STP1332: Stress Ratio Effects on Crack Opening Loads and Crack Growth Rates in Aluminum Alloy 2024

    Riddell, WT
    Resident research associate, National Research Council and senior scientist, Mechanics of Materials Branch, NASA Langley Research Center MS 188E, Hampton, VA

    Piascik, RS
    Resident research associate, National Research Council and senior scientist, Mechanics of Materials Branch, NASA Langley Research Center MS 188E, Hampton, VA

    Pages: 19    Published: Jan 1999


    Abstract

    The effects of stress ratio (R) and crack-opening behavior on fatigue crack growth rates (da/dN) for Aluminum Alloy (AA) 2024-T3 were investigated using constant-ΔK testing, closure measurements, and fractography. Fatigue crack growth rates were obtained for a range of ΔK and stress ratios. Results show that constant ΔK fatigue crack growth for R ranging from near 0 to near 1 is divided into three regions. In Region I, at low R, da/dN increases with increasing R. In Region II, at intermediate R, fatigue crack growth rates are relatively independent of R. In Region III, at high R, further increases in da/dN are observed with increasing R.

    Near-crack-tip surface closure measurements, using a new noncontact displacement method, show that Region I transient crack growth rates are a result of crack closure. Increasing fatigue surface microvoid density, coinciding with increasing da/dN for Region III fatigue crack growth, suggests that a Kmax-dependent damage mechanism is responsible for the transient fatigue crack growth rates characteristic of Region III. For AA 2024-T3, the onset of Region III correlates with a critical value of Kmax (13 MPa m1/2). Region II fatigue crack growth is unaffected by both closure and Kmax effects. In this intrinsic region, ΔK = ΔKeff Rates from this region are used to test the validity of closure measurements for Region I fatigue crack growth. Poor correlation of da/dN and ΔKeff occurs when ΔKeff are calculated from the fully open load obtained by the reduced strain technique. Good correlation of da/dN and ΔKeff are found when ΔKeff is calculated from local opening loads approximately 0.1 mm behind the crack-tip.

    Keywords:

    aluminum alloy 2024, fatigue crack growth, stress ratio, crack closure, mean stress, compliance


    Paper ID: STP14961S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP14961S


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