Elastic sheets on Winkler foundations: Indentation stiffness and nonlinearities

The indentation of thin sheets on Winkler’s mattress or elastic foundations offers valuable opportunities to gain quantitative insights into the mechanical properties of both the material and its interface. However, interpreting indentation data is complicated by the interplay of plate bending, sheet pre-tension, and foundation deformation. The challenges are further amplified in recently developed nanoindentation techniques for small-scale systems, such as 2D materials and cell membranes, where indenter size, shape, and foundation nonlinearity have been found to influence the results significantly. Here, we address these challenges by investigating a generalized indentation problem involving a pre-tensioned elastic sheet on a mattress foundation, considering both punch and spherical indenters. By linearizing the Föppl–von Kármán equations and the elastic foundation under small indentation depth, we obtain a set of asymptotic solutions that quantify the effects of pre-tension and indenter geometry on indentation stiffness. These solutions show excellent agreement with numerical solutions in various parameter regimes that we classify. We also discuss sources of nonlinearities arising from the kinematics in sheet stretching and the evolving contact radius in spherical indentation. The results should be of direct use for the nanometrology of layered materials where indentation remains one of the most accessible techniques for characterizing mechanical properties at small scales.