Rates of fatigue crack propagation were determined for CT specimens taken from the weld metal, heat-affected zone, and base metal of an Alcoa 649A Process 5556 butt weld in a 51-mm (2-in.)-thick 5456-H117 plate. Crack-opening displacement measurements were taken during the fatigue crack propagation tests so that the effects of crack-opening loads and residual stresses could be determined. The use of the concept of Elber's effective stress intensity range, based on the measurements of crack-opening loads, indicates that the crack propagation rates are equivalent in the weld metal, heat-affected zone, and base metal. Furthermore, the effect of stress ratio also can be correlated using the effective stress intensity range concept.
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.
The Kvaerner-Moss spherical tank design offers significant economic advantage for the shipboard transport of liquefied natural gas. An analytical and experimental approach based on fracture mechanics concepts was used to assist the designer in providing answers to the following basic questions: (1) Might a discontinuity smaller than detectable by nondestructive inspection lead to catastrophic fracture; (2) How fast would such a discontinuity grow under fatigue loading likely to occur during the lifetime of the tank; (3) Could a fatigue crack growing part way through the tank wall precipitate catastrophic fracture before it grows through the wall and is detected as a leak; and (4) If leakage develops, how much time is available to get the ship safely to port for discharge and repair? Both “critical” locations in the tank, i.e., the highest stressed region of the membrane and the equatorial ring, are examined. Available data indicating the safety of spherical tanks fabricated of aluminum alloy 5083-0 is documented.
The results of a number of investigations on the properties of 5083-0 plate and weldments of various thicknesses have been summarized. The data show that the tensile properties and toughness of 5083-0 plate at room and subzero temperatures remain about constant or decrease modestly with an increase in thickness from 1 to 9 in. and that the properties of 5083-0 plate formed into a spherical contour by the Alcoa 570 process are essentially the same as those of asrolled 5083-0 plate. Both strength and toughness are higher at -260 and -320°F than at room temperature. The strength and toughness of 5183 welds at room temperature, -260, and -320°F are equivalent to those of the parent plate. The high toughness of these products at those temperatures has been demonstrated by tests of relatively large-scale specimens, all of which have fractured by ductile tearing; conservative engineering estimates of KIc and Kc have been made based upon these results and those published in the literature. The fatigue crack propagation characteristics of these materials have been established and found to be largely unaffected by temperature, thickness, orientation, or location in the weld. Crack growth rates are faster in moist air than in dry air.
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.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.