STP1189: Fatigue Crack Growth Rate Measurements in Aluminum Alloy Forgings: Effects of Residual Stress and Grain Flow

    Bush, RW
    Senior engineer, technical consultant, and staff engineer, Aluminum Company of America, Alcoa Technical Center, Alcoa Center, PA

    Bucci, RJ
    Senior engineer, technical consultant, and staff engineer, Aluminum Company of America, Alcoa Technical Center, Alcoa Center, PA

    Magnusen, PE
    Senior engineer, technical consultant, and staff engineer, Aluminum Company of America, Alcoa Technical Center, Alcoa Center, PA

    Kuhlman, GW
    Manager, Research and Advanced Technology-Forgings, Cleveland, OH

    Pages: 22    Published: Jan 1993


    Abstract

    An investigation to determine the causes of increased variability in fatigue crack growth (FCG) rate measurement in aerospace aluminum forgings compared to that observed in aluminum flat rolled plate was conducted. Fatigue cracks were grown at constant applied ΔK, consistent with ASTM Test Method for Measurements of Fatigue Crack Growth Rates (E 647-88a), through regions of rapidly changing grain-flow direction within thin 7075-T73 precision forgings and large closed-die 7050-T74 forgings. The local grain flow in each specimen was correlated subsequently with measured crack growth rates so that the contribution of grain-flow variability to the scatter in measured crack growth rates could be determined. These results were found to be comparable to FCG results from 7075-T7351 and 7050-T7451 plate, once residual stress biases were removed by means of closure measurement. Hence, it is concluded that the effects of microstructural features such as grain flow on FCG rate are minimal, and the inherent FCG resistances of aerospace aluminum forgings and plate are comparable. Consequently, the analytical fracture mechanics framework applicable to damage tolerance verification of plate product should extend equally to forging product forms. However, the results indicate that failure to account for residual stress effects in interpretation of forging test results can lead to erroneous conclusions about the material performance.

    Keywords:

    fatigue crack growth, forgings, flat-rolled plate, residual stress, grain flow, damage tolerance, fracture mechanics, fatigue (materials)


    Paper ID: STP24289S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP24289S


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