Analysis of Variation of Endurance Ratio of Pure Iron (Evaluation by Ratios Based on Tensile Properties and Cyclic Yield Stress)

Abstract

The endurance limit of polycrystalline pure iron is high. To understand its mechanism, tensile strength (σ_B), cyclic yield stress (σ_yc), and fatigue limit (σ_w) were evaluated for both single-crystalline and polycrystalline pure iron. The endurance ratio (σ_w/σ_B) of rotary bending test and axial loading using polycrystalline specimens ranged from 0.58–0.68. Moreover, the σ_w/σ_B of axial loading using a single-crystalline specimen indicated a high value of 1.06. Even without considering the strain rate dependence, the σ_w/σ_B of pure iron was higher than that of general-purpose steels such as low-carbon steel, general structural steel, stainless steel, and martensitic steel. Contrary to these results, σ_w/σ_yc of single-crystalline and polycrystalline pure iron showed the same value (0.881) as that of general-purpose steels. Thus, σ_yc would be more correlated with σ_w than σ_B. Furthermore, attempts were made to quantify the effect of strain rate, residual stresses, and work-hardened layers on fatigue limit. Consequently, the change in the endurance ratio (0.58–0.68) was quantitatively explained by the strain rate, residual stress, and work-hardened layer.