Damage prediction in quasi-statically indented sandwich composite structures using a two-region plate model

A model is developed for predicting indentation depth, diameter and planar delamination area for thin face sheet, honeycomb core sandwich composites subject to quasi-static indentation loading. The model is primarily based on the Ritz method of energy minimization and is restricted to thin face sheet composites where the deformation is reasonably localized. The assumed displacement function contains unknown parameters defining the peak face sheet displacement and the indentation diameter. Nonlinear plate theory is utilized, and energy contributions from face sheet bending and membrane stresses are therefore accounted for, as are the contributions from both elastic and inelastic core deformations during loading and unloading. Further, the face sheet is divided into two regions, an inner region that is delaminated and an outer region that is intact, and the radius of the boundary between them represents a third unknown parameter that is evaluated during the Ritz minimization. Model predictions are validated by comparison with experimental results for four different quasi-static indentation geometries, all of which consider sandwich plates with eight ply carbon/epoxy, quasi-isotropic face sheets and aluminum honeycomb cores. Geometrical variations are obtained by varying the core density, thickness and indentor diameter. It is shown that the model closely recreates the experimentally observed load versus displacement response and residual dent depth. Residual dent diameter and delamination diameter are generally slightly over-predicted.