In this paper Pukánszky's model for the tensile strength, originally developed for composites filled with quasi-spherical fillers, has been analyzed in terms of the Kelly-Tyson model for the prediction of mechanical strength of composites reinforced with short fibers, often used also for nanocomposites with organically modified nanoclays and carbon nanotubes. In this way it was possible to establish a direct link between Pukánszky's interaction parameter B and fundamental material parameters such as tensile strengths of the matrix and of the fibers, the aspect ratio of the fibers, and the orientation factor and the interfacial shear strength IFSS. Also it was possible to determine the minimum value of B for which it is possible to predict the tensile strength of the composite from the modified rule of mixtures, as well the maximum value that B can achieve in the case of continuous aligned fibers with the same type of matrix, fibers and interface shear strength. Moreover, a critical volume fraction, ?_crit, was defined corresponding to the minimum amount of filler content necessary for the composite strength to be greater than the strength of the unreinforced matrix, i.e. corresponding to the case ?_c = ?_m. It was also shown that for this condition B_crit ? 3.

A few examples of calculations of the IFSS, ?, from Pukánszky?s interaction factor B have been provided, using published literature values relating to nanocomposites with organically modified nanoclays and carbon nanotubes, as well as composites reinforced with short natural fibers.