Gigacycle fatigue of ferrous alloys

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 x 10(10) cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 10(5) and 10(7) cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10(7) cycles, and the fatigue endurance stress S-max continues to decrease with increasing number of cycles to failure between 10(6) and 10(9) cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 10(6) and 10(7) cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra-high-cycle fatigue (>10(7) cycles). A surface-subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.