Fatigue crack growth in austempered ductile and grey cast irons—stress ratio effects in air and mine water

A study is presented of the effect of stress ratio on fatigue crack growth in grey (GI) and austempered ductile (ADI) cast irons in laboratory air and, for the ADI, in synthetic mine water. Fatigue crack closure was measured by compliance techniques and factored out of the applied ΔK values (ΔK=Kmax−Kmin) to give effective stress intensity values. Crack growth rate modelling was then attempted for the laboratory air data using a two-parameter approach (ΔK and Kmax). This worked well for the ADI, but not for the GI, probably due to the much larger scatter inherent in the fatigue crack growth rates in the latter alloy. Trends in the observed growth rate and closure data for the two alloys are explained in terms of mechanism changes arising from microstructural/crack tip plastic zone interactions, and Kmax effects.