Cyclic stress durability testing of lap shear joints exposed to hot-wet conditions

Adhesively bonded single lap shear joints are often required to transfer fluctuating loads in service, yet the majority of accelerated durability tests are carried out using either no load, or a sustained load. A number of researchers have emphasised the need for the development of faster and more realistic techniques for comparing the durability of different adhesive-pretreatment combinations. The aim of this study was to investigate the potential of cyclic stress durability testing for fulfilling this need. Aluminium-to-aluminium epoxide bonded joints, and polypropylene composite-to-aluminium fusion bonded joints, were produced using a range of different aluminium pretreatments. Replicate joints were tested using unstressed testing, static stress testing, and cyclic stress testing, to enable the results obtained from each method to be compared. The most durable joints in the cyclic stress testing were produced using the standard aerospace PAA pretreatment. The performance gap between the aerospace PAA pretreatment and less effective titanium/zirconium-based pretreatments was much greater in the cyclic stress testing, than might have been predicted from the results of the unstressed testing. Perhaps the most interesting finding was that joints made using aluminium substrates which were primed with an amino silane coupling agent, performed very poorly in the cyclic stress testing, despite performing extremely well in the unstressed testing. The overall results of the study indicate that cyclic stress durability testing may be very effective at discriminating between different adhesive-pretreatment combinations in relatively short periods of time.