13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- treatment. Combined with the results of surface roughness, it can be found that more surface defects can be introduced to the N48 samples’ surface. So the fatigue strength is difficult to be enhanced as these defects inducing stress concentration easily. For the N48U1 samples, though high compressive residual stress and little defects were introduced, the inclusions within the sub-surface of samples still can form a fish-eye crack and hinder the improvement of fatigue strength. Also we found that as the high compressive residual stress induced by UNSM treatment the position of inclusions trend a deeper depth beneath the samples’ surface [1]. With above results we can conclude that with the high loading during fatigue test the fatigue strength is more sensitive to the surface state of samples. At the same time, though high compress residual stress can be introduced by UNSM treatment, the sub-surface cracks induced by inclusions with materials are difficult to be hindered as the reason of limited depth of enhanced zone improved by UNSM treatment. 4. Conclusions (1) With the increase of strike number, deeper UNSM processing marks can be introduced on the specimen surface; for the specimen with the higher surface hardness, little effect of UNSM to the surface roughness can be found. (2) A gradual refined grain layer can be produced by the UNSM as multiple strikes to specimen surface; the thickness of the refined layer increases with the strike number and decreases as the growth of surface hardness; the top surface grains can be refined in to nano-size and for the sample with higher surface hardness grain refinement become easier. (3) The surface hardness can be effectively improved by UNSM treatment and increases as the increasing strike number; N8 sample has a larger zone for the hardness increase as the effect of nitrides within sample. (4) Higher compressive residual stress can be found for the nitriding specimens; increase of the residual stress can be obtained for un-nitriding samples as the increase of strike number; however opposite phenomenon was found for nitriding samples and residual stress decreases with the increasing strike number. (5) The enhancement of fatigue strength mainly depends on the residual compressive stress in the surface zone produced by UNSM treatment; as the increase of fatigue strength the surface defects and sub-surface inclusions hinder the increase of fatigue strength definitely. Acknowledgements The authors thank Atsushi Onizawa of Neturen Company for assistance with plasma nitriding and DesignMecha Co., Ltd for assistance with UNSM. This study was supported by the Double Degree Program (DDP) of The University of Tokushima. References [1] B. Wu, P. Wang, Y.-S. Pyoun, J. Zhang, R.-i. Murakami, Effect of ultrasonic nanocrystal surface modification on the fatigue behaviors of plasma-nitrided S45C steel, Surface & Coatings Technology, 213 (2012) 271–277. [2] X.J. Cao, Y.S. Pyoun, R. Murakami, Fatigue properties of a S45C steel subjected to ultrasonic nanocrystal surface modification, Applied Surface Science, 256 (2010) 6297-6303. [3] C. Suh, G. Song, M. Suh, Y. Pyoun, Fatigue and mechanical characteristics of nano-structured tool steel by ultrasonic cold forging technology, Materials Science and Engineering: A, 443 (2007) 101-106. [4] A. Amanov, I.S. Cho, Y.S. Pyoun, C.S. Lee, I.G. Park, Micro-dimpled surface by ultrasonic nanocrystal surface modification and its tribological effects, Wear, 286–287 (2012)
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