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In this paper, factors affecting the fracture characteristics of metals at cryogenic temperatures are reviewed, and the use of fracture mechanics for predicting performance of defect-containing structures is discussed. The phenomenon of serrated yielding and the effect of phase transformations on cryogenic properties are also discussed. At cryogenic temperatures, the resistance to plastic flow of body-centered-cubic metals and most close-packed-hexagonal metals increases sharply. This increased resistance causes a corresponding decrease in fracture strength. At these subtransition temperatures, brittle fracture strength is always less than the extrapolated yield strength, and only microscopic amounts of plastic deformation precede fracture. Data from smooth-tension, notched-tension, and cracked-plate tests performed on a Ni-Mo-V forging steel indicate that the temperature and stress at which fracture occurs is strongly dependent on the severity of the test conditions—the size and acuity of defects. Since fracture strength is a variable quantity, load-bearing capacity is better evaluated using fracture toughness, a basic material parameter. Quantitative predictions of load-bearing capacity can be made using the fracture-toughness parameter with expressions relating to toughness, defect size, applied stress, and relative geometry.
cryogenics, metals, defects, fracture toughness, fracture mechanics
Wessel, E. T.
Fellow engineer, Westinghouse Electric Corp., Pittsburgh, Pa.