A Mechanism for Environmental Failure of Adhesive Bonds

Kinloch and coworkers have developed a model to describe the mechanics of environmental failure of adhesive joints. They have proposed a three-stage mechanism  

Uptake of a critical concentration of water in the interfacial region. 

By measuring water uptake, the diffusion coefficient and equilibrium concentration of water for the bulk adhesive were obtained at different temperatures. A value of 37 kJ/mol was also calculated for the activation energy of diffusion. A value for the plane-strain stress intensity factor, Kic, for the bulk adhesive was obtained using compact tension specimens. Tensile butt joints were prepared from mild steel blocks bonded with the epoxy adhesive and the fracture stress determined as a function of time of exposure to water at the different temperatures. An activation energy of 32 kJ/mol was calculated for joint failure, in close agreement with that obtained for the diffusion of water. This supports the view that the processes responsible for loss of joint strength are controlled by water diffusion. It was found that joints exposed to 20°C/55% RH showed no reduction in strength, even 

The use of Auger and XPS analytical techniques enabled the locus of failure to be determined. This was found to be interfacial between the metallic oxide and the epoxy adhesive at the edges of the joint, becoming cohesive within the adhesive layer towards the center of the joint. The relative amounts of interfacial and cohesive type failure depended on the length of time that the joint was immersed in water. 

The assumption was then made that the amount of water in the outer debonded zone of the joint exceeded the critical water concentration, while in the central zone, still unaffected by environmental exposure, it was below the critical level. The extent of interfacial debonding that occurred on environmental exposure can be considered equivalent to the environmental crack length, a, needed to relate the stress intensity factor, Xj, to the fracture stress, ay, of the joint (Eq. 5). 

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