Fracture energy in structural adhesive joints of composite-aluminum under adverse environments conditions

This work analyzes the degradation of composite-aluminum adhesive joints when they are exposed to the weathering and environmental pollution in Madrid for a long period of time. Two adhesives (epoxy and polyurethane) and several surface treatments for adherends have been considered. End-notched flexure bending tests have been performed to evaluate the loss of mechanical properties (failure stress and fracture energy) of adhesive joints that were exposed to the weathering and environmental pollution. Tests results have shown that the environmental degradation of the adhesive leads to a loss of mechanical properties in the adhesive joints. Considering the relative percentage, the reduction of failure stress in the polyurethane is higher than in the epoxy (31.9% for the polyurethane and 21.1% for the epoxy). Similarly and considering relative percentage, fracture energy reduction is 42.0% for polyurethane and 41.5% for epoxy. Likewise, tests have shown that the loss of mechanical properties does not decrease linearly with the time when the samples have been exposed to the weathering. This reduction occurs during the first few weeks. In summary, tests results have allowed to conclude that adhesive joints with epoxy resist the environmental pollution better than the adhesive joints with polyurethane.