The effect of mean stress on fatigue life and fatigue limit was investigated for S355 NL+N ferritic-pearlitic steel. Changing stress ratio from R=0 corresponding to pulsating regime to R = -1, which is in conformity with alternating regime, the fatigue life was changed when applying the mean stress for the same stress amplitude. Although this phenomenon is generally very well known it is important to investigate it further regarding to commonly applied material. Research results of this topic could help to structure designer better utilize fatigue behaviors of materials as S355 steel which is commonly used for constructions of welded structures. Since this steel is favored for production of structures, it is extremely important how the alternating stress ratio affects their service life. The experimental results show that with increasing mean stress of loaded S355 steel and growing stress ratio, the fatigue limit diminishes and fatigue life is getting shorter. Then for a given value of the limited amplitude the number of cycles up to failure with increasing stress ratio decreases. This effect is much more substantial when the limited amplitude approaches to the fatigue limit for a given value of the stress ratio. Discussed experimental results help to optimize mechanical design of steel welded structures.Keywords: Fatigue; Mean stress; Ferritic - pearlitic steel; Fatigue limit; Mean stress; Stress ratio
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