Maximum RPM until burst for disk?

[blainiac]

Hello everyone!

I was wanting to know if something like this was possible. I read (Wikipedia) that the moment a cylinder will burst is when the hoop stress is equal to the cylinder's tensile strength. It also appears that the thickness of the material doesn't matter. I don't see Young's Modulus being used here either.

If I had a thin fiberglass (ρ = 1850 kg/m^3, σ = 260 MPa) disk 0.5 m in radius, it appears ~7,159 RPM is the max before bursting.

Questions:

1) Is this the correct equation to use for finding the burst speed?
2) Is there a definitive material properties sheet online to ensure I'm using the correct numbers?
3) Is there a good 'safe' percentage of the burst speed (like factor of safety) to stay under while operating something like this?

I was going to study the boundary layer on a rotating disk at high rotational speeds, and wanted to be safe before setting everything up.

 

[JRMichler]

Get a copy of Formulas For Stress And by Roark. This book is currently in the 8th edition, and the title has changed slightly. Any edition will do. It has the correct formulas for spinning disks.

Also here is a good source for safety practices: http://www.sandia.gov/ess/publications/SAND2015-10759.pdf. It's geared to larger spinning disks than you have, but the safety factor information will be useful for you. You absolutely positively MUST properly shield your high speed disk. Pay particular attention to the axial loads from a burst event. And even more attention to the potential amount of energy released in a worst case burst event.

 

[Nidum]

Can you tell us more about the purpose of this experiment and your proposed design for the test rig ?

There are many factors involved in designing discs to run safely at high rotational speeds . If we had a more complete idea of what you are trying to do then we could give you more detailed answers .

You absolutely positively MUST properly shield your high speed disk.

Yes - and if you have no technical knowledge then you should get all your design ideas and calculations checked by a professional engineer with appropriate experience before trying to run this experiment .

 

[Nidum]

ffwd to 1:30

 

[blainiac]

Get a copy of Formulas For Stress And by Roark. This book is currently in the 8th edition, and the title has changed slightly. Any edition will do. It has the correct formulas for spinning disks. Also here is a good source for safety practices: http://www.sandia.gov/ess/publications/SAND2015-10759.pdf. It's geared to larger spinning disks than you have, but the safety factor information will be useful for you. You absolutely positively MUST properly shield your high speed disk. Pay particular attention to the axial loads from a burst event. And even more attention to the potential amount of energy released in a worst case burst event.

Thank you for the information. A very similar equation I presented was found on page 752 of a PDF version of the book I found regarding the burst speed. Safety will be a priority when doing these tests. Thank you for the information again!

Can you tell us more about the purpose of this experiment and your proposed design for the test rig?
There are many factors involved in designing discs to run safely at high rotational speeds . If we had a more complete idea of what you are trying to do then we could give you more detailed answers. Yes - and if you have no technical knowledge then you should get all your design ideas and calculations checked by a professional engineer with appropriate experience before trying to run this experiment.

The eventual purpose is to study the boundary layer around a very fast rotating disk (with tip speeds exceeding the speed of sound in air). At this point, the main concern I have is just to ensure that a very thin (0.125 mm) fiberglass (G10) sheet accurately balanced and cut into a disk shape with a radius of 0.5 m could even reach ~6,500 RPM. I was thinking the boundary layer will still develop regardless of the disk thickness, the same RPM could be reached, and it will reduce the energy in case of a burst event.

I've definitely seen that video you've posted, and the set up I'm proposing would be almost identical (except spinning vertically instead of horizontally so I won't be in the debris path upon explosion.

Using the CD as a test case to see how accurate the posted equation is, I get this:

30,000,000 = 1,190 * 0.06^2 * (2 * π * x / 60)^2 = ~25,170 RPM, which is close to the 23,000 RPM.

 

[Nidum]

A disc that thin spinning at high speed could easily become unstable and fail prematurely .

I really must caution you against actually trying this experiment until you have either gained a more in depth understanding of high speed rotating disk design or have found an expert in the subject to mentor you and check your work .

 

[blainiac]

Nidum,

I was afraid it might become unstable near the burst speed. Instead of building the device, I think I'll steer more towards CFD just to be safe. I'm not sure how accurate it would be since there's a lot of wiggle-room with boundary conditions and mesh refinement, but with all the caution against building it, I'll play it safe. I don't think I'm knowledgeable enough to build something like this, especially from scratch.