Mode I and mode II fracture behavior in nano-engineered long fiber reinforced composites

Delamination still remains one of the common concerns in continuous fibers reinforced polymer composites. In the literature, the incorporation of nanoconstituents into constitutive composites plies has been suggested with the aim of improving interface toughness. In this study, vertically aligned carbon nanotubes forests (VACNTs) are transferred at composites interfaces, and mode I (DCB tests) and mode II (ENF tests) fracture tests carried out on composite samples. Microscopic analysis of reference and nano-engineered composites help to understand fracture behavior and toughness. While the crack path in ENF samples is observed at interlaminar VACNTs-resin interfaces, the crack path in DCB samples is intralaminar, located within the unidirectional carbon fiber plies. These microscopic observations give explanation for unstable crack propagation in nano-engineered ENF samples and the unchanged toughness from the nano-engineered to the reference zone in DCB samples.