Structural health monitoring of nanocomposite adhesively bonded joints using electrical impedance variations

Abstract

The effect of multi-walled carbon nanotubes (MWCNTs) on the mechanical performance, electrical percolation threshold, and structural health monitoring capability of adhesively bonded joints was investigated. Epoxy adhesives reinforced with MWCNTs were fabricated in two configurations: randomly dispersed and electrically aligned using an external electric field. The specimens were tested using double cantilever beam samples. The electrical percolation threshold was determined via dielectric relaxation spectroscopy. Furthermore, the electrical response was monitored during Mode-I fracture testing through impedance measurements. The results indicated that the alignment of nanotubes significantly reduces the electrical percolation threshold to approximately 0.1 wt% or lower, whereas this value is about 0.5 wt% for randomly dispersed systems. The optimum mechanical performance was observed at an MWCNT content of 0.3 wt%, corresponding to an increase in fracture energy of approximately 264% and 400% for the randomly dispersed and aligned specimens, respectively. Impedance measurements revealed a clear correlation between crack propagation and electrical response. While randomly dispersed systems exhibited a larger impedance increase, aligned specimens showed a more stable and gradual response due to the preservation of conductive pathways. Importantly, alignment enables the transition to conductive behavior below the mechanically optimal concentration, allowing simultaneous achievement of high mechanical performance and effective damage sensing. These findings demonstrate that controlled MWCNT alignment provides a promising strategy for the development of multifunctional adhesive joints with enhanced mechanical properties and reliable SHM capability.