The Use of Spectral Methods for Assessing Durability at Post-processing Stage Requires Justification

In some massive constructions, like off-shore platforms, it is sometimes difficult or impossible to register the loading process during many years of exploitation. Knowing the input random processes characteristics, it sometimes seems promising to estimate the output characteristics using the structural dynamic equation. Following this approach, the spectral densities are employed into longevity estimation. Many methods for estimation of the fatigue damage thought the spectral densities are being still developed. The question here arises: is it reasonable to use the spectral approach in case of post-processing stage of the machines when the prototype exists and the loading process realization could be possibly recorded? It is worth mentioning, that the ultimate goal of many authors of those methods is to achieve the results, similar to those, which are easily obtained directly from the RAINFLOW method. In the paper, the real loading process is analyzed. The aim of this paper is to show some contradictions of the spectral approach. In this paper, the short review with an analysis of the accuracy of the spectral method is presented. To show the fact, that some of the assumptions of spectral methods are doubtful, the method of the digitally modelled process is introduced. Most of all, the author wants to get the answer from the spectral methods adepts, why do they propose such a controversial way of estimation.