STP1211: Crack Density and Fatigue Lifetime of Metals Under Variable Amplitude Loading

    Bomas, H
    Senior research scientist and head of institute, Stiftung Institut für Werkstofftechnik, Bremen,

    Mayr, P
    Senior research scientist and head of institute, Stiftung Institut für Werkstofftechnik, Bremen,

    Pages: 9    Published: Jan 1993


    Abstract

    During cycling with constant and variable amplitudes, the density of the fatigue cracks of a copper alloy, a mild steel, and an austenitic steel was measured. Cycling with constant amplitude showed that the crack density increases with rising stress amplitude and advancing number of cycles. Cycling with variable amplitudes showed that the lifetime depends on the sequence of loading. Numerous changes from high to low amplitudes lead to a short lifetime. This effect depends on the material and is very pronounced for the copper alloy. The crack density was calculated by an addition model, which is based on the constant amplitude data, and was compared with the measured crack density. Lifetime was calculated after the method of Miner. For all metals investigated there exists a unique relation between the ratio measured crack density/calculated crack density and the realized damage sum after Miner. The crack density is always higher than expected when the realized damage sum is lower than the theoretical value of 1. This indicates that the influence of the sequence of loading on the lifetime bases on an alteration of the crack initiation rate in variable amplitude loading compared with constant amplitude loading and that crack coalescence plays an important role in failure of the materials investigated.

    Keywords:

    metals, fatigue, crack density, lifetime, variable amplitude loading, Miner's rule


    Paper ID: STP15082S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP15082S


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