Carbon fibre and core choice: PVC or Corecell

Hi there,

I'm just asking for help on clarifying a core choice question for a F32 trimaran build.

From having read various posts I was looking to follow RX's advice to use:

"corecell at the bottom and all above waterline, use the much cheaper PVC."

SAN-PVC-PET foam core https://www.boatdesign.net/threads/san-pvc-pet-foam-core.66283/page-2

However in an earlier exchange in response to Fallguy saying:
"so, you are agreeing there is a mismatch of Gurit corecell and carbon"

RX replies:
"Mid modulus laminate (Eglass/epoxy) is more compatible with SAN."

Vacuum Infusion Advice https://www.boatdesign.net/threads/vacuum-infusion-advice.63166/

So, I'm left a little confused. Obviously I'm happy to forego Corecell as it's expensive, but of course want to maximise impact resistance in high risk zones aware that using carbon significanty reduces skin thickness.

For these high risk zones, keels, bows, and perhaps outer faces of the amas, I'm thinkng of adding an inner layer of kevlar and an outer layer of glass to the general laminate schedule.

From interior to exterior:

300g 0-90 carbon
12/13mm: CorecellM or Mycell PVC
300g Kevlar
300g 0-90 Carbon
200g twill glass : increase abrasion and puncture tesistance

The O-90 carbon has a tensile strength: 4900 MPa, modulus: 230 GPa.

Mycell cross-linked PVC 80 / Corecell 80
Compressive strength MPa 1,43 / 1.16
Compressive modulus MPa 68 / 78
Tensile strength MPa 2,60 / 1.74
Tensile modulus MPa 137 / 98
Shear strength MPa 1,13 / 1.15
Shear modulus MPa 29 / 34
Shear elongation at break 20% / 57%

I think it also makes sense to increase the core density to 100kg/m3 in those areas where impact is more likely....

So my question is, is it still best practice to use corecellM?

 

Will respond shortly and summon rx

 

The problem with carbon fiber and foam cores and epoxy is the elongation at break do NOT generally line up.

This means the core and the bonding agent can bend some, but the carbon cannot.

And so, let’s say you bump a dock. Or, you encounter some pounding forces on the hull at sea. Or beaching, etc. The carbon can break because it cannot elongate as much as the other two items in the layup.

I am not qualified to critique your layup as much, but I see some things that don’t make much sense.

The kevlar is really questionable in a layup. I used kevlar in one location, buried about 4-6 layers in on beaching keels, in case I ever had a bad grounding incident to help prevent going through the keel; otherwise, it is best to avoid it. It is generally not repairable and only used to protect a hull bottom in a grounding. Due to your very thin skin; the chance of going through is higher and repair more certain and more impossible.

The corecell M is really a great product. It is a requirement for colored boats near the equator or super hot climates, but also post-cure is required there. I like it better than divinycel as it seems like resin uptake is less to me.

Be careful with your stack; it seems really unusual to me and you need it engineered.

@rxcomposite does this work for a living, so I’m not sure he will want to offer you a remedy, but probably he will offer critique

I am not a conposites engineer; just a builder. Your use of twill over carbon over aramid is really not what I would want on a F32. But the plan for corecell is good. I would probably recommend you don’t get too precise about the waterline being a demarcation line to transition to pvc. I would probably use the M a bit further. I used pvc for deck shoes and m200 at points of known stress. But this is a job for engineer…I believe your f32 is a bluewater vessel, so don’t guess eh.

 

Also, if you can share the f32 section, or a part of it, I’d be glad to offer my advice on transistion. I have a vague memory the hulls are round, so you also don’t want to be thermoforming different products; that is likely trouble.

 

Thanks for your feedback Fallguy.

Farrier recommended when using a thin carbon skin "to use an extra layer of a light 200g glass cloth to be used in areas subject to impact and abrasion, such as hull outer sides, and below waterline on hulls."

I can see that using kevlar is a tricky one! Certainly from my initial research on the forum here, I was shying away from using it due to the difficulty to make repairs. However, after reading Mike Water's excellent website, thought it would perhaps be judicious to add it to the stack:

Trimaran Design Materials https://smalltridesign.com/Trimaran-Articles/Kevlar.html

Used to "help prevent going through the keel". Absolutely. Okay, a difficult repair would be needed. What's the other choice?. A heavy grounding leading to core and inner layer perhaps compromised. I don't see that being an easier or less messy and stressful repair if the hull's been breached....

Farrier uses vertical thermoformed foam strips in a female mold glued together before being laminated.

Attached is a picture of float and main hull section. .

 

If you intend to combine cores, I recommend all cores from the bottom to above waterline and extended to their chine top are continuous and homogenous.

This way you are not dealing with core variation in thermoforming.

Or, if the hull will be dark and ever sail near the equator; do it all in corecell M.

let’s see what rx says about the layup … the aramid seems odd to me…you could put aramid on the inside of the hull along the bottom as the nearest to the core, but I’m not a fan of it for this vessel.. it’d make more sense to run a single 8” wide tape of 1708 along the bottom over the 200g glass…it looks like there is probably a tabbing spec there as well

200g twill behaves a bit odd in the layup and so does aramid ~ the aramid likes to float above the resin and is always best under another layer of regular glass, ime

the twill is easy to run too dry; so be sure to use bleeder release or something to keep some resin in if you plan vac

 

Thank you Fallguy for the acknowledement but it seems that although I was referenced twice, the layup schedule seems contrary to everything I said and was designed thru wishful thinking.

The schedule cannot even be analyzed as the carbon fiber mentioned "O-90 carbon has a tensile strength: 4900 MPa, modulus: 230 GPa." seems to be intermidiate modulus Toray T700G fiber properties. What resin was used? What is the resin percentage? Pure Carbon fiber was used?

So sorry cannot comment.

 

Okay, so my original post asked two questions. One was the continuity of cores and the other the addition of kevlar.

For the first question I referenced RX's advice to mix cores to take advantage of corecell's superior impact resistance under the waterline. And if so, did he still consider there to be a potential mismatch between Corecell and some carbon fibre?

The specific fibre I intend to use is:

300 g/m² Bidirektional 0/90° Carbonfibre/ SP-B305/945C
Construction: 0/90°
Fibre: HT-Faser
Weight: 300 g/m²

Fallguy thinks not to mix cores in such a fashion. I'm still not sure why thermoforming different cores can be a problem. The vertical strips of core are thermoformed individually. Screwed from underneath to battens on the mold to keep their shape and then glued to the adjacent panel before laminating. I can see that different cores of different properties may not interact very well together if they have different properties bound between the same skins, but I don't get the problem with thermoforming different cores. What am I missing?

For the layup schedule, RX you've never been asked about it before? I don't know what you're talking about when you say "contrary to everything I said"? Forgive me if I haven't read every post you've written....

For the primary laminate schedule Ian Farrier specified 600g 0-90 glass or if using carbon to substitute weight by 50-40%, with the possible use of a light glass cloth protection to the exterior in high impact zones. Extra weight is of course specified for the keel.

I was just looking at what you guys thought about belt and braces with addition of Kevlar underneath the glass and carbon next to the core.

Fallguy thinks no to Kevlar and add extra glass to the keel.

Fallguy, do you think that 200g plain cloth would be better than twill for the extra exterior protective layer?

Okay RX, wishful thinking because you think that the laminate schedule that Farrier gave was too light? Wishful thinking because the addition of 300g Kevlar next to the core is not going to be sufficient to protect the core from a heavy grounding or collision?

It would be really helpful to have some specifics ....

Nothing is made yet. That's why I'm here, to get your guys' advice before placing an order for materials.

To answer RX's other questions:

An infusion low viscosity resin to be used, perhaps from Sicomin. Any advice on specific specifications to look for on the data sheet if going for the above carbon fibre?

Resin percentage, going for vacuum bagging so looking at 40-45%

 

The core change will create a stress riser since they have different properties. Whether that is a problem would need a proper structural analysis. Does the designer indicate different cores or are you modifying it?

 

No, the designer doesn't recommend different cores. Only putting it forward as a possibility having read RX's post in the following thread:

SAN-PVC-PET foam core https://www.boatdesign.net/threads/san-pvc-pet-foam-core.66283/page-1

"Actually, there is very little difference between corecell and PVC in terms of shear specific (shear to weight ratio) which is what you need in core sizing. The difference comes in impact resistance. Corecell is much more forgiving and deflects in unison with the FRP panel. PVC is much more stiff and transfer the load from impact point to opposite side causing delamination. Good practice is to use corecell at the bottom and all above waterline, use the much cheaper PVC."

 

It's not the first time I've heard of people recommending this strategy. Hence why wanting to check-in.

 

I think you are approaching the issue backwards. Instead of looking at a design requirements and specifying materials that fulfill them, you are choosing a material and then trying to make it fit. In other words, you have a square peg and try to make it fit into whatever shape hole there is. If you randomly change the design structure, it most likely end up in failure.

 

Okay, so in your opinion better to stick with a single core.

Do you have any thoughts on Kevlar use to protect the core?

 

I'm astonished because that's precisely what I always do: I select a material, apply it to a panel, and see if it meets the minimum requirements for that panel. If it doesn't, I change the material. I no longer do it haphazardly, based on feeling, but by analyzing the properties of each material on my database to select the one that, in principle, seems likely to meet the requirements.
Also, when designing with composite materials (not just plastics), I always consider that the position of that material in the stack is crucial in determining whether it's suitable or needs to be changed.
It seems I'm completely wrong. I need to rethink my procedures.

 

Using two different cores doesn't make any sense, in my opinion. The core is placed near the neutral axis (where stresses are very low) because it's a material with low direct tensile strength. If you use two cores, it's very likely that one of them, the one furthest from the neutral axis, will be working beyond its capacity.
What do you think can be achieved with two cores?