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Chapter 7 – Application of Graphene composites: Raman Strain Sensors 7.4. SUMMARY Information on strain over a large structure (e.g. bridge) is crucial both during its design and in service. In addition to high accuracy, single-point, deformation measurements, there is a need to be able to measure local strain at multiple points over a structure. Whilst this can be achieved with electronically-based sensors (e.g. resistance strain gauges made from Cu-Ni alloy), every point of interest needs to be individually wired, leading to significant amount of infrastructure. Thus it is preferable to measure the strain using optical measurements, e.g. by using Raman spectroscopy, especially on carbon-based composites as their Raman bands are strain sensitive. Graphene is an ideal candidate for Raman active coatings because: 1) Its 2d nature suitable for coatings (each atom in graphene is sensible for change in its environment, whereas CNTs are 1d and has less surface area), 2) Higher inherent band shift per unit strain of the Raman 2D band, and 3) The highest intensity of graphene’s most strain sensitive band (2D band). FLG/epoxy composite coatings were initially investigated as a Raman active strain sensor by following the changes in the 2D band with applied strain. Little or no strain sensitivity was observed for coatings made from flakes having a length shorter than the critical length of graphene (i.e., < 3 μm). Whereas for flakes longer than the critical length, several factors such as the processing history, loading of graphene, defects in the graphene and alignment of flakes within the composites could influence the shift rates of the band. Graphene coatings were also prepared from both MC (top-down approach) and CVD methods (bottom-up approach) and were successfully demonstrated as Raman- based strain sensor. The CVD based coatings were found to have residual compression stresses which relaxed over the first 2 to 3 deformation cycles. These results suggest that sensors made from transferred graphene should be cycled by an external strain as a conditioning step during manufacture to ensure stable and accurate readings while in use. Moreover, a hysteresis loop (interfacial damage) and its upshift were observed in MC graphene coatings in the absence of residual strains. These MC graphene samples, also exhibited a strain hardening behaviour with a higher shift rates and thus a more effective modulus than CVD graphene. In general, the shift rates of MC graphene are at least ~ 35 232PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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