NEAR-THRESHOLD FATIGUE BEHAVIOR OF FLAKE GRAPHITE CAST IRONS MICROSTRUCTURES

The present paper attempts to describe a study concerning the influenc e of mean stress or R-ratio and microstructural variation, percentage polygonal ferrite, on the near threshold fatigue crack extension chara cteristics of a flake graphite cast iron. Basically significant effect s of R-ratio and microstructure on the fatigue threshold stress intens ity range, Delta K-th, were observed. Also at intermediate Delta K lev els the flake cast iron microstructure exhibited a much higher slope i n the two-parameter fatigue crack growth relationship than those obser ved in ductile low alloy steels. It was also observed that at low R-ra tio the Delta K-th values of the flake cast irons were significantly l arger than those recorded for iron-based and low alloy steel materials . The high levels of Delta K-th were explained in terms of roughness i nduced crack closure and crack deflection processes which resulted fro m the very rough topography of the fatigue fracture surfaces. Finally the role played by the graphite constituent in the fatigue fracture pr ocesses prevalent in cast iron microstructures was discussed.