Plane bending low-cycle fatigue investigations of 45-steel

The results of low-cycle fatigue investigations of the 45-steel, which were conducted under plane bending conditions are presented in this paper. Fatigue tests were carried out for five levels of the total strain epsilon_ac = 0.04, 0.02, 0.01, 0.008, 0.005. Three different heights of specimens (H=8 mm, H=12 mm, H=16 mm) were used for evaluating the influence of strain gradients in specimens. Strains in specimens were measured in a zone of maximal strains (extensometer). Test results are shown on fatigue curves and cyclic strain curves. The analysis of fatigue curves shows the influence of the specimen height on fatigue life. The maximal values of fatigue life were found for the specimen of H=16 mm height, whereas the minimal values appeared for the specimen of H=8 mm height. The comparative analysis of fatigue life under conditions of axial and plane bending shows the essential differences between the results obtained. These differences come up to 300% for large
values of strains. Taking into consideration the shapes and flows of cyclic strain curves and static bending curves the dependence of cyclic properties of the tested steel on the total strain amplitude on the lateral surface of the specimen was noticeable. Basing on the stress curves in bended specimens analysis, the influence of their parameters on the fatigue life was observed. Three parameters: the tangent gradient, the plastic gradient and the thickness of plastic zone for the stress curves analysis were proposed.