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Fatigue Performance of Large Bolts

TWI Core Research Project 1155/2021


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. One of the fundamental aspects which controls the fatigue strength of the connection is the fatigue strength of the bolts themselves, which was the focus of this project.


  • Review the literature in order to document the background to the recommendations in existing fatigue design standards relating to bolts and bolted connections
  • Carry out tests to determine the fatigue performance of large bolts in air and in corrosive seawater environments in order to check whether the current recommendations and thickness correction for threaded fasteners apply to large, galvanised bolts


Tests were performed both in air and in an artificial seawater spray environment.

The bolts tested were fabricated by three manufacturers, with different thread manufacturing routes.

Fatigue tests were carried out with a high mean stress of 60 or 70% of the specified minimum yield strength of the bolt.



As expected, the bolts tended to fail on the loaded flank of the first engaged thread (see Figure 1.).  Galvanised bolts whose threads had been manufactured by rolling had a lower fatigue performance than uncoated bolts with the same thread manufacturing route.  The fatigue performance of galvanised bolts in artificial seawater spray was below their performance in air, but the galvanised coating offered some protection against the corrosive effect of seawater spray, because the fatigue life was only a factor of 1.8 lower than in the in-air performance.


Users of M72 galvanised bolts can have confidence that the thickness correction in the BS 7608 and DNVGL RP C203 fatigue design guidance documents is applicable when considering the fatigue performance of large bolts. However, the thickness correction recommended in Eurocode 3 and DNVGL ST 0126 over predicts fatigue life, based on the results in this test programme.

The thickness corrected DNVGL RP C203 Class G, or the thickness corrected BS 7608 Class X-20%, design curves are most suitable for describing the fatigue performance of galvanised bolts in air, but the equivalent curves in seawater with cathodic protection (CP) would predict lower fatigue lives than were obtained from the galvanised bolts tested in this programme.

The limited number of test results obtained in this test programme suggest that it would be reasonable to reduce the fatigue life of the in-air DNVGL Class G or BS 7608 Class X–20 % design curves by a factor of two, to calculate the expected fatigue life of galvanised bolts subjected to a seawater spray environment, provided that the galvanised coating remains intact.


This project was funded by TWI's Core Research Programme.

You can read the Report Abstract for this project, as well as download the full Industrial Member Report version if you are a TWI Member company.


Figure 1. Fracture faces of a cracked M72 bolt
Figure 1. Fracture faces of a cracked M72 bolt
Avatar Dr Carol Johnston Consultant

Carol Johnston is a Consultant in the Fatigue Integrity Management section at TWI. Since 2009, she has been carrying out research and running a range of projects related to the fatigue performance and structural integrity of welded joints. Her main area of work is on validation fatigue testing of welds using the resonance fatigue testing technique, and providing consultancy on fatigue design of welded joints.