Abstract
The effect of a small artificial defect (i. e., a drilled hole, the diameter ranging from 40 to 200μm) on the fatigue strength of high strength alloy (WT80C) was investigated. The experimental results were discussed in comparison with those on other marterials (i. e. carbon steels).
The conclusions are summarized as follows:
(1) Like carbon steels, the effect of hole size on the fatigue strength of WT80C changed according to the maximum length l0 of non-propagating cracks which are observed at the fatigue limit of the unnotched specimen. Thus, the length l0 is the measure to evaluate quantitatively the notch effect of the defects. The values of l0 is∼100μm for annealed low carbon steel(S10C), ∼50μm for annealed medium carbon steel (S45C), ∼60μm for WT80C, ∼20μm for quenched and tempered S45C and ∼20μm for quenched S45C. The notch sensitivity of these materials increases in the written order. If the hole diameter of the holed specimen of quenched or quenched and tempered S45C is greater than l0, the fatigue strength decreases remarkably. Accordingly, the fatigue strength may be improved by controlling the defect size smaller than l0. However, even if the control of defect size is accomplished in WT80C, such a control alone is insufficient for the improvement of the fatigue strength, because the decrease of fatigue strength of the holed specimen is due not only to the notch effect but also to the cyclic softening.
(2) In the fatigue tests of geometrically similar but different size specimens (D=6, 12mm) with a small artificial hole (d=40∼200mm), the fatigue strength of the large specimens was a little lower. The size effect is considered due to (1) the difference in specimen diameter and (2) the difference in hole diameter, but the latter is more effective.
