Need stiff light material (like Beryllium)
Need stiff light material (like Beryllium)
(OP)
Hello --
I have an application that requires a 8" diameter x 3" high disk with supporting ribs and other features. For proper performance the disk must be both stiff and light. The disk vibrates so fatigue is important. Beryllium seems perfect but is too expensive. Aluminum would be suitable but it is not stiff enough. Because of the part's complex shape carbon fiber is probably not suitable. In any case, a prototype must be produced quickly.
Thanks for any suggestions.
Don Culp
I have an application that requires a 8" diameter x 3" high disk with supporting ribs and other features. For proper performance the disk must be both stiff and light. The disk vibrates so fatigue is important. Beryllium seems perfect but is too expensive. Aluminum would be suitable but it is not stiff enough. Because of the part's complex shape carbon fiber is probably not suitable. In any case, a prototype must be produced quickly.
Thanks for any suggestions.
Don Culp





RE: Need stiff light material (like Beryllium)
http://www.berylliumproducts.com/WhyBeryllium.aspx
If not this, then there likely isn't an easy answer. Most metallic alloys have similar E/ρ ratios.
RE: Need stiff light material (like Beryllium)
RE: Need stiff light material (like Beryllium)
RE: Need stiff light material (like Beryllium)
Al-Lith and beryllium are the only structural metallics with a specific modulus much more than 108 inches. (Al-Lith gives about 1.25x108 inches, pure beryllium 6.3x108 inches according to MIL-HDBK-5 properties). Brush Wellman do beryllium-aluminum products that might be a bit cheaper than pure Be. Specific modulus 2.8–3.8x108 inches.
The only way I know of to get the sort of specific modulus given by beryllium is to use ultra-high modulus (UHM) carbon fibre composite. I'm not sure what that might cost. More ordinary carbon fibre with good design (just putting in fibres every which way should give about 1.24x108 inches) should match or even just about exceed the specific modulus of Al-Lith and be vaguely competitive on cost. Some intermediate solution such as a high modulus (HM) carbon fibre might come out about right.
RE: Need stiff light material (like Beryllium)
RPstress - Is there any carbon fiber that is similar to a filled plastic -- i.e., so that I can just take a block and start machining, rather than building up from layers? If so, where can I obtain such material?
Where can I obtain aluminum-lithium in the size that I need (about 8" dia x 3" thick)?
RE: Need stiff light material (like Beryllium)
Alcoa is the leader in Lithium-Aluminum Alloys so i would contact them.
Here is paper on one of the Lithium-Alloys produced by Sandvik.
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RE: Need stiff light material (like Beryllium)
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RE: Need stiff light material (like Beryllium)
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RE: Need stiff light material (like Beryllium)
CoryPad's suggestion of Al-Be alloys is worth considering, but you'll need to note a few things about that material.
Al-Be has magical properties for sure. It has a MoE similar to steel with a density less than aluminum. But the downside is that it is very difficult to fabricate and is ferociously expensive.
The most practical way to fabricate parts from Al-Be is by casting. But the parts must be investment cast in a controlled atmosphere, since the material is very reactive with oxygen. Al-Be casting alloys also cost about $300/lb (at least the last time I checked).
Some Al-Be bar, sheet or plate is available. But the sizes are pretty limited since these forms must usually be made by sintering Al-Be powders.
If you use an Al-Be casting, be sure to apply the appropriate knockdown factors to your fatigue analysis for casting defects.
Good luck.
Terry
RE: Need stiff light material (like Beryllium)
Victrex produce PEEK filled with carbon. You may not need an exotic plastic like PEEK, but if you're using wildly exotic fibre the cost of the plastic (maybe $50/lb) won't matter too much.
Dave Adkin at Victrex has been quite helpful in the past ( dadkin@victrex.com ).
Overall I wouldn't hold out too much hope of obtaining such ultra-high modulus fibre in a thermoplastic. You might be better off trying to make a filled thermoset yourself if you could persuade MCA or Cytec to supply some fiber. MCA at least list some chopped and milled UHM carbon ( http://www.mitsubishichemical.com/dialead.php ).
http://www.victrex.com/
http://www.mitsubishichemical.com/
http://www.cytec.com/
On the topic of beryllium, note that it's toxic. There was a bit of a fuss a few years back when we wanted to use Cu-Be bushes with just 2% Be. (This was ok in the '80s).
RE: Need stiff light material (like Beryllium)
ht
Harold
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RE: Need stiff light material (like Beryllium)
Thanks to all.
Don Culp