Time–temperature-dependent response and analysis of preload relaxation in bolted composite joints

Although mechanically fastened joints are generally considered a mature technology, significant problems still exist with their use, among which the preload loss in pretensioned bolts is a major one. This paper has investigated two key issues in structural durability design for mechanically fastened composite joints, i.e. temperature–time-dependent behavior for viscoelastic preload relaxation and prediction method for its long-term performance. Towards this purpose, a two-phase experimental program was conducted consisting of short- and long-term joint relaxation tests. Phase I monitored the short-term fastener preload relaxation (up to 36 h), with a special self-made bolt strain sensor. Phase II utilized the time–temperature superposition principle to establish a method of using short-term relaxation curves to construct the long-term master curve (accelerated test). It was found that there was a significant loss of bolt preload when temperature and initial preload increased, and the relaxation rate of composited joint was much larger than that of the metallic joint. Based on the strong temperature dependence of bolt relaxation, accelerated characterization was conducted for long-term behavior. Furthermore, to quantitatively relate the relaxation data to a model equation, a new preload prediction model was developed based on the creep total-strain theory. Comparisons of the prediction model and accelerated test results indicated that the improved model provided better accuracy than existing models such as Shivakumar–Crews’s and Hook-Norton’s models for describing the long-term preload behavior.