The commonly encountered defects in welding of 9 percent nickel steel and aluminum alloy 5083-0 for liquefied natural gas tanks are described, and their significance with respect to fatigue crack propagation and fracture in ship tanks is discussed. Fracture mechanics testing and analysis have been applied. Diagrams describing the growth in depth and length of surface cracks in welded joints as a function of long term distribution of dynamic stresses for North Atlantic service are presented. The possibilities and limitations of the presently used nondestructive testing methods are treated.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.