Assessment of ultra-high cycle fatigue behavior of EN-GJL-250 cast iron using ultrasonic fatigue testing machine

High cycle fatigue comprising up to 107 load cycles has been the subject of many studies, and the behavior of many materials was recorded adequately in this regime. However, many applications involve larger numbers of load cycles during the lifetime of machine components. In this ultra-high cycle regime, other failure mechanisms play, and the concept of a fatigue endurance limit (assumed for materials such as steel) is often an oversimplification of reality. When machine component design demands a high geometrical complexity, cast iron grades become interesting candidate materials. Grey cast iron is known for its low cost, high compressive strength, and good damping properties. However, the ultra-high cycle fatigue behavior of cast iron is poorly documented. The current work focuses on the ultra-high cycle fatigue behavior of EN-GJL-250 (GG25) grey cast iron. An ultrasonic (20 kHz) fatigue testing system is developed and instrumented for testing at ambient temperature, and hourglass shaped cast iron specimens with circular cross-section are designed to be excited in resonance. The temperature of the specimen is measured using a pyrometer and controlled by ventilating air. The displacement at the free end of the specimen (which is linked to stress at the reduced section) is measured using an accurate laser sensor, whose analysis requires careful test preparation and data analysis. The high resonance frequency allowed to characterize the S-N behavior of the cast iron of interest within a matter of days, and repeat the tests to quantify the natural scatter in fatigue resistance. The obtained results will be used in a research project that aims to optimize the design of machine components with respect to weight and durability.

Publication year:2019

Accessibility:Open