PSI - Issue 2_B

A. Giertler et al. / Procedia Structural Integrity 2 (2016) 1207–1212 Author name / Structural Integrity Procedia 00 (2016) 000–000

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[Giertler and Krupp (in press)]. The difference is particularly evident when looking at the respective zones by high resolution SEM, cf. Fig. 5 a. Even at stress amplitudes in the range of the fatigue limit (≈ N =10 8 cycles), areas have formed with a strong development of local plastic deformation within the microstructure. On closer inspection of the surface of the specimen these areas have been identified as a crack nucleus, cf. Fig 5b. 4. Conclusion For the investigated steel 50CrMo4 two values of the fatigue limits were identified for two different test frequencies of 20kHz and 95Hz. The difference can be attributed to a strong influence of the test frequency on the fatigue limit for the material in the tempered condition. Particular attention was paid in the development of local plastic deformation during the investigation of the fatigue mechanisms in the VHCF regime. Therefore, a test setup was developed, in which the damage evolution can be followed in-situ by high-resolution thermography. In a series of load increase tests a direct correlation between the stress amplitude and microscopic damage evolution was established. The first development of local plastic deformation was observed at a stress amplitude of σ a =396MPa. This value corresponds well with the cyclic yield strength of σ cyc =400MPa determined the incremental step test. The presented test setup seems to be a promising alternative, to represent the damage processes within the microstructure and to allow simultaneous visual inspection. The direct comparison of the stress amplitude with the sample temperature allows to graphically determine the fatigue limit. The estimated value of σ FL =483MPa corresponds with the in the experiments under constant amplitude loading determined value of σ FL =490MPa. The combination of electromechanical testing machine and high-resolution thermal imaging enables the early detection of local plastic deformation during fatigue loading. In addition, surface analysis have shown by means of scanning electron microscopy have shown, that even after 10 8 cycles crack nucleation occurs. For a safe fatigue-resistant component design in the VHCF, regime it may be practicable to restrict the load during operation below the stress value of σ a =396MPa as an initiation value for local plastic deformation during the load increase test. Acknowledgement The German Ministry of Education and Research (BMBF) and the Robert BOSCH GmbH is gratefully acknowledged for financial support of this work and providing specimen material. References Kunio, T., Shimizu, M., Yamada, K., Enomoto, M., Yoshitake, A., 1979. The role of prior austenite grains in fatigue crack initiation and propagation in low carbon martensite. Fatigue & Fracture of Engineering Materials and Structures 2, 237–249. Kunio, T., Shimizu, M., Yamada, K., Sakura, K., Yamamoto, T., 1981.The early stage of fatigue crack growth in martensitic steel. In: International Journal of Fracture 17, 111–119. Zhai, T., Jiang, X., Li, J., Garratt, M., Bray, G., 2005. The grain boundary geometry for optimum resistance to growth of short fatigue cracks in high strength Al-alloys. International Journal of Fatigue 27, 1202–1209. Wagner, D., Ranc, N., Bathias, C., Paris, P. C., 2009. Fatigue crack initiation detection by an infrared thermography method. Fatigue & Fracture of Engineering Materials & Structures. Taylor, G. I., Quinney, H., 1934. The Latent Energy Remaining in a Metal after Cold Working. In: Proceedings of the Royal Society A. Mathematical, Physical and Engineering Sciences 143 (849), 307–326 Cayron, C. 2006. ARPGE: A computer program to automatically reconstruct the parent grains from electron backscatter diffraction data. Journal of Applied Crystallography 40(6), 1183–1188. Kitahara, H., Ueji, R., Tsuji, N., Minamino, Y. 2006. Crystallographic features of lath martensite in low-carbon steel. Acta Materialia 54(5), 1279– 1288 Takeuchi, E., Furuya, Y., Nagashima, N., Matsuoka, S., 2008. The effect of frequency on the giga-cycle fatigue properties of a Ti-6Al-4V alloy. Fatigue & Fracture of Engineering Materials & Structures 31(7), 599–605. Yang, B., Wang, G., Peter, W. H., Liaw, P. K., Buchanan, R. A., Fielden, D. E., Yokoyama, Y., Huang, J. Y., Kuo, R. C., Huang, J. G., Klarstrom, D. L., 2004. Thermal-imaging technologies for detecting damage during high-cycle fatigue. In: Metallurgical and Materials Transactions A 35(1), 15–23. Grange, R. A., (1971). Effect of microstructural banding in steel. In: Metallurgical Transactions 2(2), 417–426. Giertler, A., Krupp, U., (paper in progress). Determining microstructural fatigue damage in the very high cycle fatigue regime by means of infrared thermography. Scripta Materialia