Planning and Interpretation of Fatigue Tests

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Abstract
The significance of the scatter of results of fatigue tests and the inadequacy of the conventional procedures of their planning and interpretation is discussed. An attempt is made to derive the cumulative frequency distribution function P(N) of the fatigue life, N, at any particular stress amplitude, S, from considerations of statistical theory, which, as a first approximation, leads to a logarithmic-normal distribution function of N. Results of rotating-beam and reversed torsion-fatigue tests performed on steel, aluminum, and copper, when represented on logarithmic-normal probability paper, are shown to approximate the theoretically derived distribution. From the so plotted cumulative distributions P(N) of the fatigue life of aluminum at a number of stress amplitudes, S, a complete, three-dimensional S-N-P relation is obtained.

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