# Material for flywheel and carbon fibres

## Main Question or Discussion Point

hi all!
Are composites like carbon fibres (CFRP) the best material for flywheel?
There is no doubt on its high strength to density ratio.
Regarding the safety consideration, is flywheel made of composites safer than that made of other materials? I've read through some articles claiming that it should be safer in the sense that composite materials would break into many small pieces once broken. Is this a well-proven fact?
Also, regarding the manufacturing of flywheel using composites, is it sill difficult? Where are the difficulties?
All in all, what material would you consider to be the best material for flywheel?

By the way, can anyone tell me what "toughness" actually means? Is titanium alloy tough? are composites tough?

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## Answers and Replies

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Astronuc
Staff Emeritus
I can't answer all the questions as I am not familiar with flywheel design, but certainly getting the maximum strength-to-density ratio is the way to go.

As for toughness, toughness is the ability to resist fracture. Fracture mechanics (FM) uses stress intensity as a property to quantify toughness. Much of FM is based on the maximum size flaw that can exist in an object without further propagation.

Here is a good source for use of carbon composites in fly-wheels:

http://www.utexas.edu/research/cem/archive_J_A_S_04.html

This paper might be of interest regarding NDT of CFRC's - http://space-power.grc.nasa.gov/ppo/projects/flywheel/papers/SPIE_4336-03.pdf [Broken]

Flywheels are
made from exotic stuff, like composites of epoxy resin and carbon fibre. A carbon composite is four times stronger than steel, but only one quarter the weight. Jack Bitterly, and his team at U.S. Flywheel Systems, have come up with a flywheel roughly the size of a small bicycle wheel - but it can spin at 55,000 revolutions per minute (rpm), so it can store heaps of energy.
- http://www.abc.net.au/science/k2/moments/gmis9733.htm [Broken]

There are quite a few articles on U.S. Flywheel Systems, but I can't seem to find a website. Perhaps another company bought them or they changed their name.

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Astronuc said:
I can't answer all the questions as I am not familiar with flywheel design, but certainly getting the maximum strength-to-density ratio is the way to go.

As for toughness, toughness is the ability to resist fracture. Fracture mechanics (FM) uses stress intensity as a property to quantify toughness. Much of FM is based on the maximum size flaw that can exist in an object without further propagation.
So are composites tough materials? alo, titanium alloys, are they tough?

Astronuc
Staff Emeritus
I believe carbon composites are reasonably tough, but I can't give you a number at the moment.

As for Ti-alloys:

Here's a little background on Ti-6Al-4V, one of the most common Ti-alloys -
http://www.azom.com/details.asp?ArticleID=1547 - but there are no fracture toughness numbers.

A check of the Ti section of ASM's Metals Handbook indicates that Ti-6Al-4V has the greatest fracture toughness of a set of common Ti-alloys. With an alpha-beta forged treatment fracture toughness of Ti-6Al-4V is about 46 ksi$\sqrt{in}$ (50 MPa$\sqrt{m}$) and in the beta forged state has a toughness of about 60 ksi$\sqrt{in}$ (66 MPa$\sqrt{m}$). The trade off is that is has slightly lower YS, about 145 ksi (1000 MPa).

You might find this site of interest - a course on composite material design - http://www.mse.mtu.edu/~drjohn/my4150/
Unfortunately - no numbers on toughness.

What are the drawbacks of using composites for flywheel?
I don't really know how to manufacture a flywheel. Is the manufacturing of flywheel using composite difficult?

"When the tensile strength of a flywheel is exceeded the flywheel will shatter, releasing all of its stored energy at once; this is commonly referred to as "flywheel explosion" since wheel fragments can reach kinetic energy comparable to that of a cannon shell. Consequently, traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. Fortunately, composite materials tend to disintegrate quickly once broken, and so instead of large chunks of high-velocity shrapnel one simply gets a containment vessel filled with red-hot sand (still, many customers of modern flywheel power storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel). Gulia's tape flywheels did not require a heavy container and reportedly could be rewound and reused after a tape fracture."

From http://en.wikipedia.org/wiki/Flywheel_energy_storage

Is it true and why?