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

M Kachit (1) , J. Courbon (1) , X. Boulnat (1) , J. Adrien (1) , C. Verdu (1)

1 MATEIS - Matériaux, ingénierie et science [Villeurbanne]

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.

Mots clés: Multi-axial fatigue life; Phase shift angle; Crack orientation; Principal direction; Cast iron

Domaines: Mécanique des matériaux [physics.class-ph]

M Kachit, J. Courbon, X. Boulnat, J. Adrien, C. Verdu. Non-symmetrical fatigue life of nodular graphite cast iron under non- proportional multi-axial cyclic loading. International Journal of Fatigue, 2019, 125, pp.128-137. ⟨10.1016/j.ijfatigue.2019.03.044⟩. ⟨hal-02906567⟩