Non-symmetrical fatigue life of nodular graphite cast iron under non-proportional multi-axial cyclic loading

Abstract

Non-proportional axial-torsional loading fatigue tests on nodular cast iron revealed a non-symmetrical behavior of the fatigue life relatively to the phase shift angle δ = 90° between axial and torsion loads. This behavior was in agreement with fatigue crack orientation investigated using optical and electron microscopy. Elasto-plastic finite element model (FEM) were used to assess the crack orientation behavior and it could explain this non-symmetrical behavior by the non-symmetrical values of both σI and σI / σII ratio relatively to δ = 90°. FEM revealed that for the δ < 90° loads, principal stress was applied essentially at critical planes of low angle ϕn, thus occurrence of tensile cracks mode was higher for δ < 90° than for δ > 90° load, which reduces the fatigue life. σI / σII ratio strongly influenced the dominant crack mode. When the applied loads were torsion σI/σII = 1, cracks were observed to occur, equally, in modes I and II. Presence of the non-propagating failure (mode II) significantly increased the fatigue life. Conversely, for high phase shift, where σI / σII ≫ 1, crack mode I dominated and crack driving force remained high during the whole fatigue cycle, inducing a lower fatigue limit.