Coupling between the electrical and mechanical response of a material is a phenomenon termed piezoresistivity. This coupling phenomenon can be used to develop strain-sensitive ‘smart’ materials that are able to self-monitor their own strain and internal damage state.
One way of achieving this aim in polymer composites is through the introduction of electrically conductive carbon nanostructures into a nonconductive polymer. Upon inclusion of enough carbon nanostructures, the polymer composite becomes electrically conductive—a phenomenon known as electrical percolation. Upon percolation, changes in strain, humidity, temperature, and other external excitations yield changes in the electrical conductivity of the nanocomposite, and their correlation can be used to develop self-sensing smart materials.
Piezoresistivity and the capability of polymer composites filled with carbon nanostructures to electrically sense their own deformation and damage is discussed in a Review article by Francis Aviles et al., which covers topics all the way from the initial efforts in using carbon black and graphite as fillers to the latest research on carbon nanotubes and graphenic fillers.
The current applications of these materials are expected to provide the automotive, aerospace, transportation, and energy industries with the next generation of smart materials. Especially for thermosetting polymers, the applications of these nanocomposites are rapidly evolving toward self-sensing of strain and damage in fiber-reinforced, multiscale hierarchical composites, which can be used in virtually every industrial application of composites that demand high-load-bearing capacity and self sensing.
The applications of carbon filled elastomeric materials, on the other hand, focus more on sensors for human motion, soft skins, smart wearable sensors, and robotics, whose main requisite is large deformations. Aspects such as nonlinearity, cycling reproducibility, undesired viscoelastic response, and hysteresis, however, are practical issues that need to be resolved toward the further development of commercial devices based on these materials.