13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- for a,t = 1.2 % to a maximum stress amplitude in the range of the tensile strength f = 569 MPa of the solution-annealed austenite. Due to an increase of the dislocation density, formation of deformation-induced ´-martensite, development of intrusion and extrusions at the specimen surface and finally the formation of micro- and macro-cracks a continuous increase of the change in the mean value of time of flight tofmean from the beginning of the fatigue loading up to specimen failure was observed at ambient temperature (Figure 5). However the electromagnetically activated ultrasonic amplitude peakmean shows no significant changes until the formation and propagation of micro- and macro-cracks (comp. Figure 4c with Figure 5). Because tofmean is more sensitive to microstructural fatigue effects, which lead to an increase of tofmean until specimen failure, it isn’t possible to detect the state of macro-crack formation and propagation. The change in the mean value of the ultrasonic amplitude peakmean shows in contrast to tofmean a decrease at the fatigue state of macro-crack formation and propagation. Thus it is possible to detect specimen failure at an earlier fatigue state. Figure 5. Changes in specimen surface topography at defined fatigue states at ambient temperature: N = 100 slip bands (a), N = 400 micro-cracks (b) and N = 900 macro-crack (c) 3.2. Evaluation of the fatigue behavior without deformation induced ´-martensite formation Figure 6 shows the development of the stress amplitude a (Figure 6a), the change in the mean value of time of flight tofmean (Figure 6b) and the change in the mean value of the electromagnetically activated ultrasonic amplitude peakmean (Figure 6c) versus the number of cycles N for fatigue tests at 300 °C. The cyclic deformation behavior of the investigated metastable austenite at this temperature is characterized by initial cyclic hardening, followed by cyclic softening before the final stress amplitude drop, associated with the propagation of a fatigue crack. At 300 °C no martensite formation occurs and compared to ambient temperature the stress amplitude is significantly lower. In the tofmean, N-curves generally three sections over the fatigue life were observed. In the first section tofmean shows similar to a and tests at ambient temperature an initial increase caused by dislocation hardening. The second section follows with a decrease of tofmean due to cyclic softening. The third one shows a secondary increase of tofmean, which is mainly affected by the development of intrusions and extrusions at the specimen surface and finally the inititation and propagation of fatigue cracks (Figure 6b). In analogy to the investigations at ambient temperature, the change in the mean value of the ultrasonic amplitude peakmean shows in contrast to tofmean a decrease at the fatigue state of macro-crack formation and propagation.
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