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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Further increase in the length and density of grown CNTs in the case of CNTCF-30/epoxy composites increases the interphase thickness to 8 µ m. It is now clear that CNT grafting increases the size and stiffness of the interphase in CFRP composites. Still, the effectiveness of CNT grafting in improving the average mechanical properties of CFRPs is determined based on the extent to which load is transferred from matrix to fiber. To this end, single fiber micro droplet debond test is performed with both HCF/epoxy and CNTCF/epoxy composites to evaluate the effect of CNT grafting on the interfacial shear strength of CFRP composites. 3.4 Interfacial shear strength (IFSS) The load or stress transfer from matrix to fiber critically depends on the strength of the fiber/matrix interface. Interfacial strength in turn is governed by the wettability between the fiber and matrix. As we have already seen positive effects of CNT grafting on wettability, micro-bond test is performed here to assess the effect of CNT grafting on the interfacial shear strength (IFSS) in CFRPs. Because of poor handling and brittleness, CNTCF-30 fibers are again not included in this analysis. Figure 6 shows the maximum debond force F max as a function of varying embedded length l e of polymer micro-droplets for each fiber/matrix composite. The results of debond test are fitted well with linear interpolation, indicating direct proportionality between the peak load and embedded length of the micro-droplet. Therefore, IFSS of each composite is evaluated from the slope of linear fits shown in Fig. 6.

Figure 6. Maximum debond force ( F max ) versus embedded length ( l e ) for epoxy composites reinforced with: (a) unsized (HCF) and (b) CNT grafted carbon fiber (CNTCF-15). (c) Interfacial shear strength (IFSS) of each composite.

Figure 7. SEM micrographs of single fiber filaments after debond test: (a) unsized (HCF) and (b) CNT grafted carbon fiber (CNTCF-15) debonded from epoxy micro-droplet. Inset shows corresponding optical image of drop on fiber before testing.

As compared to the IFSS of HCF/epoxy composite (31.7±1.8 MPa), the IFSS of CNTCF-15/epoxy composite is 49±2 MPa, indicating a nearly 55% improvement. This increase in IFSS is because of good wettability of HCF with epoxy matrix (Bedi et al., 2016) which is further enhanced due to increased fiber surface area in the presence of grafted nanotubes on CF surface. Hence, better wettability and stronger interfacial shear strength is obtained in CNT based hybrid CFRP composites processed with epoxy matrix. The surface of debonded fiber is now analyzed using SEM (Fig. 7), throwing some light on the observed phenomena of IFSS for different fiber/polymer composites. The presence