Geometrical imperfection sensitivity on in-plane compressive modulus and strength of honeycomb core and deviations from isotropy

Abstract

Honeycomb core sandwich may suffer from face/core debonding. Candidate debond fracture test specimens such as End-Notch Flexure (ENF) and Mixed Mode Bending (MMB) contain unbonded regions of the sandwich where the core is required to carry in-plane com-pressive load. In-plane modulus and compressive strength of honeycomb core are not pro-vided by core producers and must be estimated from unit cell models assuming perfect hexagonal cells. In this study it is shown that geometrical imperfections, such as deviations in the angle between cell walls and curved cell walls have a strong influence on modulus and strength because of deviations from transverse isotropy. A range of Nomex honeycomb cores were tested under in-plane compression. Modulus and strength ratios (L/W) were found to vary from 0.3 to 3.24 and 0.96 to 1.8, respectively. Analysis based on unit cell models verified that deviations of the inclined cell wall angle, θ, from 30 ^◦ is a major cause for orthotropy. Rounded corners and curved walls were analyzed using finite element analysis. Both principal elastic moduli of the core (E _L and E _W ) were reduced by the curvature while the Poisson’s ratios remained largely unaffected. Implications of the results for design of MMB and ENF fracture test specimens are discussed.