Strain and crack growth sensing capability of SWCNT reinforced epoxy in tensile and mode I fracture tests

Since the invention of Carbon Nanotubes (CNTs), the self-sensing capability of Multi-walled Carbon Nanotubes (MWCNT) based nanocomposites has been widely analyzed, whilst Single-Walled Carbon Nanotubes (SWCNTs) have been studied less extensively. Moreover, brittleness of epoxy based materials made them vulnerable to crack propagation, necessitating the monitoring of crack initiation and propagation. While research activities on strain monitoring of CNT based epoxy subject to tensile and flexural tests are present in the literature, works on the crack growth sensing capability under fracture toughness tests have not been reported. The present paper was therefore aimed to investigate the piezoresistive characteristics of epoxy-based nanocomposite loaded at different SWCNTs contents i.e. 0.25, 0.5 and 0.75 wt%, under tensile and mode I fracture toughness tests in order to compare and correlate the developed microstructures, including well-dispersed CNTs and aggregates, with the electromechanical properties of the epoxy based nanocomposites. SEM and FESEM analysis were conducted to investigate the fracture surface morphology and the state of the CNTs dispersions. A uniformly dispersed CNTs as well as proper cohesive bonding were obtained at CNTs content of 0.25 and 0.5 wt% whereas CNTs loading of 0.75 wt% resulted in excessive amount of aggregates accompanied with weak CNTs/epoxy bonding. Tunneling effect between neighboring CNTs was the dominant conductive mechanism, responsible for achieving proper sensitivity. For tensile tests, at a relatively low strain (less than 1%), the highest sensitivity was obtained at a SWCNTs content of 0.25 wt%, however showing a nonlinear trend in normalized resistance versus strain, i.e. the sensitivity increased by increasing strain. In fracture test, the SWCNT-doped materials were found to be capable of detecting damage initiation and extension by showing an abrupt increase in normalized electrical resistance upon the onset of crack growth while the piezo-resistivity before failure was either linear or nonlinear depending on the CNTs loading.