Failure, adhesive joint flexibility

Designs deliberately incorporating flexible rubbery inserts between two stiff adherends are well known. For example, cyanoacrylate adhesives are successfully used to bond spectacle lenses to frames through an intermediate rubber layer. Without the rubber to dissipate peel and cleavage loads, the joint between the lens and the metal frame would be readily over-stressed, resulting in premature failure. 

Czamocki and Piekarski s) used a nonlinear elastic stress-strain law for three-dimensional failure analysis of a symmetric lap joint. Taking into account the variation of Poisson s ratio with strain within the adhesive, the authors concluded that the failure of the adhesive layer originates in the central plane of a joint (at the front edge). It was also observed that the joint width did not have any effect on the stress peaks in the central plane and that the application of a weaker but more flexible adhesive resulted in higher load-carrying capacity and lower stress concentrations in the adherends. 

These are generally termed adherends. A bonded joint may consist of one or more types of adherend material. Where the adherends are materials of a very different nature, e.g. metal to plastic, or metal to glass, the adhesive must also act as a transition layer. For example, where a large difference in thermal expansion coefficient exists between adherends, more flexible adhesives are normally used to allow differential expansion without risking failure. 

The polyvinyl acetate emulsions were designed for adhesive and coating use rather than for sealants. The homopolymers have little flexibility and are water sensitive. High levels of solvent type plasticizers (e.g. dibutyl phdialate) are used to obtain the required flexibility, but after application die plasticizer volatilizes out on aging. This results in a loss of flexibility in the sealant and failure through cracking if used in areas subject to joint movement. 

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