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    The Use of Notched Compact-Type Specimens for Crack Initiation Design Rules in High-Temperature Water Environments

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    This report describes the techniques used and results obtained for fatigue crack initiation of notched compact-type (CT) tests of SA333 Gr.6 carbon steel in air or in autoclaves mostly at 288°C (550°F), 1.05 kg/mm2 (1500 psi) oxygenated water conditions. The fatigue crack initiation life Ni was quantitatively determined from a cycle-by-cycle crack length measurement as the number of cycles of produce 7.6 × 10−2 mm (0.003 in.) of growth from the notch root. Results for several parameters of the experimental program (including temperature, environment, oxygen content, frequency of cycling, notch radius, R-value, and ΔK) are reported. A majority of the tests were run in 288°C (550°F) water at 8 ppm O2 with 5.1 × 10−2 mm (0.002 in.) notch radii, and showed a severe degradation in initiation life, especially at a frequency of 0.00021 Hz (0.0125 cpm), compared with 288°C (550°F) air tests. On the other hand, at 0.021 Hz (1.25 cpm) and 8 ppm O2, decreasing the temperature to 232°C (450°F) restored the life to that of air. Reducing the oxygen level to 0.2 ppm in 288°C (550°F) water was also beneficial. These latter two findings are preliminary because of limited testing.

    The test results were interpreted in terms of conventional procedures in which a notch root pseudostress amplitude Sa was calculated for each test and compared with the ASME fatigue data curve for carbon steel. The quantity Sa was obtained from the applied stress intensity range (ΔK) and Neuber notch analysis procedures to account for elasticplastic notch root conditions. The worst-case notch concept was introduced to account for the observed maximum in the fatigue notch factor kf with decreasing notch radii and for the environmental notch sensitivity. For the present study, the worst-case notch radius ρ was 0.165 mm (0.0065 in.).

    A procedure is described for calculating Sa and Ni from crack growth data. This procedure assumed a crack to be equivalent to the worst-case notch and provided a good prediction of the ASME fatigue data curve from the ASME crack growth curves in air, both for carbon steel. By using this approach and a conservative threshold stress intensity value for fatigue crack growth in 288°C (550°F), 8 ppm O2 water, a value of Sa was determined below which crack initiation will not occur in this environment, regardless of defect sharpness or frequency of loading. This value of Sa was consistent with the notched CT crack initiation results obtained.


    corrosion fatigue, fatigue testing, fatigue notch sensitivity

    Author Information:

    Prater, TA
    Corporate Research and Development, General Electric Company, Schenectady, N.Y.

    Coffin, LF
    Corporate Research and Development, General Electric Company, Schenectady, N.Y.

    Committee/Subcommittee: G01.09

    DOI: 10.1520/STP44823S