Damage initiation mechanism of a nodular cast iron under the very high cycle fatigue regime

Abstract

Fatigue behaviour in the Very High Cycle Fatigue (VHCF) of 1010 cycles was investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = 1. A high-speed and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels. The S-N curves obtained show that fatigue failure occurred beyond 109 cycles, fatigue limit does not exist for the cast iron. The temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Fatigue initiation takes place always at the surface graphite or subsurface void; the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition. Local temperature increased sharply is related to the crack initiation. [Received 25 April 2006; Accepted 25 January 2007]