coefficient of thermal expansion 1

[carbongr]

Hi,
What is the CTE of a carbon fiber - epoxy structure (tube)?
What should i do in order to manufacture this tube with almost zero CTE?
Thanks in advance

 

[RPstress]

See thread thread327-253642 for some clues.

To get a genuine zero CTE should be possible (in one direction, at least), but may require high modulus fibres.

Classical laminate theory should give a prediction that's not a mile out. You can use The Laminator (

http://www.thelaminator.net/

) or similar CLT analysis software to find a layup with theoretical 0 CTE and then do a few trials.

One problem is measuring a low CTE. Even to use CLT to get a reasonable estimate of a laminate's CTE you need some basic CTE info for a single ply of whatever you're laying up. This is quite hard to measure. A laminate with a very low CTE is even harder to measure. I assume that if you have a requirement for a low CTE you have some way to check it.

A 0/90 (or +-45) symmetric layup of unidirectional Toray M55 at 50-60% fibre volume fraction should give well below 1 microstrain/°C in all directions.

 

[carbongr]

Thank for the reply.
High modulus fibres is not an issue.
The CTE of this tube should be as close as possible to zero at the longitudinal direction (across the length of the tube). I believe UD fabric at around 0 degrees should work, however i am struggling to find a proper resin with as low as possible CTE.
Can anybody suggest a solution?

 

[RPstress]

CTE in the fibre direction of carbon/polymer tape with a decent (50%-60%) fibre volume is most definitely fibre-dominated.

Most polymers will have a CTE of between 50 and 80 microstrain/°C, but their low modulus (2500-4500 MPa) means that the fibres dominate (CTE maybe -0.25 microstrain/°C, modulus (HS fibre) ~235,000 MPa).

A practical value in the 1-direction for UD HS carbon/epoxy is maybe +0.25 to +0.5 microstrain/°C and about 30 microstrain/°C in the 2-direction. The 1-direction values are so low that for *most* purposes it can be treated as zero.

To get a genuine 'zero' CTE for an optical bench or something you have to have an idea of what actual values over what temperature range are needed and then make a 0° UD laminate or ones with all +-angle plies (say, +-5°, +-10°, +-30°) and test.

 

[Compositepro]

Here is some useful information on CTE of Toray fibers:

http://www.torayca.com/techref/index.html

 

[Demon3]

Adding fillers will lower the CTE, as stated. However, you will not get zero because the fillers have a finite CTE, just it's much lower than that of the polymer.

To get genuine zero or close to zero CTE you need specialty fillers with zero CTE (they do exist).

Chris DeArmitt

http://www.phantomplastics.com

Consulting to the plastics industry

 

[RPstress]

Oddly enough you can actually get a negative (or zero) CTE out of a +-theta laminate when the fibres have a small but positive CTE.

A paper (called "Negative thermal expansion of laminates") with an explanation on P. 1 should be here:

http://www.springerlink.com/content/g2k87w3r67311236/fulltext.pdf

Another called "Thermal expansion characteristics of PEEK composites" has good evidence that it happens in real life:

http://www.springerlink.com/content/lnx4317436781u35/fulltext.pdf

(See fig. 13, the laminates seems to have about -2 microstrain/K between +-20° and +-30°; the AS4 fibres at 20°C have a longitudinal CTE of about +0.2 microstrain/K from fig. 2.)

If you're cunning and use a mix of fibres you can get a zero-CTE in all directions (for a limited temperature range) like these guys from Kodak:
"Near zero CTE carbon fiber hybrid laminate" patent,

http://www.freepatentsonline.com/5993934.pdf

 

[Demon3]

Thanks RPstress. I only work with regular particulate filled materials, i.e. not multilayer laminates so you taught me something there. Thanks!

Chris DeArmitt

http://www.phantomplastics.com

Consulting to the plastics industry

 

[Hansmeister]

carbongr needs to define what he means when he asks "near zero cte for a tubular structure"?

Is is ± .10 ppm/F, ± .01 ppm/F, or ± .001 ppm/F ?

The cost to produce the product and have it accurately measured increases significantly with each increasing order of precision.

Typically, M55J with aerospace grade prepreg resin is a good starting point for the material selection.

Are there additional requirements for strength, stiffness, weight, inclusion of end fittings into the final number, thickness limitations, etc.?