## 1 Introduction  
Timber trucks are commonly used to transport logs on both forest and public roads. Transportation is the most expensive part of the timber production process. The relationship between the volume of timber cargo and its actual weight can be difficult to accurately determine. According to statistical studies, up to 60-80% of timber truck sets in European countries can be overloaded. This means that the timber cargo is heavier than can be carried by the truck within the cargo volume. In such cases, the permissible total weight of a timber truck set can be exceeded by up to 50% [1]. The possibility of significant overload makes it advisable to include it in the design process of truck components. It is particularly important to determine the actual load history for fatigue calculations. In general, the assessment of the fatigue life of structural components is based on determining stress amplitudes and mean stresses and considering S-N diagrams of the materials. The load history for the design process can be accepted with constant amplitude according to standardized load histories (SLHs) or based on measurements under service loads. For load cycles that are not fully reversed, mean stress correction is required [2]. Problems related to fatigue life prediction of constructional elements were raised by authors of numerous publications. [PERSON] provided a historical outline of fatigue analysis and described current trends in assessing the fatigue life of engineering structures under service loads [3]. Classification criteria of fatigue failure and stress state components for multiaxial load conditions were also discussed. [PERSON] and [PERSON] [4] and [PERSON] [5] presented problems related to the numerical implementation of multiaxial high-cycle fatigue criterions in structure optimization. An original method of calculating fatigue life of a soldered construction node based on the two-parameter fatigue characteristics and the linear cumulative damage hypothesis was developed by [PERSON] [6]. [PERSON] et al. presented findings in [7] on the fatigue behavior of single multiple-riveted lap joints used for aeronautic structures. Both the fatigue life and critical crack size were analyzed. [PERSON] and [PERSON] delivered analytical formulas in [8] for determining stresses in the contact area and proposed a methodology for fatigue life assessment of rolling bearings by applying the multiaxial high-cycle fatigue hypothesis. In [9], [PERSON] presented a numerical evaluation of the fatigue load capacity of the cylindrical crane wheel. The application of multiaxial high-cycle fatigue criteria for analyzing the subsurface rolling contact fatigue of structures working in contact conditions was discussed. Studies by [PERSON] et al. [10] and [PERSON] et al. [11] referred to the fatigue analysis of truck frames under service loads. In turn, dynamic loads resulting from the actual road roughness were considered in the truck frame fatigue analysis by [PERSON] et al. [12]. Experimental tests are crucial for fatigue failure analysis. These tests may concern the collection of load history data or the execution of fatigue tests for specified load sequences. For example, [PERSON] et al. presented findings on the fatigue life assessment of a wheel loader [13]. The history of external loads was obtained on the basis of experimental tests involving strain gauge measurements and the load identification model. A cumulative damage theory and a structural stress method were applied to determine the fatigue life of welded components. In the study by [PERSON] et al., the method of fatigue strength evaluation of a bogie frame was proposed [14]. Analysis included calculations of cumulative fatigue damage and experimental verification on a full-scale test rig. In addition, an increase in the dynamic factor was assumed in the following fatigue test sequences. Meanwhile, a fatigue life prediction of the end beam of a freight car bogie was developed in the study by [PERSON] et al. [15].