Abstract
A method of fatigue reliability evaluation for structure loaded by a random process σ(t) during operation is proposed in this paper. For this process construction of probabilistic fatigue diagram is proposed, giving a relationship between the parameters of random loading process and probabilistically interpreted fatigue life of the structure. In this diagram it enables, by computational way, to construct the limit line for the requirements on structure service, represented by the required (prescribed) fatigue life and allowable probability of premature fatigue fracture occurrence. The limit line allows to determine the values of limit parameters of the σ(t) loading process, exceeding of which means that the structure already does not respect the requirements put on its operation and becomes unreliable from the fatigue point of view. Reserve of the operational σ(t) process parameters in relation to their limit values is expressed by the value of safety in loading. In order to guarantee a reliable operation of the structure, the value of safety must not decrease below the minimal requested value. By this value the area of reliable operation is allocated in the probabilistic fatigue diagram.
The proposed methodology respects the stochastic character of loading process and material properties and, using the proposed probabilistic fatigue diagram, allows to evaluate different operational loadings in view of fatigue reliability for the given structure or to assess already existing structure for the given loading.

Key words
fatigue life, random loading, fatigue reliability, safety, probability of failure, operating load conditions

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