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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Fig. 3. Mode-II load(P)‒displacement(δ) curves (30 cm). (a) NPC case; (b) PC case (Bilisik et al (2019 a).

Fig. 4(a‒b) exhibited compliances (mm/N) versus the cube root of crack length (mm 3 ) for all satin 1/4 interlaced composites for non-precrack and precrack cases. All satin interlaced composites for precrack condition were lightly steeper considering to the non-precrack. Their data were perfectly fitted regression line as they were implied by their R‒squared values (0.9998‒ 0.9963). After the compliance calibration coefficients (A and m) were obtained, they were considered to find the G IIC toughness values of all the composites.

Fig. 4. Compliances versus crack length (30 mm) curves for all composites. (a) PC case; (b) NPC case.

Fig. 5 presents the average G IIC outcome in all composites for NPC and PC conditions. In PC case, the G IIC of the carbon and para-aramid nanostitched structures (CS-CS-N and CS-TS-N) were better compared to the base (CSU) due to addition of the nanostitched filament TOW and MWCNTs. Further, the G IIC of the carbon and para-aramid stitched structures (CS-CS and CS-TS) were lightly enhanced considering the base (CSU) due to addition of the stitched filament TOW. The carbon stitched fiber was more effective compared to the Twaron ® filament TOW due to stiffness properties of the PAN fiber and fiber-matrix adhesion in interface regions, in particular interlacement zone for each predetermined unit cell. It was analyzed that nanostitching and stitching enhanced the interlayer toughness of all the base and nano composites. Nevertheless, the mode-II values of nano (CSU-N) and base (CSU) composites were close to each other due to local MWCNTs agglomeration in the resin or filament TOW in the axisymmetric fabric surface. On the other hand, the mode‒II of PC for all composites were higher compared to the NPC cases. This was because of the nonlinearities as a form of heterogonous resin distribution in the starting of the crack region. The principle mode-II toughness behavior of the nanostitched composite was the interlayer resin failure (almost symmetric). The failure type was predominantly shear hackles around the cohesive matrix zone and filamentary region. The crack propagated between warp layers as a form of matrix layer fracture and it moved through the cross‒section of the filling filament TOW. The MWCNTs probably slow down the crack propagation and they suppressed the stress clustering around the fractured matrix. Concurrently, nanostitching suppressed the layer separation in stitching district during the through-the-thickness crack growth. The nanostiched filament bundle failure was not obtained. This was probably because of nonlinear interlaminar shear transfer during stress distribution. Thus, this was caused to severe matrix breakages. In the crack tip, carbon nanotubes in the fiber and matrix diminished the stress clustering as a form of filament-matrix debonding; filament or nanotubes pull-out; filament to filament or filament-nanotube stick-slip or sliding action and friction.