Effects of small defects on the fatigue strength of steel and ductile iron under combined axial/torsional loading

Fatigue tests under in-phase combined axial and torsional loading were carried out on annealed carbon steel specimens containing an artificial small hole and smooth ductile iron specimens having graphite nodules as inherent small defects. Based on the observational result that the fatigue limit is determined by the threshold condition for propagation of a mode I crack emanating from small defects, a criterion for fatigue failure of defect specimens under combined stress is presented on the basis of fracture mechanics. Excellent correlations between the criterion and experimental results are shown.