Some Factors that Influence the Fatigue Behaviour of Alpha-Iron

Abstract

A preliminary study has been made of the relationship between dislocation substructure and the rate of nucleation and propagation of the fatigue crack in α-iron, by applying the stress to the annealed metal in different ways, or by making specimens from material that had been cold worked, hot rolled, or deformed by creep. Applying the full stress to the annealed metal at the first cycle, as compared with gradually building up the stress over a few thousand cycles, greatly reduced the life to fracture and considerably changed both the dislocation substructure, as revealed by transmission electron microscopy, and the nature of the slip-bands observed on the specimen surface. In some cases the yield point of the cold-worked or hot-rolled material was still falling when fatigue fracture occurred. Surface cracking remaining after creep was the significant factor reducing subsequent fatigue life. Neither the substructure nor the voids produced during creep seemed to influence fatigue-crack nucleation or propagation.