On the Feasibility of Large-Scale Resonant Fatigue Testing to Study Bolt Force Relaxation

Abstract

Abstract Offshore wind towers are generally connected with the foundations by a bolted joint. Bolt preload relaxation and loosening result in regular inspections and maintenance. This project describes results of both static and dynamic bolt relaxation tests on a large-scale bolted flanged joint. The tests are carried out on a resonant bending fatigue test setup. These tests serve as a feasibility study to demonstrate this testing method is suitable for efficiently studying the bolt behaviour. The bolts are preloaded up to 90% of their yield stress. Static relaxation between 5 and 28% of yield stress was measured over a period of 20 days. The dynamic testing is spread over 6 days with a total of 1.5 million cycles resulting in a maximum relaxation of 7% of yield. The dynamic test program consists of loading blocks with different amplitudes and intermittent pause periods. Additionally, the Schmidt-Neuper relation between tube shell stress and bolt force is experimentally investigated during the dynamic tests. Initially, the measured bolt forces are lower than predictions by the Schmidt-Neuper relation. However, bolt forces increased significantly after an overload test.