“Corrosion Fatigue: Recent Developments and Future Needs”
W. J. O’Donnell, Journal of Marine Design and Operations, No. B10, 2006
Keywords: high temperature fatigue, crack growth rates, S-N fatigue strengths, carbon steel, low alloy steel, fatigue design life, da/dN crack growth, stainless steels
High temperature (above 300°F, 149°C) water has been found to greatly accelerate fatigue crack growth rates in carbon and low alloy steels, and to reduce their S-N fatigue strengths quite significantly. Current Code Fatigue Design Curves such as the ASME Boiler & Pressure Vessel Code curves are based entirely on data obtained in air. While a factor of 2 on life was applied to the air data to account for environmental effects, the actual effects have been found to be an order of magnitude greater in the low cycle regime. A great deal of work has been carried out on these environmental effects by talented investigators worldwide.
The ASME Code Subgroup on Fatigue Strength has done extensive work on the development of new fatigue design methods and curves to account for high-temperature water environmental effects. This effort is intended to formulate proposed new environmental fatigue design curves which maintain the same safety margins as existing fatigue design curves for air environments.
Tests conducted 25 years ago by General Electric showed that both welded and non-welded materials exhibit significantly reduced fatigue performance in elevated temperature water environments. In the 1980’s several laboratories worldwide were generating fatigue crack growth rate data in water environments. This effort was organized by the International Cooperative Group on Cyclic Crack Growth Rates (ICCGR) and its successor the International Cooperative Group for Environmentally-assisted Cracking (ICG-EAC). Most of this data was included in the EPRI (Electric Power Research Institute) Database on Environmentally-assisted Cracking (EDEAC). Eason, et al, made extensive studies of the EPRI database. Data showing crack growth rates which were a factor of 2 or so faster in water than in air, and which were independent of the strain rate, were considered to represent general corrosion.
Such data were largely covered by the factor of 2 on fatigue design life for environmental effects in the Code. Cracking under conditions where growth rates were 10 to 50 times faster than in air, and strongly dependent on cyclic rate, was referred to as environmentally-assisted cracking (EAC). Conditions which produced high crack growth rates included high sulphur content, high dissolved oxygen content in the water (for carbon and low-alloy steels), low cyclic strain rates, elevated temperatures, and high stress intensity ranges.
The transition threshold conditions from low general corrosion crack growth rates to EAC were determined using specimens tested at constant AK or AJ. Cyclic rate conditions change crack growth rates by an order of magnitude. It became apparent that modifications of the ASME Code fatigue design method for pressure vessel steels were needed to quantify the effects of elevated temperature water environments. Similar da/dN crack growth rate acceleration and S-N degradation effects have more recently been measured in stainless steels.
Resources
– NACE – Corrosion Fatigue
– ASTM Standards on Fatigue and Fracture
President, Bill, Sr. serves as a Contributing Member of the ASME (BPV III) Working Group on Fatigue Strength, and has co-authored numerous papers on design, fatigue and fracture.
O’Donnell Consulting performs engineering design and analysis to ASME Code including B&PV Section VIII Division 1 and 2 – as well as the Power & Process Piping Codes. Give us a call to discuss your engineering challenges.
