Ultrasonic fatigue testing on notched and smooth specimens of ultrafine-grained steel

Ultrasonic and conventional electromagnetic resonance fatigue tests were conducted on notched and smooth specimens of ultrafine-grained steels. The notched specimens developed no internal fractures but showed a fatigue limit. In this case, ultrasonic fatigue testing showed good agreement with the electromagnetic resonance. The smooth specimens showed internal fractures after ultrasonic fatigue testing, but not during electromagnetic resonance testing. In the smooth specimens, ultrasonic fatigue testing showed a longer fatigue life. These results show that the frequency effect is negligible in surface fractures of notched specimens but not in smooth specimens. The frequency effect causes a change in type of fracture from surface fracture to internal fracture. Concerning the gigacycle fatigue properties, the ultrafine-grained steel shows a fatigue limit with notched specimens, whereas the fatigue limit can disappear with smooth specimens because of the internal fracture.