The Investigation of Preload Relaxation Behavior of CFRP Bolted Joints Under Thermal-Oxygen Environment: Modeling and Experiments

In this paper, the effects of interference-fit sizes and service environment temperature on the preload and relaxation of CFRP bolted joints are investigated based on an ultrasonic monitoring method. Specimens of different interference-fit sizes were subjected to insertion, preloading and preloading force monitoring for up to 200 h. To describe the preloading relaxation response of CFRP bolted joints, a comprehensive relaxation mechanics model is proposed. Experimental results demonstrate that this model accurately describe the variations in bolted preloading force under interference-fit conditions and thermal-oxygen environments. During the preloading process, a portion the axial force in interference-fit bolted joints is dissipated by interfacial frictional force and the magnitude of the frictional force is influenced by the interference-fit sizes. The interference-fit will lead to a tightly coupled interface, causing interface friction between the bolt-shank and the joint-holes, which can lead to a weakening transformation ability from tightening torque to axial force. Compared to clearance-fit condition, interference-fit can suppress the preloading relaxation effect of CFRP bolted joints to a certain extent. With an increase in interference-fit percentage (from 0% to 1.2%), the preloading relaxation coefficient rises from 94.4% to 95.7%. The additional interfacial friction effectively suppresses the creep deformation of composites. However, with an increase in the service temperature, the relaxation behavior of preloading forces in CFRP bolted joint significantly intensifies. As the environmental temperature rises from 25 ℃ to 150 ℃, the preloading relaxation coefficient decreases from 95.0% to 79.8%. High-temperature environments can lead changes in the material properties of composite and interface friction characteristics, even potentially leading to damage.