The flatwise compressive properties of Nomex honeycomb core with debonding imperfections in the double cell wall

Mechanical properties of Nomex honeycomb core are governed by not only its global dimensions, cell topology, material properties and proportion of the aramid paper and phenolic resin, but also possible manufacturing imperfections, such as the debonding between the two aramid paper sheets in the double cell wall. To account for the layered feature of the cell walls and the bonding conditions between aramid paper sheets, a three-dimensional unit cell model was proposed and developed in this study. The aramid paper sheets, the phenolic resin coating, the adhesive between the aramid paper sheets, and their bonding relationships were all explicitly modelled in accordance with their actual geometry and material parameters. The model was validated by comparing the predicted load-displacement curves and failure modes with the test results. The effects of representative bonding imperfections on both the collapse load and the related displacement of the honeycomb core under flatwise compression were evaluated. Through the analyses, it was found that the debonding imperfections have significant effects on the mechanical behaviour of the honeycomb core and that with the same debonding area the debonding at the outside edge of the adhesive printing line is the most critical. It was also found that debonding fracture may occur if adhesive is not strong enough or the debonding imperfection area is large.