I've got a JP "single thruster" board to repair.

It's got a crack 2/3 across the bottom through the rail and ends just short of the front foot pad.
The board still feels solid, but I didn't want to exert too much force in case I increased the damage.
I started sanding away the damaged area on the bottom with a small right angle grinder, expecting to right through the sandwich.
But to my surprise the crack disappeared after only about 1mm of sandwich foam removal.
There's no way I can imagine ordinary PVC doing this, a crack this bad would normally go right through the sandwich, normal PVC foam is brittle.
I've used airex on a few boards in the past and it's very tough and pliable, you can bend it round the tightest rail without preheating or tapping the edge.

www.3accorematerials.com/en/markets-and-products/airex-foam/airex-r82-resistant-dielectric-foam

So is this airex foam, and it's absorbed whatever force has cracked the fibers?

If so I'll be confident I don't have to dig any deeper to do a good repair.

So anybody out there got any clues???

I have never seen a crack into the sandwich layer be only partial thickness. even with Corecell that has exceptional elongation at break (better than PVC so I use it if able) and even if that breaks it's complete. very strange happenings

when do we see your new board?

Maybe I'll dig down a bit and see what the underneath fiber looks like just to be sure.
But there's no crease, and there's no give around the crack, which you would expect if the sandwich has failed completely.
I suspect that Airex is better than corecell, you can tie a narrow slice in knots without it breaking

I'll try and get some pics later

The "flexible" airex is brownish R63 linear pvc, it's tough easier to work but cost a lot more than airex (herex) C70 cross link pvc. For custom sailboard in Europe, most used 5mm c70 for bottom and 3mm R82 for top and rails in 80 to 100 kg/m3 range.

Yesterday, I cut a slice across where the crack had been, and it came out in 1 piece. The crack only penetrated the foam about 20% of the way through.
This is incredibly surprising to say the least!

I think it isn't Airex R82 wich is more brown, look like corecell yellow color ?

That looks pretty much like my Airex.
Brilliant stuff.
When I was real flush I built my complete boards from it.
Now I only use it for the front 300mm and back 300mm, the sections which are most difficult to build/shape etc. Corecell for the remainder.

the colour is deceptive, it's covered in sanding dust so it's much more grey.
There seems to be two layers, the underneath layer is brighter and a bit more orange, (could be corecell)
So you need to look at that small spot on the side, that isn't covered by dust.
If it's not airex, it's equally tough!
That crack went 3/4 of the way across the board, to only go into the foam a mm is amazing

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decrepit said..
I've got a JP "single thruster" board to repair.

It's got a crack 2/3 across the bottom through the rail and ends just short of the front foot pad.
The board still feels solid, but I didn't want to exert too much force in case I increased the damage.
I started sanding away the damaged area on the bottom with a small right angle grinder, expecting to right through the sandwich.
But to my surprise the crack disappeared after only about 1mm of sandwich foam removal.
There's no way I can imagine ordinary PVC doing this, a crack this bad would normally go right through the sandwich, normal PVC foam is brittle.
I've used airex on a few boards in the past and it's very tough and pliable, you can bend it round the tightest rail without preheating or tapping the edge.

www.3accorematerials.com/en/markets-and-products/airex-foam/airex-r82-resistant-dielectric-foam

So is this airex foam, and it's absorbed whatever force has cracked the fibers?

If so I'll be confident I don't have to dig any deeper to do a good repair.

So anybody out there got any clues???

If I understand correctly we are talking about a compression crack. Clearly the fibre matrix has less compression ability than any PVC, when it comes to elongation they are quite similar in max length before breakage depends much on the fibre though. But if compression than you it is likely that inner fiber layer is affected as well.

Isn't it the bottom skin that buckle under compression so the fiber outside buckle and break but the inside layer localy take tensil and resist ending propagation of stress in sandwich foam ? Same way sometimes single skin of surfboards buckle, fold but don't break.

there's no sign of buckling, no crease at all. looks like the fibers have snapped under tension.
The crack is directly under the front foot.
Buckling normally appears between the feet.
My theory is it was a hard nose first landing, this would stretch the bottom and compress the top, but the top is only a small area from the rail to the foot pad, and doesn't look compressed either.

Ok. I supposed the deck rails crack first under compression, so tensil strengh increase in the bottom skin while it bend up to break, first the outside layer because its farthest and more stretch.
You need to take care to the rail and deck repair.
As mechanical engineer i had to work a lot on thin skins bending parts, buckling instability always the initiatior of break.

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lemat said..
Ok. I supposed the deck rails crack first under compression, so tensil strengh increase in the bottom skin while it bend up to break, first the outside layer because its farthest and more stretch.
You need to take care to the rail and deck repair.
As mechanical engineer i had to work a lot on thin skins bending parts, buckling instability always the initiatior of break.

Good assumption but it doesn't look like it. Again on the deck there's no evidence of buckling, and it's still feels very solid either side of the crack. It is a carbon/kevlar deck, so it could spring back into shape, but I'm sure it would no longer feel solid if there was internal damage.
The rail is a mess and the damage goes through to the core foam, but the underneath deck sandwich looks OK on the bottom.

This is all very weird. I've repaired a few heavily impacted boards in my time and never seen anything like this.

Thanks everybody for your help, but it's all a bit late now, it's coming to the bogging- fairing stage of the repair.

buckling made the deck lam (just the CK) crack jussssst, and in doing so it pushed down into the airex, like a vee. Springback then made the cracking CK pull in tension across the upper surface of the airex, thus initiating a tear in the airex, but only surface.


Hows that for a theory?


I have one at work with a deck delam (routed out) if you want to look (but you have to take it)

Thanks Mark, I'm reasonably confident my repair will hold, I don't think it's a big issue if it doesn't

Oh I'm just trying to get rid of one lol