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    Fracture of High-Strength Sheet Steel Specimens Containing Small Cracks

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    A procedure is described for introducing small surface cracks into high-strength steel sheet specimens. The effect of crack size on tensile strength of the specimens is illustrated for two high-strength steels with comparable yield strengths but different levels of fracture toughness at room temperature. The effect is also illustrated for one of the steels at lower temperatures, down to −280 F, covering a wide range of fracture toughness for this material. The results are also discussed in relation to an equation developed by G. R. Irwin which expresses the stress intensity factor for a semi-elliptical surface crack in terms of the length and depth of the crack. This is used to calculate a lower limit for the Kc fracture toughness value from the initial crack dimensions, Knom. The calculated values of Knom are almost independent of the crack dimensions over a wide range, indicating a relationship between initial crack dimensions and slow crack growth. It is shown that the effect of a small surface crack is more drastic than might be expected from the results of tests on centerslotted specimens, such as are used to determine Kc values of sheet materials. The results of tests in which water or india ink was introduced into the cracks showed that these fluids had a distinctly detrimental effect on the fracture strengths of the steels used.

    Author Information:

    Srawley, J. E.
    U. S. Naval Research Laboratory, Washington, D. C.

    Beachem, C. D.
    U. S. Naval Research Laboratory, Washington, D. C.

    Committee/Subcommittee: A01.19

    DOI: 10.1520/STP49620S