Veneer skin adds what??

General Discussion
1

I have been doing a lot of research on this site and others on composite sandwich construction. It is clear that balsa and some other woods have some definite advantages as a core material over foam, but I am having trouble understanding why wood veneer makes such a great skin material.

What exactly is it adding to the typical epoxy skin that makes it so worthwhile? Compressive strength, tensile strength. If it is only one of these and not both than maybe it should only be on one side of the board and not both.

Or is it flexural strength? and if so how are you positive it is adding this? Its not easily measured.

If anyone could give me a better understanding of veneer’s advantages, it would be greatly appreciated.

I am making up plans for my first vacuum bagged board and I am starting to question why some of the materials are used in the places they are. Questions are good, but only if you can find the answers. ;]

2

It is clear that balsa and some other woods have some definite advantages as a core material over foam, but I am having trouble understanding why wood veneer makes such a great skin material.

skin core material or board core material?

wood “veneer” being great skin mtl?

what you might be looking for is its structural benefits relative to other contruction methods…a balsa cored skin over what? it all depends…your inquiry confuses me

3

Sorry if I caused confusion, I am new to composites and may not be using the terms correctly. My question is really focused on sandwich composite theory in general and I have tried to use the terms skin and core in that way, no other. Skin referring to the outer layer of a simple sandwich composite, the layer that is generally suppose to take the bulk of the compressive or tensile stress. Core referring to the inner layer of a simple composite, generally a lightweight material that has good shear strength.

Here is a link to site that visualizes what I am trying to put into words. http://www.oneoceankayaks.com/Sandcore.htm

In a typical surfboard the foam is the core of the composite sandwich and the fiberglass is the skin.

In Bert’s vacuum bagging example, EPS foam and Dcell are the core, and epoxy/glass is the skin. No wood veneer is used in the board at all.

However, many of you appear to be making boards following Berts vacuum bagging example but you make the skin with wood veneer/epoxy/glass

And my question is what role is the wood veneer serving that the epoxy wasn’t capable of doing by itself?

4

Or, (not trying to add confusion here) maybe I am looking at this the wrong way.

Maybe the wood veneer isn’t improving the properties of the skin at all. Maybe the wood veneer is improving the properties of the core? (shear strength)

The epoxy/glass is still acting as the skin in that it takes the majority of the compression or tensile stress.

5

Veneer skin adds what? It looks good!

6

veneer is truly a suckfull word…makes me think of cosmetics…

skin core is more accurate

both materials make excellent skin cores…

dcell is a foam (pvc) that fatigues over time…

wood/balsa has spring return and doesnt fatigue or if it does much more slowly

balsa is cheaper than dcell (i think)

balsa is easier to get (ship to house)

balsa more laborious and harder to work/construct cleanly

balsa looks nicer when its done

learning this stuff will take time…read…read…construct…read some more…

http://www.swaylocks.com/forum/gforum.cgi?do=post_view_flat;post=169367;page=1;sb=post_latest_reply;so=ASC;mh=25;

construct more…learn…after a few years you might get the hang of it…

…kidding

:wink:

7

youre missing something…the skin core has resin/cloth on both sides…everything is optional tho…i dont think/work with blinders and neither should anyone else…but hey thats just me…

8

The Balsa and D-cell is put in the middle of the “sandwich” skin to even out and spread the loads over a greater surface, that’s what makes these boards so hard to ding and snap.

Since the loads are spread out You can also get away with using a very light foam core material, which makes for a light board as well.

I’m no expert in this area so please correct me if I’m wrong…

9
Quote:
In Bert's vacuum bagging example, EPS foam and Dcell are the core, and epoxy/glass is the skin. No wood veneer is used in the board at all.

Not quite. While EPS & d-cell are both cores, they’re not cores together. The d-cell is the core of the skin and the EPS is the core of the board, between the 2 skins. Skins are not epidermis, they are composite panels that get their strength from separating the layers of glass with something. 2 layers of 4 oz glass separated by a skin-core make a stronger overall skin than 2 layers of even 8 or 10 oz laminated together with no space in between.

Same as how a truss roof (rafter/ceiling joist combination) of 2x4 lumber when properly constructed will be both stronger & lighter than a conventional roof stick-built from 2x8 joists & 2x6 rafters. Same as how a steel I-beam with a taller web can use lighter guage steel and still be stronger than a heavier I-beam with a shorter web.

The web or the truss supports (diagonals & hardware) separate the outer components just as a sandwich core separates 2 layers of glass. Pretty simple engineering, really, the concepts have been around for centuries.

Quote:
However, many of you appear to be making boards following Berts vacuum bagging example but you make the skin with wood veneer/epoxy/glass

And my question is what role is the wood veneer serving that the epoxy wasn’t capable of doing by itself?

The wood - whether balsa or plain veneer or d-cell or corecell or anything is separating the 2 glass layers to make a composite skin. Greater strength & less weight at the cost of more materials & more complicated construction (and there’s the rub, commercially).

Now of course, the thicker the sandwich core is, the more you separate the glass portions of the composite skin, and the more strength you achieve (until a point of diminishing returns where your skin’s core also becomes compressible and you begin to lose the benefits of transfering energy - impact/pressure - from one layer through the core to the other layer). Your other point of diminishing returns is in the strength:weight ratio, because the thicker your skin’s core is, the thinner your board’s core will be, if you want to maintain outer final dimensions. And the real weight savings is in that lightweight inner core.

[Not to mention that if you make it too strong, you lose flex, but you were really asking about skins & cores, so we should stay away from that for now - there’s a thread on flex with all kinds of information.]

So you have balances to weigh. Veneer is a lot thinner than balsa or d-cell so it weighs less, but provides less separation of the glass layers in EACH skin, so its less strong. But still stronger than 2 layers of plain glass - even with epoxy - laminated together.

The Surftech ‘woodies’ have inner glass, then d-cell, then wood veneer, then outer resin without glass cloth. Not really even a sandwich, at least not a symmetrical one. They could be very, very strong, if they even just put a layer of 2 oz glass on over the veneer. But not cost-effective, I guess. So they break.

I’ve now built boards with 3/16" balsa & 1/8" balsa as sandwich skin cores, with 1/8" d-cell as a sandwich core, with 1/8" d-cell + 1/16 veneer" as a skin core, and I’m working on one with Oneula’s bamboo. The 3/16" balsa is the strongest by far, but also the heaviest. As it should be, on both counts, by all I’ve explained above…

Hope that helps.

10

Benny is right. Eventhough the skins have cores, they are still the skins.

The very nature of composites is to put the right materials where they will do the most good. A traditionally constructed surfboard is a core and skin composite, but it has it’s short comings. Just having fiberglass skins doesn’t give you very much pressure dent protection. The structural part of the fiberglass is wasted in the width-wise direction, and not enough in the length-wise direction. Also, the polyurethane core isn’t the most efficient core material, as it’s being used to add stiffness instead of being used just to support the two skins. This requires a higher density core, which adds weight without very much structural properties.

So you could add more fiberglass to get the pressure dent properties or other properties, but that would add weight and add more to other structural properties that you don’t want, like being too stiff. Too bad, because in my opinion fiberglass is one of the best skin materials.

By making the skin a composite you can dial in the properties you need. A core skin composite of fiberglass and a thin high density foam, like D-cell, gives you the dent resistance only where you need it.

Using a very thin veneer with fiberglass on both sides isn’t really a core and skin composite, because the veneer is too thin to separate the skins enough to contribute structurally as a core. But it does add structurally on it’s own and may be beneficial in providing dent resistance and other structural properties.

Balsa, on the other hand, is used as a core for the core and skin composite skins. It may have the same dent resistance and density as the D-cell, but it also adds structure along it’s grain. So in my opinion, it one ups the D-cell. On the other hand, depending on your design, it can make the board too stiff.

Even though I seem to be concentrating on dent resistance, I don’t want to take away from the other properties that wood can provide. For example, you can go with a much lighter core when using wood.

It’s just about knowing what each material contributes, and using it where it’s needed most, and adjusting how much is needed to get the desired attributes.

11

Thank you for the explanation. I’m following the logic now. Before I was just picturing the board as a way too simple single sandwich construction, when in reality it is a multiple sandwich construction board.

The wood veneer is the core of one sandwich, that is acting as the skin of a larger sandwich. Now that makes sense considering the mechanical properties of wood. Thanks again.