To guarantee the safety of steel structures it is important to deal properly with the problem of representation of exploitation loading. For metal fatigue testing and design, the proper choice of random (irregular) loading type is very important. The principles of random loading are discussed and some alternative approaches with their pros and cons are shortly reviewed in the paper. As a sound decision for random, but taking into account some specific features of the exploitation loading process, the target Markov method is proposed. According to this method, the important information of the real random processes in the form of the turning point is used for filling the square Markov matrix (analysis phase) and later on, with employing the random number generator, serves as a source for creating of the so-called replicas. The replicas are the random trial for numerical estimation of longevity scatter. Due to the fact, that all these manipulations are performed with the aim of metal fatigue investigation, some important processes’ characteristics for fatigue, like irregularity factor, fullness factor and machine part longevities were compared. Some important suggestion for the future development of this method, that is taking into consideration the sequence of the events, is discussed.

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Estimation of the scatter of the durability at the second stage of fatigue, namely at the stage of crack propagation is a problem of scientific and obvious practical importance: machines operate according to their technical condition, which means monitoring of the actual crack length during their service life. The limits of the spread of the strength values at the stage of crack propagation in aluminum samples are studied using published data and a previously developed model. In view of the great importance of this problem, a special simulation model was used to generate the extrema of a random sequence based on target Markov matrices. On the one hand, this simulation method guarantees the characteristic traits of real sequence in exploitation (TWIST standard in this example). On the other hand, it contains reasonable randomness — these two parts together provide an opportunity to study the variability of the crack growth rate. For the simulation experiment, literature data on aluminum and steel samples were used along with popular fatigue crack growth models (Paris, Foreman and Willenborg models). In addition, the Miner’s summation rule was quantitatively tested to estimate the crack growth resistance coefficient under various loads. The agreement with the literature experimental data is shown. Preliminary data on the effect of the type of loading (random or block) on the durability are given on the basis of scientific literature data. The proposed simulation method can be useful for testing various models. It is also intended to develop an experimental design for laboratory testing in the future.