Breaks in corecell

All I have is intuition Rob, and sometimes even it is wrong. But the bondline is solid epoxy and would have different character than the core itself. And if you were to impart point load on the join; it would break easier at the epoxy not the core one inch away. Well, actually between the epoxy and core, right?

Where I struggle with it is in application. If someone builds a curved hull with 10" wide panels as ondarvr said, it is going to have a ton of core bondlines and ties would be impossible.

But consider a vessel 30' wide by 100' long on the WL. If the bottom has a bondline that stretches for the full 30 or the full 100'; the vessel would have a weak spot there or potentially many when limited by panel sizes and running panels the same way... And so staggering and ties would make a lot of sense in vessels where panel limits resul in full panels in the hull. But most boats are longer than panels, so very interested in why this isn't in core processing guides.

But I could be wrong. I already have some hull panel joins near a bulkhead. I think I'll just add some glass there.

Maybe rx can shed a little more light on the practical application of these.

Especially if this would ever be done on a panel or hull section narrower than panel dimensions. Typically 4' say...

 

What RX is talking about and what your question is about are two different things.
RX is referring to a design that uses this to achieve certain goals.
You are talking about making panels with small pieces of foam. As pointed out it can be a cosmetic issue due to print through with light laminates not a structural problem.
If you want to minimize print through then glue the foam with say 4:1 spheres:silica bog. If the core is a neat fit then dont bother the resin will fill this.
The bond line is stronger than the core so in fact helps tie the two laminates together to a small degree, so improves things not weaken.
When not gluing, stapling with composite staples keeps it together ( or at least gives me piece of mind) that the core gaps will remain super thin while impregnating the top laminate, for cosmetic reasons only.

 

I kind of figured this was the case because the applications seemed for larger vessels.

I did not understand the bond lines would be stronger even in my small parts.

It seems like in a large vessel; one would need to take different approaches to panel joins.


Thanks

 

OK to butt join if the adhesive shear strength is stronger than the foam. Required is 6.7 to 10 MPa while foam core is about 1.0 to 1.2 MPa. There is a lot of adhesives like 3M, Araldite, ect but the most popular is to use epoxy resin filled with micro balloons. I don't have the specs of Core Bond but I know it is made up of ortho resin (low shear strength) and loaded with micro and uses a BPO paste as a catalyst. If shear strength is low, scarfing is needed.

If slitted foam core is infused, the gaps are filled with resin with resin/adhesive being stronger than the core itself. The resin also bridges the inner and outer skin acting as a shear tie.http://www.ericgreeneassociates.com/images/Core_Materials_and_Adhesives.pdf

There was an article in PB magazine that basically said "Most new boats are structurally compromised even before it hits the water". Take a look at the Shear and Moment diagram. Vertical shear is greatest at the point of support. When designing, the strength is calculated under the base of each bulkhead/frames but when lifting or supporting the boat, the support span is maybe 4 to 10X apart. This increases local loads tremendously and the initial calculation may not be enough.

 

Nothing I can find in the ISO standard for small watercrafts but it seems they have issued a safety notice. From ISO..

"The values shall be taken as 85 % of the mean value or this mean value minus two standard deviations,
whichever is lesser, but need not be taken less than the minimum value (i.e. the lowest value obtained for all
the tested samples).
D.1.3 Non-tested core material properties
Where the mechanical properties of sandwich cores have not been verified by tests, the respective properties
shall be taken from Table D.1.
Mechanical properties need not be taken at less than the manufacturers’ specified minimum values. However,
most cores can exhibit significant variations in density and this will directly affect the mechanical properties.
NOTE The connection (glueing, lamination, etc.) between core and skins for a foam density % 120 kg/m3 to
150 kg/m3 may not be straightforward and may require specific procedures."