The dynamic mechanical properties of unidirectional glass‐fiber‐reinforced polyester measured along the fiber direction were recently investigated. In the same polyester, the type of organosilane coated on the glass fiber, the amount of organosilane, the fiber volume fraction, and the fiber diameter affect the value of the loss tangent, tan δ, at the glass‐transition temperature of the glass‐fiber‐reinforced polyester. The interfacial shear strength and the tan δ at the glass‐transition temperature of the glass‐fiber‐reinforced polyester show good correlation suggesting that the latter can be used to characterize the quality of the interphase. Factors affecting the glass‐transition temperature and the application of Zorowski and Murayama's equation in the characterization of the interfacial adhesion are also discussed.

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The interface between the fibers and the polymer matrix controls the properties of fiber composites and has been the subject of much study. Recently, special techniques have been developed for single fiber pull‐out experiments on production fibers, which make it possible to obtain data on the frictional forces which govern sliding after the interface has fractured, as well as the adhesion strength of the interface. Tests on glass in polyester and epoxy resins show that the work of fracture of the interface is much smaller than that of the resin, and that the shrinkage pressures of these matrices, when fully postcured, are approximately the same (about 20 MPa). Coefficients of friction at the interface are 0.6 for the polyester and 1.0 to 1.6 for the epoxy. The carbon‐epoxy interface yields at shear stresses as high as 60 MPa, instead of fracturing, and the coefficient of friction during sliding is about 0.4.