13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental Investigation on Effects of Various Factors on Very High Cycle Fatigue Property for Spring Steels Taku Miura1 , Tatsuo Sakai2*, Takayuki Sakakibara3, Shingo Mimura3 Takanori Kuno3, Shoichi Kikuchi2 and Akira Ueno2 1Graduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, 525-8577 Japan 2 College of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, 525-8577 Japan 3 Research & Development Division, Chuo Spring Co., Ltd., Miyoshi, Nishikamo, Aichi, 470-0225 Japan * Corresponding author: sakai@se.ritsumei.ac.jp Abstract In order to investigate the effects of hardness, residual stress and loading type on the fatigue property for spring steel(SUP7), two kinds of tempering temperature, three kinds of surface finishing (grinding, electrolytic polishing and shot-peening) and two kinds of fatigue test (rotating bending and axial loading) were accepted. Specimens in harder (lower tempering temperature) series showed higher fatigue limit; however, those in shot-peened series showed approximately same fatigue limit as grinding finished series, although the former has compressive residual stress higher than that in the latter. In the case of same tempering temperature and surface finishing, specimens in rotating bending series showed higher fatigue limit comparing with the axial loading. This fact can be attributed to the difference of critical volume participating in the crack initiation. Fracture surfaces of all the failed specimens were carefully observed by a scanning electron microscope(SEM) in order to examine the fracture mode. Two types of interior initiated fracture, with and without inclusion, were found. The latter was found only in axial loading and had granular structure whose angle of slope to the loading direction was 60 degree. In the case of specimens with residual stress (grinding and shot-peening), the depth of interior fracture origin in rotating bending was shallower than that in axial loading. In both grinding and shot-peening series, the crack has initiated beneath the surface layer with compressive residual stress, and, the fatigue limits of these series were thus almost same to each other. Keywords Spring steel, Very high cycle fatigue, High cleanliness steel, Duplex S-N property, Fish-eye 1. Introduction During the long history of the study on Fatigue of Metals, a number of experimental data have been accumulated and a lot of fundamental aspects on the fatigue of metallic materials have been reported by many researchers[1-5]. Among them, one of the most important aspects is the fact that ferrous metals such as structural steels indicate the clear fatigue limit at the number of stress cycles less than 107 [1,6]. However, non-ferrous metals such as aluminum alloys have no fatigue limit such that the S-N curve tends to decrease continuously in the very long life regime longer than 107 cycles[4]. In recent years, practical structures such as railway wheels and rails, offshore structures, energy conversion and transportation systems have been used in a long term, sometimes, beyond their original design lives due to economic and environmental considerations. In such a circumstance, unexpected failures have been reported at stress levels lower than the fatigue limit even for structural components made of ferrous metals being assumed to have a distinct fatigue limit. The fatigue behavior of structural steels in the very high cycle regime longer than 107 cycles has become an important subject to ensure the long-term safety of the actual structures in the various areas of industries[4,5]. From this point of view, many researchers have carried out fatigue tests in the very high cycle regime for various kinds of metallic materials in the last decades[4]. Thus, it is known that the high strength steel indicates "duplex S-N characteristics" consisting of S-N curves for both the surface-initiated fracture and the interior-initiated fracture. According to the conventional works[4], the interior-initiated fracture is usually caused by non-metallic inclusions inside the material in very
RkJQdWJsaXNoZXIy MjM0NDE=