## 1 Introduction
Validation and Calibration: It is important to validate and calibrate the algorithm against experimental data or real-life case studies to ensure its accuracy and reliability. This may involve performing fatigue tests on representative samples or comparing the algorithm's predictions with known fatigue failures based on maintenance data. Bearing in mind the above factors affecting fatigue life, new and more accurate algorithms for determining fatigue life are being developed. These algorithms are built to take into account all factors, creating a universal way of solving the problem. Nevertheless, each case is different and even the most universal algorithm needs to be adapted to the current case. The reason for using the algorithm for the current needs is primarily the speed of calculations and their accuracy. A decision should be made regarding:  
* durability estimation methods i.e., the stress or strain approach,
* domain in which the calculations will be performed, i.e., in time or frequency,
* whether adjustments to the impact of notches on fatigue life will be necessary,
* whether \"pure material\" fatigue or fatigue of joints, such as welds or bolted components, determine durability,
* whether the influence of external factors, such as temperature change or corrosive environment, should be taken into account,
* from where they will be taken or by what method stresses or strains will be determined. In order to present the complexity of this issue, two cases of determining the fatigue life of structures are presented below, which differ significantly from each other, and the basis for their determination are classic stress fatigue characteristics, i.e., SN curves for basic material. The compressor suspension system, analysed as the first example, is a key component of air suspension systems, which are commonly used in vehicles to absorb shocks and maintain driving stability. It is obvious that this system is subjected to random vibrations and its performance is highly dependent on the material properties of the suspension components, such as shock absorbers. Accurate prediction of the dynamic response and fatigue life of this system under operating conditions is essential for its reliable operation over time. The analysis, including both experimental measurements and numerical simulations using the finite element method (FEM), allows modelling of the system response to random vibrations, determination of stresses, and finally leading to fatigue life assessment. The second example is a Motorcycle Transport Device that is subjected to a different set of loading conditions, mainly related to the dynamic forces encountered during towing operations. Since the vibrations of the device were recorded under real conditions, the load characteristics were known. The fatigue analysis of the Motorcycle Transport Device focuses on the effect of power spectral density (PSD) simplifications on the calculated fatigue life. The simplification of the load power spectral density can significantly reduce the computational effort but raises questions about the accuracy of the fatigue life estimates. How different PSD simplification methods affect the prediction of fatigue life of the Motorcycle Transport Device are examined.