13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Fig.5. General fracture crack behavior It is not surprising at all to see that multiple crack behavior is able to occur only for relatively high stress level tests, whereas simple crack condition is more likely to happen for low stress levels. One important finding related to this fracture behavior is that the transition between multiple to single crack appears around fatigue strength of 106 cycles, regardless testing frequency. In other words, this fracture behavior is not directly related to the stress level, but is mainly governed by the fatigue strength at a specific fatigue test condition. As fatigue strength of S15C steel is dependent of the testing frequency, the stress level where takes place this fracture behavior transition tends also to increase when testing frequency is increasing. 3.3. Stress-strain hysteresis Stress-strain behavior in the present study has been studied for 0.2, 2, 20 and 140 Hz loading frequencies; as such a study is not possible in the case of ultrasonic method. Stress signal was directly recorded from testing machine load cell, whereas strain was measured from strain gauge system introduced before. As failure of fatigue specimen occurs almost all the time at the extremity of central cylindrical shape due to a slight stress concentration factor Kt=1.04, crack propagation does not influence measures on strain gauge at the center. General evolution of stress-strain hysteresis loop of S15C studied here is presented in Fig. 6. In this diagram, hysteresis loop’s area Ah from some fatigue tests is plotted against the number of cycles. One can clearly see on Fig. 6(a) that an increasing general trend occurs. In other words, as those tests are stress controlled, S15C is generally softening when fatigue goes. This result can be explained by the fact that this material is as rolled, and so has a hardened initial condition. In addition, it is obvious that increasing trend becomes dramatic as the stress level increases. This result is natural, since cyclic softening phenomenon becomes usually more remarkable when material is subjected to higher stress amplitude. One can note that softening get to saturation at a certain number of cycles and then a slight cyclic hardening behavior is observed, as hysteresis loop’s area tends to decrease. This phenomenon is particularly obvious for high stress tests.
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