13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 8. Correlation between time of flight (tof) and total-strain t) at ambient temperature (a) and T = 300 °C (b) In analogy to - t-, and tof- t-hysteresis, the relationship between the electromagnetically activated ultrasonic amplitude (peak) and total-strain ( t) was investigated in fatigue tests at ambient temperature (Figure 9a) and 300 °C (Figure 9b). Generally an increase/decrease of t leads to decrease/increase of peak. The peak is a physically based data, which in comparison to the above mentioned values, describes most sensible the occurrences of macro-crack and less sensible the change in dislocation density and arrangement or initiation of micro-cracks. Hence critical values of the change in the electromagnetically activated ultrasonic amplitude can be defined for structural health monitoring applications, which correlate with macro-cracks. Figure 9. Correlation between electromagnetically activated ultrasonic amplitude (peak) and total strain t) at ambient temperature (a) and T = 300 °C (b) 3. Conclusions In total strain controlled fatigue tests at ambient temperature and T = 300 °C the fatigue behavior of the austenitic steel AISI 347 was investigated by conventional stress-strain-hysteresis ( - ) measurements and by continuous, in-situ ultrasonic measurements generated with electromagnetic acoustic transducers (EMATs). Far reaching cross effects of EMAT and mechanical strain
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