Abstract: Experiments are conducted to determine the gigacycle fatigue behavior of nodular cast iron GS51 by ultrasonic fatigue test system. Specimens are tested at 20kHz and 35Hz with two different stress ratios (R=-1, R=0.1). The S-N curves at cycles in the range from 10 ⁴ to 10 ¹⁰ are obtained. The experimental results shows that fatigue failure can occur over 10 ⁷ cycles, and that the fatigue endurance stress continues to decrease with increasing cycles to failure between 10 ⁷ and 10 ¹⁰ cycles. Comparison of experimental results tested at the frequency either 20kHz or 35Hz shows that there appears no difference on fatigue behavior below 10 ⁷ cycles. That is, ultrasonic frequency does not evidently affect the fatigue properties of nodular cast iron GS51. Scanning electron microscopy is used to examine the fatigue crack section, it is found that most fatigue cracks initiate from the non-uniformly distributed globular graphites in the subsurface and the voids in the interior of the material.

Key words: nodular cast iron, fatigue crack, S-N curve, ultrasonic fatigue test, void