Sandwich Cores for the Future
Decreasing weight while increasing strength is always critical, from airliners to future space missions to Mars. Research in sandwich cores today may lead to radical improvements in the future.
The search for lighter weight, stronger materials to construct airplanes and spacecraft remains as important as ever. But it takes a long time for new materials in aerospace structures to make their way into production designs. Capital is one reason; it is harder to take risks when it costs so much to develop new composites. Another is the long approval process from regulatory agencies due to safety constraints.
“Aerospace is an interesting case because there is a lot that you can do with composites that is being done in [just] a few cases today,” explained Anthony Vicari, Lead Analyst for Advanced Materials for Lux Research, in an interview with Aerospace & Defense Technology.
One of those materials that is well established but arousing increasing attention in the aerospace industry is sandwich materials, according to Vicari.
“A sandwich panel design can be used to avoid local buckling in panels loaded in shear and Euler buckling in panels with compressive in-plane loads, which simplifies the structure and structural analysis. It is also particularly suitable to stiffen a panel that is subjected to out-of-plane loading or torsional loads,” Malcolm Foster, Chief Engineer for GKN Aerospace, explained to A&DT.
Foster went on to explain some of the disadvantages. One is that sandwich cores can absorb water vapor, and condensation accumulates in the cell under constant cycling of air pressure and temperature that aircraft structures are exposed to. “It is possible to seal the cores to prevent this, but this adds weight that [may] offset the benefits of core over a monolithic layup,” he said.
The sealed air inside the honeycomb cells exerts a pressure in the reduced ambient pressure at altitude, leading to potential cycle fatigue. Manufacturing issues may include inability to handle full 100-psi autoclave pressure as well as telegraphing issues with honeycomb and co-cured skins, among others. Honeycomb does not play well with resininfusion processes.
“Honeycomb is an added expense. It requires careful placement in the layup, and the layup is complicated by having to fit around the core. All of this is difficult to integrate with automated fiber deposition processes and, therefore, it drives you towards manual processes,” he said.
However, sandwich composites may well get a boost as a number of companies are looking to exploit new manufacturing technologies to expand the range of cores and skins alike, overcoming some of these problems to get lighter and stronger structures.
Sandwich Construction Innovations
“At GKN Aerospace we are working on new ways to create custom cores for complex shapes that would not normally be possible to form from a pre-fabricated sheet,” explained Foster. “There is still a lot of innovation in foam cores—for example, in-mold foaming (IMF) of structural foams has the potential to allow greater design freedom and wider use of foam. Previously, IMF was only available for non-structural cores such as expanded polystyrene.”
NASA announced in April 2015 the selection of three proposals to develop and manufacture ultra-lightweight core (ULWC) materials for future aerospace vehicles and structures. All three focus on advanced sandwich construction.
“Standard composites gives us 30% weight savings, but for the Mission to Mars we really need to find a way to further lightweight our vehicles and structures—our ultimate goal is to achieve 40% weight savings, even 50% weight savings over conventional aerospace materials such as aluminum,” John Vickers, Associate Director for Materials Processing for NASA, told A&DT. “Sandwich cores have a very widespread applicability to space systems. Sandwich construction in our opinion is the most weight optimal, especially in crew habitats.”
He believes industry can develop sandwich construction that can handle structural loads as well as the conventional metal skin and stringer designs often found in aerospace.
“The criteria for evaluation of the three approaches over conventional ones, and against each other, include weight savings, crush strength, and mechanical structure test,” explained Azlin Biaggi-Labiosa, Project Manager for the Nanotechnology project at NASA, under which these contracts are implemented.
The three companies selected for contracts include HRL Laboratories, ATK Space Systems, and Dynetics, Inc. Phase I awards of the solicitation are valued up to $550,000, providing awardees with funding for 13 months to produce 12×12×1-in ultra-lightweight core panels. Technologies selected to continue to Phase II will demonstrate the ability to scale up to 2-ft by 2-ft by 1-in and ultimately to produce 10-ft by 11-ft by 1-in ULWC panels, with NASA providing up to $2 million per award for up to 18 months.
As with most new technologies, we should not expect these to be flying anytime soon. NASA does expect applications outside of space flight.
“The work that we are talking about today, with the ultra-lightweight core, is at a low technology readiness level (TRL),” explained Vickers.
“This technology, if it goes all the way to Phase II [including] our ground testing, would be at TRL 6,” added Biaggi-Labiosa, referring to the nine steps NASA uses to rate maturity of a technology. A TRL of 9 means the technology has actually flown in space.
Adapting the Proven for the New
The approach taken by one of those winners is a case study in adapting proven technologies in new ways. The proven technology that HRL Laboratories is exploiting is UV curing of polymers. UV light is used to solidify a liquid resin point by point to create highly complex patterns (of plastic). Stereolithography 3D printers use the same underlying technique, but HRL uses a self-propagating waveguide process that enables the cores to be made 100 to 1000 times faster.
A template in plastic is formed and then coated with a metal such as nickel by electroplating. The template is then etched away, leaving the final core. The key is order and structure at small scales.
“These are structured, cellular materials or architected materials in a micro-lattice structure,” Dr. Tobias Schaedler, the lead researcher from HRL on the NASA contract, explained to A&DT.
He noted that sandwich structures provide high torsional and bending rigidity at low weight, resisting forces perpendicular to the surface. “But there are only two types of cores, honeycomb and foam,” he said. Foam is cheaper, but not as stiff or strong. That leaves honeycomb as the only choice today for high-performance sandwich composites.
The lattice distributes stress in many directions where a honeycomb can only do it normal to the face-sheet. HRL demonstrated that making the core using a truss lattice structure makes for a stronger material, especially in shear. This means it is better than honeycomb core in resisting sliding forces along the surface of a material and in bending, according to Schaedler. Currently, HRL lattice cores are made from nanocrystalline nickel faced with carbon-fiber-reinforced plastic (CFRP).
“The performance of the material is a combination of the structure, where honeycomb and foam would be inferior, with the increased strength of the material the truss lattice is actually made of,” he said. The NASA project will use an unspecified but lighter and stronger nanocrystalline metal.
Schaedler was quick to point out other distinct advantages using what is, at its heart, a 3D-printing technique to form cores. “The technique can grow compound shapes and curves because you can grow the structure into the shape desired,” he explained.
No machining is needed. Also, the density of the lattice can be adapted to match local stress – less dense where less strength is needed, higher density where it is needed.
While sandwiched honeycomb structures make for strong, lightweight aircraft, they are particularly bad at blocking low-frequency noise, like aircraft engines, according to Dr. Yun Jing, Assistant Professor of Mechanical and Aerospace Engineering at North Carolina State University.
Working with the Massachusetts Institute of Technology, Jing helped pioneer an approach unique in its simplicity—add a sound-insulating rubber membrane. Using a thin lightweight membrane covering one side of the honeycomb structure, like the skin of a drum, soundwaves bounce off rather than passing through.
According to Jing, at low frequencies—sounds below 500 Hz—the honeycomb panel with the membrane blocks 100 to 1000 times more sound energy than the panel without a membrane. His team has measured sound transmission losses (STL) consistently greater than 45 dB up to 50 dB.
“This research was prompted by the needs of the airline industry,” said Jing in an interview with A&DT. He also noted that it might be difficult to retrofit existing aircraft. “I think it has to be considered when a new aircraft design is started,” he said. “We are in conversations with several companies [examining] the possibility of commercializing this technique and are actively looking for partners to commercialize it.”