The effects of surface preparation and curing procedures on load carrying capacity and fatigue life of adhesively bonded lap joints

Bonded joints are widely used in industries such as automotive, wind turbine blade, and aircraft because of their ability to adhere dissimilar materials. It is significant to study manufacturing factors such as surface preparation and curing procedures in order to improve load carrying capacity and durability of the joint. In this paper, the influences of these procedures are investigated on the strength of metal-to-metal, metal-to-composite, and composite-to-composite bonded lap joints subjected to quasi-static and fatigue four-point bending tests. Sandpapering and acid-etching are selected as mechanical and chemical surface preparation methods, and three curing procedures of no post-cure, post-curing with gradual, and sudden cooling are applied. Results showed that, for similar conditions, metal-to-composite and metallic joints achieved the highest and lowest values of maximum quasi-static loads, respectively. Besides, for all joints types, mechanical surface preparation method achieved higher maximum quasi-static load and fatigue life in compare to chemical one. Maximum quasi-static load in the metal-to-composite joint showed less dependency to gradual cooling than with metallic one, and in the composite joint depends strongly to curing procedure. It is also revealed that fatigue life of the metal-to-composite joints improved for gradual cooling after post-curing. Finally, it is concluded that the metal-to-composite joints have superior quasi-static and fatigue behavior in compare with others, and the mechanical surface treatment and second curing procedure are suitable. Microscopic observations of failure surfaces showed that the metal-to-composite joints led to the cohesive failure mode which is appropriate for future repairing purposes of a bonded joint