Viscoelastic and thermo-mechanical piezoresistive behavior of reduced graphene oxide-coated glass fiber reinforced polymer-based sandwich structures under flexural loading

This study investigates the time-dependent and thermo-mechanical piezoresistive behavior of glass fiber-reinforced polymer (GFRP) coated with partially reduced graphene oxide (rGO) -based honeycomb sandwich composite structures subjected to flexural loading. Sandwich composites with unidirectional rGO-coated GFRP facesheets were tested under monotonic loading at various temperatures, as well as in stress relaxation and cyclic bending conditions. The electromechanical response was strongly influenced by fiber orientation and loading conditions. The isothermal tests on 0° Nomex^® core samples confirmed a negative temperature coefficient, with the thermal coefficient of resistance decreasing from −0.00862°C^-1 at 35°C to −0.00687°C^-1 at 45°C, and showed that elevated temperatures delayed the fractional change in resistance (FCR) and reduced the peak stress by ∼10.6% from 25°C to 35°C and by ∼27.6% from 25°C to 45°C, while preserving the sensing functionality. Stress relaxation tests conducted using both Nomex^® and aluminum cores, revealed orientation-dependent FCR trends i.e. the 0° configurations exhibited an early increase of ∼0.4% for the Al core and ∼1.15% for the Nomex^® core within the first 30 min before plateauing, while 90° configurations showed a continuous reduction in FCR, ∼−0.1% over the relaxation period, due to matrix relaxation. Cyclic loading of the Nomex^® core samples highlighted reversible resistance changes, with baseline drift of ∼10% in the 0° configuration compared to ∼0.18% in the 90° configurations confirming greater visco-piezoresistive effects in the latter. A visco-piezoresistive model, based on Burgers’ viscoelastic model, accurately captured the time-dependent FCR and stress variations under both relaxation and cyclic conditions. The findings of this study advance the understanding of the long-term piezoresistive response of multifunctional sandwich composite structures, supporting their application in structural health monitoring systems.