FATIGUE STRENGTH PREDICTION OF DUCTILE IRONS SUBJECTED TO COMBINED LOADING

The fatigue strength of ductile iron is influenced by small defects such as graphite nodules and microshrinkage cavities existing in the structure. Rotating bending and combined axial and torsional fatigue tests were carried out on round-bar smooth JIS FCD400, FCD600 and FCD700 specimens. A criterion for multiaxial fatigue failure of specimen with small defects is presented based on the observational results that the fatigue limit of specimens subjected to in-phase multiaxial stress is determined by the threshold condition for propagation of a mode I crack emanating from small defects. Using this criterion and the area parameter model, a method is proposed that enables one to predict the lower bound of fatigue limit of ductile irons by quantifying the effects of small defects and matrix structures without a fatigue test. The experimental results are in good agreement with the results predicted by the method