13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- indication that dislocation structure changes due to fatigue loading at 20 Hz and 20 kHz is not equivalent. A possible explanation of this fact is the time allowed to dislocations to move is necessarily different for a same material fatigued under 20 Hz and 20 kHz conditions. As a consequence, we can assess that fatigue frequency has an effect on cyclic behavior of S15C steel. Nevertheless, other results are needed to conclude in this way. It is particularly expected to undertake direct dislocations observation in order to be able to detect such a discrepancy in dislocations substructures. 5. Conclusions (1) Frequency effect on fatigue properties of S15C low carbon steel has been reconfirmed. Fatigue strength tends to increase when testing frequency is increasing. (2) Strain rate has a clear effect on yield stress value of S15C steel. This phenomenon is involved in the trend described before and explains the slight change of fatigue strength between 0.2 and 140 Hz. However, it is not sufficient to explain the S-N properties gap found from ultrasonic method. (3) Comparison of stress-strain hysteresis loops and EBSD observations indicate a frequency dependence in cyclic behavior. This fact urges us to consider the cyclic hardening / softening properties as one other main phenomenon involved in frequency effect of S15C steel. (4) Some other works have to be conducted to finalize the study of frequency effect on S15C steel. Particularly, further experiments on cyclic hardening / softening behavior will be undertaken, like dislocation observations. References [1] M. Kikukawa, K. Ohji, K. Ogura, Push-Pull Fatigue Strength of Mild Steel at Very High Frequencies of Stress Up to 100 kc/s. J. Basic Eng. T. ASME D, 87 (1965) 857–864. [2] T. Yokobori and T. Kawashima, Acoustical Fatigue with Special Emphasis on Ferrite Grain Size Dependence of Fatigue Strength. J. of the Japanese Society for Strength and Fracture of Materials, 4 (1969) 19–16. (In Japanese) [3] S. Setowaki, Y. Ichikawa, I. Nonaka, Effect of Frequency on High Cycle Fatigue Strength of Railway Axle Steel. Proceedings VHCF-5 (2011) 153–158. [4] C.E. Feltner, C. Laird, Cyclic Stress-Strain Response of F.C.C. Metals and Alloys-I. Acta Metall., 15 (1967) 1621–1632. [5] L. Landgraft, J. Morrow, T. Endo, Determination of the Cyclic Stress-Strain Curve. Journal of Materials, 4 (1969) 176–188. [6] N. Tsutsumi, A. Shiromoto, V. Doquet, Y. Murakami, Effect of Test frequency on Fatigue Strength of Low Carbon Steel, J. Jpn Soc. Mechanical Eng. A, 72, 715 (2006) 317–325. (In Japanese) [7] N. Tsuchida, H. Masuda, Y. Harada, K. Fukaura, Y. Tomota, K. Nagai, Effect of Ferrite Grain Size on Tensile Deformation of a Ferrite-cementite Low Carbon Steel. Material Science and Engineering A, 488 (2008) 446–452. [8] J.R. Hancock and J.C. Grosskreutz, Mechanisms of Fatigue Hardening in Copper Single Crystals, Acta Metall., 17 (1969) 77–97. [9] H. L. Huang, A Study of Dislocation Evolution in Polycrystalline Copper during Low Cycle Fatigue at Low Strain Amplitude, Materials Science and Engineering A, 342 (2003) 38–43.
RkJQdWJsaXNoZXIy MjM0NDE=