Push-pull and plane bending fatigue tests in the high cycle fatigue region were carried out on annealed polycrystalline pure aluminum. The relationship between fatigue-induced dislocation structures and macroscopic hardening (unidirectional and cyclic) was investigated in the whole process of fatigue before the initiation of macro-crack.
The stress amplitude σa and the plastic strain range Δεp tended to saturate at the early stage of fatigue in the both strain control- and stress control-tests. On the other hand, the Vickers hardness HV, the dislocation bundle width BW and the dislocation density ρ increased gradually until the initiation of macro-crack. Such behaviors of σa and Δεp can be explained from the dislocation shuttling model for the mobile dislocations, and the behaviors of HV, BW and ρ from the increase of the immobile dislocations. Furthermore, it is shown that the relationship between HV and ρ is expressed as
HV-HVO=C3(ρ-ρ0)h
where HVO and ρ0 are the values of HV and ρ at the annealed state, respectively, and C3 and h are the experimental constants.
