Microstructural Effect Controlling Exhaustion of Ductility in Extremely Low-Cycle Fatigue

A low-cycle fatigue test was carried out under push-pull loading on an annealed low-carbon steel. Measurement of residual fracture ductility ɛ_FR was also performed after a given number of strain cycles with a special interest in the relationship between the exhaustion of ɛ_FR and the development of fatigue damage. Results show that the change in value of ɛ_FR with strain cycling in a low-cycle fatigue regime including an extremely low-cycle regime is controlled by the following basic mechanisms : (i) work hardening of the material, (ii) development of surface cracks and (iii) internal crack originating from the fracture of pearlites. The effects of these mechanisms on the exhaustion of fracture ductility (ɛ_FR) depends on the level of plastic strain range and on the number of strain cycles involved. The fatigue life in the extremely low cycle regime is primary controlled by two basic mechanisms such as work hardening of the material and increase of internal micro-voids.