NDE by pressurisable structural hollow sections?

This has been used for many years on some helicopter blades. The blade is pressurized, a gauge is permanently attached, and if the pressure ever goes down, the blade is immediately replaced.

I believe some small aerobatic aircraft with tube frames use the same system.

John

Thanks John It's so absolutely obvious, and you explain where it is used. Why isn't it used "everywhere"? What am I missing? What problem(s) stand in the way of using this on general structure?

The aircraft tubular frame is exactly the general idea I was thinking of... But not widely used, as ever I've encountered / not encountered(?).

Sorry to be dim, but I haven't seen what stops it being widely used.

As I mentioned, I was thinking of this for a lifting gantry structure out of square and rectangular hollow section.

Rich S

The pressurized tube frame for aircraft is mentioned in some of the EAA literature. IIRC (and I haven't read it for years), they pressurize with nitrogen to avoid corrosion.

This requires, of course, communication among all of the tubes. Where you have a T-junction or a cluster, holes must be drilled to allow the gas to pass from one tube to the next.

This would require about 2 or 3 drilled holes, in my case. Say 3mm /

1/8th-inch diameter. Is "nothing". Metal thickness ("large") and stationary non-exchanged air (corrosion ceases as oxygen consumed - sub-neglible consequence) means no issue in this regard, in the case I am thinking of.

Steel would be S355 (51ksi (50ksi)) - an ordinary though higher-range general structural steel. Steels would be general Square Hollow Section and Rectangular Hollow Section.

If the test was reliable, you could use lighter structure (lower wall thickness) at higher stress, giving more payload, where availability of payload is restrictive in this case I am thinking of. The reason I am motivated to sketch-out a custom solution...

It seems such a "no-brainer" that my mind tells me - there must be something regarding my first-principles conjectured idea which actually prevents the method from working well, for it to not be widely used.

I was looking to do beam-loading (bending, "moment") calculations and finite element analysis seeking moderately high stress when in use. Predicated on there being this cheap readily-performed frequently-applied integrity test.

Hence the persistent question - what is it I'm missing here?

Does the aircraft application work well? Can you give some links to look-up - and quote?

Rich S

Here's one mention of it in a forum post:

"Been there done that, You almost always have to cut out a section of tube so you can get inside and clean the crud out with a rotary wire brush and some heat before you can effect the repair. I much prefer the eastern European system of pressurizing the fuselage frame with inert gas and putting a pressure guage on the aircraft that the pilots and AMTs can monitor."

Here's another:

"Some aircraft and some race cars that used tube frame structures have the tubes interconnected by small holes, and the tubing system is sealed and pressurized Usually with dry nitrogen). A pressure gauge is used to monitor the structure. If the pressure drops off you know the structure has cracked. This obviously means that every weld must be pressure tight, but if properly done that is case. What can be done in that case is to pump down the frame with a vacuum pump so that there is no air (and moisture) inside the tubing, and then it is pressurized with dry nitrogen and you are pretty much good for life."

That's all I can find on short notice. If it were me, I'd call the EAA. They have a lot of experience in dealing with steel-tube corrosion.

Hi again Ed. You are a star! Thanks. Rich S

I'm glad if that little bit is a help, Rich.