Mode-I fracture toughness and surface morphology evolution for contaminated adhesively bonded composite structures

This study explores the effect of environmental contamination level on mode-I failure and fracture surface evolution of adhesively bonded composite joints. We examined a common adhesive/adherend material system (Hysol EA9394/Hexcel IM7-G/8552) exposed to a typical aviation hydraulic fluid (MIL-PRF-87257) at varying contaminant concentrations. The macroscopic bond line fracture toughness measurement by double cantilever beam test exhibited a progressive deterioration with increasing contaminant concentration. Even a trace level (3 µg/cm2) resulted in a 15% reduction of average bond line strength and 30% drop in bond line toughness. An Arrhenius-type correlation between interface toughness and contaminant concentration was observed, with potential to predict the bond line toughness deterioration level. Increasing the contaminant concentration changed the fracture surface morphology from ductile-to-brittle failure mode, driven by diminished plasticity within the adhesive layer. Finite element method employing cohesive surfaces was also utilized to rationalize the experimentally observed trends. The combined experimental and numerical results suggest that the bond line contamination deteriorates the interfacial toughness through two mechanisms: (i) weakening the interfacial bonding strength, and thereby reducing the intrinsic interfacial toughness, and (ii) shielding the plastic energy dissipation within the adhesive layer, and leading to a reduction in the overall effective bond line toughness.