PSI - Issue 14

Harpreet Singh Bedi et al. / Procedia Structural Integrity 14 (2019) 168–175 Harpreet S. Bedi, Prabhat K. Agnihotri/ Structural Integrity Procedia 00 (2018) 000–000

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3.3 Interphase Size and Stiffness The findings of wettability analysis show that CNT grafting has the potential to enhance the mechanical properties of CFRP composites. In order to verify this, nanoindentation tests are carried out on composites processed with HCF, CNTCF-15 and CNTCF-30 filaments. This helps us to quantify the effect of CNT grafting on the size and stiffness of interphase in hybrid CFRP composites. Figure 5a compares the representative nano-indentation response of HCF/epoxy and CNTCF/epoxy composites. As expected, the P- δ curves of both the composites overlap when indent is carried out at the fiber location (Fig. 5a). However, the lower peak–load displacement at interphase region of CNTCF/epoxy as compared to HCF/epoxy composite shows the improvement in interphase stiffness after incorporating CNTs in the composite. This reveals the existence of a region surrounding the fiber that has properties different from that of fiber and matrix (Gao and Mäder, 2002). Thus, by incorporating CNTs on the fiber surface we can easily tailor the properties of the interphase and hence that of the whole composite.

1 Figure 5. Nanoindentation response of HCF/epoxy and CNTCF/epoxy composites. (a) Representative load P vs. displacement δ curves for the nanoindents done at the location of fiber, interphase and matrix. (b) Variation of elastic modulus E across the interphase region for different composites. Corresponding to each of the indents carried out along a straight line (as shown in Fig. 2), elastic modulus E is extracted from the slope of the unloading part of P- δ curves. The variation of E across the interphase region starting from fiber to the matrix can be seen in Fig. 5b. The modulus of HCF/epoxy composite drops suddenly from its peak value at fiber (~56 GPa) to its lowest at matrix (~3 GPa). Whereas, a gradual reduction in modulus is observed in CNTCF/epoxy composites (Hernández-Pérez and Avilés, 2010). This corresponds to the existence of an interphase region in CNTCF/epoxy composites as compared to their non-CNT counterparts. The steady decrease in interphase properties prevents stress concentration around the CNT grafted fiber, facilitating efficient load transfer from matrix to the fiber (Sui et al., 2017). Stiffer nanotubes protruding outwards from fiber into the matrix make the interphase stiffer and that is why the P- δ curve for CNTCF/epoxy lies above that of HCF/epoxy composite (Fig. 5b). However, when the CNT growth time is doubled from 15 to 30 min, the length and density of CNTs increases to the extent that they get entangled with each other and prevent matrix infiltration in the CNT network (Bedi et al., 2018; Lee et al., 1999; Thostenson et al., 2001). This reduces the interphase stiffness in composites reinforced with CNTCF-30 fibers (Agnihotri et al., 2011) as compared to CNTCF-15 fibers (see Fig. 5b), which is why the E vs. x curve for CNTCF-30 lies below that of CNTCF-15. Still, the curve for CNTCF-30 is above that of HCF showing stiffness imparted to the interphase by the grafted nanotubes. In addition to this, the plots in Fig. 5b can be used to evaluate the size of the interphase region. The range of x in which E varies from fiber modulus ( E fiber ) to matrix modulus ( E matrix ) provides a direct measure of the thickness of interphase. Based on this, the interphase size is found to be 2 µ m for HCF/epoxy composites, which increases by 150% to 5 µ m for CNTCF-15/epoxy composite.

1 Reprinted from “Carbon, 132, Bedi, H. S., Tiwari, M., Agnihotri, P. K., Quantitative determination of size and properties of interphase in carbon nanotube based multiscale composites, 181-190, 2018”, with permission from Elsevier.