Numerical modeling and analysis of the damage characteristics of a honeycomb core within a sandwich panel

Sandwich panels increasingly replace metallic components in the aerospace industry. The sandwich panel configuration that is being considered in this research project is composed of carbon fiber composite facesheets encasing a honeycomb core. The honeycomb core is made from a fibrous sheet of material constructed into hexagonal cells. The core is relatively compliant compared to the composite facesheet, but together in a sandwich panel they result in a structure that has both desirable strength to weight and stiffness to weight ratios. As these composite structures are integrated into the design of aircraft, an understanding of their failure modes and an accurate prediction of their structural integrity when damaged are essential. The objective of this research is to understand how individual failure mechanisms contribute to the mechanical response of the overall material system. Damage mechanisms associated with sandwich panels include failure of the composite facesheets, crushing of the honeycomb core and disbonding at the interface between the composite and the core. To understand how such damage affects the response of the material system, a finite element model incorporating the dominant failure modes was created. Simulations of the sandwich structure needed to be computationally efficient and accurately demonstrate the failure characteristics that were observed experimentally. Experimental results showed that damage in the honeycomb could initiate without any visible damage to the facesheet, so the analysis of the honeycomb was especially important to the integrity of the model. In order to account for the damage in the honeycomb, a novel crushing zone model was developed to capture the failure response of the core. Facesheets were modeled using a plane stress orthotropic material formulation, and the Hashin-Rotem (1973) damage criterion was used to account for the initiation and propagation of the failure within the composite. The finite element model (FEM) was created and analyzed with the ABAQUS/Standard software package. FEM results show that failure of the core is present in shallow indentations and low energy impacts. Core failure will initiate prior to the failure of the composite and will always present a larger damaged area, which illustrates the importance of analyzing the material