Damage mechanics-based model for the deformation response and failure of composite honeycomb core structure

Abstract

The development of phenolic resin-based Nomex hexagonal Honeycomb (HC) structures is of great interest in recent years for low density, low in-plane, and high out-of-plane stiffness values achieving stable deformations over a wide range of structural geometries. Nonlinear elastic behavior covering the large geometric deformation is the critical issue in analyzing the mechanical response and failure of the cellular core structure. A useful approach for modeling such complex behavior is to replace the cellular structure with an equivalent homogenous material that represents identical mechanical behavior for the respective HC structure. This research aims to develop a Representative Cell (RC) model for the Nomex HC core and subsequently replace that with a homogenous orthotropic material of equivalent elastic properties utilizing the homogenization approach. A series of experimental testing was performed on the HC core to identify the nine elastic constants comprising both inplane and out-of-plane properties. The single unit cell structure is selected based on the parametric analysis through compression testing of the hexagonal HC cores with different cell geometries to compare the compression strength and energy absorption capacity. The selected cell geometry with 3.2 mm cell size, 12.7 mm height, and 0.05 mm paper thickness is used to develop a meso-scale solid element RC model to show the mechanical deformation under out-of-plane compression and shear loading. The constituent orthotropic material model along with Hashin damage parameters was used as input for phenolic resin-based Nomex paper in ABAQUS finite element analysis software. A direct homogenization method was employed to develop a homogenized equivalent homogenized honeycomb core (EHC) model. The model is examined to assess the predicted equivalent elastic properties against the stiffness matrix obtained by experimentation. The comparative analysis for the Nomex HC structural characterization showed that the geometric configurations, specifically the relative density and cell aspect ratio (height/cell size), greatly influence the mechanical properties. The optimum values obtained for the elastic moduli and compression strength were 126.5 MPa and 4.01 MPa, respectively, with a relative density of 0.056 and a cell aspect ratio of 3.96. Compared with the experimental testing results from compression loading, the developed damage mechanics-based RC model demonstrated less than a 2% difference in the collapse/compression strength and elastic moduli of the selected HC core. The EHC model was verified using a threepoint bend loading condition. The predicted flexural strength compared to the measured data had a minimal variation of only 4%. The developed EHC model can be effectively used to predict the mechanics of deformation and failure properties in the complex sandwich structures. The damage mechanics-based methodology presented in this research work could be implemented for complex structural parts in the aerospace and transport industry for reducing the need of extensive experimental testing eventually minimizing the developmental cost and time.