Short carbon fibers (5 mm long, 15 micrometers 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.
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