Is it possible to calculate the maximum pressure a PET container can hold?.
We need to make some assumptions to keep it a little practical.
To do a complete finite element?) stress analysis on the hole bottle shape is outside my capability's.
I need a more practical approach.
In practice (Dutch) bottle weakest point seems a close race between the bottle neck and bottle body.
As an illustration of this a picture of a pressure tested bottle. The bottle was pressurized with 14 to 12 bar for about 8 hours. (pressure dropped due to expansion of the bottle and / or maybe a
small leak) It did not rupture but you can see some of the strain effects. The top part of the bottle body has expanded from 88 mm diameter to 96.2 mm., the label on the bottle is torn caused by this
expansion. Part off the neck is not transparent anymore and is partially colored white. It's still a guess what part would rupture first.
Lets assume that the bottle wall is the most critical. What would the theoretical maximum pressure be?. For laminated PET material (Dutch bottles are made from this) I found on several sites a
strength specification of 80 MPascal or 8000N/cm^2 which is the same.
For a sealed, (non exploding ;-) ) bottle the forces caused by the pressure difference working on the bottle walls are compensated by the counter forces of the PET So the outwards force working on
the cross area must be compensated by the inwards working force from the two combined walls.
Wall thickness on a bottle is normally on the average 0.75 mm (again for Dutch bottles) gives a max. pressure of 13.7 bar / ~200 psi.
(The bottles I tested all held out 14 bar) Wall thickness may vary a lot in a single bottle. I've seen a bottle with wall thickness variation between 1.5 and 0.5 mm in a single bottle.
A pressure vessel with a double wall thickness (two bottles combined) should hold out 26 bar / ~370 psi.
Provided that the stress is equally divided over both bottle walls.
But what about the bottle neck part?.
The picture of this blown top gives a good indication where the weak points are.
This is the result of a pressure test at about 16 bar whit a shortened reinforced bottle body, so the top would blow first. It looks as if the bottle is torn away at the position where (in the big
photo of the complete bottle) the PET has turn white.
Reinforcing the smallest diameter of the neck looks to me as a good guess to increase the maximum pressure.
Conclusion of this exercise?. I did this calculation to see if my building plan for the double walled 20L rocket could handle the minimal desired design pressure of 20 bar.
It looks it can be done, provided the bottle neck of the top and bottom parts are reinforced well enough.
