高強度鋼の疲労特性に及ぼす応力比の影響 Effect of Stress Ratio on Fatigue Properties of High-strength Steels

Fatigue tests were conducted for three heats of 1800 MPa-class spring steels under various R ratios. For comparison, similar fatigue tests were conducted also for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels showed fish-eye fracture, eliminating conventional fatigue limits, and the fish-eye fracture origins were oxide, TiN and matrix itself, respectively. On the other hand, the conventional steels never showed fish-eye fracture and clearly showed fatigue limits. Fatigue strength of these steels was monotonously decreased according to increase of the R ratio, when the fatigue strength was evaluated with stress amplitude. However, the degradation of the fatigue strength was smaller than that expected from a modified-Goodman line, and the best fit line was obtained by drawing σW(R=−1)–σT line. When the stress ratio effect was evaluated with a function of (12R)γ, the estimated γ value was 0.5. In these results, difference was negligible between the spring and conventional steels. This meant that the stress ratio effect could be evaluated in conventional manners even in case of high-strength steel showing fish-eye fracture, regardless of the fish-eye fracture origin types.