A STUDY OF THE EFFECTS OF LOADING AMPLITUDE AND INDUCED STRESS RATIO ON FATIGUE LIFE OF A NODULAR GRAPHITE CAST IRON STRUCTURE UNDERGOING MULTI-AXIAL PROPORTIONAL CYCLIC LOADING

The influence of loading conditions on fatigue life and stress concentration, on ferritic nodular graphite cast iron material, undergoing multi-axial proportional cyclic loading, is investigated experimentally and numerically. The studies are conducted under a controlled maximum amplitude stress (induced Von Mises stress) of 158 Mpa which is close to the fatigue limit of the studied material. Good agreements are reported between numerical and experimental results. It is found that fatigue life and Von Mises stress concentration depend strongly on cyclic loading amplitude ratio ‘R'. Fatigue life is less for proportional loading than for simple torsion or simple axial loading. The ultimate number of cycles, in the studied interval of ‘R' (-1 to +1), increases in a linear fashion. It has a minimum value at R equals to -1. This effect is directly related to the maximum Von Mises stress. The latter increases in a linear fashion as R increases from -1 to +1, and exhibits a maximum value at R equals to -1. Also, it is found that fatigue life and Von Mises stress are highly dependent on the induced stress ratio (σ/τ). Von Mises stress increases as stress ratio increases. In a reverse manner, fatigue life increases slowly as the ratio /increases. This result concludes the strong effect, on fatigue life, of the presence of the axial load within the multi-axial cyclic loading. In fact, axial loading favors the mode I crack which propagates rapidly perpendicularly to the axis of axial loading.