Effects of Preload Relaxation on Structural Mechanical Properties of Bolted Joints of CF/PEEK Composite and Aluminum Alloy

Taking the connection structure of CF/PEEK composite material and aluminum alloy bolt as the research object, the influence of preload attenuation on the mechanical properties of the connection structure under static load was analyzed, the failure mode and failure mechanism of the joint were revealed, and the strain field variation during the tension process was analyzed by digital image correlation technology. The results show that as the preload increases, the ultimate load of the connection structure shows a trend of first increasing and then decreasing. A moderate increase in preload helps to enhance the ultimate load, while excessive preload leads to a decrease. The relaxation of preload reduces the contact pressure at the joint, leading to a decrease in the ultimate load. The failure modes of the CF/PEEK joints are characterized by fiber fracture, delamination fracture, and surface layer debonding, while the aluminum alloy exhibits bending deformation and crushing failure. The trend of maximum negative strain is consistent with the dependence of ultimate load on preload; the preload relaxation does not change the strain distribution characteristics but leads to an increase in both maximum positive and maximum negative strains.