Lifetime simulation under multiaxial random loading with regard to the microcrack growth

Generally, most machine parts are loaded with a combination of different variable forces and moments which often causes a state of multiaxial stress in the fatigue critical areas of the parts. In the most cases, a non-proportional cyclic multiaxial state of stress occurs. Compared to the in-phase loading, a multiaxial loading with a phase shift between the stress components and a load ratio of τa/σa≈0.5 between tension/compression and torsion leads to a significant influence on the fatigue lifetime. The reason is the changing direction and rotation of the principal stresses during one cycle. In this paper, a model designed to simulate the damage process based on the growth of microcracks under the influence of cyclic loading is presented. The crack growth is initially dominated by shear stresses leading to microstructurally short cracks (stage I) and continues to grow under the influence of normal stresses (physically short cracks). The results of the lifetime estimation generated by means of the new concept on the basis of microcrack growth are compared and verified with those experiences obtained from multiaxial fatigue testing.