Monitoring self-sensing damage of multiple carbon fiber composites using piezoresistivity

The change of electrical resistance in small bundles of multiple carbon fibers and multiple unidirectional carbon fiber/epoxy composites with applied tensile strain has been investigated. The electrical resistance of bundles initially increases relatively slowly in a stepwise manner with increasing strain due to fracture of peripheral fibers. This regime corresponds to the linearly increasing part of the load-strain curve. At higher strain, a progressive fracture of inner fibers in the bundle associated with flat region of load-strain curve leads to concomitant sudden rise of resistance. When the whole sample undergoes major failure, the slope of the load-strain curve becomes negative while the relative resistance increases abruptly to infinity. In strands of carbon fibers slightly impregnated with epoxy the change of resistance is affected by the thickness of epoxy layer surrounding the fibers. We demonstrate that volume fraction of fibers as well as initial number of fibers in the epoxy determines the piezoresistance properties of the specimen. Broken fibers can come into electrical contact with unbroken fibers and thus participate to the overall resistance of the specimen. As a result the dependency of relative resistance versus increasing applied strain presents stepwise behaviour also in the high strain region that is attributed to fiber fracture. In contrast to the bundles of bare carbon fibers, the stress-strain curves of the composites demonstrate monotonous linear increase in both low and high strain regions. The relative resistance goes to infinity when all remaining unbroken fibers undergo fracture.