Phenomenological Investigation of Nomex® Core Damage Mechanics in Honeycomb Sandwich Panels under Transverse Impact and Quasi-Static Loading

Abstract

The onset of damage on Nomex® paper based honeycomb core was investigated through a series of quasi-static and impact tests on different size sandwich coupons. Flatwise out-of-plane compression tests on core coupons were conducted to characterize the physical phenomena leading to local fracture. The extent of core damage was also quantified by performing local indentation impact tests with blunt metal tips. In all types of tests, the input energy was selected such that exposed composite skins exhibited minor residual dent. The flatwise compression tests revealed that the phenolic resin rich fillet columns surrounding the intersecting edges of adjacent Nomex® cells contribute to the core out-of-plane stiffness. As the out-ofplane displacement increases and the peak load capacity is reached, phenolic resin columns experience local fracture due to buckling resulting in a sudden decrease in stiffness and strength. During flatwise tests, the position of the localized damage can vary along the sandwich core thickness depending on the inherent flaws that the imperfect cells have. On the contrary, core damage is concentrated closer to the impacted skin for local indentation and impact tests. Finally, the effect of skin bending stiffness was taken into account by impacting specimens with different layup skin configurations. Local indentation impact tests on stiff face sheets (13 woven plies) left no residual dents or externally visible damage for kinetic energies even up to 28 J, despite the presence of crushed cell walls.