Nucleation and growth of small surface cracks in aluminium alloy AMg6 as related to discontinuity of the fatigue curve

N. M. Grinberg, V. A. Serdyuk, A. M. Gavrilyako, D. V. Lychagin, E. V. Kozlov

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    Abstract

    The nucleation and growth kinetics of small cracks are investigated through measuring their lengths at the surface and their depth on the aluminium alloy AMg6 (6.5% Mg) at stress amplitudes above and below the discontinuity in the fatigue curve in air and in vacuum at 140 K. Work hardening curves are given along with metallographic photographs. The dislocation structure of the alloy is investigated. It is shown that microcracking occurs at intermetallide inclusions due to the piled up dislocations around them. The stages of small crack growth correlate with the periods of the hardening curve. Small cracks have an elliptic form with the axial ratio 0.32 under all loading conditions. The growth rate of small cracks does not correspond to the regularities characteristics of the growth rate of large cracks. The dependences dl/dN = f(l) and dl/dN = f(σa) are plotted for small cracks. The growth rate of small cracks is essentially dependent on the applied stress and, to a lesser extent, on crack lenght. The discontinuity in the fatigue curve is accounted for by a jump-like increase in the growth rate of a small crack in the low-amplitude region as compared to that in the high-amplitude region. The reasoning for the phenomenon is provided based on a quantitative study of dislocation density of each substructure component and the metallographic distribution of secondary fine cracks.

    Original languageEnglish
    Pages (from-to)370-376
    Number of pages7
    JournalInternational Journal of Fatigue
    Volume13
    Issue number5
    DOIs
    Publication statusPublished - 1991

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    Keywords

    • AMg6
    • crack growth
    • surface cracks

    ASJC Scopus subject areas

    • Mechanical Engineering
    • Mechanics of Materials

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