Recently, International Maritime Organization (IMO) reinforced the regulations for nitrogen oxide (NOx) and sulfur oxides (SOx) toward the reduced emission. To avoid serious environmental issues, the need for liquefied natural gas (LNG) fuel tanks arose for new constructed vessels. LNG fuel tanks are typically fabricated with low temperature materials that exhibit excellent fatigue performance at cryogenic temperature. In manufacturing of LNG fuel tanks, high manganese steel is known to be cost effective compared to other low temperature materials such as 9 wt.% nickel alloy steel, SUS304L and INVAR alloy. Therefore, extensive efforts are carried out to investigate the fatigue performance of high manganese steel. However, such investigations are yet limited to the constant amplitude load case only.
In order to reflect loading or unloading condition of LNG vessels in terms of fatigue performance, this study investigates the fatigue crack growth characteristics of high manganese steel under overload condition. The main parameters considered are the overload ratio (OLR) and the mean stress. We employ the Wheeler approach for the estimation of fatigue crack growth characteristics. In general, the Wheeler approach consists of crack length, plastic zone size and shaping exponent. Among three parameters in Wheeler approach, we assess the relationship between OLR and shaping exponent and suggest a modified equation for the shaping exponent considering the mean stress effect. In order to predict the effect of overload, the estimated fatigue life based on the Wheeler approach with the modified equation was compared to that with a constant amplitude load.