Transition of Fracture Mechanism of Ductile Cast Iron in Extremely Low Cycle Fatigue.

Abstract

Low cycle fatigue tests were carried out over a wide range of plastic strain levels including an extremely low cycle fatigue (ELCF) regime on ferritic ductile cast iron under push-pull loading conditions. To evaluate the internally accumulated fatigue damage, the measumement of the density of the materials was also taken after a given number of strain cycles. Results show that ( 1 ) in the ELCF regime, the generation and the coalescence of microvoids, which originated from debonding of the graphite-matrix interface inside the material, lead to the final fracture of the specimen ; ( 2 ) in this case, the local density of the specimen, at the region where fatigue damage is concentrated, decreases gradually with strain cycling and its most drastic change occurs at the final stage of fatigue. This drastic decrease in density at that stage is caused by the rapid coalescence of microvoids inside the material.