energy and young's modulus of single-wall carbon nanotubes calculated from electronic energy-band theory

Abstract: The strain energies in straight and bent single-walled carbon nanotubes (SWNT's) are calculated by taking account of the total energy of all the occupied band electrons. The obtained results are in good agreement with previous theoretical studies and experimental observations. Young's modulus and the effective wall thickness of SWNT's are obtained from the bending strain energies of SWNT's with various cross-sectional radii. The repulsion potential between ions contributes the main part of Young's modulus of SWNT's. The wall thickness of the SWNT comes completely from the overlap of electronic orbits and is approximately of the extension of the $\ensuremath{\pi}$ orbit of carbon atom. Both Young's modulus and the wall thickness are independent of the radius and the helicity of SWNT, and insensitive to the fitting parameters. The results show that continuum elasticity theory can serve well to describe the mechanical properties of SWNT's.

Thermal Conductivity and Elastic Modulus Evolution of Thermal Barrier Coatings under High Heat Flux Conditions

Abstract: Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO2-8% Y2O3 coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.

Flexural behavior of concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars

Abstract: Concrete members reinforced with glass fiber-reinforced polymer (GFRP) bars exhibit large deflections and crack widths compared with concrete members reinforced with steel. This is due to the low modulus of elasticity of GFRP. Current design methods for predicting deflections at service load and crack widths developed for concrete structures reinforced with steel may not be used for concrete structures reinforced with GFRP. This paper presents methods for predicting deflections and crack widths in beams reinforced with GFRP. To use the effective moment of inertia for concrete beams reinforced with GFRP bars, the effect of the FRP reinforcement ratios and elastic modulus of FRP were incorporated in the exponent of Branson's equation. In addition, an equation based on flexural stiffness of GFRP reinforced concrete was developed to predict deflections. Based on this investigation and past studies, a theoretical correlation for predicting crack width was proposed. Six concrete beams reinforced with different GFRP reinforcement ratios were tested. Their measured deflections and crack widths were compared with the proposed models. The experimental results compared favorably with those predicted by the models.

Mechanical properties of single-brin silkworm silk

Abstract: Mechanical tests were performed on single brins of Bombyx mori silkworm silk, to obtain values of elastic modulus (E), yield strength, tensile breaking strength, and shear modulus (G). Specimen cross-sectional areas, needed to convert tensile loads into stresses, were derived from diameter measurements performed by scanning electron microscopy. Results are compared with existing literature values for partially degummed silkworm baves. The tensile modulus (16 ± 1 GPa) and yield strength (230 ± 10 MPa) of B. mori brin are significantly higher than the literature values reported for bave. The difference is attributed principally to the presence of sericin in bave, contributing to sample cross-section but adding little to the fiber's ability to resist tensile deformation. The two brins in bave are found to contribute equally and independently to the tensile load-bearing ability of the material. Measurements performed with a torsional pendulum can be combined with tensile load-extension data to obtain a value of E/ that is not sensitive to sample cross-sectional dimensions or, therefore, to the presence of sericin. The value of E measured for brin can be used together with this result to obtain G = 3.0 ± 0.8 GPa and E/G = 5.3 ± 0.3 for brin. The latter value indicates a mechanical, and therefore microstructural, anisotropy comparable to that of nylon. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1270–1277, 2000

Elastic modulus of polypyrrole nanotubes

Abstract: The first measurements of the tensile elastic modulus of polypyrrole nanotubes are presented. The nanotubes were mechanically tested in three points bending using atomic force microscopy. The elastic tensile modulus was deduced from force-curve measurements on different nanotubes with outer diameter ranging between 35 and 160 nm. It is shown that the elastic modulus strongly increases when the thickness or outer diameter of polypyrrole nanotubes decreases.

Bending strength and toughness of heat-treated wood

Abstract: The load-deflection curve for static bending and the force-time curve for impact bending of heat-treated wood were examined in detail. The effect of oxygen in air was also investigated. Sitka spruce (Picea sitchensis Carr.) was heated for 0.5–16.0h at a temperature of 160°C in nitrogen gas or air. The dynamic Young's modulus was measured by the free-free flexural vibration test, the static Young's modulus and work needed for rupture by the static bending test, and the absorbed energy in impact bending by the impact bending test. The results obtained were as follows: (1) The static Young's modulus increased at the initial stage of the heat treatment and decreased later. It decreased more in air than in nitrogen. (2) The bending strength increased at the initial stage of the heat treatment and decreased later. It decreased more in air than in nitrogen. (3) The work needed for rupture decreased steadily as the heating time increased. It decreased more in nitrogen than in air. It is thought that heat-treated wood was more brittle than untreated wood in the static bending test because W12 was reduced by the heat treatment. This means that the main factors contributing to the reduction of the work needed for rupture were viscosity and plasticity, not elasticity. (4) The absorbed energy in impact bending increased at the initial stage of the heat treatment and decreased later. It decreased more in air than in nitrogen. It was concluded that heat-treated wood became more brittle in the impact bending test becauseI 12 andI 23 were reduced by the heat treatment.

Ductility of bulk nanocrystalline composites and metallic glasses at room temperature

Abstract: Mechanical properties of bulk Zr60Cu20Pd10Al10 nanocrystalline composite and Zr55Ni5Cu30Al10 metallic glass were measured by compression tests at room temperature. The Zr60Cu20Pd10Al10 as-quenched alloy obviously exhibits plastic strain while no distinct plastic deformation is recognized in the Zr55Ni5Cu30Al10 metallic glass. Moreover, the plastic strain increased by increasing the volume fraction of nanocrystals and achieved maximum value in the early stage of the nanocrystallization. High-resolution electron microscopy showed that, different from the microstructure of Zr55Ni5Cu30Al10 metallic glass, nanocrystals with main grain sizes of about 2 nm were embedded in the amorphous matrix of the bulk Zr60Cu20Pd10Al10 alloy which showed the maximum plastic strain.

Mechanical properties and non-homogeneous deformation of open-cell nickel foams: application of the mechanics of cellular solids and of porous materials

Abstract: The mechanical properties of open-cell nickel foams are investigated for the range of densities used in industrial applications for energy storage. The obtained Young’s modulus, compression yield stress and tensile fracture stress are compared to the predictions of models based on periodic, Penrose and Voronoi beam networks. It is found that Gibson and Ashby’s model [L.J. Gibson, M.F. Ashby, Cellular Solids, Cambridge University Press, Cambridge, 1998] provides the proper scaling laws with respect to relative density for almost all investigated properties. The strong anisotropy of the observed overall responses can also be accounted for. The two-dimensional strain field during the tension of a nickel foam strip has been measured using a photomechanical technique. Non-homogeneous deformation patterns are shown to arise. The same technique is used to obtain the strain field around a circular hole in a nickel foam strip. The observed deformation fields are compared to the results of a finite element analysis using anisotropic compressible continuum plasticity.

Microscale material testing of single crystalline silicon: process effects on surface morphology and tensile strength

Abstract: The mechanical properties of single-crystalline silicon are measured by uniaxial tension tests from microscale beam specimens patterned by four different common silicon etchants — KOH, EDP, TMAH and XeF2. SOI wafers are used to prepare test samples, which are 3–5 μm thick, 20–100 μm wide, and 6 mm long beam specimens; these are monolithically mounted on a temporary frame. A uniaxial tension test has been designed to accommodate microscale test requirements such as sample handling, sample alignment, and friction elimination. Stress and strain are measured using a commercial load cell and a laser interferometry system, respectively. Young's modulus of silicon in the 〈110〉 direction is measured to be 169.2±3.5 GPa, very close to the widely accepted value of 168.9 GPa obtained from a macroscale sample by an ultrasonic method. The fracture strength in the 〈110〉 direction is measured to vary from 0.6 to 1.2 GPa, and is apparently affected by the etching process employed to make the microscale specimen. As surface defects are expected to be the main factor determining the strength of the specimen, surface morphology is examined not only as a function of etchants but also as a function of mask-to-crystal direction misalignment after KOH etching. In the case of samples prepared by KOH etching, measured fracture strengths are 0.94 and 0.72 GPa from samples with 0° and 2° misalignments, respectively.

Micromechanical Modelling of Wood and Fibre Properties

Abstract: Wood is a material with mechanical properties that vary markedly, both within a tree and among trees. Moisture changes lead to shrinkage or swelling and modify the mechanical properties. In the present study both experimental and numerical work concerning the stiffness and the hygroexpansion properties of wood and of fibres and variations in them is presented. The experimental work involves both characterizing the structure of wood at the microstructural level and the testing of clear-wood specimens. The experiments at the microstructural level provide valuable information concerning the cellular structure of wood, information needed for modelling wood on the basis of its microstructure. Deformations in the microstructure due to loading, as characterized by use of a SEM, was also studied. The longitudinal modulus of elasticity, three hygroexpansion coefficients and the density along the radius from pith to bark in the stem were determined by the testing of clear wood specimens. The longitudinal modulus of elasticity and the three shrinkage coefficients were shown to vary considerably along the radial direction of the stem. Models based on the microstructure for determining the stiffness and shrinkage properties of wood are proposed. The models investigated include the chain of modelling from the mechanical properties of the chemical constituents of the cell wall to the average mechanical properties of a growth ring. The models are based mainly on results of the experiments that were performed. Models of the microfibril in the cell wall as well as models of the cellular structure of wood were developed with the aim of determining the stiffness and shrinkage properties of wood from simply a few key parameters. Two models of the cellular structure of wood were investigated. In one of these, the structure was composed of irregular hexagonal cells, whereas in the other the cell structures were obtained from micrographs. Parametric studies performed by use of the hexagonal cell model are presented. The results of these studies showed the parameters governing the stiffness and the hygroexpansion properties of wood to be the microfibril angle of the S2-layer, density and the properties of the chemical constituents. An introductory study of the nonlinear behaviour of cell structures was also carried out. The results of numerical analyses of the deformations in cell structures that occur in compression loading in the radial and tangential directions are presented. The mechancial behaviour of chemically unaltered fibres of simplified geometrical shape was also studied in a preliminary way by means of micromechanical modelling. Three-dimensional finite element models of straight fibres of undeformed and of collapsed cross-sectional shape were involved. Both the force-displacement relationship and the moisture-induced deformations needed for characterization the behaviour of the fibre were determined. The results of simulations of the stiffness behaviour of fibres revealed two unique coupled deformation modes: coupling between extension and twist and coupling between in-plane bending and out-of-plane shear deformation. The deformation modes obtained were shown to be dependent on the value of the microfibril angle in the S2-layer. (Less)

Anisotropic stiffness parameters and their measurement in a stiff natural clay

Abstract: The framework of anisotropic elasticity has been used to develop relationships between various anisotropic stiffness parameters found in the literature. It is shown that they are functions of the two Young's moduli and two Poisson's ratios that describe a cross-anisotropic soil, but not of the independent shear modulus. Multiple drained triaxial stress path excursions have been performed on 100 mm dia. samples of natural Gault Clay from Madingley in a stress path cell. Anisotropic parameters at small strains along different stress paths are reported and compared. It is shown that there are some advantages in performing tests at constant vertical and constant horizontal effective stress. Horizontally mounted bender elements on the same trixial samples have enabled the two anisotropic elastic shear moduli to be measured. Combining results from both sets of tests has enabled all five independent cross-anisotropic elastic parameters to be estimated. The Gault Clay at Madingley is highly anisotropic, although ...

Imaging the elastic properties of coiled carbon nanotubes with atomic force microscopy

Abstract: Coiled carbon nanotubes were produced catalytically by thermal decomposition of hydrocarbon gas. After deposition on a silicon substrate, the three-dimensional structure of the helix-shaped multiwalled nanotubes can be visualized with atomic force microscopy. Helical structures of both chiralities are present in the nanotube deposits. For larger coil diameters ( .170 nm), force modulation microscopy allows one to probe the local elasticity along the length of the coil. Our results agree with the classical theory of elasticity. Similar to the case of straight nanotubes, the Young modulus of coiled multiwalled nanotubes remains comparable to the very high Young modulus of hexagonal graphene sheets. ations in the elastic response of the coils along their length. Our results nicely agree with the classical theory of elas- ticity. From our FMM measurements we infer a Young modulus around 0.7 TPa for the coiled MWNTs. This re- sult is consistent with the reported high values of Young's modulus for straight carbon nanotubes which has been measured via resonant motion (8,9) of free standing tubes as well as via AFM induced deformations of nanotubes de- posited on a flat surface (10) or on substrates containing micropores (11). The nanotube material containing the helical MWNTs is produced by catalytic decomposition of acetylene, carried out at 700 ± C in a flow reactor at atmospheric pressure (12). The purified carbon nanotube material is sonicated at low power in isopropanol and is deposited onto a piece of an oxidized silicon wafer with gold markers fabricated by combining electron beam lithography and lift-off techniques. The nanotube deposits are first imaged with a scanning electron microscope (SEM: Philips, XL-30 FEG), allowing one to record the position of the coiled structures. Next, the AFM imaging and the FMM measurements are performed in air with a commercial system ( Park Scientific Instruments, M5) using silicon cantilevers with a tip apex radius of curvature of about 10 nm. The elastic response of the coiled nanotubes can be probed locally with the FMM technique (7) which is illustrated in Fig. 1(a). The Si cantilever is gently tapping the sample surface at the cantilever resonance frequency fres 98 kHz with an amplitude ranging between 0.2 and 5 nm. Additionally, the vertical position of the sample is periodically modulated at a much smaller frequency fmod in the range 8 11 kHz with a modulation amplitude between 1 and 2 nm. Harmonic detection at the frequency fmod of the periodically varying interaction between tip and coiled nanotube enables one to locally probe its elasticity.

Effects of age on elastic moduli of human lungs.

Abstract: The model of the lung as an elastic continuum undergoing small distortions from a uniformly inflated state has been used to describe many lung deformation problems. Lung stress-strain material properties needed for this model are described by two elastic moduli: the bulk modulus, which describes a uniform inflation, and the shear modulus, which describes an isovolume deformation. In this study we measured the bulk modulus and shear modulus of human lungs obtained at autopsy at several fixed transpulmonary pressures (Ptp). The bulk modulus was obtained from small pressure-volume perturbations on different points of the deflation pressure-volume curve. The shear modulus was obtained from indentation tests on the lung surface. The results indicated that, at a constant Ptp, both bulk and shear moduli increased with age, and the increase was greater at higher Ptp values. The micromechanical basis for these changes remains to be elucidated.

Micromechanics-based stress-strain behaviour of soils at small strains

Abstract: A model that describes the small-strain behaviour of soils is derived using micromechanics theory. The model allows examination of the effects of fabric anisotropy, stress conditions and contact characteristics on the small-strain modulus of soils. The closed-form solutions of the small-strain modulus are presented for the case of an isotropic fabric assembly under isotropic stress conditions. A fabric tensor is used to model the fabric anisotropy, and the small-strain modulus for the case of cross-anisotropic fabric assembly under general stress conditions is numerically calculated. The effect of the contact condition between soil particles is examined by incorporating three different contact laws (the linear elastic, Hertz–Mindlin and rough-surface contact models) into the model. It was found that the numerical results using the rough-surface contact model compare well with the published experimental data, and that the model provides microscopic insight into the small-strain behaviour of soils observed ...

Determination of elastic properties of thin films by indentation measurements with a spherical indenter

Abstract: Indentation is an important method for the determination of mechanical properties of surfaces and thin films. It is well known that the measurement results from thin layers are strongly influenced by the substrate properties. For hardness measurements it is frequently quoted that the indentation depth should be less than one-tenth of the film thickness (1/10th rule). This rule is often not practicable for thickness values below 1 μm. Therefore a correction method is required that allows the separation of substrate and film properties from the load-depth data. Moreover, the calculation is complicated if plastic deformation occurs. The use of a spherical indenter allows one to remain completely within the elastic range if the indenter radius is large enough and the load is low enough. In this case a novel analytical solution for the elastic deformation of a film on a flat substrate can be used to simulate the load-depth data. With this solution the determination of Young’s modulus of thin layers is possible independent of indentation depth and film thickness. Measurement data from a UMIS-2000 indentation system for different film substrate combinations are compared with theoretical results. It is shown, that a separation of the elastic film properties is possible. For metal films on Si the load-depth data did not differ from that of uncoated substrates. This can be explained mainly by delamination.

Laser fabrication of discontinuously reinforced metal matrix composites

Abstract: Disclosed are reinforced metal matrix composites and methods of shaping powder materials to form such composites. Articles of manufacture are formed in layers by a laser fabrication process. In the process, powder is melted and cooled to form successive layers of a discontinuously reinforced metal matrix. The matrix exhibits fine grain structure with enhanced properties over the unreinforced metal, including higher tensile modulus, higher strength, and greater hardness. In some preferred embodiments, an in-situ alloy powder, a powder metallurgy blend, or independently provided powders are reinforced with boron and/or carbon to form the composite.

An improved methodology for determining temperature dependent moduli of underfill encapsulants

Abstract: Finite element analyses (FEAs) have been widely used to preventively predict the reliability issues of flip-chip (FC) packages. The validity of the simulation results strongly depends on the inputs of the involved material properties. For FC packages Young's modulus-temperature relationship is a critical material property in predicting of the package reliability during -55/spl deg/C to 125/spl deg/C thermal cycling. Traditional tensile tests can obtain the modulus at selected temperatures, but are tedious, expensive, and unable to accurately predict the Young's modulus-temperature relationship within a wide temperature range. Thus, this paper is targeted to provide a simple but relatively accurate methodology to obtain the Young's modulus-temperature relationship. In this paper, three commercial silica filled underfill materials were studied. A simple specimen (based on ASTM D638M) preparation method was established using a Teflon mold. A dynamic-mechanical analyzer (DMA) was used to obtain the stress-strain relationship under controlled force mode, storage and loss modulus under multi-frequency mode, and stress relaxation under stress relaxation mode. A simple viscoelastic model was used and an empirical methodology for obtaining Young's modulus-temperature relationship was established.

Quantitative determination of Young's modulus on a biphase polymer system using atomic force microscopy

Abstract: Atomic force microscopy (AFM) is currently used to investigate polymer surface morphology, to obtain roughness parameters or to map the qualitative differences of surface properties. Some previous studies have attempted to determine quantitatively the elastic surface properties, but the difficulty with AFM is that the contact geometry is not very well known, due to the complexity of the mechanical system composed of the cantilever-tip set and the solid surface. We propose here a relative method for measuring the Young's modulus E of a polymer surface by AFM indentation, involving a calibration step obtained from a set of standards constituted by pure polymers with known modulus. Contact stiffness, indentation at peak load and shape of unloading curve are obtained for each reference polymer, leading to a linear relationship between E and a function of these parameters. This calibration curve allows the unknown Young's modulus values of the different phases at the surface of a biphase polymer system to be determined. The force volume mode was used to record force curves. Compared to classical indentation techniques, the force volume mode gives the advantage of imaging surface domains exhibiting elasticity differences. Thus, the elastic modulus can be quantified with a spatial resolution on a nanometric scale.

A modeling and experimental study of the influence of twist on the mechanical properties of high‐performance fiber yarns

Abstract: Little data exist on how twist changes the properties of high-performance continuous fiber yarns. For this reason, a study was conducted to determine the influence of twist on the strength and stiffness of a variety of high-performance continuous polymeric fiber yarns. The materials investigated include Kevlar 29®, Kevlar 49®, Kevlar 149®, Vectran HS®, Spectra 900®, and Technora®. Mechanical property tests demonstrated that the initial modulus of a yarn monotonically decreases with increasing twist. A model based on composite theory was developed to elucidate the decrease in the modulus as a function of both the degree of twist and the elastic constants of the fibers. The modulus values predicted by the model have good agreement with those measured by experiment. The radial shear modulus of the fiber, which is difficult to measure, can be derived from the regression parameter of experimental data by the use of the model. Such information should be useful for some specialized applications of fibers, for example, fiber-reinforced composites. The experimental results show that the strength of these yarns can be improved by a slight twist. A high degree of twist damages the fibers and reduces the tensile strength of the yarn. The elongation to break of the yarns monotonically increases with the degree of twist. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1938–1949, 2000

Elasticity of high-volume-fraction fractal aggregate networks: A thermodynamic approach

Abstract: The elastic modulus of a colloidal aggregate network is dependent on the amount and spatial distribution of mass, as well as particle properties including size, shape, and particle-particle interactions. At high volume fractions, the elastic properties of a network of close-packed particle flocs is dependent on the strength of the interfloc links. A previously developed weak-link fractal scaling theory relates the elastic constant ~K! of the network to the volume fraction of solids ~F!, namely K;F. In this paper, we extend this theory to include a pre-exponential factor and obtain an exact expression for relationship between the Young’s modulus and the volume fraction of solids.

Comparison between a plasma arc light source and conventional halogen curing units regarding flexural strength, modulus, and hardness of photoactivated resin composites.

Abstract: The plasma arc curing light Apollo 95 E (DMDS) is compared to conventional curing lights of different radiation intensities (Vivalux, Vivadent, 250 mW/cm2; Spectrum, DeTrey, 550 mW/cm2; Translux CL, Kulzer, 950 mW/cm2). For this purpose, photoactivated resin composites were irradiated using the respective curing lights and tested for flexural strength, modulus of elasticity (ISO 4049), and hardness (Vickers, Knoop) 24 h after curing. For the hybrid composites containing only camphoroquinone (CQ) as a photoinitiator (Herculite XRV, Kerr; Z100, 3 M), flexural strength, modulus of elasticity, and surface hardness after plasma curing with two cycles of 3 s or with the step-curing mode were not significantly lower than after 40 s of irradiation using the high energy (Translux CL) or medium energy conventional light (Spectrum). However, irradiation by only one cycle of 3 s failed to produce adequate mechanical properties. Similar results were observed for the surface hardness of the CQ containing microfilled composite (Silux Plus, 3 M), whereas flexural strength and modulus of elasticity after plasma curing only reached the level of the weak conventional light (Vivalux). For the hybrid composites containing both CQ and photoinitiators absorbing at shorter wavelengths (370–450 nm) (Solitaire, Kulzer; Definite, Degussa), plasma curing produced inferior properties mechanical than conventional curing; only the flexural strength of Solitaire and the Vickers hardness of Definite reached levels not significantly lower than those observed for the weak conventional light (Vivalux). The suitability of plasma arc curing for different resin composites depends on which photoinitiators they contain.