13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- The flat sheet samples are glued to an adapter which is connected via a thread to the testing machine. To avoid flexural vibrations, which have been observed in first experiments, samples are damped by submerging their lower part in glycerin during cyclic loading (cf. Fig 1(a)). In-situ monitoring of the sample vibration (Fig. 1(c)) shows a clean harmonic spectrum which is free of bending oscillations. All fatigue tests were performed at a frequency of around 20 kHz at room temperature and zero mean stress. For further analysis, polished cross sections were prepared by embedding the samples in epoxy resin. The surface has been grinded with sand paper up to grade 1000 and subsequently polished with diamond paste. Indentation measurements were performed using a Berkovic type indenter. Fracture planes were analyzed in detail by scanning electron microscopy (SEM). 4. Results 4.1 Effects of riblet rolling The cold working of the sheets during riblet forming lets expect the occurrence of work hardening as well as a surface-near stress. Therefore micro-indentation tests were performed on cross sections of flat as well as on riblet structured material. An indentation load of 5 mN was used. Figure 2. Microhardness measurements at clad material with and without riblet structure: (a) As-received (non-structured) material, F = 5 mN. The hardness is plotted against the distance from the cladding surface. (b) Structured material, F = 1 mN. The hardness is plotted against the distance from the riblet top. (c) Hardness of the cladding layer against the indentation load. (d) Polished cross-section of a structured sample. The measurements in Fig. 2(b) were performed in direction of the red arrows.
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