In the previous works, the criterion for the fatigue strength of unnotched specimens under combined stress was proposed and treated separately from that of notched specimens, but it has been found that the former criterion is a special case of the latter. It has been also found that as long as the torsional fatigue strength is determined carefully, the same criterion can be applied to the results of both fracture and nonfracture tests, although the criterion was originally proposed for the case where the fatigue strength is defined to be the stress below which no 0.1mm slip-band cracks initiate. Slip-band cracks were found fractographically to correspond to the so-called stage I cracks. From several experimental results appeared in the literature, the criterion has been confirmed to be also applicable generally to the rolled steel products in spite of the fact that rolled metals possess inevitably the anisotropy due to rolling. Finally, it was concluded that the proposed criterion alone can be used successfully to estimate the fatigue strength under the combined stress state of bending and torsion, within experimental errors, for both ductile and brittle materials instead of using different empirical formulae which had been adopted previously.
