Effect of Strain Ageing on Fatigue Crack Propagation in Smooth Specimen of Low Carbon Steel

Abstract

Precracked specimens of a low temperature quenched rimmed steel were fatigued in rotating bending. Stage-II precracks were introduced by cyclic stressing at a overstress level to the length of 500μm. The specimens were aged under cyclic stressing at understress levels for 10⁶ to 10⁷ cycles (dynamic strain ageing) or statically aged at 100°C for 40 minutes or at room temperature for 24 to 48 hours corresponding to 5×10⁶ to 10⁷ cycles and for 6 weeks (static strain ageing), and then fatigued at the same overstress level as before.
The main results obtained are summarized as follows:
(1) Dynamic strain ageing contributes to the decrease in propagation rate but hardly to the increase in critical propagation stress of the precrack.
(2) Static strain ageing at room temperature for 24 to 48 hours influences both the propagation rate and the critical propagation stress to the same extent as dynamic strain ageing.
(3) Static strain ageing at room temperature for 6 weeks and at 100°C for 40 minutes makes a greater contribution to the decrease in propagation rate and the increase in critical propagation stress than the above other strain ageing treatments.