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Wednesday, February 8, 2012

Adhesive Joint and Bonding in FRP

Image below shows a series of typical bonded joint configurations. Adhesive joints in general are characterized by high stress concentrations in the adhesive layer. These originate, in the case of shear stresses, because of unequal axial straining of the adherents, and in the case of peel stresses, because of eccentricity in the load path. Considerable ductility is associated with shear response of typical adhesives, which is beneficial in minimizing the effect of shear stress joint strength. The response of typical adhesives to peel stresses tends to be much more brittle than that to shear stresses, and reduction of peel stresses is desirable for achieving good joint performance.

From the standpoint of joint reliability, it is vital to avoid the condition where the adhesive layer is the weak link in the joint, i.e. that the joint be designed to ensure that the adherents fail before the bond layer whenever possible. This is because failure in the adherents may be controlled, while failure in the adhesive is resin dominated, and thus subject to effects of voids and other defects, thickness variations, environmental effects, processing variations, deficiencies in surface preparation and other factors that are not always adequately controlled.

This is a significant challenge, since adhesives are inherently much weaker than the composite or metallic elements being joined. However, the objective can be accomplished by recognizing the limitations of the joint geometry being considered and placing appropriate restrictions on the thicknesses the adherents for any given geometry.
Adhesive joint / bonding types

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