This paper deals with the micromechanics of advanced composites. The characteristics of individual fibers, matrix, and the fiber/matrix interface are the most important factors for evaluating fiber-reinforced composite materials from a micromechanical viewpoint. To this purpose, we
first developed a testing device with which the load-elongation curve of a monofilament can be obtained. Filament diameter was measured by the aid of laser diffraction. Using these devices, we have tensile-tested several kinds of monofilaments and accumulated data. The strength distribution
agreed fairly well with the Weibull distribution. The fiber/matrix interaction was also examined with a single fiber embedded model. By measuring a fragment length, an ineffective length was calculated. This paper will provide the basic data to the second report in which the strength of unidirectional
composites is predicted by means of computer simulation.