# composite vacuum chamber

Discussion in 'Fiberglass and Composite Boat Building' started by jim lee, Sep 20, 2009.

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### jim leeSenior Member

Not a boat, but the same as a submarine. I'd like to build a composite vacuum chamber. For now, two hemispheres radius R with a small return flange to set them together. My question, how does one calculate the lay up necessary to not collapse under one atmosphere of pressure?

-jim lee

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### Eric SponbergSenior Member

You should consult an engineer to apply the appropriate engineering calculations to the task at hand--spherical plate strength and deflection. The thickness-to-diameter will be important, as will the strength of the fibers and resin that you use. The loading is easy--14.7 psi surface pressure. The rest is more difficult, but the appropriate composites and/or mechanical engineer can get you an answer.

Eric

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### jim leeSenior Member

Well, this is why I posted it. Most likely there will be an engineer around here that understands this kinda' thing.

-jim lee

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### waikikinSenior Member

Hi Jim, what are you gunna use it for? Regards from Jeff.

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### jim leeSenior Member

Its just an experiment I've been dying to try for a long time. I wanted to see how light I could make one. But I'd no idea how to calculate the stresses involved or the laminate schedule necessary.

-jim lee

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### SamSamSenior Member

I have no answers but it would seem the more perfect the sphere, the thinner it could be, as forces would be more evenly distributed. Any flattish areas or lumpy areas would be weak points, and any force from the outside, like being bumped by something such as a corner, would have to be taken into account. Also the size of the 'chamber' would seem to have an effect on the thickness of the walls.

7. ### Guest625101138Previous Member

Shell stress for thin shell on isotropic material is:
Diameter/ (40 x thickness) in MPa

You will need a triaxial layup of course. It does not need to be very thick because it is not a very high pressure.

Have you ever tried a compression failure on an egg by holding it longways between your palms and trying to break it by squeezing. Will take a huge load if evenly applied. Spheres are very good shapes for handling compression forces.

You will need some margin to allow for lack of ovality and variation in thickness. Any point load for supporting it will also be significant stress raiser.

Rick W

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### jim leeSenior Member

MPa? Sorry, this isn't my normal stomping ground so I'm not familiar with the units. Guessing 1,000 pascals?

-jim lee

9. ### Guest625101138Previous Member

M means 10e6. So a million pascals. Normal air pressure just happens to be very close to 0.1MPa.

The compressive strength of mild steel is around 400MPa.

Rick W

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### jim leeSenior Member

I guess I'm still too far out of my pond. I can't do the jump from shell stress to composite layup schedule.

Thanks though..

-jim lee

11. ### Guest625101138Previous Member

I expect anyone would find a composite layup for a spherical shell to be very complex. It almost needs to be spun as a cloth would inevitably buckle in the layup and this reduce the compressive strength dramatically.

You might get away with narrow tape and wrapping it in such a way to cover the entire surface in a triaxial pattern.

The stress I provided can occur in any direction so you need triaxial layup to get fibres arranged efficiently. Also from my own measurements and test data for composite panels you find that fabric layups fail in compression around half the rated tensile strength. However this is in flat panels or single axis log and the failure is usually caused by buckling. I cannot wrap my brain around how this relates to a sphere because a sphere is an inherently strong shape but I would not exceed half the rated tensile strength before applying any safety factors for stress raisers.

Given the strength of these materials it would not need to be very thick relative to the size of the sphere. As an example take a look at an incandescent light globe. These are under vacuum.

Rick W

12. ### Guest625101138Previous Member

I thought of another common example of a sphere under pressure. Or more accurately part of a sphere. Have a look at the bottom of a common pressure pack can. You will see that it is hemispherical. If it was a flat bottom of the same thickness it would belly out and most likely tear at the seam. Being hemispherical it has much low stress than if it was flat.

Rick W

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### jim leeSenior Member

I think I'll try it from the other end. I can calculate the lbs/sq ft I can allow for the weight of the skin. I'll come up with a composite that's a tad less and just see if it works.

-jim lee

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### TarekJunior Member

Just out of curiosity, Jim. What radius are you considering?

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### rxcompositeSenior Member

Jim,

Rick is correct and is leading you the right way. But if i were to do it, I would filament wind it and go more than triaxial. I assume you are building a spherical shape so all stresses are in multiple direction. Using filament winding will eliminate fabric overlap.

I would say all stresses on the "skin" or plate is compressive in all direction with just a little bit of shear in the neutral axis. The inner part skin, with a lessr radius than the outside will compress more. The difference of contraction with the outside layer and the inner layer will cause it to slide past each other and cause horizontal shear.

I guess you will have to learn composite to learn the lay up schedule or you can experiment by progressively building up the layer while continously increasing the vacuum at as you build up (and cure) after each layer.

Rx

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