EFFECTS OF MEAN STRESS ON THE THRESHOLD FATIGUE CRACK EXTENSION RATES OF 2 SPHEROIDAL GRAPHITE CAST IRONS

This paper is concerned with the influence of R-ratio on the fatigue threshold DELTA K(th), in two spheroidal graphite cast irons. The microstructures, viz, ferritic and pearlitic, exhibited significant and consistent R-ratio effects on DELTA K(th). Intergranular failure facets were observed and they tended to reach a maximum amount when the reversed plastic zone size approached grain size dimensions. Reasonable correlation of experimental data required the increment of crack closure effects in the analytical model. At R-ratio values approaching zero, both microstructures showed DELTA K(th) values that were markedly greater than those recorded for steels even when the data was corrected or normalised to a constant yield stress and grain size for a specific microstructure. The presence of graphite nodules may cause an increase in crack closure which in turn promoted high DELTA K(th) values.