Engineering structures often work under operating conditions in which the character of a load undergoes permanent change (different service-loading regimes). Such an operation is very frequent for transportation equipment, earthmoving machines, airplanes, etc. Considering that the service-loading process (time history of σ(t) stress or ∈(t) strain) at particular operating regimes exhibits in a majority of cases the stochastic nature, the question arises as to how to estimate the fatigue resistance of the structure under such service conditions. For this reason, a method was developed to evaluate such service from the viewpoint of fatigue reliability and to estimate the resulting fatigue lifetime in terms of probability interpretation.
The result of life calculation is the distribution function (DF) of the residual fatigue lifetime for the given loading regime. This DF reflects the loading history (sequence and nature of the previous loading regimes) and the random character of the interacting material and loading parameters. The DF obtained is in a good agreement with the experimental results. Finally, illustrative examples that reflect practical applications of this theory are presented.