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In multiaxial fatigue, a combination of random loads produces a complex state of stress at a point. These loads may be independent or partially related. Generally, the principal plane at the stress point varies with time which causes difficulty in assessing fatigue damage. This paper examines multiaxial stress using conditioned spectral analysis which, in certain circumstances, reduces the loads to a set of independent loads each of which is associated with a principal plane, allowing a separate assessment of fatigue damage with respect to each principal plane.

A mathematical and statistical relationship between descriptions of random service loading in terms of time series, rainflow counts and vibrational characteristics is presented. Data from durability tests is analyzed in terms of the relationships and the statistical nature of the data is highlighted.