Initial tests have been performed in a continuing experimental study to determine selected mechanical properties, at high strain rates, of an epoxy and of composite materials consisting of the epoxy filled with aluminum powders. These composites are examples of the large variety of polymer-matrix/particlefilling composites in general, which are widely used in military and civilian applications. The properties of such composites can be tailored for specific applications through appropriate choices of constituent materials, the proportions of the constituent materials, and the sizes of the particles. Especially in aerospace structural applications, the composites are exposed to complex, temporally varying loads. Therefore, the mechanical properties of such composites’ high strain rates are of increasing importance.

These Stress-vs.-Strain Data and associated fracture-surface micrographs were obtained in flexural tests of specimens of two composites containing differently sized particles.

The epoxy used in this study is made from a bisphenol- A resin, known by the trade name “Epon 826,” with a diethanolamine hardener. Two composites were examined in the initial tests. In both composites, the proportion of aluminum powder was 46 volume percent (equivalently, 65 weight percent). The powders in the two composites are known by the trade names “Valimet H2”and “Valimet H3.” The particle sizes, specific surface areas, and mass densities of the particles were determined by use of a light-scattering technique, a static volumetric technique, and pycnometry, respectively. The results of these determinations were the following: The H2 powder was found to have a specific surface area of 1.522 ±0.093 m2/g, a particle size of 3.479 ±0.042 μm, and a mass density of 2.720 ±0.012 g/cm2; the H3 powder was found to have a specific surface area of 1.145 ±0.032 m2/g, a particle size of 5.425 ±0.078 μm, and a mass density of 2.688 ±0.009 g/cm2.

Specimens of the neat epoxy and the two composites were subjected to static flexural tests and to dynamic compression tests at various strain rates from 10- 3 to 105 s-1. In tension, as loaded in the flexural tests, both composite materials failed at the interfaces between the aluminum particles and the epoxy. Relative to the composite containing the larger (H3) particles, the composite containing the smaller (H2) particles exhibited lower flexural failure stress (see figure). It has been conjectured that this difference could be attributable to the difference in the number of particles per unit volume or to the greater oxide component in the smaller particles.

In the compression tests, the smallerparticle composite proved to be consistently stronger than the larger-particle composite. This difference is attributed to the greater number of particles per unit volume and the consequent greater constraint on the flow of epoxy in the smaller-particle sample. The differences in the compression and tension behaviors of these materials have been interpreted as signifying that the particle/matrix interfaces are weak, relative to the matrix.

In addition, as part of a continuing effort to develop constitutive equations for composite materials like these, data from the dynamic compression tests of the neat epoxy were compared with predictions of the Hasan-Boyce model — a theoretical model that has been shown to fit experimental data fairly well in prior studies of epoxies. The data and the predictions of the model were found to be in qualitative agreement. At the time of reporting the information for this article, an effort to optimize the parameters of the model for the epoxy used in this study was in progress.

This work was done by Jennifer L. Jordan, D. Wayne Richards, and Jonathan E. Spowart of the Air Force Research Laboratory, and Brad White and Naresh N. Thadhani of the Georgia Institute of Technology.


This Brief includes a Technical Support Package (TSP).
High-Strain-Rate Tests of Epoxy/Aluminum-Powder Composites

(reference AFRL-0048) is currently available for download from the TSP library.

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