Non-symmetrical fatigue life of nodular graphite cast iron under non- proportional multi-axial cyclic loading
Résumé
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 σ I and σ I / σ II ratio relatively to δ = 90°. FEM revealed that for the δ < 90°loads, principal stress was applied essentially at critical planes of low angle ϕ n , thus occurrence of tensile cracks mode was higher for δ < 90°than for δ > 90°load, which reduces the fatigue life. σ I / σ II ratio strongly influenced the dominant crack mode. When the applied loads were torsion σ I /σ II = 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 σ I / σ II ≫ 1, crack mode I dominated and crack driving force remained high during the whole fatigue cycle, inducing a lower fatigue limit.
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