13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- (a) Specimen for servo-hydraulic type machine (b) Specimen for electro-magnetic type machine (c) Specimen for ultrasonic type machine Fig.2. Shape and dimensions of fatigue specimen 3. Experimental Results 3.1. S-N data S-N diagram of JIS S15C steel used here is depicted in Fig. 3. One can see a clear difference of fatigue strength between ultrasonic 20 kHz fatigue test results and other results from servo-hydraulic and electro-magnetic machines. Fatigue limits found under these four fatigue tests conditions are 248 MPa, 200 MPa for 20 kHz and 140 Hz respectively, and a similar fatigue limit of 185 MPa has been found in the case of servo-hydraulic fatigue tests at a testing frequency of 2 and 20 Hz. This kind of large discrepancy between fatigue tests performed in usual testing frequency range and ultrasonic frequency has been also found in several other literatures as Kikukawa et al.[1], Yokobori et al.[2] or Setowaki et al.[3] for different other kinds of low carbon steels. Fig.3. General S-N property of S15C steel
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