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The paper deals with fatigue strength properties and damage modes of aC40 carbon steel (normalised state) subjected to fully reversed torsion. Both smooth andnotched specimens with four different V-shaped notches are tested. In all notchedspecimens the notch tip radius ρ is kept constant and equal to 0.5 mm while the notchdepth changes from geometry to geometry. Fatigue crack nucleation and propagationphases are discussed in detail as well as the influence of microstructure on low/highcycle fatigue behaviour.