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    Some Observations on Mixed-Mode Fatigue Behavior of Polycrystalline Metals

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    Fatigue cracks are almost invariably one of two kinds, either Stage I (shear type) or Stage II (tensile type). The orientation, length, and lifetime duration of these cracks are dependent on: the loading system; the artefact geometry; and the material microstructure.

    All of these factors are therefore important in the fatigue analysis of a component or a structure, but especially so when mixed-mode loading (such as tension plus torsion) and mixed-mode cracking occur. However, the prior loading history and/or the history of crack growth is frequently ignored in both theoretical and experimental studies in which a crack or defect of a given size is extended by a given loading system.

    This paper records some results and some difficulties in our attempts to understand both mixed-mode loading and mixed-mode crack growth and their interactions in polycrystalline metals. In particular two regimes are considered in this study. Firstly, long crack growth thresholds under both mixed Mode (I and II) and (I and III) as well as fatigue crack propagation rates and branching criteria are discussed. Under such conditions, crack propagation behavior necessarily requires the problem of crack face interference to be addressed. Secondly, short crack behavior under multiaxial loading is studied and it is shown that multi-axial fatigue endurance can be predicted from Stage I and Stage II crack growth laws, which sometimes require crack coalescence to be taken into account.


    fatigue cracks, mixed-mode loading, polycrystalline metals, crack propagation, multiaxial loading

    Author Information:

    Miller, KJ
    University of Sheffield, Sheffield,

    Brown, MW
    University of Sheffield, Sheffield,

    Yates, JR
    University of Sheffield, Sheffield,

    Committee/Subcommittee: E08.01

    DOI: 10.1520/STP14252S