Bolt fatigue data for updating design rules

A need for fatigue data to update design rules for bolted joints in bridges and offshore structures has led to a test programme in TWI's Engineering Department. Although fatigue design rules for bolted joints exist in BS 5400 Part 10, they appear to be based on relatively sparse data. The Department of Transport and TRRL are therefore concerned about the need for better data to update the rules relating to onshore applications such as bridges.

Equally, bolted joints are often used to make offshore repairs, and a number of them have suffered fatigue failure. When reviewing its guidance on design of bolted joints, the Offshore Division of HSE discovered that there were limited reliable data on the fatigue performance of bolts in air and none at all on how they performed in seawater. Bolts used offshore generally have higher yield strengths than normal bolts but are more susceptible to hydrogen embrittlement, with resulting loss in fatigue strength.

The organisations are therefore supporting a two year programme of work at TWI which will examine the fatigue behaviour of the bolts and generate data which will help in revision of guidance and standards. There are three main areas associated with the fatigue performance of bolted joints where more data are vital:

The fatigue strength of the bolts themselves when a bolted joint is loaded with the bolts in tension;
The fatigue strength of the bolts when a bolted joint is loaded with the bolts in shear;
The fatigue strength of a high strength friction grip (HSFG) bolted joint assembly failing in the plate.

Tests on the fatigue strength of the bolts loaded in tension will study effects of the bolt material and its static strength, the level of pre-load, the form of the threads, the bolt diameter and the corrosive environment. Some tests to study the fatigue strength of bolts in shear will be carried out on finger tight joints to eliminate friction effects. No data or design guidance could be found on this failure mode.

There are already some test data on the fatigue strength of HSFG bolted joints. Doubts still remain about the fatigue strength of long connections of this type, so some comparative tests will be made on long and short joints. The test programme should enable realistic design guidance to be formulated on the fatigue strength of bolted joints for land based and offshore structures.