B Strain on a foil sheet separating two compartments in a cylinder

[TickTackBlock]

TL;DR Summary

A foil separating spaces with different pressure, vs possible difference of it separating a space with vacuum and a space with an atmosphere.

Hi. Lets say i have a cylinder, capped at both ends. It is cut in the middle and, say a titanium foil of 15 um thickness is pressed in between the 2 halves creating 2 spaces. It is pressed fairly hard, and a soft grip, "perfect" seal is created in someway but not right at the inside of the cylinder wall. There it is simply pressed loosely against both cylinder walls but without cutting the foil. Outside of the cylinder is normal earth conditions.

The cut edges of the cylinder wall on the inside are straight, the edges are graded but not at all gently rounded. The cylinder walls are also made out of titanium but are thick TLDR: a possible break (in possible followup question scenarios) would most likely happen on the foil where it meets the edges of the inside of the cylinder wall.

My inquire lies in the comparison of scenarios.

Scenario 1:
In one side of the cylinder there is an atmosphere with 1 bar of pressure, the end cap on that side has actually come of and it is open to the outside normal earth conditions, but lets say it is exactly 1 bar at this moment.

The other side of the cylinder has nitrogen gas inside it with a pressure of 2 bar.

Scenario 2:
In one side of the cylinder there is a near a vacuum with a continuous process connected to the space with an effort to create and keep this near vacuum. what amount of pressure it has, that will be compensated for on the other side of the foil.

And what do you know, now the end cap on the other side has come off and right now there is 1 bar (+ the same amount of pressure in the near vacuum) out side.


Questions:

Will the strain on the foil be the same in both scenarios? Is there a difference in being pushed towards a lower pressure atmosphere to being pushed towards nothing? if so, what and why.

Lets say i want to try if my foil holds between vacuum and 4 bar. Then could i really, for absolutely sure simply use 1 bar and 5 bar instead?

 

[Baluncore]

Welcome to PF.

Will the strain on the foil be the same in both scenarios?

Yes. The strain is due simply to the differential pressure.

Lets say i want to try if my foil holds between vacuum and 4 bar. Then could i really, for absolutely sure simply use 1 bar and 5 bar instead?

Yes.

It seems like you are working with shock tubes, where the inert driver gas is initially separated from the reactant chamber by the foil rupture disc.

There is available literature on shock tubes using foil diaphragms to trigger the reaction. The foil thickness can be selected to burst at the required differential pressure.

There is a picture of a shock tube here.
Google Earth 51°34'47.22"N, 0°51'28.68"E

 

[TickTackBlock]

Hi, thank you.

However much i am now curious about shock tubes and their use - it is not what I am doing no.

Are we absolutely sure there is no difference in what the foil is experiencing here even if i were to accept that the formula for the force applied to it would be the same? There being "nothing" on one side in one instance, and something on both sides in the other instance, does it influence the foils ability to remain intact in some other way?

 

[Baluncore]

There being "nothing" on one side in one instance, and something on both sides in the other instance, does it influence the foils ability to remain intact in some other way?

There are chemical and thermal effects that may influence the structure of the foil. Those differ in their effect on the foil.

If there are other foil barriers, then there may be sympathetic failures, due to shock waves in the cylinder taking out secondary foils.

Without a better description of the experiment, or some idea of the failures you are anticipating, or experiencing, I cannot guess what might be happening.

 

[TickTackBlock]

There is indeed several other possible culprits. Sorry i can't say more.

Thanks again, feel safer about one worry now anyway.