Although many test specimens have been developed to measure fiber-matrix interfacial strength, they typically suffer from a lack of reproducibility and produce data with coefficients of variation greater than 10 percent. This paper reviews methods thus far employed to evaluate fiber-matrix interfacial strengths including correlation of data from the various test methods.

The stress distribution at a fiber-matrix interface subjected to loading must be known if one is to calculate interfacial fracture stresses due to external load application. Thermal residual stresses due to elevated curing temperature of polymer specimens also must be considered. Equations to predict the shear stresses at fiber ends and the radial and tangential stresses around the interface will be presented. Experimental photelastic determinations of internal stresses will be compared with the theoretical predictions.

Experimental techniques for measuring interfacial strengths for single fibers will be discussed. Examples of strength measurements for glass and boron fibers embedded in polymer matrices will be presented. These strengths have been measured as a function of the specific nature of the interface to evaluate the role of a fiber surface treatment in enhancing this interface strength. The fiber or rod bond strength specimens usually consist of a single embedded fiber or partially embedded rod in a polymer matrix which is failed by loading the matrix in the first case or by loading the rod in the latter case.

Interfacial tensile strengths can be measured using a curved neck specimen, and shear bond strengths can be measured most effectively by a fiber embedded totally within a rectangular bar. A compression load is applied parallel to the fiber axis. A technique related to determination of interfacial toughness of joints by a double cantilever cleavage bar will be discussed and the results compared to strength measurements of similar systems. Finally, measurements on actual fiber composites (horizontal shear strength) will be discussed as a means of measuring fiber-matrix interfacial strength.