Forming of large-area metallic riblet surfaces

在第一个资助期内，研究重点是确定工艺限制和影响工艺参数的肋几何形状在肋轧制过程中，以及对肋板的结构和成形行为的调查：相关的工艺限制是薄和宽箔的板平面度，从技术和经济的角度来看，也是线结构辊的寿命。对板形精度的影响可以定性地描述为取决于平均正应力与平均流动应力之比kw/kfm的基本关系。在进一步的板材成形过程中，迄今为止研究的相对“厚”（h 0 = 1，0 - 1，6 mm）板材中的小（s = 100 µm）肋的各向异性行为较小，主要是由于实际加载横截面的大小与载荷方向不同。在不破坏肋结构的情况下制造了具有单轴曲率的肋板。关于主动减阻的用途，在第二个资助期内，研究必须扩展到“更薄”的板，即s/h 0> 1和h 0 <0，5 mm的尺寸。除此之外，初步调查表明，薄板的形状填充行为发生了变化，而且薄板平整度存在相当大的困难。考虑到后来的应用，肋轧制工艺应扩展到金属基复合材料（GLARE），并应开发形成具有双轴曲率的零件的概念。

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.