Issue 48

H.S. Bedi et alii, Frattura ed Integrità Strutturale, 48 (2019) 571-576; DOI: 10.3221/IGF-ESIS.48.55 574 However, at a particular loading rate, composites reinforced with CNT grafted carbon fiber show enhanced IFSS than the ones having just unsized fiber (Fig. 3c). Compared to HCF/epoxy composites, increments of 32% and 30% are observed for IFSS of CNTCF/epoxy composites at 0.05 and 5 mm/min crosshead speeds, respectively. Moreover, the elastic interaction between the reinforcing fiber and matrix governs the strain rate dependence [1]. Thus, changes in IFSS are expected when the fiber/matrix interfacial interaction is modified by incorporating grafted CNTs in the composite system. One of the reasons behind this observation is the increase in fiber surface area [16] due to CNT grafting, which improves the wettability of CNT grafted carbon fiber with epoxy matrix [17] and hence the mechanical properties of the hybrid composite [14]. Therefore, CNT grafting proves to be effective in improving the shear strength of CFRPs at all loading rates considered in this study and facilitates their usage under situations of low and/or high strain rates. One more advantage of using grafted carbon nanotubes is the resistance that they offer to brittle failure in CFRP composites. The outwardly projecting grafted CNTs mechanically interlock with the surrounding matrix and offer frictional sliding after fiber gets debonded from the matrix [13], consequently preventing catastrophic failure either by avoiding or delaying the crack propagation. Figure 3 : Maximum debond force ( F max ) versus embedded length ( l e ) for epoxy composite reinforced with: (a) unsized carbon fiber (HCF) and (b) CNT grafted carbon fiber (CNTCF). (c) Interfacial shear strength (IFSS) of various composites at different load speeds. Figure 4 : SEM micrographs of single fiber filament after debond test: (a) unsized and (b) CNT grafted carbon fiber debonded from epoxy droplet. Inset shows the optical image (Dino-Lite, Taiwan) of drop on fiber before testing. In order to get some idea about the observed differences in IFSS of CFRPs reinforced with or without CNTs, SEM