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 g lass 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 interfac ial properties and thicknesses of the coating. Flexural tests performe d along the fiber axis with different span-to-depth ratios confirm tha t the critical ratio depends on the fiber surface modification. In the case of composites with mono-epoxy coated fibers, the tensile/ compre ssive and shear stresses decrease as the coating thickness increases. However, for the composites with diepoxy coated glass fibers, the tens ile/compressive and shear stresses exhibit a maximum as the thickness increases. Evidence of the fiber surface modification is provided by a nalysis of the fracture surfaces of all of the composites. Dynamic mec hanical analysis shows that the unidirectional composites with di-epox y-coated fibers and with a thickness-to-fiber radius ratio of 1.2% app ear to have stronger interfacial bonds. Different damping behavior is observed for the coating thickness as for the type of coatings. The wi dth of the cr-transition also shows a variation with fiber coating thi ckness indicating some changes in the matrix stoichiometric ratio or i n the network structure. (C) 1998 Elsevier Science Ltd. All rights res erved.