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Chapter 7 – Application of Graphene composites: Raman Strain Sensors the solution processing of as-supplied aggregated powder might not have separated the layers well-enough before the addition of epoxy into the mixture. This would result in thicker graphene which would account for and hence the observed reduced strain sensitivity. 2. Length of the flakes: It has been shown that the length of flakes plays a major role in achieving efficient reinforcement as well as shift rates and hence strain sensing ability.32, 124 The critical length of graphene to achieve effective reinforcement as calculated by Shear-lag theory is > 3 μm (more preferably > 30 μm).4, 32, 124 It can be explained from Figure 7.6 that when the flakes are larger, the strain in the flake matches the strain in the matrix and therefore resulting in good stress transfer. But if the flakes are smaller, the strain in the flakes cannot build up to the matrix strain and hence leasing to poor stress transfer. In other words, the edge effect of graphene dominates in such composites and the graphene effectively acts as a short reinforcement resulting in less or no band shift (strain sensitivity). Figure 7.6: Representation of flake strain with respect to flake size using Shear-lag theory As a result of solution processing (dispersing as-supplied/produced FLGs in acetone), the < 𝐿 > of FLG decreases (Table 7-2). Especially for FLG-5 and FLG-20, the size decreased considerably. It is not surprising to see no shift rates for the FLG-0.2, FLG- 0.5 and FLG-1/epoxy composites, as their average sizes are smaller than the critical length. The small shift rate of ~ -1.3 and -3.8 cm-1/% strain observed for FLG-0.5 and FLG- 1, respectively, at the same spot could be attributed to regions where relatively larger flakes might have dominated. It is surprising to see no shift rates for FLG-5 and FLG-20 despite the average length (as-supplied) is higher than the critical length of reinforcement. There are number of possible reasons why this might be the case: 205PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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