PSI - Issue 35

Kadir Bilisik et al. / Procedia Structural Integrity 35 (2022) 210–218 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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The shear properties were not enhanced by the inclusion of multiwall carbon nanotubes in the aramid/phenolic composite structures. The strength of aramid nanocomposite (TBU-N) was slightly decreased (9%) compared to the TBU. However, nanostitched composites (TB-TS-N) were hardly greater (3.33%) than the TB-TS. It was found that the addition of the MWCNTs in TBU insignificantly affected to the composite shear properties. Moreover, delamination resistance of nanostitched aramid structure showed extraordinary improvement compared to the pristine because of skewed shear fastening phenomena generated residual effect on the intralayer matrix in which through-the-thickness reinforcement prevented layer-to-layer separations during matrix degradation. Also, MWCNTs/phenolic aramid composites exhibited poor resistance to delamination because of absence of binder fiber reinforcement and weak bonding between matrix/fiber and nanotubes. Yet, nanostitch composite (TB-TS-N) shear strain was 18.07% greater compared to the stitched composite (TB TS), whereas it slightly (6.60%) higher than TBU composite. On the contrary, aramid nanocomposite (TBU-N) was 42.88% lower compared to the base (TBU) composite. This indicated that nanostitching generally increased the shear strain of the aramid composites probably due to through-the-thickness fiber reinforcement hold the layers before catastrophic failure occurred. The nanostitched (TB-TS-N) and nano (TBU-N) composite shear modulus increased (14.4%) and (15.39%) compared to the base (TBU), respectively. It was realized that nanostitching slightly improved the aramid composite shear modulus. 3.2. Fracture toughness (Mode-II type) results The G IIC (toughness) and P (maximum load) results for all aramid structures are exhibited in Figure 5(a-b). As illustrated in Figure 5, the nanostitch yarn and stitch yarn improved the fracture load of all nanostitch and stitch aramid structures due to out-of-plane fiber replacement. It was found that the fracture loads of the nanostitch structures were marginally higher compared to the the nano and pristine structures. The nanostitch (TB-TS-N) composite G IIC was 12.50% higher than TBU-N, and it was 20.73% greater compared to the TBU. However, the stitch composite (TB-TS) was 7.32% higher than those of the TBU-N and TBU. On the contrary, the TBU-N was similar to the pristine composite. Generally, nanostitching enhanced the toughness of aramid structure. But, no significant difference was identified between TBU and TBU-N. Mode-II behavior of aramid/phenolic composite structure was increased because of binder placement at the decrement of its shear properties. Nevertheless, interlaminar and intra-fiber openings of nanostitch aramid composites were restricted. This was because of the nanobinder introduction in that individual nanotubes were uploaded by means of filamentary bundles. Furthermore, the toughness mechanism in the nanostitch composite structure was a symmetric matrix layers fracture. Shear hackle marks on the coherent resin region of the fiber assemblage was appeared. Crack growth was propagated in the vicinity of the intra- and inter-warp-filling TOWs border in that the matrix was fractured around each interlacement of the individual filling or warps diameters.

Fig. 5. (a) Mode‒II maximum average loads and (b) fracture toughness of all developed nanocomposite structures.