13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- In addition, one can see that there are some fatigue strength differences between tests performed in the range from 0.2 Hz to 140 Hz. The fatigue properties are equivalent for high stress tests in this frequency range. However, as the stress level decreases, some fatigue life differences can be seen. In a general way, the higher is frequency, the higher becomes the fatigue strength. Particularly, even though tests performed at 2 and 20 Hz reveals the same fatigue limit, a relatively higher fatigue limit is found for 140 Hz tests. Nevertheless, those fatigue strength differences are far smaller than compared with ultrasonic fatigue test results. 3.2. Fracture surface observation (a) f=20 kHz, σa=255 MPa, Nf=1.25×10 7 (b) f=2 Hz, σa=228.8 MPa, Nf=1.25×10 5 (c) f=140 Hz, σa=244.4 MPa, Nf=7.29×10 4 Fig. 4 Typical examples of fracture surface Fracture surface observation reveals that, for all the fatigue tests conducted up to failure, fracture has been initiated from the specimen surface, even under ultrasonic fatigue loading. This fact is usual for this kind of low strength steels, where number of inclusions, and so main factor of fish-eye fracture occurrence, is low. Figure 4 shows some typical examples encountered in the present study. Two kinds of fracture conditions have occurred. On the one hand, Fig. 4(a) presents a fracture behavior initiated by only one crack. On the other hand, Figs. 4(b) and (c) depict a clear multiple cracks conditions before specimen’s failure. In order to gather such these data, Fig. 5 is an S-N diagram, which makes clearly distinction between simple and multiple cracks behavior found.
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