Experimental investigation on the significance of the conventional endurance limit of a spheroidal graphite cast iron

Fatigue tests were performed on a spheroidal graphite cast iron in four point plane bending under constant stress amplitude and block loading conditions. The microstructure of this material has a 'bull's eyes' appearance, i.e. the spheroids of graphite are surrounded by ferrite and these nodules and ferrite zones are included in a pearlitic matrix. Scanning electronic microscope observations were carried out at different fractions of life for constant stress amplitude loadings above and below the conventional endurance limit. Non-propagating micro-cracks were observed at a stress level equal to the conventional endurance limit. These observations showed that another limit can be defined below the conventional endurance one, i.e. one below which micro-cracks were not observed to initiate in the matrix. These cracks were found to arrest at the ferrite/pearlite interface when the material was tested below this new limit. This concept was used to rationalize fatigue results from tests with loading in blocks above and below the conventional endurance limit.