13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- the VHCF-regime. Fig. 5 shows the fracture surface of a clad sample after fatigue at relatively high load amplitude. Multiple crack initiation sites, both at the bare side and in the cladding (cf. red arrows in Fig. 5(a)) can be clearly seen. A detailed view on exemplary crack initiation sites are shown in Fig. 5(b) and (c). A polished longitudinal section of the same sample (Fig. 5(d)) illustrates multiple cracks starting at the surface of the cladding layer and crack growth into the substrate. Failure of structured samples has been observed in two different lifetime regimes (cf. Fig. 3):(i) below 107 cycles and (ii) above 108 cycles. Fig. 6(a) gives an overview on typical fracture surfaces. Samples 1 and 3 represent the lifetime regime (i), whereas samples 2 and 4 are representative for the VHCF-regime (ii). Specimens with shorter lifetime show multiple crack initiation sites. In both cases the final fracture has its origin on the structured side close to the surface. For samples with longer lifetime, only one initiation site has been observed. In the case of sample 2 this is located on the structured side, for sample 4 on the backside, which does not have a cladding layer but a bare, smoothened surface. It is interesting to note that the lifetimes of both samples are similar to the lifetime of non-structured, bare sheets. 6. Conclusion Fatigue experiments were performed on Al 2024 sheets with and without a pure Al cladding layer. Cold rolling of a riblet structure on the cladded side induces -if any- only slight work-hardening within the scatter of hardness measurements performed by microindentation, whereas a slight increase of compressive surface residual stress was observed after rolling. Fatigue lifetimes determined for the bare material match literature data quite well. For cladded material without riblet structure, a reduced lifetime is observed which is attributed to surface scratches which could not be removed from the thin cladding by grinding and polishing. Riblet structures produced by cold rolling do not significantly affect fatigue life of the clad material. Bare sheets show crack initiation inside the material. Clad samples show crack initiation in the cladding layer surface and subsequent crack growth into the substrate. In general at N < 107 multiple crack initiation sites are observed, whereas in the VHCF regime only single crack initiation sites are found. Clad specimens show crack initiation both on their cladded and on their bare surface which complies well with the similar lifetimes observed for both specimen types. Acknowledgements The present work was funded by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the DFG Research Unit 1779 “Active Drag Reduction”. The authors would like to thank H. Mayer and R. Schuller for helpful discussion on the experimental setup. Furthermore we thank T. Romans and J. Pöblau for the structuring of the sheets. References [1] D. W. Bechert, M. Bruse, W. Hage, J. G. T. Van Der Hoeven, and G. Hoppe, Experiments on drag-reducing surfaces and their optimization with an adjustable geometry, Journal of Fluid Mechanics 338, 59 (1997). [2] S. Klumpp, M. Meinke, and W. Schröder, Friction Drag Variation via Spanwise Transversal Surface Waves, Flow, Turbulence and Combustion 87, 33 (2011). [3] H. Mayer, M. Fitzka, and R. Schuller, Ultrasonic fatigue testing of 2024-T351 aluminium alloy at different load ratios under constant and variable amplitude, presented at the 5th Int. Conf. Very High Cycle Fatigue, Berlin (2011).
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