Forming of large-area metallic riblet surfaces

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In the first funding period, the research focused on determining process limits and influencing process parameters on the riblet geometry during the riblet rolling process as well as on investigating the structural and forming behavior of riblet sheets: The relevant process limits are the sheet flatness for thin and wide foils and from a technological and economical point of view also the lifetime of the wire structured roll. The influences on the shape accuracy can be described qualitatively by fundamental relations depending on the ratio of average normal stress and mean flow stress kw/kfm. The anisotropic behavior of the so far investigated small (s = 100 µm) riblets in relatively "thick" (h0 = 1,0 - 1,6 mm) sheets during further sheet forming processes is minor and mostly due to the size of the de facto loaded cross section that differs with the load direction. Riblet sheets with uniaxial curvature were manufactured without damaging the riblet structure.With regard to the use for active drag reduction, in the second funding period the investigations have to be extended to "thinner" sheets, meaning dimensions with s/h0 > 1 and h0 < 0,5 mm. The forming behavior as well as the behavior during activation is then expected to depend on the riblet orientation. Besides that, preliminary investigations showed a modified form filling behavior for thin sheets and considerable difficulties with sheet flatness. With the later application in mind, the riblet rolling process shall be extended to metal-matrix composites (GLARE) and concepts for the forming into parts with biaxial curvature shall be developed.