Damage characterization of quasi-statically indented composite sandwich structures

The nature of quasi-static indentation damage is studied in aluminum honeycomb core sandwich panels with eight ply, quasi-isotropic, graphite/epoxy face sheets. Parameters that are varied include the core thickness, core density, face sheet layup, and indentor diameter. The majority of induced damage is in the vicinity of the barely visible threshold. The permanent dent in the panel is found to be always larger than the contact area of the indentor, and specimens with denser cores exhibit smaller dent diameters for a given dent depth. Regardless of specimen layup, delaminations occur essentially only at the 3rd, 5th, 6th, and 7th interfaces. Stiffer cores, either in terms of a higher density or, for those cores considered, a smaller thickness, result in lower dent depths, smaller dent diameters, and more face sheet delamination for a given indentation event. The manner in which core orthotropy influences the size and pattern of the delaminations is shown to depend on face sheet layup. Regardless of the core, larger delaminations occur in face sheets that contain 90 angle changes between the adjacent plies in comparison to those that contain only 45 angle changes. Results are compared to those previously reported in the literature, and mechanisms that are related to plate boundary conditions are described that reconcile what would appear to be conflicting results obtained by different studies. Findings are then discussed in the context of choosing the most damage-resistant structural configuration and how this translates to damage tolerance.