Adhesion strength evaluation and improvement for composite/metal interfaces

Abstract

The failure in an adhesive-bonded structure starts at the interface, and the interfacial fracture is of interest whenever adhesion between different materials is concerned. One of primary factors limiting the application of adhesive-bonded joints to structural design is the lack of a good evaluation tool for adhesion strength to predict the load-bearing capacity of bonded joints. In the present study, the adhesion strength of a composite/metal interface is evaluated using fracture mechanics characterization of interfacial fracture. Adhesion strength improvement is also investigated using both chemical and physical surface property modification. The failure of a composite/steel bonding is analyzed by correlating the applied energy release rate with interfacial toughness at the corresponding phase angle. The interfacial fracture toughness of the composite/steel bonding was measured over a wide range of mode-mixities with single-leg bend test specimens. Anisotropic/isotropic interfacial cracks with frictional contact along the crack surfaces are specially treated to characterize crack growth under mode II loading condition. The stress intensities at the anisotropic/isotropic bi-material interface corner are also implemented to estimate the interfacial strength. The applicability of fracture mechanics concepts to adhesive-bonding is validated by comparing the critical stress intensities or energy release rates of lap joints with the measured fracture toughness, and the asymptotic solution is compared with FEA results to investigate the range of applicability. Adhesion strength improvement by introducing silane-based interphase formation is investigated. The silane-based interphase formation between the epoxy resin of the composite and the steel surface improved both the adhesion strength and fatigue life of co-cure bonded lap joints by chemical bonding introduced at the interface. The effect of surface structure modification on adhesion strength is investigated...