Uncertainty in fatigue loading: Consequences on statistical evaluation of reliability in service

► Models to describe fatigue life variation with respect to the applied stress.
► Applied stress considered as a random variable with its own uncertainty.
► Evaluation of the errors of in service reliability and in service fatigue life.
► Exact equations for error evaluation.
► Applied stress uncertainty consequent to ASTM standard requirements.

A design-by-reliability approach is ever more required and accurate design inputs are needed to meet reliability targets while reducing costs. Different models have been proposed in the literature to describe fatigue life variation with respect to the applied stress by assuming the applied stress as a deterministic independent variable and the fatigue life as a dependent random variable.In the paper also the applied stress is considered as a random variable with its own uncertainty and the procedure to evaluate the error on the estimation of parameters usually adopted in service reliability assessment of structural components is shown. Exact equations are proposed for some special cases and an illustrative example showing the reliability errors originated by applying the ASTM recommendations is given.