Abstract
The low cycle push-pull loading fatigue test was carried out on medium carbon steel. It is well known that the low cycle fatigue life can be evaluated by the Coffin-Manson relationship expressed by the equation ΔεPαNf=c, where ΔεP is the plastic strain range, Nf the cycles to fracture and α and c are material constants. The exponent α ranges from 1.4 to 2.0 in most metals, but was determined to be 2.2 for the particular steel used here. We investigated the crack initiation and propagation in an attempt to explain this value, based on the conclusion of previous investigations that the Coffin-Manson relationship is equivalent to the propagation law for small cracks. The present material has inclusions of various sizes, some of them larger than the mean pearlite size. When a crack initiated from an inclusion, the initial crack length measured from the surface depended on the inclusion size. This is the reason for the high value of α. Since the crack propagation rate for a given stress amplitude is almost independent of the initial crack length, the low cycle fatigue life can be calibrated as if the crack initiated from a pearlite. α then becomes 1.66 which is within the range of general cases. The fatigue life of metals in which cracks initiate from an inclusion can then be predicted using the adjusted values of α and c, if the inclusion size is known.
