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Chapter 7 – Application of Graphene composites: Raman Strain Sensors Precision Group.328, 330 In these techniques, photoelastic sensitive coatings based on PS/epoxy resin are applied over complex structures and a polariscope measures the strain-induced birefringence to form a full-field strain map with resolutions from 10 to 20 μ𝜀 (see manufacturer‘s data sheet in reference328, 330). Bragg diffraction gratings are another option but require a complex fibre optical lay-up to achieve sufficient point resolution over a large area.435 A promising rival to these optical systems is the development of Raman active coatings, where a strain active material is coated onto a device and the strain is read using a hand-held Raman spectrometer. This technique relies on the sensitivity of the Raman bands to the bond strain; tension causes frequency downshift (phonon softening) while the compression causes frequency upshift (phonon hardening). The development of the Raman strain sensors has been previously limited due to the lack of hand-held spectrometers and the development of suitable active coatings. Raman systems, however, are now available based upon solid-state lasers for under £10k. Earlier studies on Raman active coatings focused upon using carbon nanotubes. Wagner et al. used SWNT as Raman sensors in glass fibre polymer composites to measure the stress concentrations around defects and demonstrated that the nanotubes could be used to produce a two-dimensional mapping of the strain distribution in the composite.436, 437 Sureeyatanapas et al. demonstrated that SWNTs could be incorporated into the sizing of glass fibres to give a strain sensitive coating.437 It was found that functionalized SWNT showed higher strain sensitivity due to a better interface with the sizing. Li et al. demonstrated the use of nanotube as macro-scale strain sensors in Bucky papers by both Raman and electrical measurements.438 Following this work, Vega et al. developed SWNT-epoxy coatings as strain sensors by simultaneously following Raman spectroscopic and electrical impedance measurements.439 The impedance and Raman response of these coatings followed each other in single-cycle deformation experiments. Whereas, when subjected to cyclic deformation, the Raman method gave information about the interfacial damage.439 It has been suggested by our group4, 32 and others,269, 272 that graphene would be the ideal Raman active coating and superior to CNTs due to 196PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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