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Chapter 3 â Raman Spectroscopy of Graphene and Graphene-based Strain Sensors ðð = (ðð ð) Equation 3.4 ðð Where, ðð is the tensile strength of the fibre, ð is the diameter of the fibre and ðð is the interfacial shear strength between matrix and fibre. The critical length of graphene to achieve effective reinforcement as calculated by Shear-lay model is > 3 Ξm (more preferably > 30 Ξm). It can be explained from Figure 3.9g 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 leading 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. One way to enhance the stress transfer is by increasing the interfacial adhesion by means of functionalising the graphene surface or the edges.4, 32 Doing so, reinforcement could be even achieved from smaller flakes (Equation 3.4) as a result of better interface. Jiang et al. carried out similar interfacial stress transfer studies on MC SLG on PET substrate by applying strains up to 7 % (stretching).288 They have found that in their system, the failure mechanisms occurs during straining and relaxing (unloading) by shear sliding and buckling, respectively. The calculated ISS by Shear-lag theory was 0.46-0.69 MPa and is comparable to Gong et al.32, again suggesting a weak interface. The critical tensile strain for interfacial sliding was found to be 0.3 %. The critical compressive strain for onset of buckling is around ~ -0.7 %,288 which is similar to the experimentally observed critical buckling strain by Tsoukleri et al.269 and Androulidakis et al.289 for sandwiched graphene systems. Recently Anagnostopoulos et al. studied the interfacial stress transfer in systems MC SLG deposited on SU-8/PMMA substrate.290 They have studied the local strain distribution across a graphene flake with 100 nm steps. Such small distances, compared to ~1 Ξm steps by Gong et al.32, have allowed them to investigate the stress transfer at the edge of the flake in more detail. They have reported that the observed maximum ISS (0.4 MPa) does not occur at the edge of the flake, rather ~2 Ξm from the edge, deviating the Shear-lag theory.290 However, they attribute this behaviour mainly to the presence of doping and residual compressive strains (from fabrication step) near the edges. 87PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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