13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- The results are presented in Fig. 2. Measurements on cladded sheets (Fig. 2(b)) show a hardness of (59 ± 6) HV0.005 for the cladding layer and of (182 ± 26) HV0.005 for the substrate. Measurements on structured samples were performed according to Fig. 2(d). The riblet structure is completely located in the cladding layer and does not affect the substrate. Since the riblets are quite narrow a smaller indentation load of 1 mN was applied to reduce the lateral size of the indentation. In spite of the strong local deformation during riblet forming, a nearly homogenous hardness of (75 ± 13) HV0.001 has been determined along the riblets and in the underlying part of the cladding. To investigate the influence of indentation load on the measured hardness, a series of indentation tests has been carried out in the cladding layer by performing 10 indentations at each of 6 forces in the range between 1 mN and 50 mN. The results are given in Fig. 2(c). A clear indentation-size effect is observed. Comparing the hardness values for 1 mN and 5 mN in Fig. 2(c) with cladding hardnesses in Figs. 2(a) and 2(b), it can concluded that the apparently increased hardness of the riblet structure can be completely attributed to the indentation size effect and the influence of work-hardening induced by the rolling process for riblet forming is negligible compared to the scatter of the indentation measurements. The amount of surface residual stress, induced by the riblet rolling, was measured by X-ray diffraction using the sin2(Ψ)-method. Prior to riblet rolling, the residual stress was (-11 ± 8) MPa. Due to riblet rolling a change to (-29 ± 7) MPa was noticed. 4.2 Fatigue experiments The results of the fatigue experiments are summarized in Fig. 3 and compared to literature values. Solid blue circles indicate results for bare Al2024 smoothened by grinding with sand paper ( grit 600) prior to testing. The lifetime of these samples compares well with literature results taken from Refs [3,7] ensuring the reliability of the present measurements: Neither the adhesive connection of the samples to the testing machine, nor the thin sheet sample geometry that necessitate damping of flexural vibrations, have an influence of the sample lifetime. Figure 3. S/N graph of the fatigue lifetimes obtailed in the present work and from literature [3,7,10].
RkJQdWJsaXNoZXIy MjM0NDE=