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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1. Introduction The interaction and extent of bonding between fiber and polymer decides the overall performance of the fiber reinforced polymer composites (Qian et al., 2008). Stronger the interfacial interaction at the fiber/matrix interface better the composite properties. The strength of interaction primarily depends on the contact area available at fiber/matrix interface and the ability of the matrix to wet the fibers (Li et al., 2013). Therefore, a lot of efforts have been made to enhance the interfacial area on fiber surface as it provides us a handle to tailor the average properties of the composite. With an aim to increase the bond strength at the fiber/matrix interface, researchers have integrated various types of nanoparticles on the fiber surface (Tang et al., 2013). One such way to improve the interfacial characteristics in fiber/epoxy composites is to grow carbon nanotubes (CNTs) directly on the surface of carbon fiber (Li et al., 2016). The improvement in the properties of fiber/polymer composites due to CNT grafting is attributed to the formation of an interphase region around the fiber in the presence of grafted nanotubes (Bedi et al., 2018). The interphase region in CNT based hybrid composites helps in efficient load transfer between fiber and matrix through an enhanced interfacial area, mechanical interlocking and localized stiffening of the fiber/matrix interface (Qian, Greenhalgh, et al., 2010). Accordingly, it becomes necessary to thoroughly investigate the effect of CNT grafting on the size and mechanical properties of the interphase in carbon fiber reinforced polymer composites (CFRPs). To this end, various micro-scale experiments are conducted on single carbon fiber reinforced epoxy composites with and without grafted CNTs. CNTs are grown on the fiber surface using chemical vapour deposition (CVD) method. In the beginning, contact angle analysis is carried out to evaluate the effect of grafted CNTs on the wettability of carbon fiber with epoxy polymer. Afterwards, the size and stiffness properties of the interphase are quantified based on nanoindentation tests. CNT growth time is varied from 15 to 30 minutes inside the CVD reaction tube to see the scaling of interphase size and stiffness with varying length and density of the synthesized CNTs. This is followed by calculating the interfacial shear strength between the fiber and the matrix using micro-bond testing. Finally, scanning electron microscopy is used to illustrate the different failure mechanisms in CFRP composites reinforced with and The as-received carbon fabric (Hindoostan Technical Fabrics, India) is first heated at 450˚C to make it free from the sizing layer applied on it. A bunch of 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. The details of preparation of catalyst solution and the dip coating process are described by Agnihotri et al. (2011). Now, the catalyst coated CFs are subjected to the reactive environment of argon, hydrogen and acetylene gases inside a CVD reaction chamber (Technos Instruments, India) as per the procedure detailed by Bedi et al. (2016). In different CVD runs, acetylene is flown for 15 and 30 min. so as to vary the length and density of grown CNTs. As a result, two types of CNT grafted carbon fibers are prepared having CNTs grown for 15 min (15CNTCF) and 30 min (30CNTCF). After CNT growth, the reactor is cooled down in the presence of argon and CNT grafted fibers are taken out of the reactor tube. Single fiber composites are prepared with both HCF and CNTCF by embedding them in the matrix mixture of epoxy/hardener (1/2 by weight). Curing of composites at room conditions for 24 hours is followed by post-curing at 55˚C for 4 hours. In order to get flat composites with fine surface finish, post-cured composites are polished using emery papers and diamond polishing. This helps in accurate characterization of the micro-scale features of the interphase region. 2.2 Wettability analysis The wettability of unsized carbon fiber (HCF) and CNT grafted carbon fiber (CNTCF) is investigated with epoxy polymer using drop-on-fiber method. Both the ends of a single CF filament (unsized or CNT grafted) are first attached to a stand with the help of a quick setting adhesive (see Fig. 1a). As soon as a polymer drop comes in contact with a fiber filament, it disintegrates into a number of smaller micro-droplets as shown in Fig. 1a. The shape as well as the contact angle of these micro-droplets with HCF and CNTCF filaments is assessed using First Ten Angstroms without grafted nanotubes. 2. Materials and methods 2.1 Synthesizing carbon nanotubes on fiber surface and preparing composites