‘Microlattice’ Aircraft Materials for Saving Weight — Maybe

A new ultra-lightweight material promises to save weight in aircraft structural applications. Its proponents are not very forthcoming about the downsides of the new material. For now, be wary of heady promises until independent testing results are compiled and made public.

“We are revolutionizing lightweight materials,” proclaimed Dr. Bill Carter, a materials manager at HRL Laboratories in Malibu, CA.

The material is said to be the world’s lightest, approximately 100 times lighter than the Styrofoam used in disposable coffee cups. According to HRL, the material is a nickel-phosphorous alloy that is coated onto an open polymer structure. The polymer is then removed, leaving a “microlattice” metallic structure that is mostly 99.99% air. In many respects, it is similar to bone in the human body, very rigid on the outside and very lightweight on the inside.

According to HRL, the approach “combines ultra-stiff and ultra-strong materials (such as nanocrystalline metals) that provide higher strength than conventional materials with highly optimized truss architectures that enable unprecedented degree of freedom to tailor the mechanical performance.” (See HRL press release at http://www.hrl.com/news/

2015/1005) The trick is to fabricate a lattice of interconnected hollow tubes with a wall thickness of 100 nanometers, or 1,000 times thinner than a human hair. (See HRL press release at http://www.hrl.com/hrlDocs/pressreleases/2011/prsRls_111117.html)

A sample of the new 'microlattice' structure is shown balanced atop a dandelion puffball. Large panels of the ultra-lightweight material are envisioned for airliners

A sample of the new ‘microlattice’ structure is shown balanced atop a dandelion puffball. Large panels of the ultra-lightweight material are envisioned for airliners

Boeing owns HRL and is looking at the new lightweight material for both space and aviation applications. For airliners, the material is being considered for floor, side and ceiling panels to save considerable weight.

The new material would be sandwiched between carbon fiber composite facesheets. Before application to airliner cabins, the “microlattice” structure must be scaled up to achieve realistically-sized panels. This year a core panel of just 12″ by 12″ is the development goal. If successful, a follow-on effort will attempt a 2′ by 2′ panel, for a fourfold increase in size. Then the process will be expanded further to develop a 10′ by 11′ core panel of the ultra-lightweight material — 110 square feet. This larger panel is necessary to determine if the material can be produced in large enough pieces to use as structure in the cabin of an airliner. In other words, a 100-fold increase from the 1-square-foot panel attempted this year is necessary to determine if the new material is practical.

The ultra-lightweight material can be fabricated into a variety of shapes; the sample shown in the middle suggests its usage in cabin floors and sidewalls

The ultra-lightweight material can be fabricated into a variety of shapes; the sample shown in the middle suggests its usage in cabin floors and sidewalls

The material is promised to hold its shape under pressure. For example, if used as cabin flooring, the “microlattice” will not bend and give a “bouncy” feel to the floor.

Note that initial application is not envisioned for flight-critical assemblies, such as engine pylons, flaps or wings. A healthy respect for a conservative approach is evident.

Note also that this development is not to improve safety, but to save weight. The last great initiative to improve structural safety was the “damage tolerant” ethic, in which added strength had to be added to account for the worst possible fatigue crack imparted on the structure during the manufacturing process (See http://www.efatigue.com/training/Chapter_2.pdf).

Many questions come to mind about full-scale application of the ultra-lightweight material in airliners. Here are a few items for which answers are presently unavailable:

  • How will the sandwich panels incorporating the super-lightweight materials be attached to underlying structural members, such as stringers and ribs? Whole chapters have been written about various fasteners used in production.
  • Will the interaction of markedly different materials, such as bending, compression, tension, create new problems that are not anticipated at this early stage in “microlattice” development?
  • What is the durability of this new ultra-lightweight material when exposed for extended periods to high levels of humidity, especially a salt water atmosphere associated with operations in tropical climates? Recall that current composites must be painted/protected against moisture intrusion.
  • How resistant is the new material to spilled fuel that pools and catches on fire?
  • Is the new material immune to the intense heat of electrical arcing, which may occur in circuits routed nearby?
  • How resistant is the new material to maintenance mishaps, such as damage from a dropped tool or a technician stepping on it?
  • How will damaged ultra-lightweight material be repaired?
  • Above all, complicated systems, such as this material with its ultralight mettallic latices which are too small for flaws to be detected visually, have a tendency to produce unexpected outcomes. Be wary of the heady promises of this new ultra-lightweight material; much is promised but in-service experience is presently lacking.