Piezoresistive strain sensors in the form of short carbon fiber composites

Short carbon fibers (5 mm long, 15 mu m diameter) in polymer and cement matrices provide piezoresistive strain sensors that have their DC electrical resistivity change reversibly upon strain. For fibers (5-10 vol.%) in an epoxy matrix, the resistivity increases upon tension and decreases upon compression, due to the change in proximity between adjacent fibers in the composite; the gage factor (fractional change in electrical resistance per unit strain) is up to 97 under compression and up to 24 under tension. For fibers (0.2-0.5 vol.%) in a cement matrix, the resistivity increases upon tension and decreases upon compression, due to the change in contact resistivity between fiber and matrix; the gage factor is up to 700.