UC San Diego

Sandwich structural elements consist of two metallic or non-metallic laminated skins separated by a lightweight thicker core layer, maximizing in this manner weight efficiency and structural performance. From the available core configurations, phenolic resin impregnated Nomex® paper honeycomb cores are highly preferred in aerospace industry because of their lower density, excellent thermal resistance, and curved shape adaptability.

Despite the inherent benefits of Nomex® paper sandwich structures, their impact response is still questionable. Towards this direction, a series of simulated hail ice impact tests was performed on flat carbon fiber sandwich panels via gas gun test apparatus to assess Nomex® paper core damage. Impact test results revealed significant core crushing in the form of cell collapse without any externally visible signs of damage on the impacted skin surface.

Since high-velocity impact tests on sandwich panels are by nature highly complex, the out-of-plane response of over-expanded Nomex® cores was phenomenologically characterized through quasi-static flatwise compression tests on stabilized sandwich core coupons. Localized detachment and crushing of the phenolic resin accumulation regions between adjacent cell walls, were the main damage modes that triggered honeycomb collapse.

X-ray Computed Tomography (CT) scans were also employed to assess damage modes on the interior cells of tested sandwich core coupons. Given several sets of X-ray CT scan 3D data, an automated B-spline image-based reconstruction scheme was developed to numerically capture scanned honeycomb core geometries. Different applications of the proposed reconstruction scheme are going to be presented. Finally, the contribution of the phenolic resin accumulation regions on the out-of-plane response was investigated through numerical modeling of the reconstructed B-spline geometry. To this end, constituent material properties were determined through uniaxial tensile testing on in-situ phenolic resin impregnated paper coupons as well as bare aramid paper coupons. Numerical simulations on sandwich cores without the phenolic resin accumulations yielded significant reduction of the collapse limit, validating in this way the experimental observations.