On Wed, 09 Aug 2017 12:29:49 +0100, Richard Smith
wrote:

John Halpenny writes:

On Tuesday, August 8, 2017 at 6:50:19 AM UTC-4, Richard Smith wrote:
Hi all

For a structure like a crane, is it ever a designed
fitness-for-purpose / planned Non-Destructive Examination technique to
make it entirely from Structural Hollow Section, fully-sealed
(eg. ends blanked-off), so that the voids can be pressurised through a
valve, with cracks detected by leaks?
Immersed in water to see bubbles; watch for pressure fall over time;
etc.

I was thinking of this for a structure I was sketching and designing,
to prove it has no through-thickness cracks anywhere.

What it lacked in not detecting cracks before they become
full-thickness, it would gain from testing the entire structure 100%,
not requiring any preparation (surface or otherwise), no sensitivity
to testing orientation of flaws / defects which do exist (like
dye-pen, but unlike mag-part) - and cheap so you could test often /
before any critical use.

???

Rich S

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.

--
Ed Huntress