The presence of a corrosive environment may accelerate fatigue crack propagation in structural steels. If pitting conditions exist, development of corrosion pits will introduce additional points of stress concentration at which cracking may develop. Both these mechanisms are likely to reduce fatigue strength below that in an inert environment.
The effect of a marine environment on fatigue of joints in ferritic steels has been extensively investigated (1) and the results incorporated in fatigue design rules. Corrections to the expected fatigue performance are usually quoted in terms of an environmental reduction factor (ERF), which is the ratio of fatigue endurance in air to that in the marine environment. For example, in the UK, BS7608 (2) suggests an ERF of 2 for unprotected joints exposed to sea water. In addition the S-N curve is assumed to have a constant slope irrespective of stress range, rather than an endurance limit at low stress range as in air. A more recent review of fatigue design guidance for offshore structures (3) suggests an environmental reduction factor of 3 for unprotected joints. Reference 3 also provides guidance on the value of ERF under cathodic protection. Other environments may be more aggressive and specialist advice should be sought - contact Structural Integrity at TWI.
Where direct contact with the environment is prevented by a protective coating the fatigue strength can be taken as that in air. However, it must be certain that the coating will remain sound for the expected life of the structure.
References
- UK Offshore Steels Research Project - Phase II Final Summary Report, OTH 87 265, HMSO, 1987.
- BS 7608:1993, Code of Practice for Fatigue Design and Assessment of Steel Structures, BSI.
- Fatigue Background Guidance Document, Report OTH 92 390, UK Health and Safety Executive, 1992.