Piezoresistive characterization of epoxy based nanocomposites loaded with SWCNTs-DWCNTs in tensile and fracture tests

Strain sensing capability of carbon nanotubes (CNTs) doped epoxy under tensile and flexural tests were broadly studied, while the piezoresistive behavior of nanocomposites in precracked specimen, that is, in fracture toughness tests, have not been addressed deeply in the literature. Therefore, in the current paper, both the strain and crack sensing capabilities of single wall carbon nanotubes-double wall carbon nanotubes (SWCNTs-DWCNTs) doped epoxy subjected to tensile and mode I fracture tests were investigated. Different CNT loadings including 0.5 and 0.75 wt% were used to compare the effect of various states of CNTs dispersion, including uniformly dispersed CNTs and aggregates, on electromechanical properties. Results showed that specimens loaded at 0.5 wt% of CNTs possessed proper tensile strength, fracture toughness, piezoresistivity, and sensitivity. A nonlinear trend in normalized resistance change with respect to strain was noticed under tensile test resulting from tunneling effect which induced nonlinearity in the electrical signals. During fracture tests, prior to crack propagation, different trends in piezoresistivity as a function of displacement were observed, depending on the CNT loading, that is, linear and nonlinear behavior at CNT content of 0.5 and 0.75 wt%, respectively. The nanocomposite could simultaneously monitor damage initiation and extension with the onset of crack growth by showing abrupt increase in normalized resistance, and two different failure modes can be distinguished including abrupt and crack extensions. It was concluded that any abrupt jump in normalized resistance could be used as an indicator for damage initiation in the specimen.