Interphase modification in unidirectional glass-fiber epoxy composites

The flexural properties of glass-fiber epoxy unidirectional composites have been studied as the surface of the glass fiber is changed. The glass fibers were treated with two different coatings: a mono-epoxy and a di-epoxy. Both were deposited on the fibers by a solvent process at several coating concentrations in order to produce different interfacial properties and thicknesses of the coating. Flexural tests performed along the fiber axis with different span-to-depth ratios confirm that the critical ratio depends on the fiber surface modification. In the case of composites with mono-epoxy coated fibers, the tensile/ compressive and shear stresses decrease as the coating thickness increases. However, for the composites with diepoxy coated glass fibers, the tensile/compressive and shear stresses exhibit a maximum as the thickness increases. Evidence of the fiber surface modification is provided by analysis of the fracture surfaces of all of the composites. Dynamic mechanical analysis shows that the unidirectional composites with di-epoxy-coated fibers and with a thickness-to-fiber radius ratio of 1.2% appear to have stronger interfacial bonds. Different damping behavior is observed for the coating thickness as for the type of coatings. The width of the cr-transition also shows a variation with fiber coating thickness indicating some changes in the matrix stoichiometric ratio or in the network structure. (C) 1998 Elsevier Science Ltd. All rights reserved.