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    Factors Influencing Fracture Toughness of Sheet Alloys for Use in Lightweight Cryogenic Tankage

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    Several factors affect the fracture toughness and tensile strength properties of sheet alloys. Some of these are described for selected alloy conditions tested at 75 and −423 F. Included are: (1) the influence of rolling temperature and reduction per pass on the properties of air-melted Inconel X and AISI 301 given heavy total reductions, (2) the influence of normal commercial cold rolling to heavy reductions on consumable electrode vacuum melted AISI 301 and on air-melted AISI 310, V-36 and Inconel X, and (3) the influence of interstitial content on the properties of annealed 5Al-2.5Sn titanium. The 1-in. wide sharp-edge-notch test was used to evaluate the relative resistance to brittle fracture. Extremely high tensile strength values can be achieved by heavy rolling (≥50 per cent) of AISI type 301 at subzero temperatures. However, very brittle behavior is associated with these conditions. Furthermore, preliminary results do not indicate a definite toughness advantage associated with any particular combination of rolling temperature and total reduction. Of the alloys heavily cold rolled under commercial conditions, AISI type 301 appears to offer the best combination of strength and toughness at both room temperature and −423 F. Regarding the influence of interstitials on the alpha titanium alloy, it is clear that interstitial contents within the currently “normal” range can reduce the fracture toughness at cryogenic temperatures. Nominal values of fracture toughness (neglecting slow crack extension) have been used to illustrate the practical significance of sharp-edge-notch tensile data in the design of cryogenic tankage. This is accomplished by calculation of the critical stresses necessary to cause catastrophic propagation of cracks of different size. When the design is based on the cryogenic mechanical properties, this approximate analysis indicates a low interstitial alpha titanium alloy to be the most promising material for liquid hydrogen tanks investigated; on the other hand, if the design is based on the room-temperature mechanical properties, AISI 301 cold rolled 70 per cent (840,000 room-temperature yield strength to density ratio) is superior. This analysis emphasizes that crack tolerance is substantially reduced when the operating temperature is decreased from that of liquid oxygen to that of liquid hydrogen. This decrease in the “margin of safety” is accentuated as the strength level of the alloy is increased.

    Author Information:

    Espey, G. B.
    Research Engineer, NASA Lewis Research Center, Cleveland, Ohio

    Jones, M. H.
    Research Engineer, NASA Lewis Research Center, Cleveland, Ohio

    Brown, W. F.
    Chief, NASA Lewis Research Center, Cleveland, Ohio

    Committee/Subcommittee: A01.19

    DOI: 10.1520/STP49625S