Fatigue crack nucleation in iron and a high strength low alloy steel

It is now well established that even the initially smooth surface becomes roughened during fatigue and that these surface irregularities become cracks over many cycles. While numerous phenomenological theories and models have been proposed to explain fatigue crack initiation, it is still far from clear as to how microcracks nucleate in the first place. In this paper, the initial formation of a fatigue crack is considered as a nucleation process due to random fluctuations of a metastable assembly of defect structures generated during cycling. By minimizing the Gibbs free energy change, the critical number of cycles required to nucleate fatigue cracks is estimated as a function of strain amplitude. Data from commercial purity iron and a high strength low alloy steel are utilized to demonstrate that the model is in reasonable agreement with experimental data.