SynopsisThe stress-strain behavior of glass bead-filled polystyrene, styrene-acrylonitrile, styrene-acrylonitrile-butadiene, and polyphenglene oxide composites is studied at various temperatures below their glass transition temperature. Earlier studies of beadfilled composites indicated that the addition of filler decreases toughness and ultimate elongation. Our results show that while this is true for certain conditions, it is also possible for such composites to have higher toughness and ultimate elongation than the unfilled matrices. A deformational mechanism, involving crazing of the matrix, is proposed which explains this behavior.

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The flexural fatigue characteristics of boron fiber reinforced epoxy were shown to be strongly dependent on the aspect ratio of the reinforcing fibers. These composites were evaluated in a stiffness limited, constant maximum stress mode. The failure mechanism was found to be a combination of interfacial failure and brittle rupture of the matrix at 45 deg to the axis of the fibers. These fatigue cracks result in debonding of the outer fibers, thereby reducing the effective moment of inertia of the specimen.

The structural utility of short, glass fiber‐reinforced epoxy composities is experimentally investigated for fiber volume fractions from 0.15 to 0.5. The strength and stiffness of systems with randomly oriented fibers are compared with those of similar composites with aligned fibers. The ultimate strength of both types of material increses in a reasonably linear fashion with volume fraction up to 0.5. For all volume fractions in this range, strength of the random composites is slightly higher than the longitudinal and much higher than the transverse strength of equivalent compsites with aligned fibers. The modulus of the random system is approximately two‐thirds the longitudinal and twice the transverse modulus of the unidirectional material. The structural utility of the flow molded material is greatest in uniaxial, stiffness critical situations. The greater strength and planar isotropy of the random composites make them preferable in all strength limited or multiaxial applications.

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Material parameters which are dependent on direction of orientation can be best represented by tensors of appropriate ranks. Using tensor transformation rule and associated scalar invariants of the tensor, experimental data can be averaged and reduced to compact constants for engineering documentation and retrieval. Examples on data averaging of composite swelling coefficients and elastic compliances illustrate the simplicity and viability of this tensor averaging methodology.

This study was made to determine the effect of test variables on the strength of unidirectional composites. Specimens of glass fiber reinforced epoxy of varying geometry were tested in tension and flexure at several strain rates. Both experimental and theoretical results show that specimen geometry strongly influences strength measurements for off-axis specimens having a small length-to-width ratio. This is a result of nonsymmetrical anisotropy in the specimens which causes twisting in flexure tests and shear coupling in tension tests. For slender specimens, having a free length-to-width ratio greater than six, both strength and stiffness measurements are almost unaffected by the boundary conditions of the test. A normalization procedure was used successfully to relate the strength of an off-axis composite to its transverse strength. While changes in loading mode, strain rate, and specimen geometry may produce significant changes in absolute strength values, normalized strengths proved to be essentially invariant.