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# Final project - Thickness of PETG sheet that I'm bending into a cylinder for transparent HVAC duct?

## Final project - Thickness of PETG sheet that I'm bending into a cylinder for transparent HVAC duct?

(OP)
I'm doing my final project and it involves making a transparent duct for an HVAC setup. It's going to be between two metal HVAC ducts. I'm going to use a clear PETG sheet, wrap it around the two ends with hose clamps and thereby get a transparent duct with a radius of 6 inches.

My problem is, I've been asked to do shell calculations to see what thickness we go with (the sheet thicknesses are from 0.02" to 0.08", in increments of 0.02).

I've already done hoop stress calculations with an expected internal pressure (less than 100 kPa, so thickness needed per internal pressure is very very small), but I am lost on how to do shell calculations so that the sheet thickness is flexible enough. I want to use 0.06" honestly since the manufacturer's data sheet says " VIVAK sheet may be cold formed into circular shapes by observing the rule that the radius of curvature must be at least 100 times the material thickness. "

But engineering calculations-wise, I've been stuck on this for a while now. Should I calculate a bending moment on each side of the sheet and somehow get the thickness? Any ideas?

Thank you.

### RE: Final project - Thickness of PETG sheet that I'm bending into a cylinder for transparent HVAC duct?

It is not a thing you calculate from theory. The manufacturer has supplied you with the calculation for this material.

### RE: Final project - Thickness of PETG sheet that I'm bending into a cylinder for transparent HVAC duct?

Given the material modulus and thickness, for a given radius of curvature you can calculate the required moment. From that you can calculate the bending (surface) stresses, which can be compared to the material strength.

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