R1DER said..

Bertie said..
What we are all forgetting is the blue Dow XPS has some Polypropylene incorporated into it somehow which is why we have so many delam issues. Its not just the outgassing but the PP. If you have ever tried to stick stuff to PP you would know its almost impossible.

What Cobra are using must be using is a true XPS manufactured for this purpose, not a foam dedicated for insulation purposes that board builders have stolen and tried to use.

I was dubious when i first saw that construction but i can't say I've heard of bulk failures (or any tbh), and its used more widely now across the JP range especially in the SUPS so it must be working.

When considering the stiffness of the veneer it acts like a layer of unidirectional since the fibres run pretty straight if the grain of the wood is true. Agreed its impact strength and ability to handle compression is key in its performance.
The thickness if the wood veneer has the effect of bulking up the outer laminate which we all know will increase compression strength and stiffness of the sandwich.
0.8mm of wood is worth about 600GSM of glass to get the same laminate thickness in a wetlay which would add considerable weight.

As Decrep says Stiffness is not related to density, it is actually related to the Young's Modulus Mike.

Thanks Alex always good to have your composite and engineering knowledge on here. Do they need a layer of glass between the wood and XPS or could they have taken that layer of glass and put it on the outside and added to the other outside layer?

Very good question Mike!
No, glass is not necessarily needed on the inside of the wood.

This is one of those situation where the theory says the strength will be the same regardless of the layup sequence (provided the laminate has the same total thickness), however the only way to know for sure is to make some samples and test them. Often this wont be the case.
They might have different failure modes (hence ultimate strength) but the stiffness before failure could all be the same in their regular work loading range. So many factors to think about.

Because the XPS foam won't absorb water (or much resin) the inside glass is not required to seal the blank off from water ingress.

However, my thoughts are that the sandwiching on the wood with glass helps the bond to the XPS. Without the glass the resin under vacuum doesn't spread (it gets pushed around like a bubble) so can get trapped and pool in some areas and have little to no resin in other areas. Think of the glass as a flow media like in infusion, that acts as a cheap warranty.
We also need to remember glues (resin in this case) have an ideal thickness to achieve optimum mechanical properties. So if the resin is a bit light on in an area especially between two surfaces that wont let it soak into them a delam is almost certain.

Remember some of the early wood starboards had the wood on the deck without a layer of glass on the top, just a coat of resin and deckgrip. I think this is because the wood went into the mold and all the resin in the lam then has the ability to disperse slightly and to prevent pooling.

I know in 1 job i was doing putting a layer of glass between a steel beam and a thick carbon laminate actually increased the load the beam could handle before failure. The glass being slightly more elastic than the carbon reduced the stress raisers and stopped the carbon from delaminating. I have a feeling the glass between the XPS and the wood acts in a similar way.

I hope my ramblings make sense, its hard to put my thoughts on paper.
I may be wrong. What does everyone else think?

Bertie said..

R1DER said..

Bertie said..
What we are all forgetting is the blue Dow XPS has some Polypropylene incorporated into it somehow which is why we have so many delam issues. Its not just the outgassing but the PP. If you have ever tried to stick stuff to PP you would know its almost impossible.

What Cobra are using must be using is a true XPS manufactured for this purpose, not a foam dedicated for insulation purposes that board builders have stolen and tried to use.

I was dubious when i first saw that construction but i can't say I've heard of bulk failures (or any tbh), and its used more widely now across the JP range especially in the SUPS so it must be working.

When considering the stiffness of the veneer it acts like a layer of unidirectional since the fibres run pretty straight if the grain of the wood is true. Agreed its impact strength and ability to handle compression is key in its performance.
The thickness if the wood veneer has the effect of bulking up the outer laminate which we all know will increase compression strength and stiffness of the sandwich.
0.8mm of wood is worth about 600GSM of glass to get the same laminate thickness in a wetlay which would add considerable weight.

As Decrep says Stiffness is not related to density, it is actually related to the Young's Modulus Mike.

Thanks Alex always good to have your composite and engineering knowledge on here. Do they need a layer of glass between the wood and XPS or could they have taken that layer of glass and put it on the outside and added to the other outside layer?

Very good question Mike!
No, glass is not necessarily needed on the inside of the wood.

This is one of those situation where the theory says the strength will be the same regardless of the layup sequence (provided the laminate has the same total thickness), however the only way to know for sure is to make some samples and test them. Often this wont be the case.
They might have different failure modes (hence ultimate strength) but the stiffness before failure could all be the same in their regular work loading range. So many factors to think about.

Because the XPS foam won't absorb water (or much resin) the inside glass is not required to seal the blank off from water ingress.

However, my thoughts are that the sandwiching on the wood with glass helps the bond to the XPS. Without the glass the resin under vacuum doesn't spread (it gets pushed around like a bubble) so can get trapped and pool in some areas and have little to no resin in other areas. Think of the glass as a flow media like in infusion, that acts as a cheap warranty.
We also need to remember glues (resin in this case) have an ideal thickness to achieve optimum mechanical properties. So if the resin is a bit light on in an area especially between two surfaces that wont let it soak into them a delam is almost certain.

Remember some of the early wood starboards had the wood on the deck without a layer of glass on the top, just a coat of resin and deckgrip. I think this is because the wood went into the mold and all the resin in the lam then has the ability to disperse slightly and to prevent pooling.

I know in 1 job i was doing putting a layer of glass between a steel beam and a thick carbon laminate actually increased the load the beam could handle before failure. The glass being slightly more elastic than the carbon reduced the stress raisers and stopped the carbon from delaminating. I have a feeling the glass between the XPS and the wood acts in a similar way.

I hope my ramblings make sense, its hard to put my thoughts on paper.
I may be wrong. What does everyone else think?

Your ramblings all make sense to me as well Alex. I like the idea that the inner glass provides a even thickness of enough glue to bond. I really want to try making a board like this with XPS and glass bamboo veneer glass. As the XPS won't bend around the rails I could just glue a 25mm thick block of xps around the outline and shape this as the rail, similar to how Bert Burger of Sunova has been doing his parabolic balsa rails for the last 20 or 25 or even 30 years.

decrepit said..

R1DER said..
>>>> I could just glue a 25mm thick block of xps around the outline and shape this as the rail, similar to how Bert Burger of Sunova has been doing his parabolic balsa rails for the last 20 or 25 or even 30 years.

Be very careful doing this, the first windsurfer Bert made using this method split at the rails, there wasn't enough glass wrapping around holding it together. Bert hadn't allowed for the extra flexing load on a windsurfer, and it split the seam.

Mike I don't understand split at the seam, as in what part of the rail is the seam?

decrepit said..
The junction between bottom and rail at the tuck. There's usually no overlap with the internal cloth, and often no overlap with the top layer of outside cloth, leaving only the bottom layer at that junction. With Burt's construction. I think the deck and bottom sandwich go on before the balsa rail, so I think there were two spots on the rail without complete overlaps. The curved balsa rail, forming a beam, was stiffer than the rest of the board. My guess is there wasn't enough cloth tying the rail to the body, so it split open where the rail joined the board.

It should work if you stick the rail section on first, and do full overlaps with the external cloth. But I'd be much happier if you could figure a way to overlap the internals as well.

Ah ha thanks

Is it worth the effort to spray paint as it's just an old board?

decrepit said..
depends how well it works with the foil in.

I find I'm over pretty. The final coat of paint on mine shows up all the places where there should have been more bog. But they aren't in the business area, so I can't be bothered fixing them.

I'm more performance over looks too, but a lot of people want their repair to their $3000 board to look pretty and new, which is fair enough. I might hit it with a quick undercoat and leave it at that.

^^ which is why I do

Yay Done

decrepit said..
Good one Mike, so now to try it out.

Yep fingers crossed.

Yep, lots of reef out of the water here. Especially just in front of waves.

decrepit said..
Yep, lots of reef out of the water here. Especially just in front of waves.

Ditto with the .48m tide here.

R1DER said..
Stuffed some hand paper towel in the square hole overnight to try and wick the water out, paper was quite damp this morning.
Cut a foam block to size and bought some glue Mark Australia recommended in another post to give a try. Normally I'd mix resin and qcell to glue the block in.

I was under the impression that hard-edges are bad ?.... a qcell/resin mix would create a nice vertical edge which would stress the surrounding foam.

mathew said..

R1DER said..
Stuffed some hand paper towel in the square hole overnight to try and wick the water out, paper was quite damp this morning.
Cut a foam block to size and bought some glue Mark Australia recommended in another post to give a try. Normally I'd mix resin and qcell to glue the block in.

I was under the impression that hard-edges are bad ?.... a qcell/resin mix would create a nice vertical edge which would stress the surrounding foam.

They probably are dependent on where they are. This is such a small hole that let water in and it's not a high stress area, sometimes I fade in as decrepit says but this one I'm doing a bit different.

Cut pvc square.
Marked pvc square on board.
Routed down to bottom layer of glass so new glasses overlaps.

All dry.