Improving the In-plane Stiffness of the Hexagonal Re-entrant Auxetic Honeycomb Cores

In the present work, two new designs of honeycomb structures based on the basic re-entrant structure were presented namely, splined-reentrant honeycomb and stiffened-reentrant honeycomb. The new structures were designed in order to improve the in-plane stiffness of the honeycomb and keeping the auxetic behaviour of the structure. In this study, finite elements modelling and experimental work were carried out to evaluate the in-plane mechanical properties of the new designs. The effect of the geometrical parameters such as rib length, rib thickness and orientation of the unit-cell of the new designs on the in-plane properties was investigated. Finite element results were compared with the analytical results to verify the finite element models of the basic re-entrant structure and good agreement was obtained. The finite element results showed that the in-plane stiffness of the new designs was improved significantly compared to the basic re-entrant structure. Also, the stiffened-reentrant structure showed better enhancement of the in-plane stiffness than the splined-reentrant structure. However the auxeticity of the splined-reentrant structure is higher than the stiffened-reentrant structure. For example the modulus of elasticity of the stiffened-reentrant structure 10 times that of the basic reentrant structure in x-direction and over two times in the y-direction with lower values of the negative Poisson’s ratio. Compression tests were carried on honeycomb samples made of steel using laser cutting technique with different geometrical parameters. Test results for the three designs were compared with the finite element results and they were in a good agreement.