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Mode-II fracture of nanostitched para-aramid/phenolic nanoprepreg composites by end-notched flexure

The mode-II interlaminar fracture toughness considering the end-notched flexure method of nanostitched para-aramid/phenolic composite structures was investigated. The fracture toughness (GIIC) of the nanostitched and stitched composites exhibited a slight increase as compared to the pristine sample. Hence, the nanostitching enhanced the fracture toughness of the para-aramid/phenolic composite structures. Although the type of stitch fiber was not effective, the fabric interlacement frequency, notably prepreg Twaron nanostitched yarn and basket nanoprepreg biaxial interlaced fabric was of critical importance. The principle mechanism for raising the GIIC in the nanostitched composite structure was the interlayer resin fracture particularly as a form of slight shear hackle marks. Cracks grew around the inter- and intrayarn boundaries where the resin was fractured half way around each yarn cross-section. This is called a “zigzag crack path,” and microcracks moved to the through-the-thickness of the composite where nanostitching arrested the crack growth and suppressed delamination in the stitching zone. At the blunt crack tip, carbon nanotubes in the phenolic resin and multiple filament bundles probably diminished the stress clustering via friction/debonding/pull-out/sliding or stick-slip. Thus, nanostitched para-aramid/phenolic composite structures demonstrated better damage tolerance behavior considering the neat structure.