PSI - Issue 21

Kadir Bilisik et al. / Procedia Structural Integrity 21 (2019) 146–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Davis and Whelan (2011). Amino treated nanofiber mat and plain nanofiber mat also improved the mode‒II toughness and mechanical properties of nanocomposites, respectively Beckerman and Pickering (2015). The fiber shear in the resin and resin tensile fracture were dominant parameters for the mode‒II failure of composite Russel (1987). CNT dispersion to carbon/epoxy nanocomposite via spraying and film transferred technique illustrated that nanotubes significantly increased the fiber bridging to improve the fracture toughness Joshi and Dikshit (2012). Graphene nanoplatelets (GnPs) and the MWCNTs additions in carbon/epoxy composites enhanced its interlayer shear and fracture toughness properties because of strong bonding at nano materials/resin/filaments Srivastava et al. (2017). A model on fracture toughness was introduced based on bridging and strain energy during delamination crack growth Sun and Jin (2006). The mode ‒ II failure was quantitatively modeled. Lee found that resin properties dominated microcrack shear initiation stress and coalescence process. Resin yielding and plastic deformations were found as important parameters to control the mode ‒ II toughness Lee (1997). The fracture process during mode‒II loading was well defined by cohesive zone model especially for thermoplastic composites Reis et al. (2019). It was also reported that 3D orthogonal architecture provided a higher strength, whereas 3D angle interlock provided more energy absorption and prevented mode-II crack propagation probably due to fiber interlacement patterns Pankow et al. (2011). One of another study showed that crack growth of G II exhibited a low and unstable propagation (ductile and slow) for initial state and it showed a high and stable propagation (brittle and fast) for latter state when the infra red thermography technique was employed Perez et al. (2019). The interlacement pattern influenced the crack propagation in woven composite in particular, instable crack growth was obtained in mode‒II fractured composite Blake et al. (2012). The nanoparticle property transfer was studied by using the energy and mechanic models Batra and Sears (2007). By applying the Cottrell – Kelly – Tyson (CKT) model, nanocomposite fracture toughness was determined considering the nanotube specifications and pull-out energy Wagner et al. (2013). The restraint of stress distribution of the nanotubes in the composite was defined by using the two-scale model Romanov et al. (2015).

2. Materials and methods 2.1. Nanostitched carbon/epoxy composites

Carbon substrates (Polyacrylonitrile (PAN), Spinteks A.S., TR) were utilized to make stitched prepreg preform. The carbon and para‒aramid stitching yarns , epoxy (Araldite LZ 5021, Biesterfeld GmbH, DE), multiwall carbon nanotubes (MWCNTs) and carbon fabric were used to design the preform composites. Two dimensional (2D) carbon fabric was weaved from 12K filaments. The substrate was low order interlaced satin (1/4) patterns. The multiwall carbon nanotubes (MWCNTs, Nanothinx, GR) were considered and its specification of MWCNTs is given in Table 1. The mode II toughness test samples were manufactured as four kind preform structures. First one was base (CSU) preform which was four layer carbon fabrics. Second type was stitched (CS-CS and CS-TS) preforms. The CS-CS was four layer satin fabrics, and stitched with carbon fiber. However, the CS-TS was four layers satin fabrics, and stitched with Twaron CT yarn. Third type was base nano (CSU-N) preform. The CSU-N was four layer satin fabrics with added multiwall carbon nanotubes.

Fig. 1. Carbon/epoxy carbon nanostitched composites, actual and schematic, respectively.