13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- ring-like area hatched in Fig.15. On the other hand, such an area occurring fatigue crack is the entire cross section without the modified surface layer having the improved strength in the case of axial loading. Thus, the critical volume participating in the fatigue crack initiation in axial loading is extremely larger than that in rotating bending. The fact that the crack initiation depth in axial loading is clearly higher than the results in rotating bending in Fig.14 can be well interpreted in this concept. In the case of high strength steels such as spring steel, material defects such as inclusion and machining flaws become sensitive to the crack initiation comparing with the usual structural steels with some moderate strength. Paying a particular attention to this aspect, the number of such defects in axial loading is much larger than the case in rotating bending. Therefore, the size of the most harmful defect in axial loading becomes larger than that in rotating bending. This is another reason why the fatigue limit in axial loading becomes a little lower than that in rotating bending. 4. Conclusions In order to investigate the very high cycle fatigue properties of spring steels (SUP7 and SWOSC-V), fatigue tests were performed for specimens prepared by several different processes in both rotating bending and axial loading. Main conclusions obtained in this study are summarized as follows; (1) In every series of the specimen preparation, fatigue fracture can occur in the very high cycle regime such as 7 9 10 10 cycles regardless of the specimen’s hardness, the surface roughness and the loading type. (2) Fatigue limit in rotating bending is a little higher than that in axial loading for every series of specimens. This fact can be attributed to the difference of the stress distribution across the section, i.e. the critical volume participating in the crack initiation in axial loading is much larger than that in rotating bending. (3) At higher stress levels with shorter life, the surface-induced fracture tends to occur instead of the interior-induced fracture, but the interior-induced fracture tends to occur at relatively higher stress levels in the case of shot-peened specimen due to suppression of crack initiation on the specimen surface by high value of the compressive residual stress. (4) In the case of interior fracture mode for the conventional spring steel, an inclusion was usually found at the crack initiation site (Central portion of the fish-eye) and the FGA was observed around the inclusion. But, in the case of high cleanliness spring steel, such an inclusion was not found even in the interior fracture mode and a FGA-like facet inclined 60deg. against the macroscopic fracture surface was formed at the crack initiation site. References [1] A. Woehler, Uber die Festigkeits-Versuche mit Eisen und Stahl, Zeitschrift fur Bauwesen, 20, (1870), 74-106. [2] K. Shiozawa, T. Sakai et al., Databook on Fatigue Strength of Metallic Materials, Vols. 1-3, Elsevier Science B. V. and JSMS, (1996). [3] S. Mimura, T. Aoki, T. Sakakibara, M. Wakita, Development of High Fatigue-Proof Spring Using High Silicone Oil Tempered Wire, Transactions of the Japan Society of Spring Engineers, 46, (2001) , 1–6. [4] T. Sakai, Review and Prospects for Current Studies on Very High Cycle Fatigue of Metallic Materials for Machine Structural Use, Journal of Solid Mechanics and Materials Engineering, 3(3), (2009), 425-439. [5] T. Sakai, T. Furusawa, R. Takizawa, N. Oguma, H. Hohjo, H. Ikuno, Development of Multi-type High Efficiency Fatigue Testing Machines in Rotating Bending and Axial Loading, Proceedings of the Hael Mughrabi Honorary Symposium, TMS Annual Meeting, (2008), 69–73. [6] T. Sakai et al., Standard Evaluation Method of Fatigue Reliability for Metallic Materials -Standard Regression Method of S-N Curves- [JSMS-SD-6-08], JSMS Committees on Fatigue and Reliability Engineering, (2008).
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