Abstract
The cumulative cycle ratio is usually larger than unity, when steels are tested under gradually increased stress amplitude, and is less than unity when tested under decreased stress amplitude. This fact seems to indicate that the strengthening effect, due to the work-hardening, during the test, prevails in the case of increased stress amplitude. What, then, is expected on the steel which was heavily work-hardened beforehand, when it is tested under the same stress conditions?
With respect to this problem, the fatigue tests were carried out on the steel wires which have received different treatments, under the stepwise increasing and the stepwise decreasing stress amplitudes. The results on the heavily cold-drawn steel wire showed that the average of the cumulative cycle ratio is very close to unity under the increased stress amplitude, and is approximately 0.7 under the decreased stress amplitude. Therefore, it is clear that the heavily work-hardened material has no capacity for further work-hardening when it is subjected to the increased stress amplitude and the material is rather weakened or softened by the decreased stress amplitude. This inference was also confirmed by the examination of the hardness of specimens fractured under the respective stress conditions.
On the other hand, the results on the steel wire, which was annealed at 400°C in vacuum after the same cold drawing, showed the increase of the cumulative cycle ratio more than two, in both cases of the increased and the decreased stress amplitudes. Therefore, it is inferred that the work-hardening can still take place for this annealed one. The increase of the cumulative cycle ratio in the case of the decreased stress amplitude is probably the result of the work-hardening, that particularly was occasioned by the large stresses applied at the initial stage. However, no more confirmation of the results was available from the examination of the hardness of the fractured specimen on this annealed steel wire.
