Showing papers on "Composite number published in 1998"

Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix

Abstract: We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

Processing of Carbon Nanotube Reinforced Aluminum Composite

Abstract: Carbon nanotube reinforced aluminum (Al) composites were produced by hot-press and hot-extrusion methods. The interfacial structure between the carbon nanotube and Al was examined using a transmission electron microscope (TEM), and the mechanical properties were measured by a tensile test. TEM observations have shown that the nanotubes in the composites are not damaged during the composite preparation and that no reaction products at the nanotube/Al interface are visible after annealing for 24 h at 983 K. The strength of the composites is only slightly affected by the annealing time at 873 K, while that of the pure Al produced in a similar powder metallurgy process significantly decreases with time. These studies are considered to yield experimental information valuable for producing high performance composites.

Fabrication of biocompatible polymeric composites

Abstract: Composite materials formed from biocompatible polymer fibers and biodegradable polymers are disclosed. The heat treatment conditions for the reinforcing fibers are described so that the mechanical properties of the fibers can be retained during composite consolidation process. The processing conditions and set-ups to consolidations are constrained to the temperatures lower than fiber heat treatment temperatures. The reinforcing fibers are restrained under tension so that the minimum relaxation occurs during consolidation process.

Composite Materials: Mechanical Behavior and Structural Analysis

Abstract: Preface.- Translators Preface.- Part I: Composite Materials. Basic Features of Composite Materials. The Constituents of a Composite Material. Molding processes and Architecture of Composite Materials.- Part II: Basic Concepts of the Mechanical Behavior of Materials. Mathematical Basics. Stresses. Strains. The Elastic Behavior of Materials. The Mechanics of Deformable Solids.- Part III: Mechanical Behavior of Composite Materials. Elastic Behavior of Unidirectional Composite Materials. Elastic Behavior of an Orthotropic Composite. Off-Axis Behavior of Composite Materials. Fracture Mechanisms and Damage of Composite Materials.- Part IV: Modeling the Mechanical Behavior of Laminates and Sandwich Plates. Basics of Laminate Theory. Classical Laminate Theory. Effect of the Stacking Sequence. Mat and Cloth Reinforced Materials. Governing Equations and Energy Formulation of Classical Laminate Theory. Including Transverse Shear Deformation in Laminate Theory. Theory of Sandwich Plates.- Part V: Analysis of the Mechanical Behavior of Composite Material Structures. Cylindrical bending. Bending of Laminate and Sandwich Beams. Bending of Orthotropic Laminate Plates. Bending of Plates.

Feasibility study of a passive smart self-healing cementitious composite

Abstract: The basic concept of a passive smart-healing cementitious composite has been demonstrated, in the laboratory, to be feasible. The basic elements of this smart material include the sensors and actuators in the form of controlled microcracks and hollow glass fibers carrying air-curing chemicals. Controlled microcracking is offered by a strain-hardening engineered cementitious composite developed previously. The mechanisms of sensing and actuation are revealed through in situ environmental scanning electron microscopy observations. The self-healing effectiveness is confirmed by measurement of the elastic modulus of the composite. The elastic modulus is found to regain its original value in a repeat loading subsequent to damage in a first load cycle.

Bamboo fiber-reinforced polypropylene composites: A study of the mechanical properties

Abstract: A new type of bamboo fiber-reinforced polypropylene (PP) composite was prepared and its mechanical properties were tested. To enhance the adhesion between the bamboo fiber and the polypropylene matrix, maleic anhydride-grafted polypropylene (MAPP) was prepared and used as a compatibilizer for the composite. The maleic anhydride content of the MAPP was 0.5 wt %. It was found that with 24 wt % of such MAPP being used in the composite formulation, the mechanical properties of the composite such as the tensile modulus, the tensile strength, and the impact strength all increased significantly. The new composite has a tensile strength of 32–36 MPa and a tensile modulus of 5–6 GPa. Compared to the commercially available wood pulp board, the new material is lighter, water-resistant, cheaper, and more importantly has a tensile strength that is more than three times higher than that of the commercial product. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1891–1899, 1998

Analytical predictions for the magnetoelectric coupling in piezomagnetic materials reinforced by piezoelectric ellipsoidal inclusions

Abstract: In this paper, a number of problems of fundamental importance in a piezoelectric-piezomagnetic composite material are solved. The problems covered range from the derivation of the analytical expressions for the magneto-electro-elastic Eshelby tensors to the analysis of the magnetoelectric coupling effect which is a new property exhibited in the piezoelectric-piezomagnetic composite. In particular, when both the matrix and the inclusions of the composite are transversely isotropic with different magneto-electro-elastic moduli, and shapes of inclusions are of elliptic cylinder, circular cylinder, disk, and ribbon, closed-form solutions for the magnetoelectric coupling coefficientS are presented compactly. The resulting solutions are a function of the shape of inclusion, phase properties, and volume fraction of the inclusions. These results could provide us with insight into how a piezoelectric-piezomagnetic composite material consisting of inclusions will perform and would be helpful in understanding the magneto-electric-elastic behavior of the composite.

Tissue response to nano-hydroxyapatite/collagen composite implants in marrow cavity.

Abstract: The tissue response to a nano-hydroxyapatite/collagen composite implanted in a marrow cavity was investigated by histology and scanning electron microscopy. A Knoop microhardness test was performed to compare the mechanical behavior of the composite and bone. The ultrastructural features of the composite, especially the carbonate-substituted hydroxyapatite with low crystallinity and nanometer size, made it a bone-resembling material. It was bioactive, as well as biodegradable. At the interface of the implant and marrow tissue, solution-mediated dissolution and giant cell mediated resorption led to the degradation of the composite. Interfacial bone formation by osteoblasts was also evident. The process of implant degradation and bone substitution was reminiscent of bone remodeling. The composite can be incorporated into bone metabolism instead of being a permanent implant. For lack of the hierarchical organization similar to that of bone, the composite exhibited an isotropic mechanical behavior. However, the resistance of the composite to localized pressure could reach the lower limit of that of the femur compacta.

Encapsulation of Inorganic Particles by Dispersion Polymerization in Polar Media.

Abstract: Following a previous work (Bourgeat-Lami, E, and Lang, J, J Colloid Interface Sci 197, 293 (1998)), encapsulation of silica beads has been achieved by dispersion polymerization of styrene in an aqueous ethanol medium using poly(N-vinyl pyrrolidone) as stabilizer Silica beads, prepared according to the Stober method, were coated prior to polymerization by grafting 3-(trimethoxysilyl)propyl methacrylate onto the surface A great number of silica beads per composite particle were previously found using beads that had diameters between 49 and 120 nm In the present work, larger silica beads with diameters between 191 and 629 nm are investigated We demonstrate by transmission electron microscopy that, consequently, only a small number of silica beads are contained in the composite particles By counting the composite particles containing precisely zero, one, two, three, four, and more than four silica beads, it clearly appears that the encapsulation of only one silica bead can be obtained simply by increasing the size of the beads Under our experimental conditions, the optimal bead diameter for achieving composite particles containing only one silica bead turns out to be around 450 nm We show that increasing the silica bead size above this value results in an increased number of composite particles without silica beads In contrast, the number of composite particles with two, three, four, or more than four silica beads increases with decreasing silica bead size In addition to the above variations in composition of the composite particles, changes in particle shapes were also observed as a function of the size of the silica beads and the styrene concentration in the polymerization medium Hypotheses concerning these variations are presented Copyright 1999 Academic Press

High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite

Abstract: A unidirectional solidification method was investigated to manufacture Al2O3/YAG eutectic composites with high-temperature resistance that would make them usable at very high temperatures. We were successful in manufacturing a single-crystal Al2O3/single-crystal YAG eutectic composite with a dimension of 40 mm in diameter and 70 mm in length containing no colonies or pores. This composite also displayed excellent high-temperature strength characteristics. The flexural strength was in the range 350∼400 MPa from room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. During tensile tests above 1923 K, the eutectic composite showed evidence of plastic deformation occurring by dislocation motion, and a yield phenomenon similar to many metals was observed. In addition, the microstructure of the composite was extremely stable: after 1000 h of heat treatment at 1973 K in an air atmosphere there was no growth. The above superior high-temperature characteristics are caused by such factors as the eutectic composite having a single-crystal Al2O3/single-crystal YAG structure, the formation of a compatible interface with no amorphous phase and thermal stability, and the combined effect of a YAG phase with superior high-temperature characteristics. © 1998 Chapman & Hall

Fibril composite electrode for electrochemical capacitors

Abstract: Composite electrodes comprising carbon nanofibers (fibrils) and an electrochemically active material are provided for use in electrochemical capacitors. The fibril composite electrodes exhibit high conductivity, improved efficiency of active materials, high stability, easy processing, and increase the performance of the capacitor. A method for producing the composite electrodes for use in electrochemical capacitors is also provided.

Process-induced residual stress analysis of as4/3501-6 composite material

Abstract: The development of process-induced residual stresses in a typical graphitt/epoxy composite material during cure is analyzed in this article. An extensive material characterization study of the resin system was reported in a previous article by the authors [1] and the results of that study are used to develop a cure-dependent viscoelastic material model for the composite system. A finite-element model is developed to solve the stress analysis problem. To overcome large memory storage requirements and lengthy calculation times, a recursive formulation is used in the finite-element analysis. Cure kinetic and heat transfer modeling are evaluated independently using the finite-difference method. Process-induced residual stresses in cross-ply laminates are calculated and the results are compared with elastic solutions. The res ults of the analysis indicate that in certain cases significant residual stresses are present during the cure cycle, even before final cool-down commences. Whether a material experiences ...

Effect of etchant, etching period, and silane priming on bond strength to porcelain of composite resin.

Abstract: The purpose of this study was to evaluate the effect of etching and silane priming on bond strength to a feldspathic porcelain (VMK 68) of a composite resin (Clearfil APX). Two hydrofluoric acid etchants (2.5% and 5%) and seven different etching times (0, 30, 60, 90, 120, 150, and 180 seconds) were used to etch the porcelain specimens respectively. A self-curing bonding agent containing a silane coupler (Clearfil Porcelain Bond) was used on both etched and unetched porcelain surfaces. Etched relief patterns were observed by means of a scanning electron microscope, and the bond strengths between the photocured composite resin and the porcelain were determined. Scanning electron micrographs revealed complicated etching patterns with increased etching time periods. Shear testing results showed that the bond strength to the unetched porcelain of the composite resin was very low, and that etching periods for more than 30 seconds effectively enhanced the bond strength. Of the two etching agents applied to the unsilanated porcelain, the buffered 2.5% etchant produced higher bond strengths than the 5% etchant for all etching time periods except for 180 seconds. Silane priming was effective and critical for improving bond strength to the porcelain. Application of the silane bonding agent to the porcelain after hydrofluoric acid etching appeared to be suitable for achieving consistent bonding between the composite resin and the porcelain.

Development and clinical applications of a light-polymerized fiber-reinforced composite☆☆☆★★★♢♢♢♦

Abstract: Statement of problem. After 30 years of intermittent reports in the literature, the use of fiber reinforcement is just now experiencing rapid expansion in dentistry. Purpose. This article describes the development and use of a continuous, unidirectional fiber-reinforced composite as a framework for the fabrication of fixed prostheses. Methods. By using various matrix materials and fibers, a number of fiber-reinforced composite formulations were evaluated with the goal of creating a system with optimized mechanical properties and handling characteristics. Fiber-reinforced composite based on a light-polymerized BIS-GMA matrix has been used clinically to make 2-phase prostheses comprised of an internal glass fiber-reinforced composite substructure covered by a particulate composite. The clinical and laboratory procedures required for the fabrication and use of reinforced composite fixed prostheses are described for laboratory-fabricated complete or partial coverage fixed prosthesis and chairside prosthesis. Results. Although additional clinical experience is needed, fiber-reinforced composite materials can be used to make metal-free prostheses with excellent esthetic qualities. (J Prosthet Dent 1998;80:311-8.)

Microstructural factors controlling the strength and ductility of particle-reinforced metal-matrix composites

Abstract: A micromechanical model is developed to simulate the mechanical response in tension of particle-reinforced metal-matrix composites. The microstructure of the composite is represented as a three-dimensional array of hexagonal prisms with one reinforcement at the centre of each prism. The shape, volume fraction and state (either intact or broken) of the reinforcement is independent for each cell, so the interaction among all these factors could be studied. The tensile response of the composite is determined from the behaviour of the intact and damaged cells, the fraction of damaged cells being calculated on the assumption that the reinforcement strength follows the Weibull statistics. The model is used to determine the microstructural factors which provide optimum behaviour from the point of view of the tensile strength and ductility. The analyses included the effect of the matrix and reinforcement properties, the reinforcement volume fraction, the interaction between reinforcements of different shape and the heterogeneous distribution of the reinforcements within the composite.

Formation of bilayer Ni–SiC composite coatings by electrodeposition

Abstract: Composite plating is a method of co-depositing fine particles of metallic, non-metallic compound or polymers in the plated layer to improve material properties, such as wear-resistance, lubrication, or corrosion resistance. Bilayer Ni–SiC composite coatings were produced in this research on the base of a reported study. Before producing bilayer Ni–SiC composite coatings, effects of the particle size, the particle content, the pH of the electrolyte, the temperature, the current density, the stirring rate on the amount of SiC deposited in the Ni layer were investigated. By manipulating the current density and the plating time properly, bilayer Ni–SiC composite coatings were produced and mechanical properties of these coatings were evaluated by micro-indentation hardness tests.

Nonlinear Analysis of Composite Beams with Deformable Shear Connectors

Abstract: Composite floor systems, consisting of reinforced concrete slabs and steel girders, are common in buildings and bridges. Partial restraints at the interface of the concrete slab and the steel girder have significant effects on the response of the composite beam. A few analytical models capable of accounting for these effects are available; however, none can be efficiently used for the nonlinear analysis of composite beams. In this paper, the basic governing equations are presented for a composite beam with deformable shear connectors under small displacements. Based on these equations, a new composite beam element is developed using the force method of analysis. The performance of this new element is compared with that of a previously developed displacement-based composite beam element. Both elements consist of two beam components connected through a deformable interface. A distributed spring model is used to account for the shear deformation at the interface. Two numerical examples on the nonlinear behavior of composite beams are solved using both displacement- and force-based elements. Comparison of the numerical results shows the superior performance of the newly developed force-based element over the displacement-based element.

Prediction of thermo-mechanical properties for compression moulded composites

Abstract: We present a method to determine the thermo-mechanical properties of compression moulded composite parts. The flow-induced fibre orientation is first calculated by numerical simulation, and the resulting orientation state is used as input in a micromechanical model that predicts the thermo-mechanical properties of the part. A two-step homogenization scheme based on the grain model approach is followed. First, the properties of a reference composite with aligned fibres are estimated by means of a mixture rule between the upper and lower Hashin-Shtrikman bounds (derived by Willis). This method is in agreement with the Mori-Tanaka estimates for moderate concentrations, and gives better results for higher concentrations. Next, the properties of the composite are obtained by averaging several reference composites with different fibre directions. An example of a 3-D compression moulded composite part is analyzed and the results are discussed. (C) 1997 Elsevier Science Limited.

A Tough, Thermally Conductive Silicon Carbide Composite with High Strength up to 1600°C in Air

Abstract: A sintered silicon carbide fiber-bonded ceramic, which consists of a highly ordered, close-packed structure of very fine hexagonal columnar fibers with a thin interfacial carbon layer between fibers, was synthesized by hot-pressing plied sheets of an amorphous silicon-aluminum-carbon-oxygen fiber prepared from an organosilicon polymer. The interior of the fiber element was composed of sintered beta-silicon carbide crystal without an obvious second phase at the grain boundary and triple points. This material showed high strength (over 600 megapascals in longitudinal direction), fibrous fracture behavior, excellent high-temperature properties (up to 1600 degreesC in air), and high thermal conductivity (even at temperatures over 1000 degreesC).

Manufacturing of composite structures with a built-in network of piezoceramics

Abstract: A manufacturing method was developed for integrating a network of distributed piezoceramic actuators/sensors onto laminated carbon/epoxy composite structures. The network of built-in actuators/sensors is used to monitor the health of the host composite structure by acquiring information about the condition of the structure throughout its service life. The manufacturing method applies the printed circuit technique to fabricate a thin flexible layer with a network of piezoceramics. It is used as an extra ply that is either inserted into or bonded onto the surface of a composite laminate to give it actuating and sensing capabilities. This layer that provides the added functionality is referred to as the “SMART Layer” (Stanford Multi-Actuator-Receiver Transduction Layer). Using the developed method, several SMART Layer prototypes have been fabricated and embedded inside carbon fiber composite laminates successfully. Tests were conducted on composite panels integrated with the SMART Layer to validate the design and the manufacturing procedure.

Ceramic ferroelectric composite materials with enhanced electronic properties BSTO-Mg based compound-rare earth oxide

Abstract: Ceramic ferroelectric composite materials comprising barium strontium titte/magnesium and oxygen-containing compound composite further doped with rare earth (lanthanide) oxides. More particularly, these inventive composites are comprised of Ba 1-x Sr x TiO 3 /Mg--O based compound/rare earth oxide composite, wherein x is greater than or equal to 0.0 but less than or equal to 1.0, and wherein the weight ratio of BSTO to Mg compound may range from 99.75-20 wt. % BSTO to 0.25-80 wt. % Mg compound, and wherein said rare earth oxide additive comprises less than about 10 mole percent of the composite. The rare earth oxides of the composite include all oxides of the lanthanide series elements including scandium and yttrium, as well as combinations thereof. The magnesium-based compound may be selected from the group consisting of MgO, MgZrO 3 , MgZrSrTiO 3 , MgTiO 3 , and MgCO 3 . This new class of composite materials has enhanced electronic properties including: low dielectric constants; substantially decreased electronic loss (low loss tangents); increased tunability; increased temperature stability; decreased sintering temperatures; and low curie temperatures. The electronic properties of these new materials can be tailored for various applications including phased array antenna systems, capacitors, transmission wire, wireless communication, and pyroelectric guidance devices.