A comparison of laboratory-conditioned and naturally-weathered bonded joints

Adhesive bonding is often the most desirable joining method in terms of structural efficiency and cost and is being used in an increasing number of civil and military applications. One of the main stumbling blocks to the further use of adhesive bonding is the lack of confidence in long-term durability, particularly in humid environments. When designing bonded joints, the effect of ageing is usually accounted for by subjecting the joints to artificially high levels of loading or extreme environmental conditions for short times prior to testing. However, the relationship between the results from these accelerated tests and actual long-term ageing is poorly understood. The Defence Evaluation and Research Agency (DERA) has what is probably a unique set of experimental data for a wide range of adhesively-bonded joints subjected to both accelerated ageing in the laboratory and long-term outdoor exposure. In this paper the relation ship between the naturally aged and accelerated aged joints is explored. It is concluded that no simple correlation can be made between joints aged in different environments. The best that accelerated ageing can achieve is to eliminate those adhesives likely to perform badly in conditions of high humidity and to indicate those systems likely to perform well.