PSI - Issue 13

Adam Smith et al. / Procedia Structural Integrity 13 (2018) 566–570 Smith / Structural Integrity Procedia 00 (2018) 000 – 000

5

570

Fig. 3. Diagrams showing the progression of the fatigue crack from the weld toe for a tee-butt welded joint modelled using da/dN versus ∆ K data for steel in (a) air and (b) aqueous 3.5 wt.% NaCl at a very low cycle frequency. (c) A plot of number of cycles-to-failure (left axis) and corresponding environmental reduction factor (right axis) versus cycle frequency with results from the present fracture-mechanics study showing excellent agreement with experimental data reported by Kilpatrick (1997).

4. Conclusions

1. A fracture-mechanics approach has been used in conjunction with finite-element modelling to successfully calculate the fatigue life of a tee-butt welded joint under freely corroding conditions for a range of different cycle frequencies (30 Hz to < 0.001 Hz). 2. This study allowed a better estimate of the environmental reduction factor to be calculated compared with the guidance provided in the British Standard, BS 7608:2014+A1:2015, particularly for marine structures subjected to very low cycle frequencies (< 0.001 Hz) under freely corroding conditions. For very low cycle frequencies, the environmental reduction factor is estimated to reach a maximum value of about 6.7 based on mean crack-growth rates. 3. The methodology and fatigue crack-growth-rate data used in the present study can be used in the future to assess the influence of other variables on the environmental reduction factor. BS 7608:2014+A1:2015, Guide to Fatigue Design and Assessment of Steel Products, British Standards Institution 2015. BS 7910:2013+A1:2015, Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures, British Standards Institution 2015. Kilpatrick, I., Fatigue of HY Weldments, in “ Offshore Technology Report – OTO 97 066: A Review of DRA Work on Marine Strength Steels ”, Health and Safety Executive, November 1997. Kilpatrick, I., Cargil, J., 1981. Fatigue Crack Detection and Sizing in We lded Steel Structures, in “ Proceedings of the DARPA/AFWAL Review of Progress in Quantitative NDE” , October 1979 – January 1981, 567 – 575. Knop, M., 2015. Effects of Cycle Frequency, Waveform, and Electrode Potential on Corrosion-Fatigue Crack Growth in High-Strength Tempered Martensitic Steels, PhD thesis, Monash University, Clayton, Australia. Offshore Technology Report – OTH 92 390: Background to New Fatigue Guidance for Steel Joints and Connections in Offshore Structures, Health and Safety Executive, December 1999. Scott, P. and Silvester, D., The Influence of Seawater on Fatigue Crack Propagation Rates in Structural Steel, Interim Technical Report UKOSRP 3/03, AERE Harwell, December 1975. Shimodaira, M., Matsuoka, S., Masuda, H., Nishijima, S., 1990. Acceleration of Fatigue Crack Growth for HT80 and SUS304 steels in 3%NaCl Aqueous Solution under Very-Low Frequency Cyclic Loading, Journal of the Society of Materials Science Japan 39, 162 – 168. Vosikovsky, O., 1975. Fatigue-Crack Growth in an X-65 Line-Pipe Steel at Low Cyclic Frequencies in Aqueous Environments, Journal of Engineering Materials and Technology 97, 298 – 304. References