Roy L. McCullough

Generalized combining rules for predicting transport properties of composite materials

Abstract A general relationship is presented to predict the effective transport properties (electrical conductivity, thermal conductivity, dielectric constants, magnetic permeability, and diffusion coefficients) of composite materials in terms of the properties and concentration of the components and the internal microstructure. Numerous existing relationships are obtained as special cases. Filler shapes ranging from platelet, particulate, and short-fiber, to continuous fiber are consolidated within the relationship. Particular emphasis is given to isotropic composites containing particulate fillers. Simplifications appropriate for metal-filled polymers are presented. An interpretation of structural parameters is proposed which accounts for sharp transitions associated with percolation mechanisms.

The effect on the mechanical properties of carbon/epoxy composites of polyamide coatings on the fibers

Abstract An experimental investigation has been carried out to study the effect of polyamide coatings on the mechanical properties of unidirectional carbon/epoxy composites. Carbon fibers were coated with polyamide 6,6 by means of interfacial polyamidation and solution dip coating in a laboratory-scale unit developed for this purpose. The amount of coating deposited on the fibers was determined by thermogravimetric analysis (TGA); the morphology and microscale homogeneity were examined by scanning electron microscopy (SEM). Transverse tensile tests and short-beam shear tests were performed in order to assess the effect of the coating on the adhesion of the fiber to the matrix. Composites containing solution-coated fibers exhibited improved tensile and ILSS performance over those of uncoated-fiber composites, although interfacial coating induced deterioration in performance. The double-cantilever beam (DCB) test was used to investigate the effect of the two different polyamide fiber-surface coatings on mode I interlaminar fracture toughness. Initiation fracture toughness (G ICinit ) increases for both polyamide coatings. Moreover, interfacial coating causes extensive fiber bridging leading to much increased propagation values. The mechanical results are attributed to the different nature of the two polyamide coatings and to differences in the microstructure of the resulting composites.

Characterization of fiber orientation in short‐fiber composites

A method for measuring intermediate states of fiber orientation in short‐fiber composites is described. The technique is based on the observation that a representative fiber distribution, for example, the negative of a photomicrograph from a specimen, resembles a collection of diffraction apertures. Diffraction masks for a Fraunhofer diffractometer made from the representative fiber distributions produce diffraction patterns characteristic of the state of orientation. Analyses are presented which relate the features of the diffraction pattern to (i) the orientation distribution, (ii) shape, and (iii) aspect ratio of the fibers. The results from these analyses are in good agreement with experimental diffraction patterns from masks made from simulated fiber distribution patterns.

Characterization of nanoscale property variations in polymer composite systems: 1. Experimental results

A technique utilizing the indenting capabilities of the atomic force microscope is used to evaluate local changes in the response of polymer composite systems near the fiber-matrix interface. Room temperature and elevated temperature indentation response is measured for several model composite systems. Results of indentation studies are compared to finite element model predictions to understand the influence of interphase properties on the measured responses. For sized fiber systems, unexpected property variations are observed, leading to the discovery of a possible interphase formation mechanism in these systems. q 1998 Published by Elsevier Science Ltd. All rights reserved.

Characterization and analysis of the electrical properties of a metal-filled polymer

Abstract The electrical behavior of a polyester resin filled wtih aluminum powder is reported for filler concentrations ranging from 0 to 45 vol%. The a.c. dielectric properties (dielectric constant, loss index, and dissipation factor) are characterized as a function of temperature in the range 20 to 130 °C at frequencies of 1, 10, and 100 kHz. The d.c. volume resistivity is characterized at room temperature. The characterization data are used to evaluate various relationships to predict transport properties. A new relationship is shown to give significantly improved predictions by incorporating information concerning the extent of formation of chains of contacting metal particles. The microstructural parameters are obtained from analyses of photomicrographs of specimens at various concentrations of filler.

The effect of temperature on the behavior of the interphase in polymeric composites

Abstract The single-filament fragmentation method for measuring the fiber/matrix stress transfer was used for the identification of interphase perturbations. This technique is based on the measurement of the fiber length resulting from the multiple fracture of a single fiber embedded in a resin specimen during tensile loading. A series of single-fiber fragmentation experiments was conducted over a wide range of temperatures on the AS4-carbon-fiber/Epon-828/PACM20-epoxy-resin system. Critical aspect ratios, the magnitude of which is considered to be inversely proportional to the square root of the matrix modulus, showed a significant increase from ambient to elevated temperatures, at temperature levels much lower than the glass transition point of the bulk matrix. This increase was consistent with the existence of an interphase of lower glass transition temperature than the bulk matrix. A three-concentric-cylinder elastic model was employed to correlated the effect of material properties.

Kinetic and Thermodynamic Considerations Regarding Interphase Formation in Thermosetting Composite Systems

Abstract Thermodynamic and kinetic treatments are employed to analyze the role of preferential adsorption and diffusion in the formation of interphase regions in thermosetting composities. The thermodynamic analyses show that a driving force exists for the preferential adsorption of amines onto carbon fiber surfaces in amine-epoxy systems. Scaling analyses, based on kinetic considerations, lead to the identification of reaction-diffusion time regimes that control interphase formation in sized and unsized fiber systems. These considerations provide guidelines for the selection of material formulations and processing conditions that may be used to tailor behavior of the interphase region in thermosetting composites.

An Analysis of Mechanisms Governing Fusion Bonding of Thermoplastic Composites

A number of mechanisms have been proposed in the literature as contributors to the strength development at the polymer-polymer interface during fusion bonding of thermoplastic composites. Of these, healing and intimate contact emerge as fundamental mechanisms governing bonding. Intimate contact refers to the development of the amount of surface area that is physically contacted at the interface at any time, and healing describes the migration of polymer chains across the interface in intimate contact. This work provides a new theoretical development of a coupled bonding model that accounts for variability in initiation time for healing due to growth in the area in intimate contact. The generalized coupled bonding model is valid for any set of processing conditions and reduces to the proper controlling mechanism as dictated by the process. Analysis revealed a key dimensionless group, Q, that captures the coupled nature of the mechanisms governing fusion bonding. By evaluating Q, which is a function of material and process parameters, one can determine the relative contributions of each mechanism. Experimental validation of the coupled model using two different processes, tow placement and resistance welding, is also presented. An evaluation of Q for the tow-placement process indicates that both mechanisms are controlling. For this case, the coupled model demonstrates better strength predictions than the conventional healing model alone. In contrast, the resistance welding process is shown to be intimate-contact controlled, in which case the coupled model reduces to a more simplified model. The ability to rigorously determine the controlling mechanisms is of critical importance to accurately model the strength development during fusion bonding processes.

Molecular characterization of glass fiber surface coatings for thermosetting polymer matrix/glass fiber composites

Model multi-component glass fiber sizings, with formulations based upon current patent disclosures, were prepared to model the full coating packages used in commercial glass fiber manufacture. The sizings consisted of silane coupling agent, film former, and emulsifying surfactant in water and were applied to glass fibers prepared directly from molten glass. Fibers were analyzed before and after acetone extraction. The analyses of the extract solutions, with the fiber analysis, were used to determine the quantity and quality of the physically and chemically adsorbed layers. It was found that all three species remain on the fiber after extraction and that both coupling agent and surfactant concentrations in the coatings are higher than in the applied sizing. The impact of these species on the polymer composite/glass fiber interphase is discussed.

An energetics approach to the analysis of molecular motions in polymeric solids

Abstract The feasibility of certain modes of intra-and intermolecular motion in polymeric solids are examined in terms of interaction energetics and geometrical constraints. Energy methods for determining equilibrium chain structures and stable deformation structures are reviewed. A systematic approach to the specification of explicit geometrical constraints in condensed phase polymeric systems is developed by matrix techniques. Combination rules are derived which provide straightforward means of constructing deformation structures for oriented polymers. Applications of these general methods are illustrated for paraffinic systems. Structure-energy maps, based on empirically justified potential functions, are presented for assemblies of paraff inic chains. The potential energy surface in the vicinity of the minima associated with the orthorhombic crystal structure is described by a Taylors series in the unit cell parameters. Applications of this approximate representation of the energy surface to the pred...