Industry Guidelines on the Fatigue Performance of Large Bolts
TWI Industrial Member Report 1173-2023 [pdf / 690 KB]
By Carol Johnston
Threaded fasteners are used in a range of industries, for a wide variety of reasons (structural, pressure containing connections, shear connections etc). Many of the applications cause the fasteners to experience cyclic loads. Existing fatigue design guidance exists for standard sized bolts, but, when threaded fasteners need to withstand larger applied forces, much larger sizes are needed, such as the M72 bolts used in the internal ring flanges in wind turbine substructures.
As well as the fundamental fatigue performance of large bolts, there are many other aspects of bolted joints that the designer must understand in order to design a successful bolted joint that will see fatigue service.
The overall aim of this document is to provide a summary of key considerations when using large bolts in fatigue service, and apply to non-pressure containing, tension connections, and large bolt sizes.
- Bolt failures occur in industry, despite existing industry guidance, because bolted joints are complex.
- Material selection, quality control, joint design, fatigue performance and the methods used to apply and monitor preload are all key.
- To predict the fatigue performance of a bolt within a joint, the key parameter is the stress range experienced by the bolt. In the internal flange connections in wind turbine towers, relationships exist which link tower load with bolt load, but these can underpredict bolt load.
- The most accurate way to know the load that a bolt is experiencing is to measure it using strain gauges. A more practical method is the use of direct tension indicators.
- A lot of industry guidance on the performance of bolted joints already exists, much of it relates to pressure containing joints in the oil and gas industry.
The report summarises recent research and also shows how this relates to current standards and other industry reports. It includes references to other standards and documents so that members can carry out their own work to apply the knowledge to their particular situation.