Abstract
Polycrystalline aluminium specimens of 99.99 % purity were fatigued in alternate tension and compression with zero mean stress. Substructures near the tip of fatigue cracks were examined at various stages of crack propagation using an X-ray micro-beam technique. The propagation rate of the fatigue crack was also measured by a replicating method. The main results obtained are as follows.
(1) Regardless of the intensity of fatigue stress and the length of fatigue cracks, there exists a plastic zone around a fatigue crack in aluminium.
(2) A well-defined substructure characterized by the following three features is observed in the plastic zone : (i) heavily fragmented subgrains (1∼2 μ in diameter), (ii) very low excess dislocation density within a subgrain (about 108/cm2), and (iii) sharply defined sub-boundaries probably consisting of regular and simple arrays of dislocations. These characteristics are considered to be due to the generation of vacancies in large quantities and also to the stress concentration at the tip of a fatigue crack.
(3) Two kinds of plastic zones, one observed by an X-ray micro-beam technique and the other in the studies made from a macroscopic point of view, are concluded to be the same.
(4) The propagation rate of fatigue crack, dl/dN, can be related to the size of the plastic zone, Zp, by an equation,
dl⁄dN=AZpm,
where A is a constant independent of fatigue stress and m an exponent of about 2.
(5) Intimate relations between the propagation rate of fatigue crack and some structural parameters of the plastic zone were explained qualitatively by a vacancy absorption mechanism.
