Influence of deformation constraints of core cells on the bending stiffness of single-face honeycomb core sandwich

This study is focused on the influence of core cell deformation constraints on the bending stiffness of honeycomb core sandwich with a single face sheet. Constraints on the deformation of core cell walls develop in sandwich structures due to bonding of the cell walls to rigid face sheets. The constraint prevents bending of flexible cell walls and the large transverse deformation of unconstrained honeycomb core under in-plane axial loading. Such constraints elevate the effective in-plane modulus of the core to a large extent. The bending stiffness of a single-face sandwich beam with a one-sided constrained core is approached using a gradient core modulus model implemented in laminate beam theory. The model includes two parameters, that is the factor of core modulus magnification and the size of the constrained region (decay length). The one-sided constraint is shown to greatly increase the bending stiffness. Single-face sandwich beam specimens with a honeycomb core were prepared and tested in flexure. Experimentally measured bending stiffness results verify that the core constraints significantly elevates the bending stiffness, by a factor close to four. The experimental results agree with the model for a constraint decay length of about 7 mm. The core constraints must be considered in fracture toughness analysis of ENF and MMB debond tests.