PSI - Issue 35

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

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3.3. Failure morphology by SEM micrograph 3.3.1. In-plane shear

SEM micrograph of the shear fracture of nanostitch aramid composite (TB-TS-N) are exhibited in Figure 6 (a-b). It was recognized that the stitching yarn in TB-TS and nanostitching yarn in TB-TS-N structures under load were not failed. In the inside failed surface, resin-fiber failure circumference of the stitching region was found in that skewed deformations to the axial (warp) and filling were observed. In close view, some slender fibril stripping on the fiber surfaces near to stitch hole border and multiple fiber breakages were realized (Figure 6 b). This was probably due to the cohesive interactions in filament/phenolic resin as well as homogeneous resin and multiwall carbon nanotubes during shear stress transfer phenomena. Thus, resin-nanotube bonding in the fiber TOWs was not strong perhaps because of the poor boundary interfacial adhesion between the filament assemblage and matrix. Moreover, multiple branched crack path pattern was observed in the TB-TS-N nanocomposite cross-section (Figure 6 a).

Fig. 6. (a) Failed Twaron stitched nano composite cross-section (TB-TS-N) and (b) enlarge view of fractured fiber surfaces near to stitching hole (SEM images).

3.3.2. Fracture toughness (Mode-II type) Figure 7 (a-b) exhibits the micrograph of the fractured of the TB-TS-N structure. As shown in optical image and SEM micrograph in Figure 7, multiple phenolic resin failure and minor bunch of filament fracture on the TB-TS-N surface were obtained. Numerous intralayer filling to filling separation and interlaminar criss-cross crack propagation was detected along axial (warp) of the TB-TS-N structure (Figure 7 a). In a close-up view, micro matrix fracture around the lenticular filling cross-section and crack growth path around the filling to filling interlacement region as a blunt crack tip formation at TB-TS-N structure were found (Figure 7 b). We also obtained that delamination was restricted around the stitching region because of nanostitching. Catastrophic nanostitch yarn failures were not observed due to strong p-aramid filament TOWs strength. On the contrary, multiple matrix fractures and matrix/filament debonding in the TBU-N surface were identified. Considerable interlaminar crack growth in the pristine (TBU) structure was found, whereas even fiber surfaces in the TBU-N were pointed out. It was realized that resin/nano bonding, fiber/matrix cohesiveness as well as matrix/filament/nanotubes connections in the 3D preform structure were perhaps not strong because of incomplete interfacial cohesion between the filaments and matrix in that the filament TOWs had low surface energy. Resin fracture and various slender fiber strip off on the failed surface of the stitched composite (TB-TS) as well as weft side shear hackle marks on the TB-TS surface were identified. The stitching yarn probably delayed the layer splitting in the stitching zone during the crack proliferation. Interestingly, we did not observe any complete stitched yarn failure probably because of strong strength properties of para-aramid fiber.