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Chapter 3 – Raman Spectroscopy of Graphene and Graphene-based Strain Sensors Most commonly used resistance-based strain gauges are the metallic foil-based strain gauge. These foil strain gauges are produced normally from a copper-nickel alloy which are photo-etched to a desired pattern, and are encapsulated within a flexible polymer substrate (e.g., polyimide) which acts as a carrier (Figure 3.15a). The gauge is bonded to the specimen using an adhesive, usually a thin layer of epoxy resin or cyanoacrylate adhesive, hence also called bonded resistance strain gauges. Figure 3.15: Different types of foil-based bonded resistance strain gauges. a) Common strain gauge showing the grid encapsulated within polymer matrix. Basic rosette types classified by grid orientation: b) tee; c) 45o- rectangular; d) 60o-delta.323 The variations in the electrical resistance of the grid are measured as an indication of strain. The overall resistance of this type ranges from 30 to as much as 5000 Ω, but gauges with 120-350 Ω are normally used. Higher resistance gauges are mainly used for transducer applications and on composite materials. These strain gauges can be used to measure both static and dynamic strains. When the principal direction of strain/deformation/stress is known, the strain gauge can be fixed in specific direction. When the principal direction of strain is unknown or in case of biaxial strains, rosette type of strain gauges as shown in Figure 3.15b,c,d are normally used.323, 324 Foil strain gauges can be used to measure strains up to 5 % or 50000 microstrain (μ𝜀). By changing the grid metal (Constantan P alloy), strains as high as 20 % can be measured, but high cyclic strains at this strain levels tend to induce permanent resistance changes.323 The GF of the commercially available sensors are in the range of 2 to 5. Moreover these sensors are only moderately affected by the temperature changes, and have service temperatures - 269 oC to 230 oC.323, 324 Other advantages of these sensors are: Uniform characteristics within different range of gauges Small size, light weight and durable Inexpensive 97PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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