Honeycomb
A structural sandwich is a layered construction formed by bonding two thin facings to a thick one.
The basic design concept is to space strong thin facings, or skins, far enough apart with a thick core to assure the combination will be stiff, to provide a core that is stiff and strong enough to hold the facings flat with an adhesive layer, and to provide a core material of sufficient shearing resistance.
The structural sandwich panel is analogous to an I-beam, with the facings carrying compression and tension loads, as do I-beam flanges, and the core material carrying shear loads, as does the I-beam web.
The aerospace industry remains the greatest consumer of composite materials and sandwich construction, whether for civil aircraft, military jets, helicopters, aero-engines or the newer space satellites and launchers.
Faster speeds, higher altitudes and higher G forces all put immense demands on aircraft and rocket structures. Standard honeycomb cores employed in sandwich construction make stiff and light sandwich panels used extensively in the aerospace industry.
Who invented honeycomb?
Professor J.E. Gordan wrote a book called "Structures, or, Why things fall down".
In this book (pp297) he describes how a circus proprietor by the name of George May met up with the professor, who was at that time, an aircraft engineer at Farnborough in 1943 with the initial idea of gluing ribbons together to make a honeycomb structure.
This material was successfully used during WW2 and gave us the edge to win the war in the skies with the highly efficient Wellington and Mosquito bombers.
Dupont then brought the idea to the United States of America.
Hexcel licensed the expanded honeycomb concept, and was the first company to manufacture honeycomb on a commercial scale, over 50 years ago.
Disadvantages of Traditional Honeycomb
After nearly 7 decades however aerospace has gained both in speed and altitude and continues to stretch the limits of commercially produced honeycomb core technology. A fatal indication of this occurred on April 28, 1988, the skin of an aging Aloha Airlines 737 peeled off at 24,000 feet.
A crew member was lost in the resulting explosive decompression, and the passenger cabin was opened to the frigid, oxygen-poor sky. Only sheer luck, the intrinsic strength of the 737 airframe, and the skill of the pilots managed to get this aircraft back on the ground safely.
So what happened? According to the NTSB, "the cause of this accident was the presence of significant disbonding (delamination) .
Another expensive example was the failure of NASA’s experimental Space vehicle the X33.
The liquid hydrogen fuel tank failed five stages of testing because the honeycomb walls of the tank succumbed to delamination (honeycomb’s dirty little secret), at a cost of 1.33 billion tax dollars.
The Aerospace Industry is severely burdened by the problem of delamination of honeycomb panels, leading to expensive and sometimes catastrophic failures.
The Aerospace Industry is severely burdened by the problem of delamination of honeycomb panels, leading to expensive and sometimes catastrophic failures.
Delamination can only be detected by the use of slow and costly techniques. Currently, an estimated 25% of an aircraft’s average life cycle is spent on the ground undergoing lengthy inspections.
Delamination.
Delamination happens when the epoxy between the skins and the edges of the core fails to maintain a bond due to:
- The small bonding area of honeycomb cell edges to the face sheets.
Blue indicates the available bonding area for attachment of face skins
Red indicates the glue which holds the ribbons together
- The inability of closed cells to vent.
- Stress from varying air pressures and flexing experienced during flight.
- Cell expansion caused by water vapor freezing at high altitudes
- Lightning strikes turning trapped moisture into steam.
Hexaflex
Hexaflex is a new innovative core, perfectly designed to alleviate all these shortcomings, thereby reducing catastrophic failure, loss of life and costly aircraft downtime.
The benefits of Hexaflex over conventional honeycomb include:
The benefits of Hexaflex over conventional honeycomb include:
- 1000% more bonding area:
Strengthens the bond between skins, and improves peel strength - Ability to vent:
Eliminates pressure build-up, allows water vapor to be purged from the core and greatly reduces if not eliminates damage from lightning strikes.
- Ease of manufacture:
Here you can see the transitional stages of the manufacturing process of the Hexaflex matrix.
Hexaflex is fabricated from a continuous sheet of core material. The sheet is first die-cut with a repeating geometrical design.
From an overall flat shape of the die cut flat sheet as in upper left you can see the beginnings of the folding sequence as the valley and mountain folds begin to shape the overall appearance as it concertinas in upon itself.
Sheet metal cores can be folded using progressive forming techniques.
Plastics can be injection molded or vacuum formed.
Fiber based materials can be chop sprayed.
- Freedom of design:
Hexaflex can be formed to any desired shape, including spheres, cones and tubes. - Lap jointing:
High integrity lap jointing. “No ifs ands or Butts”.
- Flexibility It is able to conform to any variable surface curvature without any deformation of the matrix.
- Lightweight:
Hexaflex does not use glue in its manufacture, and uses less material overall. - Optimal engineering: Because Hexaflex is made from a flat sheet it does not have any glue or double walls to add to its weight. This weight saving can be utilised to allow foam metal plugs to be inserted into the matrix. These plugs can be of a variable density so that one can tailor the panel to optimize its performance by distributing strength only to where it is required.
By strategically inserting foam metal inserts of appropriate crush strengths into the open cells, compression and shear forces can be tailored to suit a specific application; In the bottom left one can see one side of the fully folded Hexaflex with partial placement of metal foam inserts.
Summary
The introduction of Hexaflex to the aerospace industry will increase safety, flight efficiency and aircraft longevity.
Other Applications:
- ABSORPTION PROTECTIVE STRUCTURES
Aerospace, automotive, defense, test facilities, industrial machines, marine, nuclear, rail.
- AEROSPACE
Satellites, launch vehicles, space shuttle, morph technology. - AUTOMOTIVE
Crash test barriers, door and roof panels, double-skinned exhaust manifolds, fairings, heat exchange panels, flexible fuel tank, motorcycle fairings. - AVIATION
Ailerons, cowlings, doors, flooring, flaps, radomes, rudders. - BIO-ENGINEERING
arterial stent technology, artificial extra-cellular matrix, capsid engineering. Hernia mesh prosthetics.
CONSTRUCTION architectural panels, concrete reinforcement in earthquake prone areas, false ceilings, flexible tubular structures, insulation curtains for hazardous material removal, roof panels, wall panel.
- MARINE bulkheads, bunks, covers, decks, double skinned hulls, hatches, wave energy framework.
- MISCELLANEOUS dirigibles, double-skinned oil tanks, flexible body armor, oil pipelines, piping / ductwork with interstitial space, radio frequency shielding, soil stabilization mat, solar energy panels, sound attenuation panels.
- RAIL Ceilings, doors, energy absorbers/bumpers, floors, partitions.
- RECREATION INDUSTRY athletic shoes, motorcycles, surfboards, snowboards, tent walls, toy, wake board.
- RESEARCH morphing wing concept, nanotechnology, robotics.
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