Combination and interaction of ageing parameters on single lap shear adhesive joints

Abstract For adhesives used in structural adhesive bonds, the service lifetime and environmental ageing are fundamental elements when considering safe design and ensuring the integrity of a structure over its desired lifetime. Ideally, the environmental conditions in which a bonded structure must operate can be quantified, and their effect on structural degradation estimated. It is known that, on the long term, the mechanical performance (stiffness and strength) of an adhesively bonded structure strongly depends on the environmental temperature and the amount of moisture. Both of these environmental factors adversely affect the structural integrity of an adhesive joint, and thus its durability and lifetime. This work analyses aluminium-epoxy single lap shear samples after ageing. An experimental approach is applied to improve the understanding of the mechanical behaviour of adhesive bonds when exposed to multiple environmental factors acting simultaneously. This paper extensively discusses and compares the individual and combined influence of four different environmental factors, static and dynamic loading, UV radiation, temperature and humidity, on the residual strength of a single lap shear joint (ASTM D 1002-05). A design of experiment approach is adopted to define the interaction between the different ageing parameters mentioned above. Moreover, an ANOVA procedure, carried out on various test batch results, analyses the sensitivity of the various environmental factors. The main observed mechanical characteristics are the stiffness and (residual) strength of the single lap samples. After failure, an analysis of the fracture surface yields further information on the sample condition after ageing. The results of the study, for the specific investigated test conditions, indicate that the residual strength of the single lap joint depends strongly on temperature, our observations even show an increase by a factor of 3. On the other hand, other effects may cause a reduction of mechanical properties.