A study on combined torsion and axial load fatigue limit tests with stresses of different frequencies

The aim of this paper is to investigate the high cycle fatigue behaviour of metals in the case of multiaxial loading where the stresses are of different frequencies. In particular, combined axial load and torsion fatigue tests were performed on specimens made of quenched and tempered steel. This material has been chosen because it is commonly employed for the production of mechanical components intensively stressed under lengthy fatigue loads. Two frequency ratios between the shear stress and the normal stress are examined and the resulting fatigue limits from these tests are compared to the results from tests where the shear and normal stress are of the same frequency and in-phase. Moreover, different multiaxial fatigue limit criteria, based on the critical plane concept and on the so-called integral approach, are used to analyse the obtained experimental results. In general, better agreement between predictions and experimental results is achieved by the criteria of the integral approach category.