Materials Science, Vol. 55, No. 4, January, 2020 (Ukrainian Original Vol. 55, No. 4, July–August, 2019) FATIGUE LIFE OF S355JR STEEL UNDER UNIAXIAL CONSTANT AMPLITUDE AND RANDOM LOADING CONDITIONS A. Nies ony 1,2 and M. Böhm 1 The results of fatigue tests of samples made of S355JR steel under random tension-compression with nonzero mean stress are presented. The procedure of experimental research is described. The obtained experimental results are presented with the use of Wöhler fatigue graphs. The algorithm for the deter- mination of the fatigue life uses, among other things, a rainflow cycle counting procedure, as well as the Palmgren–Miner linear damage accumulation hypothesis and six selected models that take into account the effect of mean stress on the tested durability. We also display the plots for comparison of the exper- imental and computed lives. It is indicated, which of the considered models describes the impact of the mean stress on the fatigue life of the tested material within the scatter error 3. Keywords: fatigue, mean stress, S355JR steel. Introduction The initial force affecting a structural element and even its inherent weight has a significant influence on the fatigue phenomenon. These types of stresses are called initial or mean stresses and are often overlooked by de- signers in the process of design of connections or components due to fatigue. In numerous branches of industry the adequate consideration of the mean stress plays a significant role due to the prevalence of time-varying exter- nal forces, as well as internal forces in structures and machine components [9]. We can also mention an espe- cially important and difficult case involving the correct analysis of the mean stress, i.e., the case of multiaxial load with nonzero value of the mean stress [3, 4, 6, 7]. In this case, no one has been able to propose an efficient and reliable computation model so far. Steels belong to the most common groups of structural materials used for the construction of machines, mainly due to their availability, cost, and good mechanical properties [10] or corrosion resistance [2]. The literature review shows that the fatigue tests of S355JR steel studied in our work are rare. Therefore, the main aim of the present work is to verify the developed algorithm [12] used to deter- mine the fatigue life on the basis of the original experimental results. The case of tension-compression of un- notched round specimens with different levels of mean stresses is analyzed. The computational algorithm is constructed with regard for the state of knowledge in the evaluation of fatigue life in a random uniaxial stress state. The Rainflow cycle counting procedure used together with the linear cumulative damage hypothesis by Palmgren–Miner [16] and six selected models that take into account the effect of mean stresses on the fatigue have been used in the proposed algorithm. The effect of mean stresses on the fatigue life was taken into account by transforming the stress amplitude specified for each cycle by the Rainflow method. The coefficient K [4, 14] was found for six models in order to perform this task. 1 Opole University of Technology, Opole, Poland. 2 Corresponding author; e-mail: a.nieslony@po.opole.pl. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 55, No. 4, pp. 51–56, July–August, 2019. Original article submitted March 18, 2014. 514 1068-820X/20/5504–0514 © 2020 Springer Science+Business Media, LLC DOI 10.1007/s11003-020-00333-0