Fatigue Strength of Large Bolts: Results of Tests in Air and Artificial Seawater Spray
TWI Industrial Member Report 1155-2021 [pdf / 19215KB]
By C Johnston
The fatigue strength of bolts is important to structures that experience cyclic loading. The offshore wind industry, in particular, uses bolted flanged connections in offshore wind turbine structures. The bolts used in the flanged connections joining the main tower to the transition piece are typically hot dip galvanised, M72 bolts. The joint experiences cyclic loads during service, and so the fatigue strength of the bolted flanged connection is important. The fatigue strength of the joint itself is influenced by a number of factors (including fit up, load redistribution, flange flatness etc) which affect the level and type of loads transmitted to the bolt assembles. The fundamental aspect which controls the fatigue strength of the connection is the fatigue strength of the bolts themselves, which is the focus of this project.
This report begins with a review of the literature and the background to the current recommendations in fatigue design standards. It then provides results from fatigue tests performed on grade 10.9, M72, galvanised bolts in both air and in artificial seawater spray. The test results are compared to S-N curves for threaded fasteners from fatigue design standards (BS 7608, DNVGL RP C203, Eurocode 3, DNVGL ST 0126, BS EN 13445-3, VDI 2230 Part 1 and DASt 021) and recommendations are made as to the appropriateness of these S-N curves to the fatigue performance of large bolts.
- The test results were above all of the existing design S-N curves reviewed, and so those design curves are safe to use, but there were different levels of conservatism.
- The thickness corrected BS 7608 Class X-20% and the DNVGL Class G curves were the more appropriate for predicting the fatigue performance of grade 10.9, hot dip galvanised M72 bolts.
- A reference thickness of 25mm was found to be most suitable for the thickness correction factor used with the standard S-N curves.