# Thin Walled Tube around a 180deg bend

Hey guys any help you can help me solve a problem at work. Ive been UV testing some polymide tubing and I need to know if it will survive one of our manufacturing processes post UV exposure. The OD is 21 (0.91 mm) thou and wall thickness is 3 (6.4 mm) thou. It will be loaded with approx 4kg, say 40N around a 180 degree bend/support of 50 mm in diameter. Now in looking for the principal stress am I looking for hoop stress * 2? Been a long day not sure why I am struggling with this one some direction would be lovely. Thanks boys n gals :)

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etudiant
Gold Member
The dimensions need a check, 0.91 mm OD conflicts with 6.4 mm wall thickness, although 3 thou (mil?) thickness works with 21 thou OD.
Hanging 10 pounds on a thin polyimide tube will cause it to collapse. Is that an issue?
Is the tube supposed to be load bearing or is the question about internal pressure? What is the extent and duration of the UV exposure?

Sorry about the dimensions, I was just leaving and typed it in haste. 0.021'' OD with 0.003'' wall thickness. The tube will have a mandrel line inside, collapse is not an issue as long as the tube does not fail in being loaded with the 4kg for a short period of time on both sides. I am still working out the UV exposure but worst case scenario we are looking to loose a max of 33% in original load carrying capacity. The question is about the resulting stresses as it is in the jig and how the load is distributed.

I have attached a quick sketch to give you an idea. Thank you!

PS: Attached picture should be CW 90 degrees.

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etudiant
Gold Member
So we have a plastic tube effectively holding about 20 pounds (2 x 4kg, with a 1/3rd capacity cut).
I'm still iffy on the tube dimensions, .003' is 3/1000 foot is about .91mm wall thickness and .021' then is the OD, about 6.4 mm, about a quarter inch.
If that is correct, it is a pretty puny tube, which I expect would surely stretch with the weight applied, but should hold, provided there are no sharp clips holding it which could start tears.
As long as some stretching is ok for your application there should be no problem, but if you need more margin, a 2 mm or thicker walled tube would give much more leeway.