[thedoc (OP)]

Paul asked the Naked Scientists:
   Is it possible to monitor the "skin" of an aircraft while it is in the air to monitor possible stress problems?



What do you think?

[alancalverd]

Not practically.

Skin failure from fatigue (chronic stress and strain cycles) begins with very fine cracks that can appear almost anywhere, so you would have to monitor the entire surface, either from the inside (which would compromise thermal insulation) or outside (which would destroy the aerodynamic finish). If you know where the maximum stress or fatigue is likely to occur, you should be able to design a fix rather than allow it to happen. In practice you compromise by checking critical points on the ground, and assigning a service life to critical components or the entire airframe.

Failure by acute stress cannot be predicted by monitoring. At some point outside the design flight envelope, you can almost certainly break an aeroplane by flying it wrong. 

Depressingly, the most likely time for an aircraft to fail in flight is within a few hours after routine maintenance.

[syhprum]

I don't know to what extent it applies to aircraft but there is an old adage "if it ain't broke don't fix it"

[alancalverd]

It's a difficult line to draw.

Every so often we hear of aircraft disintegrating or succumbing to gravity because some tiny widget was corroded or worn below tolerance, but it seems that fatal mistakes made during routine maintenance or minor repairs, outnumber such failures. On the other hand if we didn't undertake routine maintenance and minor repairs, the balance would certainly tip the other way.

So obviously we need to tighten up on maintenance training and quality control, and component documentation. But that can get insane: I wanted to replace the corroded headset jacks in a light aircraft. No problem, new jacks look pretty much like the ones on a guitar amplifier, cost around £5 each, and take 10 minutes to solder in. but if I fit them myself it will invalidate the aircraft insurance because it is nominally a plane that can carry farepaying passengers (even though I don't) so the jacks have to be fitted by a licensed engineer. But he can't fit any part that is not certified, and the certificate takes weeks to arrive and costs £50 per unit because it is issued, stamped, filed and forgotten, by a government-controlled process.....so I squirted a bit of switch cleaning fluid into the jacks because that counts as aircrew-permitted minor maintenance and we struggled on with crappy radio reception for another year.

If you operate an aircraft on a public transport basis, which includes light planes for daily rental, you have to take it offline for certain faults. Now a failed landing light bulb really doesn't matter if you want to use the plane for an hour's daylight training flight, so if you find the light doesn't work during your morning preflight inspection, you remember it but don't log the fault because that would take the plane offline immediately. And then you forget, and hand the plane over with "no faults" to the next pilot, and darkness falls.....

The word "just", as in "I was just going to...",  "I was just testing the....", "It's probably just a bit of dirt..." is a killer. 

[evan_au]

There have been attempts to monitor mechanical stress in live structures by passing an optical fiber through the material (or by attaching it to the surface of the material).

The fiber has tiny indents which reflect some of the laser light, allowing short laser pulses to accurately measure the mechanical stress at every point along the fiber, in real time. This could be useful for bridges, which you can't easily take apart for periodic maintenance.

It could be used for the structural beams of an aircraft (assuming you can get it to work in the extreme vibration of a flying aircraft). However, it would not be so useful for panels on the skin of an aircraft; you would need to attach the fiber after you had assembled all the panels, and you would need to detach the fiber before you could remove a suspect panel.