Crack path in torsion loading in very high cycle fatigue regime

Torsion fatigue tests have been conducted at 20 kHz ultrasonic fatigue testing systems, and compared to the torsion fatigue data generated on 35 Hz conventional fatigue test machine to determine if there are any frequency effects, for steels including D38MSV5S steel and 100C6 steel. Results indicated that the S–N curves exhibit decrease in fatigue strength beyond 107 cycles. The initiation in the Gigacycle regime is related to defects sometimes located beneath the surface which shows a competition between the maximum shear at the surface and the stress concentration under the surface, even in torsion.