Showing papers on "Young's modulus published in 1987"

Influence of structural and morphological factors on the mechanical properties of the polyethylenes

Abstract: Relations force-longueur a temperature ambiante determinees pour une serie de polyethylenes d'une large gamme de masse moleculaire et contribution moleculaire

Young's modulus of porous brittle solids

Abstract: A new equationE =E0 (1 −aP)n whereE andE0 are the Young's moduli at porosity,P, and zero, respectively, a andn are material constants, has been derived semi-empirically for describing the porosity dependence of Young's modulus of brittle solids. The equation satisfies quite well the exact theoretical solution for the values of Young's moduli at different porosities for model systems with ideal and non-ideal packing geometry. The equation shows excellent agreement with the data Onα- andβ-alumina over a wide range of porosity. Unlike the existing porosity-elastic modulus equations, the proposed equation satisfies the boundary conditions and is inherently capable of treating isometric closed pores as well as non-isometric interconnected pores. The parameters a and n provide information about the packing geometry and pore structure of the material.

Microfabricated structures for the in situ measurement of residual stress, Young’s modulus, and ultimate strain of thin films

Abstract: Two microfabricated structures for the in situ measurement of mechanical properties of thin films, a suspended membrane, and an asymmetric ‘‘released structure,’’ are reported. For a polyimide film on silicon dioxide, the membrane measurements yield a residual tensile stress of 30 MPa and a Young’s modulus of 3 GPa. The released structures measure the ratio of residual stress to Young’s modulus, and yield 0.011 at strains comparable to the suspended membranes, and 0.015 at larger strains. The ultimate strain as measured by both structures is approximately 4%.

Modelling the elastic behaviour of granular materials

Abstract: A review of the literature indicates that the elastic behaviour of granular materials is isotropic and that Poissony's ratio is constant, whereas Young's Modulus, the bulk modulus and the shear modulus vary with the mean normal stress and the deviatoric stress. A nonlinear, isotropic model for the elastic behaviour is developed on the basis of theoretical considerations involving the principle of conservation of energy. Energy is therefore neither generated not dissipated in closed-loop stress paths or in closed-loop strain paths. The framework for the model consists of Hooke's law, in which Poission's ratio is constant and Young's modulus is expressed as a power function invlving the first invariat of the stress tensor and the second invariant of the deviatoric stress tensor. The characteristics of the model are described, and the accuracy is evaluated by comparison with experimental results from triaxial tests and three-dimensional cubical triaxial tests with a variety of stress paths. Parameter determination from unloading–reloading cycles in conventional triaxial compression tests is demonstrated, typical parameter values are given for granular materials and extension of the model to soils with effective cohesion is described.

Structure-property relationships in carbon fibres

Abstract: The tensile modulus of carbon fibres is related to the preferred orientation of the layer planes, and it is easier to achieve high values with carbon fibres from mesophase pitch than from polyacrylonitrile (PAN). Tensile strength has been related to the flaw population, but with improved processing conditions, fundamental structural organisation is more important. Carbon fibres fail in both tension and compression by the rupture of layer planes in misoriented crystallites and the propagation of a crack beyond the critical size. PAN-based carbon fibres have a complex structure of interlinked layer planes which hinder crack propagation unless voids are present with extensive walls. In many cases crystalline walls are formed by catalytic graphitisation due to contaminant particles; it is these shells rather than the voids themselves which lead to premature failure. Mesophase-pitch-based carbon fibres crystallise so that the layer planes are arranged in sheets; this facilitates crack propagation and leads to lower strain-to-failure than for PAN-based fibres of similar modulus.

Effects of Cure Pressure on Resin Flow, Voids, and Mechanical Properties

Abstract: The effects of cure pressure on resin flow, compaction, void content, and mechanical properties of fiber reinforced composites were investigated. Tests were performed on lami nates made of Fiberite T300/976 and T300/934 unidirectional prepreg tape. Data are pre sented on compaction, resin flow, and void content which lend support to the validity of the Springer-Loos model. Data are also given showing the effects of cure pressure on void content and on the compressive strength and short-beam shear strength and modulus.

Blow-Off Pressures for Adhering Layers.

Abstract: : An analysis is given of the critical internal pressure P at which a circular debond blister will grow in size, in terms of the tensile modulus E and thickness t of an adhering layer, and the strength G sub a of its adhesion to a rigid substrate. Measurements of blow-off pressure are reported for adhering layers of pressure-sensitive tapes having widely-different effective modulus and thickness, and with blisters having a range of diameters. Satisfactory agreement is obtained with the theoretical predictions, suggesting that the theory is basically correct in assuming that relatively thin layers behave like elastic membranes. Attention is drawn to the unusual form of the dependence of the debonding pressure P upon the resistance Et of the layer to stretching and upon the detachment energy . Even though the adhering layer is assumed to be linearly- elastic, the markedly non-linear (cubic)relation between pressure P and volume V of the blister, or maximum height y, leads to this unusual result. The detachment energy is given by a particularly simple function of the pressure P and maximum deflection of the blister: G sub a = 0.65 Py, independent of the stiffness of the adhering layer and diameter of the blister.

Drawing of virgin ultrahigh molecular weight polyethylene: an alternative route to high strength/high modulus materials. Part 2: Influence of polymerization temperature.

Abstract: The synthesis of ultra-high molecular weight polyethylene films and the production of high strength/high modulus tapes and filaments drawn directly from the virgin polymer are described. The study particularly focuses on the effect of the polymerization temperature on the deformation behaviour of the virgin UHMW polyethylene films. These results are discussed within the framework of the entanglement concepts for deformation of weakly bonded macromolecules. The developrnent of the room temperature Young's modulus and the tensile strength with draw ratio is presented and compared with modulus and tenacity/draw ratio relations observed for melt and solution crystallized polyethylene.

Cellulose fiber‐polyester composites with reduced water sensitivity (1)—chemical treatment and mechanical properties

Abstract: Formaldehyde and di-methylomelamine were used to modify the surfaces of cellulose fibers. Composites were prepared with unsaturated polyester and treated cellulose as the reinforcing material. The tensile strength and the elongation of the cellulose fibers were determined in dry and wet conditions as well as the tensile strength and the tensile modulus of the cellulose-polyester composites. The water uptake of the composites was reduced by 46 to 52 percent. The wet strength of the composites was improved by more than 50 percent.

Elasticity of particle assemblies as a measure of the surface energy of solids

Abstract: A theory is suggested to explain the elasticity of particle assemblies. It is shown that a regular cubic packing of spheres has an effective Young's modulus that depends on the contacts between individual particles. In particular, it is noted that the effective modulus depends on the interfacial attractive energy between the spheres, and thus provides a direct method for measuring the surface energy of solids. However, most particle assemblies are neither cubic nor regular. The problem is to describe the properties of these real systems in terms of the packing of the grains. Theoretically, it is shown that the modulus of a powder compact should vary as the fourth power of the particle packing fraction. This result has been verified experimentally and has been used to determine the surface energies of zirconia, titania, alumina, and silica powders. The measured values were sometimes much lower than expected from theoretical calculations of surface energy. Experiment has shown that such discrepancies result from contamination of the solid surfaces.

Elastic behavior of open-cell alumina

Abstract: The elastic constants of open-cell aluminas were measured as a function of their relative density and cell size. The density exponent was found to be in excellent agreement with previous theoretical work that considered cell strut bending as a major deformation mode. The measured constants in the elastic behavior relationships were found to be less than previously suggested, and it was found that this factor is sensitive to the microstructure of the cell struts, especially porosity within these struts. The materials studied were found to contain soem closed cells, but these were not found to have a major effect on the elastic behavior. The values of Young's modulus and the shear modulus depended on the relative density in the same way, and this is consistent with previous theoretical work which showed that Poisson's ratio should be independent of density.

Mechanical properties and permeability of polypropylene and poly(ethylene terephthalate) mixtures

Abstract: The synthetic membranes currently used for soil stabilization and road construction are mainly made of polypropylene and of polyesters. They are used separately for each application. The polymer used has an effect on the wettability and, the permeability of the membrane. The polypropylene membranes, for instance, have a zero wettability, whereas it is high for polyester membranes. This paper reports on the mechanical properties and the permeability of mixtures of polypropylene (PP) and poly(ethylene terephthalate) (PET). The elastic modulus of the mixture was at a minimum for a 50/50 mixture. For the other compositions, the moduli gave a positive deviation as compared with the additivity equation results. This is probably due to the fact that pure PET has a fragile behavior at the temperature at which the mechanical tests were run. This 50/50 composition corresponds to the domain where a phase inversion occurs. The permeability to water vapor gave an S-shape curve that is typical of a “mixture” of immiscible polymers. The diffusion of the water molecules is controlled by the continuous phase. To compatibilize the two homopolymers, a 94/6 copolymer of PP and of polyacrylic acid was added, at various levels, to a 60/40 mixture of PET and PP: This did not affect markedly the elastic modulus. The yield stress increased, however, indicating that we had a better adhesion and that the copolymer seems to have a certain emulsifier effect, increasing the quality of the dispersion.

Determination of Young's modulus of dental composites: A phenomenological model

Abstract: The Young's moduli of isotropic dental restorative composites are determined with a non-destructive dynamic method, which is based on the measurement of the duration of the fundamental period for the first harmonic of a freely oscillating sample. Statistical analysis of these results yields a phenomenological model in which Young's modulus is given by an exponential rule of mixtures of the matrix phase and the filler phase of the composites. It is found that this phenomenological rule is substantiated empirically.

Finite element thermal stress solutions for thermal barrier coatings

Abstract: The primary objective of this investigation is the quantitative determination of stress states in a model thermal barrier coating (TBC) as it cools in the air to 600 °C from an assumed stress-free state at 700 °C. The TBC under consideration represents a plasma-sprayed zirconia-yttria ceramic layer with a nickel-chromium-aluminum-yttrium bond coat on a cylindrical substrate made of nickel-based superalloy. To account for complex geometry and material properties, the versatile finite element method was used to model the TBC. The elastic model which is an outgrowth of a previously reported one is named TBCG. It contains 1316 nodal points and 2140 elements. An improved version known as TBCGEP is capable of representing the bond coat with an elastic-perfectly plastic material. Numerical results of stresses and strains were obtained from this model by using a generic code called MARC. Actual computations were performed on a CRAY-XMP supercomputer. Based on 15 lengthy computer solutions, quantitative determinations have been made of the influence of the coefficient of thermal expansion of the bond coat, Poisson's ratio of the bond coat, and Young's modulus of elasticity. Also presented in this paper are stress states in both the ceramic layer and the bond coat of an elastoplastic solution.

Moisture in portland cement concrete

Abstract: Moisture gradients in concrete pavements cause differential shrinkage between the top and the bottom of the pavement. This leads to curling stresses in which the top of the pavement is in tension while the bottom is in compression. The magnitude of these stresses is determined by the moisture distribution, the volumetric aggregate content of the concrete, and the elastic modulus of the concrete. Pavement moisture contents were determined by field moisture measurements, laboratory measurements, and computer simulation. These indicated that substantial drying occurred only at the top surface, to a depth of less than 2 inches. The rest of the pavement remained at 80 percent saturation or higher. A typical pavement moisture distribution was determined, and using an aggregate content of 74 percent and an elastic modulus of 3.6 x 1,000,000 psi, a stress distribution was calculated. The tensile strength of the concrete at the surface was exceeded, and cracks could be expected to form to a depth of 3/4 inches. Because the tension in the concrete was concentrated near the surface instead of decreasing linearly with depth, the actual moment in the pavement caused by the moisture gradient was only 40 percent of the moment capacity of the unreinforced concrete.

Light-weight composite material

Abstract: A light-weight composite material having a laminated structure comprises at least one porous fiber layer constructed of reinforcing short fibers which are distributed in random directions and combined with each other mainly at intersections thereof by carbonized binder and at least one fiber reinforced plastic layer comprising reinforcing fibers and a matrix resin. The composite material can have a low density, high mechanical properties of strength, modulus of elasticity and rigidity.

An ultrasonic study of the elastic properties of the normal and superconducting states of YBa2Cu3O7-δ

Abstract: The elastic properties of YBa2Cu3O7- delta ceramic have been determined from ultrasonic wave velocity measurements between 5 K and 260 K. In the vicinity of the superconducting transition temperature both longitudinal and shear mode velocities show decreases with temperature, an indication of long-wavelength acoustic mode softening. Below Tc the mode velocities increase quite steeply. The elastic stiffness of this material is substantially smaller than that of ideal perovskite materials, a feature which can be abscribed to the oxygen defect, layer-like structure.

Mechanical studies of sol-gel transition: Universal behavior of elastic modulus

Abstract: The critical behavior of the shear modulus of agarose gel was studied as a function of concentration and temperature. Experimental evidence is presented that the elasticity exponent agrees with the conductivity exponent of a percolating resistor network. It is found that the value of the elasticity exponent of agarose gel is independent of the path to the phase boundary. These results strongly suggest that the elastic behavior of gel in the critical region can be explained by the scalar elasticity model of percolation theory irrespective of the mechanism of gelation.

Experimental Determination of the Elastic Modulus of Crystalline Regions of Some Aromatic Polyamides, Aromatic Polyesters, and Aromatic Polyether Ketone

Abstract: Experimental Determination of the Elastic Modulus of Crystalline Regions of Some Aromatic Polyamides, Aromatic Polyesters, and Aromatic Polyether Ketone