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Chapter 7 – Application of Graphene composites: Raman Strain Sensors 7.3.2. Cyclic deformation behaviour The 2D band of graphene was used to follow the cyclic deformation of the coatings. Upon axial strain, 2D band splitting was not observed for graphene (both MC and CVD) as reported in literature.271, 285, 445 But broadening of the 2D band was observed as shown in Figure 7.11a. The accuracy of the fit of the 2D band using the Lorentzian function is shown in Figure 7.11b. Figure 7.11: a) Change in the shape and position of Raman 2D band of MC SLG graphene during and after deformation, b) Accuracy of the curve fit by using the Lorentzian function 7.3.2.1. Sequence I: Uniform maximum strain The coatings were subjected to cyclic deformation using a four-point bending rig with the maximum strain of 0.3 % at each cycle. Figure 7.12a,d show the deformation sequence applied and the response of the 2D band position. The Raman 2D band shifts significantly with the applied strain, indicating that stress transfer occurred in the coatings (Figure 7.12b,e). Elastic deformation occurred in the MC graphene coatings, with the loading and unloading curves for the first cycle overlaying each other. The shift rate of -53 cm-1/% strain (Figure 7.12c) was obtained for both loading and unloading curve is similar to the previous studies.269, 274 As the sample undergoes subsequent deformation cycles, the linearity between the loading and unloading curve is lost, giving rise to a hysteresis loop. This behaviour can be related to the coatings undergoing damage at the graphene-matrix interface. Cyclic deformation of bulk CNT composites revealed similar behaviour due to damage at nanotube-matrix interface.446 The hysteresis loop increases in area and shifts upwards with the deformation cycle (Figure 7.13). This behaviour suggests that the coating underwent irreversible damage at the graphene-matrix interface. Furthermore at the end of each cycle, the 2D band position at 0 % strain is at 212PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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