Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
coefficient of thermal expansion 1
- Thread starter carbongr
- Start date
-
1
- #2
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 ( ) 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.
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 ( ) 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.
- Thread starter
- #3
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?
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?
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.
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
Chemical
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
Consulting to the plastics industry
To get genuine zero or close to zero CTE you need specialty fillers with zero CTE (they do exist).
Chris DeArmitt
Consulting to the plastics industry
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:
Another called "Thermal expansion characteristics of PEEK composites" has good evidence that it happens in real life:
(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,
A paper (called "Negative thermal expansion of laminates") with an explanation on P. 1 should be here:
Another called "Thermal expansion characteristics of PEEK composites" has good evidence that it happens in real life:
(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,
Thanks RPstress. I only work with regular particulate filled materials, i.e. not multilayer laminates so you taught me something there. Thanks!
Chris DeArmitt
Consulting to the plastics industry
Chris DeArmitt
Consulting to the plastics industry
Hansmeister
Aerospace
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.?
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.?
- Status