Longitudinal Compression Strength of Glass Fiber-Reinforced Composites

Abstract

The compressive performance of fiber-reinforced composites is a major concern in a variety of structural applications. The accurate determination of this property for unidirectional composites is exceedingly difficult; in general, difficulties encountered in gripping or supporting test specimens compounded by this anisotropic material's in herent sensitivity to axial alignment result in inconsistent failure loads and failure modes. Recently, however, we developed a novel test specimen which provides reliable and repro ducible compression strengths for graphite and boron fiber-reinforced composites. The specimen is a miniature sandwich beam with composite skins on either side of a neat epoxy resin core, and it is tested in a conventional IITRI test fixture. This specimen con figuration promotes true compressive failure under axial loading in contrast to all- composite coupons which fail prematurely via specimen buckling. Through stress analysis it was determined that this specimen had lower interlaminar stresses at the free edges and a lower stress concentration at the end of the gage section than the all-composite specimen. In the present study we have employed this miniature sandwich specimen to determine the compressive properties of E-glass and S-glass fiber-reinforced epoxy composites. The comparatively low moduli of these reinforcing fibers increase the difficulty in obtaining accurate compression data for the corresponding composites. Longitudinal compression strengths were obtained for specimens with unidirectional, cross-ply and fabric reinforce ments. The longitudinal compressive strengths of the unidirectional composites measured with this sandwich specimen are significantly higher than those of corresponding all-com posite specimens. The measured compression strengths for cross-ply and fabric- reinforced composites, on the other hand, are the same for both specimen types.