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 Δε
p or Vickers hardness
HV were investigated during the fatigue process.
(1) The dislocation structure induced previously by the prestrain ε
pre 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 ρ
t 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 Δε
p than that of the annealed one in the initial stage. But, as the amount of ε
pre increased the value of the Δε
p became smaller and the number of cycles to the maximum of the Δε
p became larger.
(4) The variations of ρ
t 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 ρ
t was expressed as a simple relation,
HV ∝ √ρ
t.
(6) A mean cell size
Lm at a fatigue failure point did not depend on ε
pre, but on the stress amplitude or the number of cycles to failure.
抄録全体を表示