In my vast experience of comp sands (otherwise known as searching the archives), I believe a composite sandwich works by utilizing two or more materials that are complementary to each other in at least one way while also keeping them mechanically separate. Correct? In the case of balsa skins with an eps core this mechanical separation would be the fibreglass in between right?
Just wondering how you mechanically separate fibreglass with the use of fibreglass and resin only or is this not possible?
I think you’re calling the wrong part of the object the “sandwich”
it’s not the skin but the skin/core/skin combination that’s makes the sandwich.
The composite structure of the skin layer I believe is just to allow more transference of the strength needed to the skin layer versus being provided by the core which is how the weight savings is established. The balance is more how the shell and the core work with each other to the handle the strain on the entire object from the forces being externally applied to it.
Maybe I’m wrong.
But it seems the secret is figuring the right balance between the core and the entire exoskeleton skin and rails since the strength is not being provided by a center stringer attached to a deck and bottom skin wrapped at the rails. The less exotic the skin layer the more dependant and higher density the core becomes.
It’s those combinations that I don’t think anyone has the whole book on yet.
I think you’re right on the money. In standard board construction as well as composite-sandwich-skin construction the blank is the center of the sandwich.
Standard hand laminated “PuPe” or basic hand laminated epoxy boards are technically composite sandwich structures. The fiberglass shell is the skin - one on top, one on bottom with the blank in the middle.
On composite-sandwich-skin surfboards (Sunova, Surftech, Firewire as well as other assorted “Compsands”) there are additional sandwiches in the skins themselves. In those cases the balsa, bamboo or high density foam layer in the skin is a sandwich core as well as the blank itself, arguably a triple sandwich if you will.
I believe a composite sandwich works by utilizing two or more materials that are complementary to each other in at least one way while also keeping them mechanically separate. Correct?
Incorrect. KKsurf has it right wrt stiffness/weight.
But to add on the very important micro level, its about proper placement (material dist) of good stress bearing material (typically cored or non-cored fiberglass) to areas where stress is high, and reducing/eliminating it where stress is low.
Iow, wood springers placed at the neutral plane are not worth the effort required to design and install them. But if they make you feel more special to use them, by all means do it.
Look up the tensile strength of fiberglass, understand its magnitude, then compare it to the avg tensile strength of your typical surfboard foam.
I think the mechanical speration your talking about has to do with the maintence of the deck and bottom seperation. A rather large issue and the shear strength of the interior core has much to do with it. The amount of shear a surfboard core encounters can be quite extreme and the ability of the core to keep the skins in line has all to do with how the board handles stress.
In standard PU/PE boards this is attained primarilly through foam strength (density). This is because there isn’t room (weight wise) to do a lot with your skins. In short a heavier blank will make a board more resistant to breaking because the foam itself is more resistant to flexing. So the reduction of flex will keep the skins in line. Stringers also increase stiffness and reduce breakage by reducing dynamic flex. Therefore he lack of memory in the PU foam and the rapid rate at which PE resin fatiques makes lightweight PU/PE performance very short term. Adding structure reduces performance by taking away flex and adding weight.
With a compsand your able to increase flex because the EPS core allows shear movement and the skins maintain line through use of the exo sandwich itself. In other words the skins are able to take tensile and compression loads on their own without relying on the assistance from the interior core. EPS is the perfect core for this because of it’s light weight and memory (it’s more able to move and return without breakdown). Since there are many different ways to engineer a compsand, flex and weight can be handled in much broader terms allowing for better performance and performance durability (longevity) at lighter weights.
In other words the skins are able to take tensile and compression loads on their own without relying on the assistance from the interior core.
The word ‘without’ seems a bit extreme Greg. I think it would be more accurate to say that cored skinned compsands are less reliant on the core as compared to pupe. Without some sort of core, the typical compsand would break on the first bottom turn (hollow Aviso style construction notwithstanding).
Wrt compsands, its been my experience that there is a delicate balance between skin and core. Most of my balance ‘failures’ result in stiff compsands.
I should know, I stayed at a Holiday Inn Express last night.
About time someone posted about shear. Though I would have written it differently I have nothing too add but…
Y’know, everyone who “designs” a surfboard, ought to be required to understand the basics of materials mechanics. Not necessarily on a engineering college level, where it’s usually a second-year full semester, but enough to identify and understand the basic definitions of compression, tension, shear, moment, stress, strain, and their distribution. After that add a little about non-rectangular beams, and beams of more than one material. This level of education would go a LONG way to focusing attention on what matters. In many posts, it boils down to addressing how to handle SHEAR.
It should also be required to be able to express yourself in unambiguous terms when one writes.
take a 1lb piece o’ foam semi shape it. Slap on the bottom side a jigsaw of balsa with 2oz fiberglass wetted RR (of course!). Vac. Attach my balsa rails. Do some other stuff. Repeat the slap it on step only with 3 or 4 oz. glass. Vac. Do a little more other stuff with wood working tools. Hand lam both sides 4oz or? Go surfing.
There are the mechanics. How does one improve the design?
Swaylocks readers being the exception, I don’t think very many people have a clue anyway.
Listen up at the local beach and hear the standard… “This is an epoxy board” with no mention of the blank density or in the case of a hollow, lack of. Don’t try to discuss skin-core material, inside/outside schedule, or if it even has a sandwich composite skin. It’s as if they’ve never even seen a Surftech advertisement.
“Epoxy board” is generally grounds to write the speaker off immediately but don’t make waves. Make someone happy - just smile and nod attentively.
Swaylocks readers being the exception, I don’t think very many people have a clue anyway.
Listen up at the local beach and hear the standard… “This is an epoxy board” with no mention of the blank density or in the case of a hollow, lack of. Don’t try to discuss skin-core material, inside/outside schedule, or if it even has a sandwich composite skin. It’s as if they’ve never even seen a Surftech advertisement.
“Epoxy board” is generally grounds to write the speaker off immediately but don’t make waves. Make someone happy - just smile and nod attentively.
Good post, John. We can’t expect to make studying composites a prerequisite for buying a surfboard.
The valid technologies will prove themselves in the water.
