Effects of Crack Tip Constraint on the Fracture Toughness Assessment of 9% Ni Steel for Cryogenic Application in Liquefied Natural Gas Storage Tanks.

Recently, increasing demand for the accurate assessment of the structural integrity and fitness-for-service (FFS) analysis of engineering structures has elevated constraint effects to one of the most important issues in fracture mechanics and structural integrity research. In this paper, the effect of crack tip constraints are investigated on the fracture toughness assessment of 9% Ni steel for application in liquefied natural gas storage tanks. Crack tip opening displacement (CTOD) tests were conducted using both conventional standard three-point bending (3PB) and wide plate (WP) specimens at a cryogenic temperature of -196 °C. The distribution of the stress and strain fields near the crack tip in the 3PB and WP specimens were then obtained by FE (Finite Elements) analysis. Based on both the experimental and numerical results, the parameters of the Weibull distribution were obtained to evaluate the critical Weibull stress at brittle fracture. The equivalent CTOD ratio β is defined as the ratio of the CTOD of the 3PB specimen to the CTOD of the WP specimen at the same Weibull stress. The application of the proposed CTOD toughness correction method to the WP results was then demonstrated in the context of a failure assessment diagram (FAD). It was determined that the conventional evaluation yields an excessively conservative result for WP specimens, but can be reasonably reduced by applying β.