Feasibility of multi-walled carbon nanotube/epoxy thermoset-based strain sensors for sensing in structural applications

Embedded sensors are being explored to non-intrusively monitor the loading experienced by a part either during the manufacturing process or in subsequent service. With the advancement of additive manufacturing techniques, it may be possible to print sensors directly into a part fabricated from metals, polymers, or composites. There is even more benefit if the sensor is also an integral part of the assembly. Single-walled carbon nanotubes and multi-walled carbon nanotubes are used to improve the stiffness and strength of polymeric resins. But they also offer an additional benefit of reversible piezo-resistive behavior. The objective of this study was to explore the fidelity of a signal extracted from nanocomposite samples of multi-walled carbon nanotube/epoxy resin under uniaxial loading, as a function of increasing content of multi-walled carbon nanotube. Data obtained from the multi-walled carbon nanotube networks formed correlated with the strain as a function of the mechanical loading. The gauge factors calculated for the multi-walled carbon nanotube/epoxy samples ranged from 2.651 to 5.064 as compared to the metallic strain gauge factor of 2.125 used to calibrate the samples. Under repeated mechanical loading, the initial zero-strain condition was recovered within 5 min after unloading. In samples with 5 wt% of multi-walled carbon nanotubes, the electrical conductivity and gauge factor plateaued. Furthermore, increases in the w/w% of multi-walled carbon nanotube resulted in degradation of the mechanical properties of the multi-walled carbon nanotube/epoxy resin.