STP677: Role of Crack-Tip Stress Relaxation in Fatigue Crack Growth

    Saxena, A
    Senior engineers, Westinghouse Research and Development Center, Pittsburgh, Pa.

    Hudak, SJ
    Senior engineers, Westinghouse Research and Development Center, Pittsburgh, Pa.

    Pages: 18    Published: Jan 1979


    Abstract

    This study constitutes an effort to identify underlying processes which contribute to load ratio (R) effects in fatigue crack growth. Approximate analytical expressions are developed for crack-tip strains during steady-state cyclic loading. Using these expressions, the strain history of an element of material which is being approached by the tip of a fatigue crack—growing at a constant rate of either 2.5 × 10-9 or 2.5 x 10-7 m/cycle (10-7 or 10-5 in./cycle) was calculated for load ratios of 0 and 0.8. Applying these strain histories to smooth axial fatigue specimens of 10Ni steel and 2219-T851 aluminum simulated the material's mean stress relaxation behavior at the crack tip. The number of cycles to failure correlated with the simulated growth rates. Also, mean stress relaxation characteristics in the crack-tip region qualitatively explain the load ratio effects on da/dN. This information is likely to be important to alloy development and material selection for fatigue resistance as well as for proper modeling of fatigue crack growth data at several load ratios.

    Keywords:

    stress intensity, fatigue (materials), crack growth, aluminums, steels, load ratio, stress relaxation, crack closure, crack propagation


    Paper ID: STP34915S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP34915S


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