Surface engineering and interphase damage in AA5052/hybrid carbon–Kevlar epoxy joints

Reliable bonding of lightweight metal–fibre-reinforced polymer structures depends on the local surface state and fracture processes at the adherend–adhesive interphase, yet these effects remain insufficiently resolved for hybrid carbon–Kevlar adherends. This study combines contact-stylus profilometry, Shore D surface-response mapping, digital optical microscopy, SEM–EDS fractography, digital image correlation (DIC), and an effective cohesive-layer finite-element model to evaluate AA5052/hybrid carbon–Kevlar (Al/CK) epoxy single-lap joints; symmetric Al/Al and CK/CK joints are used as experimental reference configurations. Aluminium adherends were abraded and treated with NaOH/HNO₃, whereas the carbon–Kevlar laminates were manufactured by vacuum-assisted resin infusion with a peel-ply-controlled epoxy-rich bonding surface. Mean nominal lap-shear strength followed CK/CK (18.09 MPa) > Al/CK (11.25 MPa) > Al/Al (10.11 MPa). Because adherend material, stiffness, surface preparation, and interphase architecture were not varied independently, this ranking is reported as a configuration-level outcome rather than attributed to a single surface descriptor. SEM–EDS indicated residue-retaining mixed interfacial failure in Al/Al, mixed interfacial/interphase failure with Kevlar fibrillation and aramid-associated remnants in Al/CK, and distributed fibre-bridged failure in CK/CK. In an additional Al/CK field of view, nitrogen was not detected at Al-rich and carbon-rich comparison regions but reached 26.23–28.10 wt% at two fibrillar remnants. This localized enrichment is consistent with aramid-associated transfer when interpreted with the fibrillar morphology, but nitrogen alone is not treated as chemical proof. The calibrated Al/CK FE–CZM matched the representative peak nominal stress within 1.95% and located overlap-end cohesive degradation in the same region as the DIC hotspot. The results provide a system-specific workflow for relating surface state, interphase architecture, and damage evolution; application to other aluminium alloys or fibre architectures requires independent surface characterization and cohesive calibration.