Recent studies indicate that most of the “scatter” observed in finite fatigue-life-testing of metals is an inherent characteristic of the material and does not necessarily indicate poorly adjusted machines or improper testing techniques. In the present study, groups of 17 to 57 specimens of 75S-T6 aluminum alloy were tested in rotating bending at six different stress levels. The smallest stress used was 30,000 psi and the greatest was 62,500 psi. A statistical analysis was made of fatigue-life data obtained at each of the six stresses. The distribution of the fatigue lifetimes, for each stress, was found to be approximately logarithmic-normal. The standard deviation (which is a measure of scatter in the fatigue life) was related to the stress amplitude and could be represented within the stress range investigated, by a simple exponential function. Constants of an S-N relation recently proposed by W. Weibull were evaluated for a probability of failure P = 0.50. Results of the study are summarized in two equations and in a composite S-N diagram showing lines of equal probability of failure.

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