Self-Healing System Applied to Aviation

ScienceDaily is reporting that the self-healing materials are being used in some new aircraft designs. We covered several self-healing systems in the past months, but it is nice to see it starting to find practical applications. "This simple but ingenious technique, similar to the bruising and bleeding/healing processes we see after we cut ourselves, has been developed by aerospace engineers at Bristol University, with funding from the Engineering and Physical Sciences Research Council (EPSRC). It has potential to be applied wherever fibre-reinforced polymer (FRP) composites are used. These lightweight, high-performance materials are proving increasingly popular not only in aircraft but also in car, wind turbine and even spacecraft manufacture. The new self-repair system could therefore have an impact in all these fields."

Have you seen this boy?

[ivanmarsh]

I for one welcome our flying Terminator overlords.

So...

[ZonkerWilliam]

The plane will heal itself after a crash. Great for the plane, not so much for the passengers.

One concern with this system

[Enlarged to Show Tex]

How much weight does this system add vis-a-vis the use of non-composite materials? If you use a system that weighs more than the corresponding non-composite system, you won't gain anything by using the composites in the first place...

A possible step back

[Hojima]

I'm no engineer, but wouldn't the use of new self-healing polymers be inferior to a mechanical failsafe or backups. If damage is done to an aircraft, the component of the structure that was carefully designed for a specific use is compromised. When under intense air pressure, self healing doesn't seem to make the cut. Wounds don't heal when aggravated, and bones have been known to heal badly (which could translate to a greater problem). If there is a new "healing" system that is to be used, I think it's a long way down the road before we see them implemented in commercial/military aircraft.

Re:One concern with this system

[Uncle Focker]

Any gain in weight over the normal composite material is far made up for in the fact that more and more non-composite parts can be replaced with these self-healing composite parts.

Re:Potential

[maxume]

The big benefits are in resistance to fatigue, not in tensile strength.

Re:One concern with this system

[SBacks]

"A key benefit would be that aircraft designs including more FRP composites would be significantly lighter than the primarily aluminium-based models currently in service."

Re:A possible step back

[bostonsoxfan]

I don't think that this is going to be a big step back. I think it is meant more for skins and interior panels. There is little substitute for good old Aluminum or Titanium in aircraft. This isn't meant to be a massive fix just fix dents or you know the normal wear and tear of being an aircraft. If the composite break or fracture can reseal itself it is less downtime, less cost and easier overall on the parties involved. I think this wouldn't be used for airframes. Having little gaps of liquid would make the material a little weaker and probably not as effective as a frame. How about on helicopter blades. Those things take a beating, having them self repair even to 80% would be a big plus. rather than lost the integrity of the blade.

Re:One concern with this system

[mcrbids]

Many concerns with this kind of system.

Airplanes aren't like cars; cars are mass-produced, throwaway items that seldom see more than 10-15 years of use. Yes, there *are* 30 year old cars, but they represent a rather small fraction of the actual cars in day-to-day use.

Airplanes, on the other hand, are in a different category. Airplanes are all-but hand made. They are very expensive, so it's usually cheaper to fix an existing plane than to buy a new one. I got my pilot's license in a 1971 Cessna 172 that was older than I am. This isn't a particularly old plane, C-172s go all the way back to 1955 or so, and there isn't a whole lot that changed in the plane characteristics from 1959 to 2006 - mostly just newer instrumentation and a few minor tweaks.

Since we can be fairly certain that many (most?) of airplanes made today will be flying 40 years from now, how well does this "self healing" work then? Composites are much more sensitive to extreme temperatures - how well does it "heal" at below freezing? (typical of high altitudes, as well as high lattitudes)

Aviation is very risk averse - KISS is the rule of survival! Most planes are leaned MANUALLY just to avoid the possibility that some little spring in the carburetor would die while flying over mountains to the detriment of the plane occupants.

Yes, even though I'm a technocrat, I remain a bit skeptical.

Re:One concern with this system

[Kelbear]

I suspect that the purpose is similar to that of the self-inflating tires. They keep you running until you can fix it properly. Since not all cars are equipped with flat-proof tires, it's a good idea for drivers to be acquainted with how to pull over and change a flat. However, manually patching hull cracks in mid-flight is an unreasonable expectation of a pilot, so this technology has found a niche.

Not for fibre-reinforced polymers

[Solandri]

I'm no engineer, but wouldn't the use of new self-healing polymers be inferior to a mechanical failsafe or backups. If damage is done to an aircraft, the component of the structure that was carefully designed for a specific use is compromised. When under intense air pressure, self healing doesn't seem to make the cut.

FRPs like fiberglass and carbon-fiber are composed of strengthening fibers embedded in a polymer matrix. The fibers provide the strength, the polymer holds the fibers together with each other (transfers load from one fiber to its neighbors). The initial modes of failure will be the polymer losing its "grip" on the fiber (like pulling a nail out of wood), followed by fracture of polymer that's lost its fiber reinforcement in this manner. The fiber is still there and intact, it's just lost its mechanism for accepting load from adjacent material. Initially there's enough polymer that stresses can be routed around a minor failure of this type (transferring load to adjacent fibers). But eventually you get to the point where you'll have multiple dislocations spanning between fibers, and the polymer is no longer able to transfer stresses to enough fibers to carry the entire load, eventually leading to catastrophic failure.

This self-healing mechanism essentially injects new polymer into the crack thus reseating the fiber within the polymer, sealing the polymer dislocations, and restoring the polymer's ability to transfer load between fibers. The dye to indicate a failure is to catch an inspector's attention just in case the stresses exceeded the fiber's breaking strength (e.g. from a rock or birdstrike). The presence of the dye does not in itself indicate the part is now substantially weaker than a new part (aside from the self-repair mechanism being used up).

Yes, the "healing" polymer is probably not as strong as the original polymer. But because of the nature of the failure mechanisms I've described above, any FRP already has plenty of leeway for polymer failure built into it. If it didn't, the material would be incredibly susceptible to fatigue failure after just a few load cycles.