Multiaxial fatigue behaviour of sintered steel under combined in- and out-of-phase bending and torsion

This paper discusses the fatigue behaviour of sintered steels under multiaxial loading. These steels are the Fe-1.5% Cu and the Fe-2.0% Cu-2.5% Ni, sintered at low and high temperatures, in the densities 7.1 and 7.4 g/ccm, which are used in the production of several ready-to-assemble automotive parts. Fully reversed or pulsating combined loading with constant frequency and amplitudes acting in-and-out-of phase, was applied to round notched specimens (Ktb=1.49, Ktt=1.24) in the infinite life region (10 high 4 < Nf < 2x10 high 6). The mechanics of crack initiation and propagation as well as rupture were studied using fractography and microfractography. These analyses led to a mechanical model based on a local normal stresses for the fatigue life evaluation. The fatigue life evaluation on the base of the local bending stress obtained under uniaxial loading describes the test results for in-phase combined bending and torsion satisfactorily. But the increase of fatigue strength and life by out-of-phase loading is overestimated in the case of fully reversal loading. However, for design purposes the out-of-phase loading can be neglected because of its beneficial effect in increasing fatigue life for this type of material.