13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- number of cycles (N) during total-strain-controlled LCF-fatigue tests with a,t = 1 % at ambient temperature (Figure 7a) and at 300 °C (Figure 7b). The development of stress amplitude and/or plastic strain amplitude is used usually for the characterization of cyclic softening or hardening processes (comp. Figure 4a and 6a). The change of the shape (A) of - -hysteresis loop occurs due to macro-crack formation at N = 1020 at ambient temperature and at N = 1300 in the test at 300 °C. Figure 7. Stress-strain-hysteresis at ambient temperature (a) and T = 300 °C (b) The time of flight-total-strain-relationship (tof- t) provides, in analogy to the stress-strain hysteresis, information about the actual state of fatigue of the austenitic steel. In the range of elastic-plastic material behavior, the tof- t relation leads to a hysteresis-relationship (Figure 8a) for fatigue tests at ambient temperature and 300 °C (Figure 8b). It can be seen, that an increase/decrease of total-strain ( t) leads to increase/decrease of the tof signal. Due to elastic-plastic material behavior, microstructural changes like e. g. increase of dislocation density lead to an increase of the mean value of tof and shift the hysteresis-loop to higher values with increasing number of cycles. In tests at ambient temperature further cycling leads to deformation-induced ´-martensite (Figure 4a), which correlates with an increase of the mean value of tof. Due to changes in dislocation arrangement in tests at 300 °C cyclic softening occurs after N = 183 cycles, which leads to a decrease of the mean value of tof. Furthermore the tof- t-relationship depends on micro and macro-crack initiation and propagation, which also shifts the hysteresis-loop to higher values along the ordinate. In comparison to the - -hysteresis (Figure 7), a significant change in the shape of the tof- t-hysteresis loop occurs.
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