Fatigue softening of low-carbon steel

Abstract

Low-carbon steel specimens may exhibit softening, followed by hardening, when subjected to tension-compression cyclic loading above a certain stress range. The softening has been attributed to the formation and expansion of plastically deformed zones. The work described in this paper was designed to investigate the spread of plasticity during fully reversed cyclic loading, by monitoring the strain response in each cycle. It was found that the extent of softening increases in a stepwise fashion with the number of cycles. At each step the specimen is subjected to constant positive and negative load limits and the strain-amplitude remains constant for a number of cycles. The latter condition is ‘self-imposed'. Eventually it gives way and an increase in strain-amplitude occurs. Possible mechanisms to account for this effect are discussed, and it is shown that it may be explained in terms of changes in the balance between frictional and elastic stresses in the specimen.