STP969: Constraint-Loss Model for the Growth of Surface Fatigue Cracks

    Van Stone, RH
    Senior engineer, engineer, specialist, and manager, Materials Life and Methods, GE Aircraft Engines, Cincinnati, OH

    Gilbert, MS
    Senior engineer, engineer, specialist, and manager, Materials Life and Methods, GE Aircraft Engines, Cincinnati, OH

    Gooden, OC
    Senior engineer, engineer, specialist, and manager, Materials Life and Methods, GE Aircraft Engines, Cincinnati, OH

    Laflen, JH
    Senior engineer, engineer, specialist, and manager, Materials Life and Methods, GE Aircraft Engines, Cincinnati, OH

    Pages: 20    Published: Jan 1988


    Abstract

    As surface flaws grow in high-strength nickel-base superalloys, the growth along the free surface (dc/dN) can be much slower than growth in the depth direction (da/dN), even after accounting for variations in K along the crack front. This behavior reduces the crack aspect ratio (c/a) and can result in the formation of stable shear lips at the free surface during fatigue crack growth. Trantina et al. recently performed elastic-plastic finite-element analyses of surface cracks which showed that as the applied stress approached the elastic limit, a relatively large region of constraint loss was observed where the crack front intersects the free surface. This concept was used to develop a constraint-loss modification to the Newman-Raju K solution for the surface flaw problem. This model has been applied to physically small surface crack growth in the advanced nickel-base superalloy René 95. The predicted results match the dc/dN data, which are slower than and not parallel to the corresponding da/dN-K data. This model has been combined with a dual-Walker exponent approach to model nonzero stress (R) ratios. This methodology has been used to predict the residual lives of surface flaws growing in both uniform and nonuniform stress fields within a factor of two of the experimentally observed lives. This model has also been successfully applied to small surface flaws and can predict the lives of cracks with depths (a) as small as 65 μm (0.0025 in.).

    Keywords:

    life prediction, surface flaws, nickel-base superalloys, fatigue crack growth, small cracks, fracture mechanics


    Paper ID: STP33098S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP33098S


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