Fatigue of a spheroidal graphite cast iron in the very high cycle range

Spheroidal Graphite cast Iron (SGI) or Ductile cast Iron (DI) is a very goo d material for industrial applications, and is widely applied to automotive parts because of its exceptional combination of high strength and ductilit y In the automotive industry, the required design lifetime of the component s usually exceeds 10(9) cycles. Although a few of the fatigue data for the SGI have been published in the form of S-N curves, the data in the literatu re have been limited to fatigue lives up to 10(7) cycles, because the testi ng time and the costs are too high to perform fatigue tests of more than 10 (8) cycles using the conventional testing machine. The ultrasonic frequency (20 kHz) method is a modern experimental technique to obtain numbers of cy cles as high as 10(9) within 14 h, whereas it would take about three years with a conventional fatigue testing method at 10 Hz. The objective of this paper is to determine the very long fatigue life of a SGI up to 5 x 10(9) cycles at an ultrasonic frequency of 20 kHz and at the stress ratios of R = -1 and 0. Good agreement is found with the results in conventional tests at a frequency of 25 Hz by Renault, between 10(5) and 1 0(7) cycles. The experimental results show that fatigue rupture can occur b eyond 10(7) cycles, and the stress-life curve does not generally exhibit a plateau beyond 10(6) cycles. The evolution of the temperature of the specim en caused by the absorption of ultrasonic energy is obtained. The temperatu re increases rapidly with increasing stress amplitudes. There is a temperat ure maximum between 10(6) and 10(7) cycles which may be regarded as the cra ck nucleation.