A Kellering or Keller surface is the surface of a material that has been deformed by a uniaxial stress to just beyond its elastic limit. At that point, the surface will exhibit "Lüders bands" which will grow until either the material breaks, or the stress is released.

The yielding of a metal under simple tension is a very complex event. Yielding typically occurs along planes that are about 45 degrees to the direction of the tensile force. These regions are called Lüders bands and, when they first form, they are microscopic. As the metal is deformed further, the Lüders bands grow and eventually link up with other bands, causing a significant decrease in the load required to deform the metal. Once the deformation is complete, the metal will have a rough, wavy surface that is characteristic of a Kellered surface.