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    Low-Cycle Fatigue Characteristics of Ultrahigh-Strength Steels

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    The low-cycle fatigue properties (10 to 20 cycles) of Ladish D6A steel at a yield strength of 240,000 psi were investigated at a constant value of crack-tip driving force. Two specimen thicknesses (0.160 and 0.075 in.) were studied. The data were analyzed using Irwin's steady-state model, and a correlation was found. The behavior exhibited may be divided into two classes: (1) “thick-plate” and (2) “thin-plate.” In the thick-plate behavior the plate thickness is greater than the local plastic-zone size at the advancing tip of the crack. The elastic constraint, therefore, dominates the rate of crack propagation. Because of the lower clastic constraint present in a 45-deg oblique shear fracture, the rate of crack propagation at a constant driving force is less than for the cup-cone condition. In the thin-plate behavior, the shear displacement on the Y axis dominates rather than the elastic constraint. The 45-deg oblique shear type of fracture, therefore, results in a greater rate of crack extension because the vertical shear displacements are larger than for the cup-cone type of fracture.

    Author Information:

    Carman, C. M.
    Metallurgist, Frankford Arsenal, Philadelphia, Pa

    Armiento, D. F.
    Metallurgist, Frankford Arsenal, Philadelphia, Pa

    Markus, H.
    Director, Frankford Arsenal, Philadelphia, Pa

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP44459S