Carbon or carbon/Kevlar inlay on carbon fibre boat?

[hughb]

Hi guys,

I've been searching the forums for an answer but wasn't able to find one. For a carbon fibre boat (Zonda), is it better to do a strengthening inlay with more carbon fibre layers, or using a carbon/Kevlar blend?

[Tamelesstgr]

A good quality CF inlay should provide plenty of strength and rigidity. I believe the Carbon/Kevlar is also harder to get to conform around curves, but I have never used it personally.

[ade-rc]

Carbon fiber is stronger than Kevlar fiber but will break or crack if pushed beyond its limits. Kevlar will flex more before breaking. Kevlar does not like sharp bends.

[hughb]

Thanks for the replies. I was leaning toward having a carbon/Kevlar inlay due to Kevlar's bit of 'give', which would make sense in a hard landing. However from your descriptions of it being harder to form, Keith's tutorial over at the KBB Forums, and the below video, i think i'll stick with just carbon. Among other things, the video talks about the mixing of composite materials, how each has different properties, and how this can cause problems when used together under load or stress.

https://www.youtube.com/watch?v=j7bQYEt_qtU

Interestingly, on our (real) rescue boat, it is made fully from Kevlar. The manufacturer is called 'Kevlacat'.

IMG_1070.JPG

[NativePaul]

IMO carbon/kevlar looks great but is pretty pointless, I often use kevlar or aramid as it is a great material for boats and I sometimes use carbon to add stiffness locally where needed, but if you just use a mixed weave carbon/kevlar cloth you almost get the worst of both worlds.

Structurally it is half way between the stiffness of carbon and kevlar, so nowhere near full carbon, and while kevlar is much more flexible if you flex it past carbon's breaking point you break the carbon and the hull looses all its stiffness, as you now have only half a kevlar cloth doing anything. Nice to have a kevlar bag to take your bits home in I guess, but if it had been full carbon it would have been stiffer and may not have reached the carbon's breaking point, or if you had used full kevlar, you would have had to use a little more for the same stiffness and paid a weight penalty, but it would have survived intact.

Despite the gaps in the carbon weave, carbon/kevlar forms a faraday cage and is almost as impervious to radio signals as full carbon is.

It is often as costly or more than full carbon despite even Kevlar (tm Dupont) being cheaper than carbon, and generic Aramid (same thing as Kevlar, but often more gold coloured rather than Yellow) is available at less than half the price.

Carbon, kevlar, and carbon/kevlar all have less drape than glass and are much harder to get to conform to corners with a wet layup, from my experience carbon is worse for this, but compared to the ease of glass, the difference is minor and the pain of cutting kevlar makes carbon an easier material to work with. I have not tried laying up carbon/kevlar around corners so cannot attest to its drapability, but would have thought it was as tricky as carbon if not more.

[hughb]

Awesome thankyou. That has sealed it for me, carbon it is!

Nice to have a kevlar bag to take your bits home in I guess

[boredom.is.me]

Carbon fiber is stronger than Kevlar fiber but will break or crack if pushed beyond its limits. Kevlar will flex more before breaking. Kevlar does not like sharp bends.

Digging this up to correct a misconception.

Carbon is actually the weakest (strength wise) of the three materials (carbon, kevlar, fiberglass). It's all about application.
Here is a really good video on all three cloth materials as well as a comparison to steel. https://www.youtube.com/watch?v=KHXVf0SaJpA

[bob horowitz]

Digging this up to correct a misconception.

Carbon is actually the weakest (strength wise) of the three materials (carbon, kevlar, fiberglass). It's all about application.
Here is a really good video on all three cloth materials as well as a comparison to steel. https://www.youtube.com/watch?v=KHXVf0SaJpA

Very interesting video. Something everyone using composite reinforcements should view.
Thanks,
Bob

[Stillwet]

Very interesting video. Something everyone using composite reinforcements should view.
Thanks,
Bob

This was one of many comments below the video. And thanks to all who posted here on this thread. I'm tring to learn all i can on these composites than just laying them up and moulding them.


From: one of the video comments,
Hi! I need to clear up some things!

So you are saying that High modulus Carbon fibers are essentially strands of diamonds?

After 2800°C heat treatment of mesophase pitch-based Carbon fibers they reach a mean interlayer distance of 0.342nm. Ideal Graphite's is about 0.335nm, so MPP-based Carbon fibers are nearly Graphite fibers! The carbon atoms in graphite are sp2 hybridized. That's why MPP-based CFs are good electrical conductors. Diamond bonds on the other hand are purely sp3 hybridized, the bonds are saturated so there are no electrons to transport any current. Diamonds are insulators. It is also important to note that the microstructure of carbon fibers both pitch- and PAN-based are not 100% understood to this day.

Side note: Diamond Nanothreads can be synthesized by applying huge pressure (200000bar) to benzene and slowly reducing the pressure to let the threads grow.

Also I need to ask you something about your table of material strengths. I'd be glad if I could buy materials with these strengths off the shelves lol. Where did you get the data of Kevlar + Epoxy. 3500 MPa?

Look, here is some tensile strength data of epoxy prepreg laminates from Hexcel which are basically available off the shelf:

E-Glass:
UD: 1100 MPa
Fabric: 600 MPa

Aramid:
UD: 1100 MPa
Fabric: 500 MPa

High Strength Carbon:
UD: 2000 MPa
Fabric: 800 MPa

Intermediate Modulus Carbon:
UD: 2400 MPa
Fabric: 900 MPa

I think it would have been smarter to compare laminate strengths than pure fiber strengths. Pure fiber strength data is a marketing tool! PAN-based IM fibers are marketed with strengths over 7000 MPa. The data is gathered from tensile testing of single filaments with huge measurement errors that are sometimes larger than the measurement value!!!