Indentation resistance of sandwich beams

High-order sandwich beam theory is used to model the local deformation under the central indentor for sandwich beams loaded under three-point bending. `High-order' refers to the non-linear variations of in-plane and vertical displacements through the height of the core which the model incorporates. The analysis is elastic, which is appropriate to describe the beam response up to peak load for the material combination of GFRP skins and Nomex honeycomb core which is the focus of this paper. Reasonable agreement is found between theoretical predictions of the displacement field under the indentor and experimental measurements using a beam with GFRP skins and Nomex honeycomb core. By using the model to consider the way in which different wavelengths of sinusoidal pressure loading on the top skin are transmitted to the core, a spreading length scale λ is introduced. λ, which is a function of the beam material and geometric properties, characterises the length over which a load on the top surface of a beam is spread out by the skin. Calculations of the effect of roller diameter on indentation behaviour illustrate the importance of this length scale. When λ is small compared with the roller radius R, corresponding to a flexible skin, the contact load at the roller-skin interface is transmitted relatively unchanged to the core. Conversely, when λ/R is greater than about 0.25, corresponding to a relatively rigid skin, the load from the roller is spread out by the skin and the pressure in the core is distributed over a length of the order of λ.