13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Just the scattering of our values seems to be slightly increased compared to the literature values. For the clad material (orange half squares) surface smoothening is not possible without removing the cladding layer. The surface quality therefore seems to be the for lifetime reduction compared to bare Al 2024 by several orders of magnitude. Nevertheless, some samples, tested at 90 and 100 MPa, do not seem to be influenced by scratches and exhibit a much longer lifetime which lie in the same range as for the bare material. Furthermore, sample thickness in the tested range seems not to influence lifetime. First measurements on structured samples with riblets along the sample direction (green half circles) show similar results as obtained for the unstructured clad material The fractures between 106 and 107 cycles at load amplitudes between 80 MPa and 110 MPa are is most induced by small scratches on the sample surface. Nevertheless, also run-outs above 108 cycles were observed in the same load regime. Figure 4. Fracture surface of a bare sample. N = 9.8145x108, σ max = 110 MPa. (a) Overview. (b) Higher magnification of the crack initiation site. Figure 5. Fracture surface of a cladded specimen. N = 3.38x105, σ max = 173 MPa. (a) Overview Crack initiation sites are indicated by red arrows. (b) and (c) Higher magnification of crack initiation sites. (d) Polished longitudinal section of the same sample. The fracture plane is located on the right.
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