Enhanced fracture behavior of aluminum joints with multi-walled carbon nanotube and polyvinyl alcohol nanofiber reinforced epoxy adhesives

The objective of this study is to investigate and compare two different reinforcement routes for epoxy-bonded AA2024-T3 aluminum joints: direct incorporation of multi-walled carbon nanotubes (MWCNTs) into the epoxy matrix and interleaf-based reinforcement using electrospun PVA/MWCNT nanofiber mats. The novelty lies in the direct comparison of these two approaches within the same bonded-joint framework under both shear and Mode I fracture loading conditions. Neat epoxy was used as the reference, while an epoxy with 1 wt.% MWCNT represented the direct nanotube-reinforced system. In the interleaved configurations, the epoxy matrix remained MWCNT-free and the reinforcement was provided solely by PVA nanofiber mats with or without 1–5 wt.% MWCNT. Electrospun nanofiber mats with an average thickness of approximately 0.13 mm were obtained, and the nanofiber diameters decreased from 222–403 nm to 95–192 nm with the addition of MWCNTs to the PVA solution. In single-lap shear tests, the maximum load value was 7804 N for neat epoxy, increased to 8824 N for the epoxy system containing 1 wt.% MWCNT, and reached 9437 N for the joints using a PVA nanofiber mat containing 1 wt.% MWCNT. Joint toughness increased from approximately 1204 kJ/m \(^3\) to 1954 kJ/m \(^3\) for the hybrid system with 3 wt.% MWCNT, but decreased at higher MWCNT contents, which may be associated with less uniform dispersion and local defect formation. In double cantilever beam tests, the EPCNT1 sample exhibited an average \(G_{Ic}\) increase of about 231% relative to neat epoxy.