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The interference-fit processing and split-sleeve cold expansion processing were employed on the 7050-T7451 aluminum alloy joints and fatigue test of joints were carried out. The effects of the processing on fatigue life were compared and characteristics of fatigue fracture appearance were studied by scanning electron microscope. The results show that the interference-fit bolted joints obtain longer fatigue life with interference of 0.08∼0.14mm, the longest with 0.11mm, and the fatigue life of split-sleeve cold expanded bolted joints increase with hole cold expansion of 3-5%'s growth. The maximum fatigue life is increased to 3 times that of the non- cold worked joints. The fatigue crack fractography present the characteristics of mixed transgranular fracture and material of 7050-T7451 has toughness. Interference-fit and split-sleeve cold expansion processing can also strengthen the wall of hole and make fatigue crack source from the surface of structure, not the wall of the hole.

Joints represent potential weak points in the structure, the design of the overall structure tends to follow from the design of the joint. To date, metal/composite joints are general in current aircraft structure. Some of the reasons for lower joint efficiency in composites are: brittleness which means little stress relief around the highest loaded holes, anisotropy which leads to higher stress concentration factors, low transverse strength, susceptibility to delamination, and sensitivity to environmental conditions. All of these factors together with the complexity of composite failure modes make the analysis and design of composite joints far more complex than that of metallic joints. Fatigue failure is the root cause which makes aircraft unserviceable. 75-80% of fatigue failures occur in the joints of structure, therefore researching joints is very important for improving aircraft life. Much effort has been put into solving the technical challenge for composite bolted joints.