Data are presented on the growth rate of surface flaws and through cracks in thick aluminum alloy 5083-0 plate for spherical liquefied natural gas (LNG) cargo tanks. Tests were made using 135-mm (5.3-in.) thick specimens loaded so as to simulate the bending and membrane stresses in the equatorial ring and 43-mm (1¾-in.) thick specimens loaded to simulate the membrane stresses in the bottom region of the tank. The investigation included: (a) determination of crack growth rate under ship spectrum loading; (b) measurement of leak rates through penetrated cracks; and, (c) fracture strength of the cracked specimens.
The tests demonstrated that there is a large margin of safety against the occurrence of leaks; but, if leaks should occur, the design requirement of “leak-before-failure” is satisfied. Even after penetration, the growth rate of through cracks is extremely slow. Finally, static failure of specimens containing cracks with lengths six times the thickness occurs by ductile tearing rather than by unstable crack growth.
Tests were also made to evaluate the tensile properties and the fracture toughness and fatigue crack growth characteristics of the plate and welds used for the large surface-flawed specimen. The plain and welded plate had exceptionally high toughness at room temperature, -162°C (-260°F), and -196°C (-320°F), as demonstrated by the fact that no rapid crack growth was observed in any of the 178-mm-thick notch bend fracture tests. There was no fracture instability in any of the tests, and all failures occurred by ductile tearing.