PSI - Issue 4

David Simunek et al. / Procedia Structural Integrity 4 (2017) 27–34 D. Simunek/ Structural Integrity Procedia 00 (2017) 000 – 000

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The data points are fitted by a polynomial and additionally compared to results of the software tool INARA, also see Fig. 8. A comparison of the results reveals sound accordance disregarding a/D ratios greater than 0.25. The observation of the fracture surfaces indicate a flat elliptical shape of the crack front at high crack lengths (a/D > 0.25), which concludes on intense crack growth on surface and an evolution of a more plane crack front. This phenomenon is also reported in another work, see Carpinteri et al. (2006). The deviation at a/D ratios minor a/D < 0.05 may be caused by residual stresses on the surface of the specimens, which will be investigated in detail within the prospective work.

Fig. 8 Evolution of a/c ratio at 1:3 scale specimens during crack growth

4. Conclusions and Outlook

Based on the results of fatigue crack growth tests on 1:3 scale specimens in this work, the following conclusions can be drawn:  For constant amplitude tests, a scatter in the results is observed. Although the initial testing conditions are very similar, different residual stress states may affect the threshold regime and further crack growth significantly. This effect will be investigated in detail by forthcoming residual stress measurements and additional constant amplitude tests.  Overloads at an overload ratio of R OL = 2.5 lead to distinctive retardation effects enhancing the final lifetime markedly. A comparison of the constant amplitude tests with and without overloads shows a major decrease of the crack growth rate after overloads.  Retardation of the crack growth rate can delay the number of load-cycles under constant amplitude loading by a factor of 1.6 up to 3.3. Ongoing work focuses on the experimental characterization of further overload ratios, e.g. R OL = 1.5, and oxide induced crack closure effects. In addition, the transferability of small-scale (SEB) to full-scale railway axle applications will be studied in more detail by means of analytical and numerical investigations; preliminary results on retardation modelling and 1:1 tests are reported in companion articles on EBFW 3 in this volume. Acknowledgements Financial support by the Austrian Federal Government (in particular from Bundesministerium für Verkehr, Innovation und Technologie and Bundesministerium für Wissenschaft, Forschung und Wirtschaft) represented by Österreichische Forschungsförderungsgesellschaft mbH and the Styrian and the Tyrolean Provincial Government, represented by Steirische Wirtschaftsförderungsgesellschaft mbH and Standortagentur Tirol, within the framework of the COMET Funding Programme is gratefully acknowledged.