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    Statistical Aspects of Fatigue Failure Due To Alloy Microstructure

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    The elements of a methodology are described to calculate the scatter in fatigue lifetime which arises from statistical variations in alloy microstructure from sample to sample within a uniform heat of material, and from variations in mean grain size and in the ductility of the alloy surface from heat-to-heat. The modeling is formulated for certain structural aluminum alloys for which the dominant mode of crack initiation is by fracture of constituent particles near the alloy surface. For one of these, an Al 2219-T851 alloy, small amounts of internal hydrogen at contents less than 1 ppm are found to affect surface ductility and both crack initiation and early growth. The application of models of crack initiation and short crack growth to predict the combined effects of grain size and hydrogen on fatigue lifetime is demonstrated for smooth bar specimens of two heats of Al 2219-T851. Experiments confirm predictions that fatigue lifetime is increased by decreasing both grain size and alloy hydrogen content.

    The models used to predict fatigue lifetime are discussed individually and deal with both the initiation and the early growth of surface fatigue cracks. They are evaluated by comparing predicted and measured mean and stochastic trends in rates of initiation and growth in Al 2219-T851 and Al 7075-T6. The paper concludes with a short treatment of the implication of the statistical nature of the fatigue failure process to prediction of the occurrence of failures of low probability, very early in the service lifetime.


    fatigue, microstructure, mathematical modeling, micromechanics, crack initiation, short crack growth

    Author Information:

    Morris, WL
    Rockwell International Science Center, Thousand Oaks, Calif.

    James, MR
    Rockwell International Science Center, Thousand Oaks, Calif.

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP30558S