Experimental considerations adhesive bonding

On the basis of the simple calculations of ideal adhesive bond strength given earlier, it has been suggested that bond failure in a proper adhesive joint will seldom occur at the interface. Instead, failure will occur in a weak boundary layer near the true interface, or within the weaker of the two bonded phases. Modern experimental techniques and theoretical considerations, however, indicate that all three possibilities for failure do, in fact, occur, depending on the given situation. 

Construction. Polymeric materials such as adhesives, sealants, and composites have been used considerably in the last several decades for the construction, repair, and rehabilitation of our transportation infrastructures. Even though most processes were experimental until recently, they have evolved to the point where many are now standardized and well accepted. Table 1.6 hsts several common applications for advanced polymeric materials (as well as the polymeric resins that are most commonly employed). In the construction or repair of roads and bridges, epoxy adhesives have primarily been used for bonding concrete and for bonding stiffening members or repair structures to degrading concrete facilities. 

Bulk diffusion into adhesive Joints has been studied considerably [9], however little work has focused on diffusion at adhesive interfaces, largely because of the limitations of experimental techniques. Mass uptake experiments are a convenient method to study fluid absorption in adhesives. This method has been used to study interfacial diffusion by comparing the relative rates of diffusion from non-bonded or free-standing films with diffusion into the actual adhesive joint [10,11]. A disadvantage of mass uptake experiments is that the method does not provide any direct evidence of the concentration of fluid at the interface. 

Experimental results with rubber cylinders have confirmed the general validity of equations (19) and (20). Measurements of failure loads in compression and torsion and in the presence of friction at the interface have been successfully analyzed in the same way (142). Moreover, a transition from pullout to fracture is expected when the strength of adhesion Gg is relatively large compared to Gc. The transition takes place at a critical ratio of the diameter of the embedded fiber to the diameter of the elastic cylinder in which it is embedded (147). An analysis along these lines also accounts for the brittleness of well-bonded laminar composites compared to weakly bonded ones (138). Thus, again, energy considerations account for the principal features of the strength of adhesive joints. 

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