13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- measurements enable the recording of a hysteresis relation between time of flight (tof) and total-strain ( t) as well as relationship between the electromagnetically activated ultrasonic amplitude (peak) and total-strain ( t). The tof- t-hysteresis gives, in analogy to the stress-strain-hysteresis, information about the cyclic hardening and/or softening processes, micro- and macro-crack initiation and propagation. Due to the occurrence of micro-cracks a significant change in the shape of the tof- t-hysteresis curve can be detected earlier compared to - -hysteresis measurements. The significant changes in the peak values correlate with the development of macro-cracks. This effect can be used for the early detection of critical fatigue states before final failure in the sense of structural health monitoring. Acknowledgements This research work was carried out within the framework of the R&D in the Nuclear Safety Research Program, financed by Federal Ministry of Economy and Technology (BMWi), project number: 1501379, Germany. We also thank the German Research Foundation for financial support of the work. The used EMAT probes were developed at the Fraunhofer Institute for Non-Destructive Testing (IZFP), Saarbrücken, Germany. We thank the director of the IZFP, Prof. C. Boller and his coworkers, R. Tschuncky, I. Altpeter and G. Dobmann for their very helpful support. References [1] J. Rudolph, S. Bergholz, A. Willuweit, M. Vormwald, K. Bauerbach, Methods of detailed thermal fatigue evaluation of nuclear power plant components, Mat.-wiss. u. Werkstofftech. 42, (2011) 1082-1092. [2] K.H. Lo, C.H. Shek, J.K.L. Lai, Recent developments in stainless steels, Mater. Sci. Eng. R-Rep R65 (2009) 39-104. [3] M. Bayerlein, H.J. Christ, H. Mughrabi, Plasticity-induced martensitic-transformation during cyclic deformation of AISI 304L stainless-steel, Mat. Sci. Eng. A 114, (1989) L11-16. [4] M. Smaga, F. Walther, D. Eifler, Deformation-induced martensitic transformation in metastable austenitic steels, Mat. Sci. Eng. A 483-484, (2008) 394-397. [5] M. Smaga, F. Hahnenberger, A. Sorich, D. Eifler, Cyclic deformation behavior of austenitic steels in the temperature range -60 °C ≤ T ≤ 550 °C, KEM 465, (2011) 439-442. [6] V.S. Srinivasan, M. Valsan, R. Sandhya et al., High Temperature Time-Dependent Low Cycle Fatigue Behaviour of a Type 316L(N) Stainless Steel, International Journal of Fatigue 21, (1999) 11-21. [7] H. Mughrabi, H.J. Christ, Cyclic Deformation and Fatigue of Selected Ferritic and Austenitic Steels: Specific Aspects, ISIJ International 37, (1997) 1154-1169. [8] I. Altpeter, G. Dobmann, C. Boller et al. and M. Smaga, A. Sorich, D. Eifler, Early detection of damage in thermo-cyclically loaded austenitic materials, Electromagnetic Nondestructive Evaluation XV, IOS Press, 36, (2012) 130-139. [9] H.J. Salzburger, EMAT's and its Potential for Modern NDE - State of the Art and Latest Applications, IEEE International Ultrasonics Symposium 1 (2009), 621-628.
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