STP1122: Fatigue Crack Growth from Narrow-Band Gaussian Spectrum Loading in 6063 Aluminum Alloy

    Veers, PS
    Senior Members of the Technical Staff, Sandia National Laboratories, Albuquerque, NM

    Van Den Avyle, JA
    Senior Members of the Technical Staff, Sandia National Laboratories, Albuquerque, NM

    Pages: 23    Published: Jan 1992


    Abstract

    Constant amplitude and narrow-band Gaussian loadings are applied to extruded 6063 aluminum crack-growth specimens in an effort to characterize the effective stress intensity levels during random loading. Crack-growth rates are determined for constant amplitude loadings at stress ratios (R) of 0.09, 0.3 and 0.5, and for a variable amplitude loading simulated to match a narrow-band Gaussian spectrum. Crack-opening stress levels measured by the compliance method during the constant amplitude loading are found to differ substantially for -T5 and -T6 heat treatments due to a change from intergranular to transgranular crack growth. Crack-opening load ratios correlate well with the maximum applied stress intensity factor, Kmax, for the -T5 material. The Kmax dependence leads to an effective halving of the crack-growth exponent. Calculated variable amplitude lives are much shorter when this correlation is taken into account (an acceleration effect) and show a greater difference between loading blocks condensed by racetrack filtering at threshold levels of two and four standard deviations, similar to what was observed in the tests. Crack-opening-load measurements in one specimen with the narrow-band Gaussian (variable amplitude) loading failed to detect any closure. A substantial difference in the closure behavior of nominally identical R = 0.3 tests indicates that closure may occur irregularly in the extruded aluminum. Calculated crack-growth lives, assuming no closure in the variable amplitude tests, are much shorter than the test results. Including closure in the variable amplitude loadings greatly improves the predictions.

    Keywords:

    fatigue, crack growth, crack closure, random loading, loading simulation, aluminum, crack-opening load, load condensation, racetrack filter, wind energy, test methods


    Paper ID: STP24160S

    Committee/Subcommittee: E08.04

    DOI: 10.1520/STP24160S


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