Push-pull fatigue tests at a constant stress amplitude were carried out on mild steel specimens which were annealed and pre-strained in tension to 1%, 5% and 10% strains. Some relationships between the dislocation structure and the plastic strain range Δεp or Vickers hardness HV were investigated during the fatigue process.
(1) The dislocation structure induced previously by the prestrain εpre was recovered and rearranged with stress cycling, and became a typical cell structure by fatigue in the later stage.
(2) The annealed and the 1% pre-strained specimens in which the total dislocation density ρt and HV increased during the fatigue process showed cyclic hardening. On the other hand, the 5% and the 10% pre-strained specimens in which they decreased showed cyclic softening.
(3) The pre-strained specimens showed higher values of Δεp than that of the annealed one in the initial stage. But, as the amount of εpre increased the value of the Δεp became smaller and the number of cycles to the maximum of the Δεp became larger.
(4) The variations of ρt for the annealed and the pre-strained specimens during the fatigue process were expressible by the same equation derived from dislocation dynamics.
(5) During the fatigue process, the relationship between HV and ρt was expressed as a simple relation, HV ∝ √ρt.
(6) A mean cell size Lm at a fatigue failure point did not depend on εpre, but on the stress amplitude or the number of cycles to failure.
