High modulus carbon 1

[glass99]

We are designing another fancy architectural staircase in carbon which is deflection controlled - the stresses aren't remotely close to governing the design, so would like to use a stiffer carbon fiber, ideally high modulus or ultra high modulus. The fabricators we deal with use primarily prepreg.

All four fabricators we are talking to seem to have little to no experience with anything other than standard modulus fiber. They have all said they could in theory use the high modulus material, though always seem to have trouble getting back to me about exactly how much it would cost or what the lead time would be etc. If we were to use the high modulus material it would be primarily unidirectional. Is it especially hard to get or expensive? Why does no one seem to use it much? Maybe the typical application is more strength or fatigue governed? It would be a major advantage to the structural performance of this design (and most of our designs) if we could use the stiffer materials.

 

[SWComposites]

Likely expensive.
Ultra high modulus material is likely spacecraft grade.
Call prepreg suppliers directly for availability.

 

[glass99]

Likely expensive.
Ultra high modulus material is likely spacecraft grade.

I have heard that the high modulus carbon can have an extreme range of cost because it's a relatively illiquid and small market. If a dept of defense customer has a big order, the cost goes up by 10x or something. That said, my project should not be super material cost sensitive.

It is my understanding that one major use for ultra high modulus carbon is tennis rackets.

Yes am trying to track down the Gurit rep.

 

[Compositepro]

The Dialead fammily of pitch carbon fibers includes some "low-cost" industrial grade fibers, which, I believe, is K139-16. This is a 16,000 filament tow. I've used the K-139-10, which was less than $100/lb. Being high modulus, these fibers have low strain to failure, like less than 1%. This means they are relatively brittle and therefore more difficult to process than standard modulus fibers.

 

[glass99]

these fibers have low strain to failure, like less than 1%. This means they are relatively brittle and therefore more difficult to process than standard modulus fibers.

Interesting about the Dialead material. Will look into it more.

"Brittle" confuses me a bit because all carbon fiber has a sudden failure point, but I guess that there would be higher stress on the resin because of the high modulus. Is that something which is analyzable in terms of inter-lamina shear?

In practical terms, what does harder to process mean? We will likely be using prepreg in an autoclave with a middle market fabricator who does a lot of Indy cars.

 

[Compositepro]

"Harder to process" refers to difficulties in handling the fibers. They appear to be far more fragile, in that dragging the fiber over a bar or eyelet will cause filaments to break and cause fuzzing of the fiber. This is because any drag on the tows will create waves in the filaments as they approach a contact point. These waves get squeezed into tiny loops which will break when the loop gets below a certain size. The diameter at which a loop in a filament breaks is determined by the strain to failure and the diameter of the filament.

 

[glass99]

The diameter at which a loop in a filament breaks is determined by the strain to failure and the diameter of the filament

Ok that kind of makes sense I guess. I am calculating that they will break at about a radius of curvature of 1/32" if the filament diameter is 8 micron. That's a fairly tight radius, but there could be some instantaneous conditions during handling, or if you have a fold line in your at a joggle it could exceed that.