BACKGROUND

The rising demand for new materials with higher strength to weight ratios has created a dramatic growth in sandwich composite technology. Sandwich construction employs a lightweight core that has a flexural strength and flexural modulus far exceeding that of the skin laminates alone. The most common type of core material is honeycomb, which is used primarily in the aerospace industry.

The main disadvantage of honeycomb type cores used in the Aircraft and Aerospace industries is that of delamination which can cause a catastrophic failure of the vehicle. This is caused primarily by the failure of the epoxy adhesives to maintain a bond between the facing skins and the core because of the very small bonding area that honeycomb cores edges offer.

This is further exacerbated by the fact that honeycomb type cores create pockets of trapped air within the closed cells of the core when the skins are attached. The air pressure experienced at high altitudes is much lower than the trapped air within the cells with the result that the skin is pushed away from the inner core by the air pressure. Ingress of water into already partially delaminated cells at high altitude freeze into ice particles which expand and force the skin to separate from the core. Eventually after many cyclic operations the skin will delaminate. Additionally, lightning strikes can cause entrapped moisture within the core to immediately turn to steam with catastrophic results to the integral strength of the panels.

The Aerospace Industry remains the greatest consumer of honeycomb materials, whether for civil aircraft, military jets, helicopters, aero-engines or the newer space satellite and launchers.

The Director of NASA’s Marshall Space Flight Center once stated that “our (composite) technology has not yet advanced to the point that we can successfully develop a new reusable launch vehicle that substantially improves safety, reliability and affordability.” He was referring to the failure of the composite fuel tank panels of the NASA X-33 Reusable Space Vehicle due in part to honeycomb delamination.

HEXAFLEX HONEYCOMB COMPARISON

1. SURFACE BONDING AREA

In this 3D view of Hexaflex, one can very quickly grasp the fact, purely by observation, that the hexagons have a much larger surface area available for bonding to the face sheets in comparison to the surface area of the edges available in conventional honeycomb. The blue color denotes bonding area.

Hexaflex core design has two different surface architectures. the side which is shown above, which we think of as the outside surface, and the other, inside surface, below, which as you can see, has only half as much bonding area as the outside, but none-the-less, still has a large bonding area in comparison to conventional honeycomb.

2. FAST VENTING CORE

Each cell in a honeycomb sandwich is an airtight vessel. When heated, the air in each expands, increasing the pressure. If the pressure gets too high, the film adhesive bond may fail, initiating a delamination.

Hexaflex overcomes any possibility of this occurring because it is an open fast-venting core design that prevents any pressure differentials from building up within its geometry.

Some adhesives give off gases or solvent vapors during cure, which can interact with resin systems in some non-metallic cores, or with the node adhesive in some metallic honeycombs. The entire bonding process must be checked to ensure that no reduction in mechanical bonding properties has occurred.

One could quickly purge the assembled Hexaflex sandwich panel with gases or liquids by virtue of the ventways that inherently run through the core design. (see red arrows)

These ventways could also be utilized for service runs for electrical pneumatic or hydraulic lines.

3. STRENGTH

Compression and shear forces can be tailored to suit the application by placing foam metal hexagonal cross-sectioned inserts into the blind hexagonal cells on the one face of the core material.

4. FORMABILITY

Hexaflex conforms naturally to compound curvature with its cells normal to the face surface, without the need for curving, rolling or heat forming operations. It does not suffer from cell wall damage, columnar failure, node separation or distortion of the hexagonal cells when subjected to compound curvature.

5. LIGHTWEIGHT

Honeycomb cores are heavier due to the fact that they consist of multiple ribbons of core material glued together (see red lines below) requiring one of the six cell walls of every cell in a regular honeycomb core to be double thickness.

Hexaflex core has no glue, is a single thickness throughout and is formed from a single sheet. In the event of excessive forces the core will demonstrate superior structural integrity.

6. EDGE JOINING AND STACKING

Hexaflex core material can be edge-lapped upon itself allowing hexaflex panels to be joined together with superior seam strength. Hexaflex core material can be stacked upon itself to allow efficient storage.

Edged-lapped

Nested partially deployed configuration

PRODUCTION OF HEXAFLEX

These next 2 illustrations show the steps taken to form Hexaflex from it's initial flat sheet to the final folded configuration. It is easy to imagine that Hexaflex does not have to be formed out of metal.  The intended use will to a large degree influence the choice of materials used in it's fabrication.