Strain Ageing and Fatigue Limit on High Purity Iron

Abstract

The mechanism of fatigue limit was examined on pure iron, which shows no strain ageing. The purpose of the experiments was to clarify whether the endurance limit is brought about by strain ageing or any other mechanism. The Jhonson-Matthey pure iron sheets were decarburized and denitrized in moist hydrogen for 70 days and 200 days. The former treatment gave the strain age hardenable sample and the latter the non-hardenable one. The latter sample showed also the fatigue limit at 10⁷ cycles in pulsating fatigue tests at the stress level a little higher than the proportional limit. The age hardenable sample showed the knee in fatigue limit at 10⁶ cycles at the stress a little below the tensile strength. The fatigue limit was concluded to be attributable mainly to strain ageing and hardly to work hardening.
The conclusion above was applied to understand the problem of disappearence of the endurance limit under fluctuating fatigue stress on carbon steel. The technique here used was reversion treatment to remove strain ageing. The two S-N curves -one of which was obtained under constant applied stress and frequent reversion of strain ageing, the other was obtained under two-steps program load -coincided fairly good. The fatigue limit seems to exist only when strain ageing of the material is stable. If strain ageing condition is disturbed by occasional overstressing, the fatigue limit disappears and the S-N curve becomes similar to that suggested by Corten and Dolan.
It is suggested that the fatigue limit is caused by balancing two mechanisms of hardening mainly due to strain ageing and damaging due to dislocation multiplication.