Published: 23 August 2016
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At room temperature the fatigue strength of high-purity aluminum approaches a lower limiting value in accord with the usual observations on nonferrous metals. But at 250 C its fatigue strength continues to decrease abruptly with increasing cycles of stress without exhibiting any trend toward reaching a limiting value even at 107 cycles. Whereas finer grain-sized specimens exhibit higher fatigue strengths at room temperature, grain size has no effect on the fatigue strength at 250 C. Such insensitivity of the fatigue strength to grain size is undoubtedly due to the fact that both grain sizes yield identical structures consisting of extensively disoriented polygonized subgrains which completely obliterate the original grain size following brief cycling at a given stress at 250 C.
Over the limited frequency range (25-1440 cpm) of the experiments, the fatigue strength of high-purity aluminum is insensitive to frequency at room temperature. But above about 150 C, greater numbers of cycles of a given stress can be endured at higher frequencies of test. In this range the number of cycles to failure, N, appears to be related to the temperature, T, frequency, ν, and stress, σ, through a thermal activation energy, ΔH, and the gas constant, R, according to the relationship
N =f (νeΔH/RT, σ)
where f is an experimentally determinable function. Therefore for a given stress the same number of cycles can be endured at the same value of νeΔH/RT higher frequencies of test are therefore equivalent to lower temperatures. Since fatigue fracturing in the higher temperature range probably occurs along polygonized subgrain boundaries, it is not unexpected that the activation energy ΔH was found to be that for dislocation climb processes, namely that for self-diffusion.
Daniels, N. H. G.
Research Engineer, Shell Development, Emeryville, Calif.
Dorn, J. E.
Professor of Physical Metallurgy, University of California, Berkeley, Calif.