Journal of Composite Materials

An analytical investigation has been made of gripping effects on the off-axis tensile test results for flat specimens. It is an extension of the earlier work of Pagano and Halpin [1] and aims at establishing approxi mate bounds on the accuracy of the subject test. Considered are the cases of rigid clamping with and without end rotation for selected properties which are typical of glass, boron and graphite composites. Solutions of the corresponding mathematical models are effected numerically, utilizing a computerized finite-element approach.

The results support the findings of [1] in demonstrating that length/width ratio and type of constraint can significantly modify the intended uniform stress/strain fields in the subject test. Despite this adverse effect, the analysis suggests that if strain measurements can be properly recorded, reasonably accurate test results should be obtain able for even small length/width specimens. Moreover, though the numerical results lack the generality of closed-form solutions, it would still appear that the trends estimated for the special glass, boron, and graphite systems are typical of other hard composite systems in these respective families.

The wave propagation and dispersion characteristics of libroelastic materials are considered. Such materials consist of molecules which interact by means of position as well as orientation-dependent forces, and possess mass as well as rotational inertia.

The following results hold for homogeneous libroelastic isotropic materials with a centre of symmetry: (a) Translational as well as librational waves decompose into longitudinal and transverse waves. (b) The longitudinal waves are uncoupled and the librational one is dispersive. (c) The transverse waves are coupled and the transverse translational particle motion is always perpendicular to the transverse rotational motion. These waves are dispersive.

The difference of two libroelastic constants of solid carbon dioxide has been evaluated (using Raman scattering measurements) for, what it is believed to be, the first time.

An experimental study of the uniaxial compressive stability of rectangular boron-epoxy laminated plates is presented. The plates are clamped on the loaded edges and either clamped or simply- supported on the sides. The buckling loads are determined by means of Southwell plots and are compared to analytical results obtained with a previously published method. Experimental results are presented for twenty composite plates and four metal plates. Good agreement between the experimental and analytical results is shown for both symmetrical and unsymmetrical plates, and for plates strongly aniso tropic as well as orthotropic and isotropic.

A method for characterizing the orientation of short fibers during flow of short fiber composites is described. A system comprised of approximately 50% by volume glass fibers in an epoxy resin matrix was used for this study. A glass rheometer was used to observe steel tracer fibers, which were incorporated into feed particles with the glass. By reacting the epoxy resin with a secondary amine, a product was obtained which did not react further during the measurements. The product also had a viscosity at room temperature similar to com mercial systems at processing temperatures; this made flow measure ments at room temperature possible. A technique of data reduction for the orientation measurements is described.

Early results show that, at these high fiber loadings, plug flow with little change in orientation occurs above and below the constriction in the rheometer tube. At the constriction, particles are sheared into strands. If feed particles containing pre-oriented fibers are not used, a "log jam" results at the entrance to the constriction, preventing flow. The dependence of extrusion pressure on extrusion rate is much less than one finds for unfilled systems.

The development of a submicron boron nitride coating has sub stantially improved the stability of boron filaments in metallic matrices. It is now possible to think in terms of continuous casting techniques for the fabrication of low melting metal matrix composites like aluminum/ boron. The boron nitride coating also contributes to the formation of void-free nickel/boron composites by electroforming processes, and reaction-free nickel/boron and titanium/boron composites by diffusion bonding methods. Mechanical properties of 60-70 vol. % nitrided boron/6061 aluminum composites prepared by continuous liquid infiltration are discussed. The effects of heat treatment on these com posite strengths are also considered.

A plane elasticity solution is derived for the end problem in a two- layer laminated strip. The laminae may have arbitrary elastic constants and an arbitrary thickness ratio. The solution is obtained as a series of non-orthogonal eigenfunctions, each of which varies exponentially along the axis of the strip. Each exponential coefficient is a complex root of an eigenvalue equation that results from the requirement of compatible displacements at the interface.

The general solution obtained in this paper may be used to find the stress field in a two-layer laminated strip subjected to specified self- equilibrating end loading. It thus permits the determination of the stress concentration near the end which is usually neglected by invok ing St. Venant's Principle.

A linear analysis is presented for determining the natural frequen cies of vibration of laminated anisotropic rectangular plates. The plate may consist of an arbitrary number of thin orthotropic layers, the major material-symmetry axis of each layer oriented arbitrarily with respect to the longer plate edge. An approximate solution is obtained by the Rayleigh-Ritz energy method. Numerical results are presented for fully clamped boundary conditions and compared with experimental results for symmetrically and unsymmetrically laminated plates.

In this paper a material region is considered to be an aggregate of point particles which move in accordance to Newton's Laws and inter act by means of forces that depend only on the relative distance between the particles and are derivable from a potential U. The follow ing propositions are, then, established: the continuum field equations are obtained by a smoothing assumption that allows the representation of sums by integrals; the classical constitutive equations of elastic materials are obtained when forces between particles are of infini tesimally short range; when the range of interaction between particles is of short but finite range, higher order stresses and strains are neces sary to describe the mechanical state of the material.

Shrinkage stresses produced during the curing of the matrix in a two-phase material were studied. Specimens representing a typical two-phase material made of unplasticized epoxy as inclusions and plasticized epoxy as the matrix were studied photoelastically.

The magnitude of stresses at the interface of a single inclusion in an infinite matrix during the transient period of curing depended only on the constant elastic modulus of the inclusion and on the instantaneous value of the matrix elastic modulus and it was independent of the amount of shrinkage of the matrix. In the case of a square array of inclusions in close proximity, the stress-distribution depended on both elastic moduli and the amount of shrinkage. The influence of the ratio of the space between inclusions and their diameter on the magnitude of stresses was also studied, in the case of already set matrices.

Various techniques have been proposed and developed for aligning the whisker reinforcing phase of metal matrix composites. Most of these techniques involve multi-step alignment processes, and are not suitable for economical, continuous production of metal-whisker com posites.

A laboratory scale, prototype system for the continuous dielectro phoretic deposition and alignment of micron sized, ultra high strength whiskers has been designed, developed and tested. This process consists of the deposition of whiskers onto one of two electrodes placed in a suspension of the whiskers, when a voltage is applied between the electrodes. By employing a metal matrix foil as the deposition elec trode, this technique has demonstrated that the continuous production of metal-whisker combinations suitable for fabrication into advanced whisker reinforced composites is feasible.

The volumetric loading of the reinforcing whiskers on the metal foil can be controlled by varying such process parameters as voltage, foil speed, electrode separation and geometry, and the concentration of whiskers in the supporting media. A discussion of these alignment and deposition process variables is presented.

The metal foils containing the deposited, aligned whiskers are then sandwiched into a preform structure consisting of successive layers of metal foil and fibers. Consolidation of the stacked layers into a com posite is accomplished by hot pressing and diffusion bonding. Photo micrographs of the consolidated composite, showing alignment of the whiskers and the microscopic composite structure are shown and discussed for a range of whisker compositions. Preliminary mechanical properties data for selected composite compositions are presented and discussed.

The electric permittivity of a composite can be expressed in terms of the electric permittivity of the components and the average of the electric potential on the surface of a filler particle. The expression obtained is developed in powers of the volume fraction c of filler particles. Both the term linear in c and the term which is quadratic in c . are derived explicitly for two systems: (a) the particles are spheres; (b) the particles are circular cylinders perpendicular to the external field. Two methods are used. The first is an approximate treatment which is based on successive "reflections" of the perturbation caused by one particle off a second particle. The assumption is made that each reflection can be calculated as though the perturbation of the field is seen by the particle as a homogeneous field. This approach is worked out for cylinders and for spheres. The second method is carried through only for cylinders. It is based on the use of bipolar coordinates and is rigorous. The results obtained are compared with those of two approxi mate theories published previously: that of Polder and Van Santen and that of Kerner. This comparison can be found in the last section.

Although the basic differential equations and appropriate boundary conditions for anisotropic plates have been available for many years, exact solutions are unavailable except for the case of infinitely long rectangular plates [1].

This note describes an analogy between the differential equations of skew (parallelogram) isotropic plates and rectangular anisotropic plates. Although the analogy is restrictive in applicability, it provides several exact solutions for finite length rectangular anisotropic plates. These solutions are useful by themselves and should prove especially valuable in assessing the accuracy of approximate analyses of anisotropic plates.

Recently solutions have been obtained [1, 2, 3] for the bending, vibrations, and buckling of laminated plates in which coupling between bending and stretch ing is important. However, buckling results were limited to unsymmetric angle- ply laminates under biaxial compression. Thus, there are no solutions available for the buckling of coupled laminates subjected to shear loading.

This paper is concerned with the stability of unsymmetric cross-ply rectangular plates under uniform shear. These composites consist of an even number of layers all of the same thickness and elastic properties with the orthotropic axes of symmetry in each ply alternately oriented at 0° and 90° to the plate axes. Hinge- support boundary conditions are considered. The effect of coupling is ascertained by comparing coupled solutions to those obtained by neglecting the coupling coefficients in the governing equations. Applicability of the reduced bending stiffness approximation to shear buckling of cross-ply composites is also in vestigated.

Mollard and Flemings [1] and Cline [2] produced lamellar and fibrous com posites of various lead-tin alloys by directional solidification under conditions in which free convection was suppressed. Experiments on mixtures containing a non-faceted material and a platelet-forming material showed that oriented com posites could be obtained from off-eutectic compositions by zone melting, even with convection present [3]. Pepper [4] produced irregular fibers of Be in Al by zone melting of off-eutectic mixtures. The purpose of the present work was to see just how drastic freezing conditions could be when one component was a strong fiber-former and still obtain an oriented composite. An organic system was chosen for the experiments to permit observation of the freezing interface under the microscope.