A comparison of the fatigue strength of machined and as-cast surfaces of SG iron

The fatigue strength of machined specimens of ferritic SG iron has been compared with the strength of specimens having an as-cast surface. Micropores, exposed by machining, initiated failure in the machined specimens, whereas surface irregularities or dross defects initiated failure from the as-cast surface. While the endurance of specimens in which failures initiated at surface irregularities was only marginally less than that of machined spec imens, dross defects reduced fatigue life by a factor of ten. It is demonstrated that the fatigue life of SG iron is dominated by micro-crack growth and that the effect of all types of SG -iron defects on fatigue endurance is related to defect size. Specimen endurances of less than 10 5 cycles can be predicted using a fracture mechanics approach and integrating the Paris Law from the appropriate defect size.