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In fatigue life prediction, it is common to analyze a component subjected to a load-versus-time history that varies in magnitude over time. In established methods, it is assumed that the directions of the applied loads do not change, so stresses along the same reference plane can be compared as unidimensional quantities. Put differently, these methods require that the principal stress orientation remains the same throughout the load history. This work aims to determine the fatigue life of a component experiencing a load-versus-time history that produces stresses varying in both magnitude and principal stress orientation. To accomplish this, all six components of the stress tensor are required throughout the loading history. By choosing a cut plane defined by the normal direction n, the normal stresses can be determined. The application of the rainflow counting method can then be used to convert the normal stresses into equivalent fatigue cycles.

Estimation of remaining life of an engineering component becomes an important subject for optimum utilization of resources, materials, energy, and labor. It is also useful for new components and components in current use. The paper develops analytical expressions for the estimation of remaining life for parts that satisfy the Weibull distribution. Weibull parameters established for new components are used in the expressions. The ratio of remaining life to the initial rated life is considerably large for components with small Weibull shape parameter.