The effective elastic properties of flexible hexagonal honeycomb cores with consideration for geometric nonlinearity

Flexible hexagonal honeycomb cores exhibit nonlinear elastic properties due to the large geometric deformation. To rapidly and efficiently analyze the mechanical properties of honeycomb sandwich structures, it is standard to replace the actual core structure in analyses with a homogenized core material presenting reasonably equivalent elastic properties. As such, a convenient and efficient method is required to evaluate the equivalent elastic properties of flexible hexagonal honeycomb cores. The present work develops analytical expressions based on a deformable cantilever beam under large deformation. On that basis, the equivalence expressions are improved by including the stretching deformations of the honeycomb structure on an infinitesimal section of a unit cell. Finite element analysis and experimental testing are subsequently performed for two examples of flexible hexagonal aluminium and Nomex^® honeycomb cores. Both computational and experimental results indicate that their equivalent elastic moduli have the different variation with high strain in the two characteristic orthogonal directions. The analytical predictions demonstrated that the improved analytical expressions are more suitable to flexible honeycomb cores under conditions of high strain and low elastic modulus. It is further revealed that the structure of Nomex honeycomb cores result in different forms of damage during failure along the different load directions, which lead to differing equivalent properties in the orthogonal directions when the strain limit is reached.