Stability of honeycomb sandwich panels as fresco supports: an experimental, analytical and numerical evaluation

The physical and mechanical characteristics of honeycomb sandwich panels define a specific field of application also in the conservation of artistic heritage for supporting mural paintings after removal, thanks to the advantageous ratio between their self-weight and mechanical performance. During operation and when the mural painting is handled, these panels are subject to in-plane loads that, depending on the surrounding conditions, can trigger local and global instability phenomena that can affect the integrity of the mural paintings. The objective of this study is to parametrically study honeycomb sandwich panels as they vary in slenderness to evaluate the optimal configuration based on the activation of local and global instability mechanisms. A homogenization procedure for the core of the panel, with a hexagonal geometry, allowed us to model a multilayer element consisting of the outer skins and the core. The identified mechanical characteristics were adopted in the analytical orthotropic plate model to verify the collapse mechanisms with respect to inplane compressive loading. The relationship between the loads of global buckling and local mechanisms, core shear buckling, and external skin buckling, were analyzed varying panel slenderness. Finite element analysis was also carried out on the homogenized equivalent material to determine the critical load and investigate global instability modes and compare numerical and analytical formulations. Comparison with experimental tests conducted with in-plane edge-wise compressive loading allowed us to validate the proposed methodology. Monitoring in-plane and out-of-plane deformations using an adapted dual 2D-DIC (Digital Image Correlation) technique as the load increases has defined the interaction modalities between global and local instability mechanisms that can be transferred to the mural paintings, compromising their integrity.