Influence of stress concentration and multiaxial stress state on low cycle fatigue

Torsion tests were performed on thin tubular specimens made of 34CrMo4 steel. The low-cycle curves obtained from torsional tests were compared with the ones obtained from axial tests. Experimental tests were executed on notched specimens made of the same material and the fatigue behaviour was studied. Microscopic observations and profile measurements of the fracture surfaces permitted to put in evidence the different kind of fractures and the fracture surface changes induced by the shape of the specimen and the load amplitude. The experimental tests together with a numerical analysis (conducted by using a finite element model) enabled the determination of all the useful parameters to perform a low-cycle fatigue life prediction. The calculation of the multiaxial stress and strain state near the notch was performed by the Neuber's rule and the ESED method.