An experimental investigation on interfacial behavior and preload response of composite bolted interference-fit joints under assembly and thermal conditions

This paper reports an experimental study on interfacial behavior and preload response of composite bolted interference-fit joints under assembly and thermal conditions. The bolt-inserting, tightening and preload relaxation tests were specially designed to explore the formation of interference interface, frictional behavior, torque-preload relation and their subsequent effects on preload relaxation. The interface damage was characterized by surface strains and microstructure around joint-holes. The clamping forces on both sides of joints were measured by a pair of load washers to evaluate the frictional forces. The preload loss was captured by the instrumental bolts in the 240-hour relaxation test. Results show that the insertion of bolt with a proper interference-fit size can act as cold expansion which contributes to achieving tightly coupled interface and evenly distributed interference-fit size. The transformation ability of tightening torque into preload is found to be an intrinsic feature which is related to stiffness of clamped components. Although the interfacial friction hinders the transmission of preload from the nut side to the bolt-head side and leads to unequal clamping forces on two surfaces of mating components, it facilitates to alleviate preload relaxation by reducing preload level and preventing creep deformation.