In-situ monitoring of impact dynamics, indentation, and compression after impact (CAI) on CNT-doped GFRP panels by means of electrical monitoring

Carbon nanotubes (CNTs) have been exploited in structural health monitoring applications as a novel inspection technique for fiber-reinforced plastic materials, thanks to their excellent electrical properties and piezoresistive behavior. However, investigations on the CNTs sensor network behavior during compression after impact tests and correlations between electric resistance variations and damage mechanisms are lacking. Therefore, in this study, the electromechanical behaviour of CNT percolation networks in glass fiber-reinforced plastics materials has been investigated during impact, indentation, and compression after impact tests aiming at relating electrical resistance variations with damage mechanisms, also proving the CNTs network sensitivity in presence of repeated damages. The electrical response of the CNTs networks was compared with conventional nondestructive measurements to improve the understanding of the electrical response. Good agreement between the electrical and mechanical response is demonstrated during impact tests, highlighting a shift in the electrical resistance correlated with permanent and irreversible damage. Sharp changes in electrical resistance are related to sudden variations in impact and indentation loads, which are correlated with damage mechanisms. Compliance between electrical and mechanical signals is also observed in compression after impact tests, resulting in a general decrease of the electrical resistance with increasing loading due to compressive and buckling strains. Finally, compression after impact and repeated impact tests proved the ability of the CNT-based sensor network to sense even after being damaged. These results push towards the future applicability of the CNTs-based monitoring approach for composite structures, exploiting the CNTs self-sensing capability for damage identification.