Showing papers on "Elastic modulus published in 1994"

Mechanical degradation and viscous dissipation in B2O3.

Abstract: The Brillouin light scattering technique was used to determine the complex mechanical modulus, which describes the dynamic response of molecular structures, for ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ and binary alkali borate. The effects of temperature, alkali concentration, oxygen and water vapor partial pressures on the structural developments and on the thermal activation of dissipative processes were examined. The glass transition in these systems is characterized by a discontinuity in the temperature dependence of the elastic component of this modulus. Above ${\mathit{T}}_{\mathit{g}}$, the elastic modulus decreases with a faster rate, the higher the alkali concentration. The complete structural evolution from a room temperature glass to a liquid near the boiling point was found to involve several distinct mechanisms, which become gradually activated with increasing temperature. By using a mechanical relaxation formalism, the activation energies and preexponential time constants describing the mechanical degradation, as well as the molecular rearrangements associated with each mechanism were determined. For a given system, the initial network degradation is characterized by the smallest activation energy. The motion involved in this process is that of boron atoms oscillating between triangular and tetrahedral coordination, upon exchanging one of their oxygen neighbors. During this phase boroxol rings open, without the formation of nonbridging oxygens. At intermediate temperatures the motion of alkali cations between network segments is activated, and at very high temperatures complete network disintegration takes place, leaving ionic species whose motion occurs by complete dissociation from their immediate neighbors.

Order relationships for boundary conditions effect in heterogeneous bodies smaller than the representative volume

Abstract: Previous results by Huet [J. Mech. Phys. Solids 38, 813–841 (1990)] for heterogeneous elastic bodies smaller than the representative volume and submitted to static or kinematic uniform boundary conditions are partly extended to the case of mixed boundary conditions. Apparent elastic modulus and compliance tensors denned through the energetic procedure on a single specimen are considered. Through a variational approach, it is shown that each of them is bounded on one side. For cases fulfilling the Hill condition, these apparent modulus and compliance tensors are reciprocals. This provides two-sided bounds on each one. In this case, for a body smaller than the representative volume, the apparent elasticity tensor for mixed boundary conditions falls between the tensors associated with the static and kinematic uniform boundary conditions. Illustrating examples are studied. Various possible fields of application and other extensions are considered.

Polymeric-hydroxyapatite bone composite

Abstract: A method for the fabrication of three-dimensional macroporous polymer matrices for use as bone graft or implant material was developed. The composites are formed from a mixture of biodegradable, biocompatible polymer and hydroxyapatite (HA), a particulate calcium phosphate ceramic. The method leaves irregular pores in the composite between 100 and 250 microns in size. In a preferred embodiment, implants are composed of a 50:50 poly(lactide-co-glycolide) (PLGA) polymer and reinforced by hydroxyapatite. Mechanical and histological analysis showed the matrix fabricated by this method to be structurally and mechanically similar to cancellous bone. Prior to degradation, pure polymer specimens exhibited an elastic modulus of 293 MPa and specimens which were 50% HA by weight exhibited a modulus of 1459 MPa. After six weeks of degradation under physiological conditions, the reinforcing effect of ceramic loading had diminished. Modulus of polymer matrices at all HA load levels had decreased sharply to approximately 10 MPa. Mean macro- and micropore diameters of the polymer specimens were 100 μm and 20 μm respectively and remained constant throughout degradation.

Effective elastic moduli of two-phase composites containing randomly dispersed spherical inhomogeneities

Abstract: Based on the general micromechanical framework proposed in a companion paper, effective elastic moduli of two-phase composites containing randomly dispersedspherical inhomogeneities are investigated in this paper. At variance with existing micromechanical pairwise interaction models (accurate up to the second-order in particle volume fraction ϕ), the proposed approximate, probabilistic pairwise particle interaction formulationcoupled with the general ensemble-volume averaged field equations leads to a novel, higher-order (in ϕ), and accurate method for the prediction of effective elastic moduli of two-phase composites containing randomly located spherical particles. The relevant ensemble integrals in the proposed formulation are absolutely convergent due to a “renormalization” procedure employed in a companion paper. In accordance with the analogy between the effective shear modulus of an incompressible elastic composite with randomly dispersed rigid spheres and the effective shear viscosity of a colloidal dispersion with randomly dispersed rigid spheres (at high shear rates), the proposed ensemble-micromechanical approach is extended to predict effective shear viscosities of colloidal dispersions at the high-shear limit. Comparisons with experimental data, classical variational bounds, improved three-point bounds, the second-order particle interaction model, and other micromechanical models are also presented. It is observed that significant improvement in predictive capability for two-phase composites with randomly dispersed spheres can be achieved by using the proposed method.

Flexible cube-corner retroreflective sheeting

Abstract: A retroreflective article (10) has a body portion (14) and a multiple of cube-corner elements (12) that project from a rear side (20) of the body portion (14). The body portion (14) includes a body layer (18) that contains a light-transmissible polymeric material having an elastic modulus less than 7 x 108 pascals. The cube-corner elements (12) contain a light transmissible polymeric material having an elastic modulus greater than 16 x 108 pascals. A retroreflective article of this construction can be highly flexed while maintaining good retroreflective performance.

High performance elastomeric nonwoven fibrous webs

Abstract: Disclosed is an elastic nonwoven web containing a coherent matrix of fibers formed from an extrudable blend composed of an elastomeric A-B-A-B tetrablock copolymer where A is a thermoplastic polymer block and where B is an isoprene monomer unit hydrogenated to substantially a poly(ethylene-propylene) monomer unit. The elastic nonwoven web adapted to have an elastic modulus of less than about 1.25 which is locally constant during elongation of the web up to about 250 percent. The elastic nonwoven web may also include at least one type of nonelastic fibers and/or particulate materials distributed within or on the matrix of elastomeric fibers. The elastic nonwoven web may be incorporated into a multilayer material and/or may be a component of a composite elastic material in which the elastic web is joined to a gatherable layer at spaced apart locations so that the gatherable layer is gathered between the spaced-apart locations.

Network Modulus and Superelasticity

Abstract: We discuss the elastic modulus G and swelling/deswelling behavior of networks as a function of their concentration 4 and their preparation state. Simple scaling ideas reproduce the prediction of James and Guth for the modulus of networks swollen in a 8 solvent (G - 4113) but lead to a new prediction in a good solvent (G - 4'/12). We also suggest that both fully swollen modulus and dry modulus are related to swelling in ways that are independent of the network preparation details. By cross-linking long chains at low concentration and removing the solvent, many temporary entanglements are formed that force each network strand into a double-folded treelike compact configuration. These deswollen networks are capable of stretching by enormous amounta (-1Oox) without breaking (superelastic) and have a much lower modulus than melt-cross-linked networks. The energy stored in temporary entanglements during deswelling is released upon stretching, leading to a weaker-than-linear dependence of stress u on elongation X in tension (a - All3).

Resilient modulus for fine-grained subgrade soils

Abstract: A method has been developed for the estimation of resilient modulus of compacted fine-grained subgrade soils. The method takes into account the influence of soil physical state, stress state, and soil type. The effect of soil physical state is quantified by combinations of two equations relating resilient modulus to moisture content. One equation is for paths of constant dry density and the other is for paths of constant compactive effort. The effect of stress state is determined by equations relating resilient modulus at optimum moisture content to deviator stress so that the equation parameters represent the effect of soil type and its structure. Means to estimate the resilient modulus at optimum moisture content are suggested in the absence of actual test data. Examples of applications of this method showed that it is simple and versatile and also gives consistency between predicted resilient modulus and resilient modulus test results.

The effect of indenter geometry on the elastic response to indentation

Abstract: The paper describes various analytical procedures that account for the influence of the geometry of a rigid indenter upon the measured contact compliance of a smooth perfectly elastic half space. The analytical solutions provide a means of interrelating the reaction force, P, and the displacement, h, characteristics in terms of the contact geometry and the reduced elastic modulus E*. The general form is P=gE*hn, where g and n are functions only of the indenter geometry. This relationship is incorporated into a curve-fitting procedure and used to evaluate the influence of the indenter geometry upon the computed modulus of a poly(isobutylene) rubber. The method described provides a viable means of incorporating the imperfections, associated with the geometry of an indenter, into the interpretation of contact compliance data.

Theoretical analysis and verification of ultrasound displacement and strain imaging

Abstract: Evaluation of internal displacement and strain distributions in tissue under externally applied forces is a necessary step in elasticity imaging To obtain a quantitative image of the elastic modulus, strain and displacement fields must be measured with reasonable accuracy and inverted based on an accurate theoretical model of soft tissue mechanics In this paper, results of measured internal strain and displacement fields from gel-based phantoms are compared with theoretical predictions of a linear elastic model In addition, some aspects of elasticity reconstruction based on measured displacement and strain fields are discussed >

Mechanical Properties of Partially Dense Alumina Produced from Powder Compacts

Abstract: The elastic modulus (E), the critical strain energy release rate (G[sub c]), and the flexural strength ([sigma]) have been determined for two partially dense alumina bodies produced from the same powder but with different initial densities. The mechanical properties were measured for specimens fabricated at four different relative densities. The measured elastic modulus, critical strain energy release rate, and a calculated critical stress intensity factor (K[sub c]) were observed to be linearly related to ([rho] [minus] [rho][sub 0])/(1 [minus] [rho][sub 0]), where [rho] is the current relative density and [rho][sub 0] is the initial relative density of the powder compact. With the observed linear relations for E, G[sub c] (or K[sub c]), and the assumption that the crack length responsible for failure was present in the initial powder compact and shrunk in proportion to the relative density change, a Griffith equation was constructed to estimate the strength at any relative density. This relation was in good agreement with measurements.

From static to kinetic friction in confined liquid films.

Abstract: The transition from rest to sliding contact of atomically smooth solids separated by molecularly thin liquid films was studied. The films could be deformed nearly reversibly to a large fraction of the film thickness. The modulus of elasticity and yield stress were low, considerably less than for a molecular crystal or glass in the bulk. The transition to dissipative sliding was typically (but not always) discontinuous. The dissipative stress was then nearly velocity-independent. The similar response of monolayers strongly attached to the solid surfaces, presenting a well-defined interface for sliding, suggests that the physical mechanism of sliding may involve wall slip.

New Surface-Hardened, Low-Modulus, Corrosion-Resistant Ti-13Nb-13Zr Alloy for Total HIP Arthroplasty

Abstract: To optimize the performance of total hip replacement, scientists and clinicians are seeking new materials and noncemented, press-fit designs that can improve load transfer to the bone and reduce the incidence of loosening and thigh pain. Currently used Co-Cr-Mo alloy has a relatively high elastic modulus (E = 227 GPa), which limits its ability to transfer load to the surrounding bone in the proximal calcar region. Thus to improve load transfer, designs are considered with less cross-sectional area to increase flexibility, but at the expense of fit and fill, and thus stability of the implant within the bone. Should stem loosening occur, the stem stresses may exceed the relatively low fatigue strength of the Co-Cr-Mo alloy and lead to stem breakage. To improve these conditions, lower modulus Ti-6Al-4V alloy (E = 115 GPa) is being used. More recently, a new lower-modulus (E = 79 GPa) Ti-13Nb-13Zr alloy has been developed which does not contain any elemental constituents associated with adverse cell response (i.e., Co, Cr, Mo, Ni, Fe, Al, V), and which possesses comparable or superior strength and toughness to existing Ti-6Al-4V alloy. The carefully selected Nb and Zr constituents improve bone biocompatibility and corrosion resistance compared to that of currently used implant metals. Additionally, a unique diffusion hardening (DH) treatment can be conducted during the age-hardening process of this near-beta alloy to produce a hardened surface with abrasion resistance superior to that of Co-Cr-Mo alloy. This also provides an improvement in the micro-fretting tendencies that may occur within femoral head-neck taper regions and modular interfaces of other implant designs. The present study describes the metallurgy and mechanical properties of this unique low modulus Ti-13Nb-13Zr alloy, and the heat treatments used to obtain the high strength, corrosion resistance, and surface hardening that renders this biocompatible alloy well-suited for press fit hip replacement applications. Because of the relatively lower beta transus (735 degrees C), this alloy is also much easier to net shape forge into more complex stem designs.

The influence of particle distribution on the mechanical response of a particulate metal matrix composite

Abstract: A self-consistent model, based on Eshelby's equivalent inclusion method, has been developed to predict to flow of a particulate-reinforced alloy. The model gives excellent agreement with the measured elastic moduli for Al/SiC composites. Beyond the elastic limit, the model predicts an increase in the initial work hardening rate with increasing particle content. At large strains (above about 1%) the stress-strain behaviour of the composite is parallel to that of the unreinforced alloy. The results agree well with those obtained by Bao et al. [G. Bao, J. W. Hutchinson and R. M. McMeeking, Acta metall. mater.39, 1871 (1991)] using finite element methods, indicating that solutions based on average stress fields around particles do capture the essential features of composite strengthening. However, the current treatment can be readily extended to treat the effect of an inhomogeneous particle distribution on strength. As the degree of particle clustering increases, an increase in the rate of initial work hardening is predicted. Moreover, the strengthening ratio is increased substantially by clustering.

Temperature and Chain Length Effects on Bending Elasticity of Phosphatidylcholine Bilayers

Abstract: A strong decrease of the bilayer bending elastic modulus, kc, has been experimentally established for dimyristoyl and dipalmitoyl phosphatidylcholine, when the liquid-crystal-gel phase transition temperature is approached from above. This phenomenon may be due to local-curvature-induced changes in the transition temperature. In the liquid-crystal phase, the bending modulus, kc, significantly increases as a function of the lipid chain length for dilauryl, dimyristoyl and dipalmitoyl phosphatidylcholine.

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.

Review of Elastic and Plastic Contact Conductance Models: Comparison with Experiment

Abstract: More than 450 thermal contact conductance data points obtained from isotropic conforming rough surfaces for five different materials; nickel, stainless steel, two zirconium alloys, and aluminum have been compared with the existing elastic and plastic models. For the first time data have been reduced to a dimensionless form assuming both elastic as well as plastic deformation. Normally, data were compared with either the elastic model or the plastic model assuming a type of deformation a priori. The relative merits of different models and the surface factors influencing the mode of deformation are still not clear. Hence, the aim of the present work was to compare most of the models available in the literature with themselves as well as with isotropic data. Comparison showed that generally smoother surfaces deform elastically, and rougher ones plastically. However, there are some data sets that compare well with both the elastic as well as the plastic models.

Effect of superlattice layer elastic moduli on hardness

Abstract: Epitaxial V0.6Nb0.4N/NbN superlattices have been deposited on MgO(100) by reactive magnetron sputtering. The V0.6Nb0.4N alloy was chosen for one layer, rather than VN, in order to reduce the lattice mismatch from 5.7% to 3.5%, allowing growth of superlattices with relatively planar layers. The elastic moduli of NbN and V0.6Nb0.4N are nearly equal, in contrast to previously studied high‐hardness nitride superlattices where the layer moduli differed substantially. Vickers microhardness measurements showed no measurable deviation (<4 GPa) from alloy values. The results verify that the difference in layer elastic moduli is the primary parameter determining nitride superlattice hardening.

New experiments on shear modulus of elasticity of arteries.

Abstract: Although the mechanical properties of blood vessels have been studied extensively, the shear modulus of the blood vessel wall is still unknown. New data on the shear modulus of elasticity of rat arteries and its variation with axial stretch and blood pressure are presented. The data were obtained from a new instrument designed and constructed by us to perform simultaneous torsion, inflation, and longitudinal stretching tests. It was found under physiological conditions (pressure = 120 mmHg or 16 kPa; longitudinal stretch = 1.2 relative to zero-stress state), the shear modulus of normal rat thoracic aorta is G = 137 +/- 18 kPa. The difference of shear modulus at body temperature (37 degrees C) and room temperature (25 degrees C) is within 10%. The shear modulus varies significantly with changing longitudinal and circumferential strains in proportion to the strain energy due to these strains. A constitutive equation based on a pseudo strain energy function is proposed. The vessel wall is not transversely isotropic in the incremental sense. When the rat was subjected to high blood pressure due to constriction of its aorta, the shear modulus does not vary significantly with the length of time the animal was subjected to hypertension.

Settlement of Shallow Foundations on Sand

Abstract: A new method for estimating the settlement of shallow foundations on sand is presented. The method is based on elastic stress-strain theory and is designed for use with soil data from the Standard Penetration Test (SPT). The elastic modulus is assumed to vary with mean effective normal stress and strain level. The variations of modulus with depth and the effects of footing size and shape, depth of embedment and groundwater level are introduced through their effects on the mean effective normal stress. The non-linear nature of the stress-strain relation is based on a normalized version of the Seed-Idriss curves, which relate shear modulus to strain level and relative density. SPT data are used to estimate relative density. In the general solution, the zone of influence beneath the footing is divided into sublayers and the foundation load is applied in increments. Then the incremental load solution is used to develop a simplified procedure in which the total load may be applied in one increment. The proposed method is applied to data from published case histories. The results of the proposed method are shown to compare favorably with those from other currently used methods.

Precipitation Hardening in Fe-Cu Binary and Quaternary Alloys

Abstract: The behavior of precipitation hardening in two types of Fe-Cu alloys has been investigated by means of mechanical tests as well as small angle neutron scattering measurements. The integrated intensity increased first and reached a constant corresponding to the completion of precipitation reaction, while particle radius increased monotonically with aging time, where Vickers hardness and yield stress increased and reached maxima, then decreased. The interaction force with a dislocation due to each precipitate was very small compared with the force by the Orowan mechanism. After discussion based on three possible mechanisms in terms of coherency strain, elastic modulus change and interfacial energy, the strengthening was suggested to be caused from the coherency strain effect. The first increase of yield stress during aging is attributed to the growth in size of clusters and the decrease of yield stress after the maximum is mainly related to the decrease of number density of precipitates. It was found that the loss of coherency with the matrix greatly lowers the strengthening effect, whereby the structure of precipitates changes from bcc to fcc during aging.

Reinforcing sand with strips of reclaimed high-density polyethylene

Abstract: The feasibility of reinforcing soil with strips of reclaimed high‐density polyethylene (HDPE) was investigated in this study. Strips of HDPE were mixed with Portage sand and tested to determine the California Bearing Ratio (CBR), secant modulus, resilient modulus, and shear strength. Strips were prepared at aspect ratios of 4, 8, and 12 to examine how reinforcement of the sand is affected by the length of the strips. The tests showed that reinforcing sand with reclaimed HDPE strips enhances its resistance to deformation and increases its strength. Addition of strips increased the CBR and the secant modulus, with the CBR increasing by as much as a factor of 5. The resilient modulus also increased with addition of strips, with the greatest increases (35% increase) occurring for strip a content of 3%. Direct shear tests also showed that addition of HDPE strips increased the shear strength of the sand. Increases in friction angle as large as 18° were measured at normal stresses below the critical confining st...