Issue 48

H.S. Bedi et alii, Frattura ed Integrità Strutturale, 48 (2019) 571-576; DOI: 10.3221/IGF-ESIS.48.55 572 imperative for CFRPs so as to avoid any chances of premature and unexpected failure [1]. In order to sustain the structural integrity of a composite member at varying load conditions, the fiber/matrix bond strength is an important parameter which if optimized improves the mechanical performance of the composite [2]. The average mechanical properties of CFRP laminated composites largely depend on the direct interaction and extent of bonding between the individual reinforcing fiber and the surrounding matrix [3]. Stronger the interfacial interaction better the composite properties. Interfacial bond strength can be tailored in many ways such as by texturing the fiber surface [4], modifying the matrix [5], functionalization of fiber surface, type of fiber, modification of fiber sizing [2] etc. The exceptional properties of Carbon Nanotubes (CNTs) [6] have made them a superior candidate in improving the mechanical properties of CFRPs. The potential of CNTs is harnessed in CFRPs in two ways: by preparing polymer nanocomposites [7] by dispersing CNTs in polymer matrix and by growing CNTs directly on the surface of Carbon Fiber (CF) [8]. In both the cases, bonding between fiber and matrix is affected by the presence of CNTs [3] at the interface. Interfacial strength primarily depends on the area of contact available on the fiber surface to interact with the matrix [9] which can be increased by synthesizing CNTs on the surface of fiber [10, 11]. Additionally, grafted nanotubes improve the wettability of fiber with matrix by increasing the surface energy of unsized carbon fiber [10] ultimately enhancing the ability of the fiber to attract the matrix [14] and spread it more efficiently. This leads to an improvement in stress transfer between fiber and matrix through an enhanced interfacial area, mechanical interlocking and localized stiffening of the matrix [8]. A direct measure of the interfacial bond strength can be estimated from Interfacial Shear Strength (IFSS) at the fiber/matrix interface with the help of single fiber micro-droplet debond test or simply microbond test [12]. Accordingly, debond tests are performed on epoxy matrix composites reinforced with single unsized/Heated Carbon Fiber (HCF) and CNT grafted Carbon Fiber (CNTCF) and hence IFSS is evaluated from the load-displacement data so obtained. Furthermore, the effect of load rate variation on the IFSS of both the composites is also evaluated. Scanning electron micrographs of debonded HCF and CNTCF help us to better understand interfacial failure mechanism in unsized and hybrid composites. E XPERIMENTAL Preparation of CNT grafted carbon fiber he as-received carbon fabric (Hindoostan Technical Fabrics, India) is firstly heated at 450˚C to make it free from the sizing layer applied on it. A bunch of heat treated sizing free or unsized carbon fibers (HCF) is then separated from the heated fabric and is coated with nickel catalyst for synthesizing CNTs on fiber surface using dip coating method. The details of preparation of catalyst solution and the dip coating process are described in Ref. [13]. Once the unsized fibers are coated with catalyst, they are subjected to the reactive environment of argon, hydrogen and acetylene gases inside a Chemical Vapour Deposition (CVD) reaction chamber (Technos Instruments, India) as per the procedure detailed in Ref. [14]. After the successful completion of a CVD cycle, the catalyst coated CFs gets transformed into CNT coated CFs. Surface morphology of as-received (ar-CF), unsized (HCF) and CNT grafted carbon fiber (CNTCF) can be seen in Fig. 1. Where, as-received fibers appear smooth (Fig. 1a) because of the sizing layer on their surface, the rough surface of unsized fiber helps in retaining the catalyst particles in a better way (Fig. 1b). It is clear from Fig. 1c that the synthesized CNTs uniformly cover the fiber surface. Composites are then prepared with both unsized and CNT grafted CF using epoxy as matrix as described in the next section. Figure 1 : SEM micrograph of: (a) as-received, (b) unsized and (c) CNT grafted carbon fiber. T