Micro pitting

Abstract: The nucleation of fatigue cracks from corrosion pits was investigated by conducting fatigue experiments on open-hole specimens of a 2024-T3 aluminum (bare) alloy in 0.5 M NaCl solution at room temperature and different load frequencies from 0.1 to 20 Hz. The maximum cyclic stresses applied at the hole ranged from 144 to 288 MPa and the load ratio, R , was 0.1. A specimen subjected to pre-corrosion in the NaCl solution prior to corrosion fatigue was also investigated. Pitting was found to be associated with constituent particles in the hole and pit growth often involved coalescence of individual particle-nucleated pits. Fatigue cracks typically nucleated from one or two of the larger pits, and the size of the pit at which the fatigue crack nucleates is a function of stress level and load frequency. The observations indicate that the nucleation of corrosion fatigue cracks essentially results from a competition between the processes of pitting and crack growth. Pitting predominates in the early stage of the corrosion fatigue process, and is replaced by corrosion fatigue crack growth. Based on these results, two criteria are proposed to describe the transition from pit growth to fatigue crack growth: (1) the stress intensity factor of the equivalent surface crack has to reach the threshold stress intensity factor, Δ K th , for fatigue crack growth, assuming that a corrosion pit may be modeled by an equivalent semi-elliptical surface crack, and (2) the time-based corrosion fatigue crack growth rate also exceeds the pit growth rate.

Abstract: Fatigue crack initiation and growth from artificial pits of different depths has been studied. To analyze the experimental results a simple three-dimensional fracture-mechanical model has been developed. The model shows very good agreement with experiments including for small cracks, in describing the initiation and growth of a fatigue crack emanating from a pit and in predicting the dependence of reduction of fatigue life on pit size. Based on experimental data an empirical relation between the depth of the corrosion pit and the fatigue life has been established. Also, a microradiographic method for pitting corrosion depth determination has been described.

Abstract: A review of some of the recent work on the mechanism of rolling contact fatigue (RCF) is presented. Topics covered include the appearance and classification of RCF and the processes of stra...