Choosing Core Material -- Elongation vs. Strength/Stiffness

I'm trying to choose a foam core for a skiff build, and I'm looking at two different products. One is a PVC foam very similar to D-cell H60. The other is a PE foam with 80kg/m^3 density, but it appears to be weaker and less stiff when compared to the PVC 60kg/m^3. However, the PE foam shows much greater elongation before failure, which I think is pretty important as well.

The plans call for H80, but I would like a less expensive alternative. I'm a mechanical engineer by trade, but am new to composites. I will be using epoxy resin, and the skiff is small/light (bare hull weight ~300 lbs) with a 30hp 2 stroke tiller outboard.

Thanks,

Bryson

 

Bryson,

Firstly, as an engineer, you need to understand the role of the core.
The core is a means to separate 2 layers of material, in doing so, what does this do? It increases the stiffness of the structure. The farther apart the 2 layers the more stiffness it has.
So if you separate 2 layers what is in between them...er....nothing!
So, you need a means to have these 2 layers connected, otherwise the 2 layers will behave independently, ergo, as one single layer not 2 at a separation. So the core is means of joining the 2 layers together so the whole structure acts as one complete structure, not single layers with minimal stiffness. This it becomes a 'sandwich''.

Now the core is connecting the 2 layers, this core to work effectively must be able to carry the shear load, otherwise the 2 layers will not behave as one. So the core must be able to carry shear load. So the shear strength of the core is the most important attribute of a core.

So why not use a material that has very high shear strength, say a metal?

And this gets to the final point of a core.

The weight. If you use a material that ends up greater weight, per unit length, for the core than the 2 layers, it will be a heavy structure. It then gets to the point of asking, why bother if the core is sufficiently strong enough why not take the whole loads, like any isotropic material?

Thus the core must have high shear strength properties and minimal weight; it's density.

Then you get into costs....which is a personal issue not a technical one.

 

Resistance to compression has to be considered, too, balsa certainly has good characteristics in that department, but other problems that weigh against it.

 

@Ad Hoc that's a good point -- I was definitely falling in the mindset of choosing a foam as a stand-alone material, rather than the core of a laminate. Since stiffness is way more dependent on total laminate thickness than the actual core stiffness, I guess the big question then becomes -- how much strength is really needed to carry the shear load? I think it would be tough for me as a home builder to quantify most of the loading. The only "cantilevered" area (and I'm guessing the area most susceptible to damage) would be the "flange" around the deck. I'd like the deck to overhang a few inches as kind of a last resort against spray that gets past the chines.

Elongation is a plus, simply because I'd rather have a failure of the skin that I can see and repair, rather than a failure of the core without my knowledge. Once again, though, I can't imagine any of these areas would end up deforming very much for my application. Running aground would put a good bit of stress on the transom, and dumping against a dock may stress the deck overhang, but that's about it. The hull is pretty much flat for the last several feet, so I guess hitting an oyster bar at speed could make for some interesting loading.

@Mr Efficiency -- Resistance to compression has been a concern for sure. Adding an extra layer of glass would definitely help absorb/distribute any point loads over more of the foam, but I'm not sure how much that will really help. I feel like any significant impact (especially from a pointed object like a stump or something) will be too much for most cored structure to handle anyway.

At the end of the day I realize I'm not building a rocket ship here, but I do want to do my homework before substituting any materials. I have a lot of respect for the designer and his plans, but I would also like to save $1000+ if it's not going to cause problems down the road.

Thanks guys for your help.

 

You're right, either will provide strength vastly greater than the stress loads this boat will see. If you hit something hard enough make the foam fail, so will the laminate. Amid all the other bad things you'll worry about first. Your concern about impact and point fail is why many people still choose to use plywood as core material for the lower hull.

 

This might help, still need to read and understand it myself:

https://www.hexcel.com/user_area/content_media/raw/Honeycomb_Sandwich_Design_Technology.pdf

If you want to save money in materials, look into building with wood. It's kind of an amazing material. For small boats it might even be lighter that foam core if you need impact resistance that drives the laminate thickness up.

PS: Here is a very illuminating thread that helped me to understand the basics: Balsa VS foam core - Page 2 - DIY Yachts https://diy-yachts.com/forum/viewtopic.php?f=18&t=993&start=10#p8964

 

Well, looking at simple values of H60 Foam as a core the Shear stress is given as 0.63MPa (min).
If your panel size is say 1.0m by 0.30m and you have a core of say 25mm thick....it is a straight fwd calc to establish how much shear this can take.
For your boat - Plenty!

And that's the crux.
You really need to address where your point of failure will be.
Do you want the laminate to fail or the core?....the laminate is generally the first point of contact thus, additional layers will help. Once it gets through the laminate, no matter what core you use, it'll be punctured, since the compression stress of the core is low anyway (that's why it is light too!). So i wouldn't get fixated on this issue, nor elongation - red herring in this context.
Just follow the load path...and structural design becomes easy enough.

 

Thank you all very much for your advice. Definitely needed to take it back to basics -- it's easy to lose sight of the forest for the trees with all the options out there (and opinions of many without experience). I ended up going with the PVC 60, in case anyone was curious.

 

Thanks for the update and feedback.
Rarely get post comment feedback - much appreciated

 

Attached different foam types/material available. It shows that high elongation foam has lower shear strength. Generally, you have to compute for shear strength first then choose the thickness to satisfy the equation. Low shear strength will require thicker core to reduce shear stress.

The formula is quite complex as the modulus of the "skin/face" is included in the computation. Lower shear strength can be compensated by using higher modulus for face. Say using carbon or prepreg, or just generally just increasing the fiber ratio.

There was an article which appeared in PB magazine a long time ago concerning the elongation of core with regards to impact.
1. High stiffness/low elongation foam (Balsa) transfer the impact load from the impact face to the opposite face causing delamination on both sides.
2. Low stiffness/high elongation foam (PVC) absorb the shock but in the process the core is deformed causing core failure.
3. Medium stiffness/medium elongation foam (SAN) works best by absorbing partially the impact creating damage only in the "point of impact".

Test were also done regarding the stiffness/modulus of the faces. High modulus face will absorb impact by distributing the load over a wider area. Low modulus face will absorb local impact and transfer it to the foam which will in turn determine the amount of energy it can absorb.

It is best to compute the strength of the panel using the formula that uses face modulus as another properties that satisfy the equation. Most formula provided by the foam manufacturers uses a simple formula that does not take into account the modulus of the face.

Cheaper is not always better. You either end up with a thicker core or more weight per panel.

 

ain't that the truth...!!