Mechanism of bolt pretightening and preload relaxation in composite interference-fit joints under thermal effects

The present research investigates the mechanism of bolt pretightening and preload relaxation in composite interference-fit joint structures under thermal effects. In view of the dissimilar material properties and the interfacial friction in such joints, the mechanical behavior of bolt and composites during assembly can be regarded as immediate preload response which is described by a linear elastic model, whereas the long-term behavior of composites during preload relaxation in service is considered as delayed response which is characterized by a viscoelastic model. The clamping forces on both sides of joints were captured to evaluate the preload balanced by the frictional behavior. The preload relaxation of joints with various interference-fit sizes and tightening torques under thermal effects were monitored for 240 hours to calibrate and validate the proposed model. The research revealed that the interference-fit size determined the level of frictional force which resulted in a difference between clamping forces at two sides of the fasteners. The preload first increases slowly with the growth of temperature, then decreases sharply when it approaches to glass transition temperature of matrix. The interference-fit joint behaves better in terms of maintaining the stability of preload than clearance-fit joints.