Static and fatigue interlaminar shear reinforcement in aligned carbon nanotube-reinforced hierarchical advanced composites

High densities (>10 billion fibers per cm2) of aligned carbon nanotubes (A-CNTs) are used to reinforce the interlaminar resin-rich region of aerospace-grade unidirectional carbon microfiber plies in a hierarchical carbon fiber reinforced plastic (CFRP) laminate architecture. Such nano-engineered interfaces have been shown to increase interlaminar fracture toughness and substructural in-plane strengths, and here we show a 115% average increase in fatigue life across all load levels (60–90% of static strength), with a larger increase of 249% in high-cycle (at 60% of static strength) fatigue, despite no statistically significant increase in static strength. These findings are in agreement with a numerical damage progression model developed to simulate both interlaminar and intralaminar damage in the laminates, which shows the relative insensitivity of short-beam shear (SBS) strength to the enhancement of interlaminar fracture toughness, e.g., a 50% increase in interlaminar toughness yields an SBS strength increase of less than 20%. Consistent with observations of other CNT-reinforced epoxy architectures, larger improvements in fatigue life are noted in low-stress regimes (e.g., high-cycle fatigue) vs. in high-stress regimes (e.g., static and low-cycle fatigue), indicating a transition in dominant mechanisms from high-energy dissipation caused by CNT pullout to low-energy dissipation caused by CNT fracture as stress increases.