criteria for choosing high strength carbon tapes

[shearlag]

Dear all,
Is the ratio (longitudinal extension strain)/(transverse contraction strain) is a very useful criteria for choosing high strength Uni Directional carbon Tapes (like T800 or VKU-27 or M65) ?. If so , why ?

thanks in advance
with regards
shearlag

 

[RPstress]

I don't recognise VKU-27 or M65 as fibre types. I'd be grateful if you could tell us more. (IMS65 is a Tenax IM fiber and M65 is a Hexcel BMI resin but I've never heard of VKU in any material context (not even metallic) and for once the Internet is no help.)

As far as I know the Poisson's ratio of a UD carbon fiber/resin combination is not particularly significant. In the longitudinal/transverse direction it is mainly controlled by the resin and is usually around 0.3 to 0.4 no matter what the fibre is as long as it's carbon (not even whether it's HS, IM, HM or UHM, or pitch-based vs. PAN-based makes any real difference) and usually no matter what the resin is, as long as it's a polymer. You need it for classical lamination theory analysis, but that's about it. I certainly wouldn't expect it to figure in any normal material selection between UD materials. It may be a significant property when designing layups with almost exactly zero CTE, such as for optical benches, but different materials will just mean slightly different thicknesses are needed.

For the fiber itself Toray says that as fibre anisotropy (Ef/Gf) increases compressive strength decreases (

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

Perhaps you could come back with a bit more context (for instance the source of the idea—I'd be very curious to read more).

 

[SWComposites]

Except possibly in some specialized spacecraft applications requiring unique thermal or coefficient of expansion properties, that ratio has little to no significance.

 

[shearlag]

Yes RPStress, M65 is a Hexcel BMI resin and VKU from source looks to be German material.But no much information is available.In fact I was referring to Epsilon2/Epsilon1 (not poission's ratio) similar to E22/E11 ratio.I beg an apology for not saying things correctly..Some UD tapes will have little transverse fibers to hold the strands in place.I feel this will have very less influence on strain in longitudinal direction then compared to purely UD tapes, where there are no fibers in transverse direction . Should such a criteria be significant for fibre selection ?
Thanks to u 2,SWComposites

hanks in advance
with regards
shearlag

 

[RPstress]

Surely epsilon2 = -Poisson's*epsilon1 when strained in the 1-direction?

Yes, some dry UD has a few weft fibres, though these are rarely carbon. Usually they're something cheaper and less stiff like polyester. They also tend to be quite bent compared with the 1-direction fibres, often weaving around each tow in the 1-direction. This will increase E2 and decrease epsilon2 a bit, but not usually much and the effect on a laminate is a) small, and b) not that important. It's just another number to get right.

Re BMI, it behaves very similarly to epoxy, apart from its higher Tg and usually a somewhat higher modulus for the basic resin (E is typially above 4000 MPa vs. in the 3000s for epoxy). This doesn't affect selection, as either you need the Tg or you don't pay the extra (BMIs are usually a fair bit more money than epoxies). The higher E can make the GIc look less attractive than the KIc implies, if looking at choosing a tougher resin. I don't know if GIc or KIc is more important for laminate behaviour with damage.

If prepreg, the resin with the T800 fibres might be Toray's own 3900 (or 3911 if it's the version they supply from France). This is interlayer toughened like a few others (including Hexcel's 8551-7 as used by GE for fan blades) and is very tough, with a QI compression after impact strength typically well above 300 MPa. However, it might be an old, relatively cheap resin like 914, making for a laminate which is somewhat wanting in the toughness department no matter how good the fiber is.

Can you tell us the name of the manufacturer of the 'VKU' material and what it is? Just curious.

 

[shearlag]

dear RPstress,
Thanks for the quite a good bit of information.Yes BMI's are on higher side from price consideration.Yes i have heard, Epsilon2/Epsilon1 is of importance for compression after impact. But i have not read in any technical articles.
Regarding VKU material, my boss only told me it is from German and almost equivalent to T800 from strength & stiffness point.
Once again Thank you