Lifetime calculation under multiaxial random loading with regard to microcrack growth*

Abstract

Abstract 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 most cases, a nonproportional cyclic multiaxial state of stress occurs. Compared to in-phase loading, multiaxial loading with a phase shift between the stress components and a load ratio of τa/τa ≈ 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 is designed to simulate the damage process based on the growth of microcracks under the influence of cyclic loading. 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 based on microcrack growth are compared and verified with those experiences obtained from multiaxial fatigue testing.