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An analytical model is developed to predict the loading and unloading response, as well as the residual dent diameter and dent depth, of carbon/epoxy-aluminum honeycomb core composite sandwich structures undergoing quasi-static indentation loading. The model considers damage created using spherical indenters and is valid up to the barely visible external damage threshold. The initial low load regime (until the onset of core crushing) is modeled using a combination of local Hertzian indentation of an elastic half-space and small deflection plate theory of a circular plate on an elastic foundation. For loads above those required to cause core crushing, the model uses the Rayleigh-Ritz method of energy minimization with the total system energy determined using a combination of face sheet bending energy, face sheet membrane energy and work done to the core during both elastic deformation and crushing. Degraded face sheet properties are used in the model beyond the onset of face sheet delamination, which is predicted using Griffith's energy criterion. The model is validated using experimental results for sandwich structures consisting of quasi-isotropic 8- (thin) and 16- (thick) ply carbon/epoxy face sheets and aluminum honeycomb cores. The results show that the overall mechanics of the model are fundamentally correct and reflective of physical behavior. Thus, in its present form the model shows promise as a preliminary design tool. [ABSTRACT FROM AUTHOR]

The damage resistance of composite sandwich structures with eight and 16 ply quasi-isotropic, carbon/epoxy face sheets and aluminum honeycomb core is evaluated. The external damage is induced quasi-statically, using spherical steel indentors under displacement control, to be in the vicinity of the barely visible threshold. In addition to the face sheet thickness, other parameters that are varied include the core thickness, core density, face sheet layup, and indentor diameter. The effect of these parameters on the extent of damage is evaluated using the damage metrics of dent depth, dent diameter, and planar area of delamination. When dent depth or dent diameter is considered as the damage metric, specimens containing a higher density core are always found to be the most damage resistant. When planar area of delamination is considered as the damage metric, the eight ply configuration comprised of a lower density core and face sheets containing only small ply angle changes are found to be the most damage resistant. However, this configuration is found to be the least damage resistant when this damage metric is applied to the 16 ply specimens. Rather, the best delamination resistance is provided by a 16 ply configuration with a high density core and face sheets that have ±45° ply groups at the beginning and ending of their stacking sequence. [ABSTRACT FROM AUTHOR]

This study examines the parametric effects of core density, core thickness, face-sheet stacking sequence, and indentor diameter on the compressive strength of aluminum honeycomb-core sandwich panels stiffened with eight-ply, quasi-isotropic, graphite/epoxy face sheets. The sandwich panels contained damage at the threshold of visual detectability created through quasi-static indentation with 25.4 mm or 76.2 mm-diameter spherical indentors. During compression-after-indentation testing, failure occurred due to: dent deepening followed by localized, compressive micro-buckling of fibers in the 0° plies; localized buckling of the near-free-surface sub-laminates; or unstable dent growth in the direction lateral to the applied compressive load. Regardless of failure mode or face-sheet type, the compression-after-indentation strength increased with increasing core thickness and with decreasing core density. Additionally, panels containing face sheets with the 0° plies near the mid-plane and 45° angle change between subsequent plies exhibited greater undamaged compressive strength and higher compression-after-indentation strength relative to panels containing 90° angle changes between subsequent plies and 0° plies near the free surface. The compression-after-indentation strength was found to be relatively unaffected by the indentor diameter size and the resulting variations in the face sheet and core damage. These results imply that precise representation of the damage state in models to predict the post-indentation response of sandwich panels may not be necessary in order to make accurate average residual strength predictions. [ABSTRACT FROM AUTHOR]

Two types of experiments were designed and performed to evaluate the adhesive bond in honeycomb sandwich panels. The tensile bond strength between the facesheet and the core was determined through the flatwise tension test. The fracture toughness of the bond line was measured through the double cantilever beam test. Fracture toughness values varied for different facesheet thicknesses and core materials. Toughness was also different for the bag and tool sides of the panels for all specimen types. [Copyright &y& Elsevier]