• Furtado, Carolina
• Kopp, R.
• Sinclair, I.
• Camanho, Pp
• Borstnar, G.
• Ni, X.
• Arteiro, A.
• Kalfon-Cohen, E.
• Spearing, Sm
• Wardle, Bl
• Helfen, Lukas
• Mavrogordato, Mn

Abstract

Aerospace-grade unidirectional carbon fiber reinforced epoxy standard-thickness prepreg (herein called ‘Thick’) and thin-ply prepreg (herein called ‘Thin’) composite laminates comprised of different fiber and matrix materials were reinforced in the relatively weak interlaminar regions with high densities of vertically aligned carbon nanotubes (A-CNTs), in a hierarchical architecture termed ‘nanostitching’. Presented here is a 4D experimental study of the interactive effects of A-CNT interlaminar reinforcement and ply thickness on damage initiation and progression in composite laminates, leveraging in situ synchrotron radiation computed tomography (SRCT) of double-edge notched tension (DENT) specimens to non-destructively elucidate the dominant 3D failure micro-mechanisms and how they progress. For the quasi-isotropic specimens, we find an ~9% improvement in the DENT ultimate tensile strength (UTS) of nanostitched standard-thickness laminates vs. the baseline, and no statistically significant improvement in the DENT UTS of nanostitched thin-ply laminates. The dominant damage mechanisms identified using 3D visualization and damage segmentation software were matrix cracking and fiber/matrix interfacial debonding; these mechanisms were common to all material configurations. Relatively small, sub-critical interlaminar delaminations were revealed in both baseline and nanostitched samples near the notch edges for all load steps except unloaded; however, all delaminations propagated into intralaminar regions over small distances on the order of 10 fiber diameters from the notch edge. Nanostitch was not directly observable via SRCT, and similar trends in progressive matrix damage type and extent within the notch vicinity were revealed regardless of whether nanostitch was present. Although no significant differences in damage mechanism type were found among any laminates, unique qualitative trends in damage extent and location were apparent when comparing standard vs. thin-ply technologies. Particularly, regarding ply thickness effects on damage progression, the matrix damage 3D surface area of the Thick baseline was found to be one order of magnitude higher than that of the Thin baseline at a loading of 90% UTS, indicating that the thin-ply morphology intrinsically suppressed critical matrix damage progression that always nucleated at the unpolished notch edges and remained constrained to a smaller region near the notch edge. The greater extent of matrix damage in Thick supports the more pronounced nanostitch effect on UTS observed vs. Thin. Overall, this study finds that nanostitch increases DENT UTS in standard-thickness laminates despite the presence of similar 3D damage extent for the load steps visualized here (up to 90% UTS); however, the nanostitch effect on thin-ply laminates is undetectable for DENT loading of the quasi-isotropic ply sequence tested.

Topics

• impedance spectroscopy
• surface
• Carbon
• nanotube
• tomography
• strength
• composite
• tensile strength
• isotropic
• aligned