This experimental study investigates the strain and damage sensing abilities of randomly dispersed graphene nanoplatelets (GNPs) embedded in an epoxy matrix for use in health monitoring of structural composites. Three weight concentrations of GNPs (0.1%, 0.5% and 1% by weight relative to the weight of the epoxy matrix) are considered in this study and compared with the baseline specimens of neat epoxy. The GNPs are dispersed in the epoxy matrix via ultra-sonication by mixing them in a low viscosity solvent (Acteone) and then mixing the epoxy in that mixture for another round of sonication. The solvent is extracted and the mixture is poured into hot silicone molds. A gradually rising high temperature curing cycle is used for curing: 80-120°C in 30 minutes, held at 120°C for 2 hours, 120-176°C in 30 minutes, held at 176°C for 2 hours and finally the specimens are allowed to cool until room temperature for 16 hours. Five tensile dog-bone specimens of each material type are prepared according to the ASTM standards. The mechanical properties of these materials are characterized via displacement controlled quasi-static uniaxial tensile tests using slow displacement rates. Prior works have seen enhancement in mechanical strength due to addition of GNPs to their polymer which is beneficial to structural composites. Studies have also shown a marked increase in the fracture toughness of structural epoxies with addition of GNPs. In conjunction with the mechanical testing, the electrical response of the specimen is monitored via an LCR meter which measures the resistance and reactance of the specimen.  Prior works have demonstrated that the high conductivity of GNPs imparts electrical conductivity to the whole epoxy matrix if the weight concentration is at or above percolation. This allows the nanocomposite to be more conductive that the neat epoxy. Disturbances in the GNP networks due to mechanical strain or damage causes disruption of conductive pathways formed earlier resulting in a permanent change of electrical properties. The change in resistance and reactance of the specimen is measured and correlated to applied strain. This correlation is quantified using gauge factors (GFs) for all the material types. Furthermore, Digital Image Correlation (DIC) is performed on all material types to identify the instances of micro-scale strain concentrations and damage which are detected by the GNP sensing networks due to their nanoscale length. This micro-scale damage ultimately leads to macro-scale failure and hence the ability of the GNPs to sense these is accurately assessed. The GNP-Epoxy nanocomposites are multifunctional materials as the GNPs cause strength enhancements as well as act as piezoresistive sensors for the nanocomposite. The GNPs embedded in the binder act as self-sensing elements within the structural composite and need no additional sensors. Such in-situ real-time structural health monitoring system find plentiful applications in the field of structural composites, vehicle shells, aircraft skins, and much more.


Piezoresistive Sensing Abilities of Randomly Dispersed Graphene Nanoplatelet Networks Embedded in an Epoxy Matrix

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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