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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3.2. SEM micrograph for failure analysis

The scanning electron microscope images show the warp side fracture of the base (CSU) composite in Fig. 6(a‒b). As depicted in Fig. 6, severe resin breakages in the matrix/filament boundaries were identified. Minor multiple filament bundle fractures were observed as a form of tensile and shear breakages (Fig. 6a). Kink form brittle multiple warp filament bundle breakages was obtained. After the multiple warp filaments were broken, they were skewed during mode-II loading. This led to the crack branching near to the blunt crack tip region (Fig. 6b). Thus, delamination was spread from interlayer to the angular layer-to-layer zones.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Gııc (NPC) Gııc (PC)

2 )

G IIC (kj/m

CSU

CS-CS

CS-TS

CSU-N

CS-CS-N

CS-TS-N

Base

Stitched

Nano

Stitched/Nano

Composite structures

Fig. 5. Mode-II values of carbon/epoxy composites for PC and NPC cases.

Fig. 6. (a) SEM views of fractured warp side structure (CSU); (b) fractured warp side structure near to the blunt crack tip (CSU).

The SEM images of mode‒II toughness of warp side fractured nanostitched (CS-CS-N) composite are depicted in Fig. 7(a‒b ). As depicted in Fig. 7, tensile filament failures were observed. Further, multiple filaments-matrix debonding around the fractured matrix was found. Angular severe matrix breakages near to the crack tip were identified (Fig.7a). Multiple resin fracture and some agglomerated MWCNTs in the matrix was found at the boundary of the filament to filament interface region in where several tensile filament bundle breakages and delaminated filaments were also obtained (Fig 7b). It was realized that nanostitching responded as a crack growth barrier neighbouring the stitching region and complete nanostitching breakages was not identified because of high modulus and high strength carbon and p-aramid fibres. 4. Conclusions The mode-II fracture toughness of the nanostitched and stitched composites exhibited several fold improvement considering to the control. Therefore, the nanostitch addition to the base enhanced its fracture toughness properties. Carbon stitch yarn was adequate for enhancing the fracture toughness considering to the para-aramid due to its fiber stiffness and robust filaments and resin adhesion properties. The fundamental mechanism of the mode-II behavior of the nanostitched composite was predominant the interlayer matrix failure as a form of shear hackles. The MWCNTs probably slow down the stress clustering in the